The AM Forum

Here's one from the outfield. My dipole 'works' on 80M, but not so well because the 4:1 balun gets very hot @ 300W carrier, as does a 1:1. It's tunable but very fussy. 40M is great! The MFJ does not show resonance there, or on 80M, but at least on 40 I can tune it and use it.

A gentleman at the local ham store got me to thinking that I have messed up the matching on my antenna. It is a dipole with 65FT  long wires, and it came on spools, with each half of the wire, and the center piece of ladder line feeder, would up, pre-cut. It was touted to be for 10-80M. (actually a 4 legged dipole made totally of 500-600 ohm ladder line, on 5 spools, but this is overly weird and not relevant to why it does not work and the fact of my cutting the feed section which acts as a transformer (I am told) and questions about that messing up the impedance and causing tuning difficulties on one band)

I changed the length of the ladder line feed, and also went from a 2" spacing one to a 1" one.

Assuming the antenna has a certain impedance at the center of the dipole which is for the sake of the argument 100 Ohms, and the 'stock, included' ladder line brings this down to the radios in some manner that the 100 Ohms also appeared at the radios, and I have changed this length, then could I have accidentally changed the matching to some difficult situation?

There was an article about someone putting lengths of ladder line up and using switches to bypass them until the system was tuned, or tunable.

I think you might want to go back to the original setup.
Was this a system where a tuner is not needed? Certain length wire for antenna, ladder line spacing and length determined resonance?
Fred

If the two sides of the dipole are each (roughly) 65ft long, then the impedance looking into the feed point of the dipole will be around 70 ohms on 80m. The impedance looking into the feed point of the dipole will be very high on 40m. The ladder line will transform the impedance... so the impedance looking into the ladder line at the location of the transmitter will be different from the impedance looking into the feed point of the dipole. The length of the ladder line (in wavelengths, at the frequency of operation), and the nominal impedance of the ladder line (e.g. 450 ohms), will determine this transformation. The impedance looking into the ladder line at the location of the transmitter could be higher or lower than the impedance of the dipole at the feed point.

In this case, by substituting a ladder line of a different type (possibly a different nominal impedance), and using a ladder line of a different length than the design called for... you should expect the impedance looking into the ladder line, at each frequency of operation, to be significantly different than it would be in the original configuration.

The balun (whether 1:1 or 4:1) will saturate (and get hot) if it is looking into an impedance that is much different than the impedance it was designed for.... particularly if the balun is an economy design (not enough core material).

At any given frequency of operation (e.g. 80m) you may be able to significantly improve the situation by adding additional ladder line.

Stu

Assuming you used bare 14AWG wire and 1 inch spacing, the Open Wire Line has an impedance of 329 Ohms.  2 inch spacing has an Impedance of 412 Ohms.
http://www.smrcc.org.uk/tools/OpenWire.htm

300W carrier x 4 = 1200w power.  
Few if any commercial 4:1 current BalUns are truly rated for this power.  Most that advertise as '1500w" have caveats like 'Typical SSB use, 25% duty" (I just bought these from Radio Works).  A more realistic rating would be perhaps 400w-500w at moderate impedance/swr.

You may have overheated the ferrites causing permanent changes to the permeability.  High impedance mismatches would make the situation even worse.

I have a question...

WHY would any one put a balun at the feed point of a center fed Zepp Antenna???

As AB2EZ states, the transmitter end of the open wire feeders is different for different band operation, etc, so what is called for is a flexible tuner in the shack, something that can be capable of either parallel or series tuning to cover the full range of impedances you might encounter...

My formula for this antenna is:  Cut the radiator portion of the  antenna to the length required between the supporting infrastructure,  split it in the center,  attach open wire feeders (whatever be the spacing between the two wires),  cut open wire to the length required from radiator center to the tuner location.  

Feed tuner with coax from Tx output to tuner link and tune.   It helps to use an antenna analyzer like the MFJ or one of the many available so that you do not tune the system on the air...  Pre-tune with analyzer then load rig to system...  

Most transmitters these days use a pie network in the output for matching to the antenna system so it really doesn't matter if you use 50 or 70 ohm coax to the tuner.  One other refinement in the feed from Tx to tuner is taps on the tuners link, especially if the link is fixed.  If you are running something with a variable link on the final tank, be careful in the loading process as some OT's forgot about the HV in the final area and cancelled their OT status  ::)...

Just my two cents worth from 70+ years in ham radio.  Your mileage may vary...






 

-I assumed he has the Balun at the coax/OWL junction.

It's hard to diagnose the problems without more specific info e.g.

-Operating goal? Assumed to be 80m-10m...?
-Length given as 130ft
-Height: center, ends?
-OWL length and AWG? (1" and 2" width's given as available)
-Coax type and Length?
-Tuner type/brand/model?

Cecil, W5DXP has some excellent antenna examples and discussion of transmission lines.
http://www.w5dxp.com/

Tom, W8JI has much info as well.
http://www.w8ji.com/antennas.htm

Here is some clarification:

It is the original setup, the 4 legged dipole. It has a 2:1 SWR on 40M, but never has done 80M right. I can't run more than a 50-100W carrier long-winded on 80M because the balun heats up. A drawing is attached that should answer a lot of questions and maybe point point what is wrong.

I didn't want to get into the whole 4-legged dipole discussion, because the original antenna, with the original length of feed line, would have worked as specified, and the only thing changed is the feed line.

Digression: About the 4-legged dipole, if the legs are approximately 65 feet, then would this be around 35 Ohms, since it is more like two identical dipoles in parallel?

There seems to be confusion about the thing. Here is no Zepp antenna presently, just something like a fan dipole except the fan is flat, not vertical.


The balun returned to normal and works right, after this abuse. I even tested it with a 200 Ohm load and the MFJ analyzer, and it shows 50 Ohms. (4:1 baun). It is a transformer type current balun. a 2.5KW "COMTEK Jerry Sevick W2FMI Series Current Balun"

I previously thought this antenna's open wire line was 2", but it is not, it is 1" wide I apologize for that.

The antenna as designed has a 1" wide open wire line from its center to the transmitter. This piece of wire is, was, some unknown FT long. I did not measure it. At the time I had no experience with balanced line for transmitting, only long wires and ground plane verticals.

I had to cut the original ladder line to get it into the building. At that point I continued with some insulated "450 ohm" 7/8" 18 gauge solid wire ham-style ladder line.


What I did originally, modifying the feed of the antenna, and what exists now, is so:

1.) The original 1" line comes down from the 55Ft high dipole center for 20 FT and from there slopes down to the building.

2.) There is about 60FT FT of this original wire.

3.) It goes into a switch box with a large 3PDT knife for grounding, and the 7/8" plastic ladder line comes out.

4.) The leads from the 450 Ohm ladder line go through ceramic tubes, spaced about 5" apart, into the building.

5.) The whole "5-inch" section is only 1-2 FT long.

6.) There just inside the building it is connected directly to more 7/8" ladder line and goes 20FT to the balun over the transmitter.

so that is the exact setup.

about Z, and the 1mm open wire spaced 1":

http://www.smrcc.org.uk/tools/OpenWire.htm says 470 Ohms

http://www.emclab.cei.uec.ac.jp/xiao/Wire/index.html says 471 Ohms

Have you tried the system as installed without the balun in line?  From what I see in your diagram, the tuner should be able to match the system without the balum in there if the tuners range is sufficient... Get rid of that coax out of the tuner as well when removing the balun...

The two wire legs of the dipole  are simply to increase the band width between 2/1 VSWR points and can either be strung horizontal or vertical, makes no difference in this configuration...

Do you have a schematic of that Murch 2000 box?   Seams I recall those tuners have a balun built in.   

As this was a pre-fabed antenna, what did the instructions (if any) say about usage?   

IIRC the Murch is an exact copy of The Ultimate Transmatch from a McCoy QST article from 19 and 73 or 74. It was a high pass T network without a BalUn.

85' total feedline on 80 meters will be close enough with 65' each side on the doublet to present a relatively low resistive portion of the load impedance at the shack. The 4:1 is the wrong choice which is probably why you already used the 1:1.

The Balun Designs 1:1 does work well at 50 ohms J0 at QRO.

Maybe try a Balun using donuts over coax.

An antenna current meter placed in series with one feeder at the shack should provide some indication of improvement. Use a known steady carrier for a benchmark measurement. Make a change to the system and remeasure.

The tuner has its on internal 1:4 step up balun. I have tried with that but it did not make any difference.
This is to say that the tuner will match the antenna+feedline with either its balun or the external 1:4 one. The only difference is the one inside the tuner is smaller and gets hot faster. Either way it tunes up, and when the balun starts to heat, the tuning goes off and has to be touched up. Eventually there is an odor of plastic. haha. So I do not do this.

I did not try it connected to one of the unbalanced SO-239 jacks, as that would not have been right at all.

The 2000A and B manuals is attached as one. I forget which I am using..

The instructions didn't say much of anything about the antenna, except basic, the manual is attached. I'll review it now also. I do not have a 9:1 balun for 450 to 50 Ohms.

On page 4 it says not to put it over metal such as roofs.. OK well it's ends on one side are 10-20 FT above a metal roof. I have no control over this.  It also says the ends should all be the same height. This was not quite possible, but they are within a few feet of each other.. There is no radial system. the tower guys are steel cable, but not close to the dipole elements.

here's a blurb vis the internet archive, from the badly OCR'd text of '73', September 1963. There's another from 1968.
So that's when the antenna was popularly sold. Amazing to have found two NOS in the box. I gave one to my tower man.

LOOKING FOR
THE WEAK ONES?

The TEKRAD Mark V antenna has an enormous capture area compared to quads and yagi antennas . , , really pulls in the weak ones, and you work what you hear!
Missing DX off the side of the beam? The Mark V is omnidirectional so you know where the band is open at any time.

Windstorms worry you? The Mark V is impervious to wind and wiU stay up while the quad is going through the neighbor's roof.

Like to work all bands? Sure you do, and the Mark V will instantly put you on any band from 80M to lOM, hori-zontally polarized, with high efficiency and low VSWR. Make your transceiver
act like a 2KWPEP rig, loudest on the

The Mark V antenna comes to you complete, ready to use, cut to length with 70 feet of feedline for only $32.50 postpaid. All you have to furnish is a mast or tower about 50-60' tail.

Order foday while ffte bands are ftof*
W4JM/Ratf W4SIW/Bil[

TEKRAD, IHC
RO. BOX 57 VALPARAISO, FLORIDA 32580

Sounds like the usual antenna snake oil ...

Grant NQ5T

Can someone educate me on the effects of the 4 wires on the antenna side of the system?
I'll call it a fan dipole with all 4 wires of equal lengths.
Is this the magical part of the antenna?
Fred

1. We know the 'capture area' claim is not supported scientifically.  Anyone who has ever built a dipole from magnet wire knows that.

2.  My meager attempt with EZNEC shows that a 'double vee' dipole, level at 55ft, with 45degr angle between elements is omnidirectional with a high angle. 

Best thing to do here is use a real balanced open wire tuner like a Johnson matchbox or build one, they are simple to make.  The total system efficiency will be much better, no balun cores to heat up.  Baluns do not work well trying to couple reactive loads like those presented by this antenna system.

There is no magic with the fan dipole , it will be a bit more broadbanded due to the increase in effective diameter of the "fanned" dipole elements but the pattern and gain are that of a regular dipole.

I'm clueless, but from where I sit, IF you have 65ft on either side, the solution is far easier than all this going in and out of stuff to match it.

What I'd do is to extend or cut the 65' dipole to resonate on the part of the band you want, then feed it with co-ass. Add in an additional 40m (tuned for resonance) section (new wires) and you have nothing else to do.

You could keep the Murch in bypass mode except when you move too far up or down the band and the SWR gets above 1.5:1 or so.

I suspect that the dual elephants, I mean elements, will add bandwidth, and certaintly will if you stagger tune them, one a little higher and one a little lower than the other...

I know that OWL is very popular here, and unless yr running major powah and you don't have large diameter (high-powah) co-ass, it's probably not necessary.

YMMV.

                  _-_-bear

I agree with Chuck, matter of fact if you read my first response (#4 from the top!) and absorb what I stated, I think I said basically the same thing...

That Murch box is okay for a coax feed system or long wire antenna BUT THAT'S ALL...

If you are stuck with that antenna as installed and have no other recourse, do as Chuck says, get a Match Box...  No real need for the big KW, a 250 version is okay if not running high level class C plate modulation in your final, IE, low level modulated exciter to linear amplifier .  The variable caps in the 250 M.B. are 3KV spacing !

If, per chance, the box don't have enough range  (they are a bit limited) you can use a bit of coil stock as a balanced  auto transformer (high Z or low Z feed to the coil from the box and reverse to the open wire feeder, use open wire to feed the xformer, very complicated  8))...

Oh yes, regarding the fanned elements.  In the horizontal  mode it gives you  a semi omni pattern if spread 40 degrees or so... If the dipoles are installed over/under style you retain the topical dipole patterns on the various bands...

I've said enough, I rest my case... ::)   ;D

Nothing magical about the fan. It may yield more bandwidth on some bands. But if you are using open-wire and a tuner, I'd don't see the point.

If you are going to bother with a fan design, cut the wires for some other band. Then feed the thing with coax and skip the tuner and balun mess. Cut one set of legs for 40 meters. Now you can use the thing on 80, 40 and 15 meters with NO tuner or balun.

Any horizontal dipole at 55 feet will be a high angle, essentially omnidirectional radiator on 80 meters.

Using ONE antenna, I agree with using the balanced open wire / tuner  OR multiple dipoles fed with the same coax.  They all have simplicity in common.  Forget the baluns and gimmicks.


Another simple method, my favourite, is independent dipoles.  Put up a dedicated dipole for 75M fed with coax - and a dedicated 40M dipole fed with coax.  Nothing easier than this and it will work well.  You also get to control your vertical angle and favored direction. For local work, a 40M dipole at 35' will always be better than a combo-dipole at 60'.   And vice-versa, you want that 75M dipole at 60' for local work. "Local" being 0-400 miles on 75M.  (The 40M dipole will cover 15M too)


A more advanced system would be to put up a pair of simple coax-fed phased 75M dipoles at 60', spaced 60' apart. Phase them + - ~100 degrees or in-phase to have  NE, SW directions - or local high angle coverage. Nothing better than that. Do the same thang on 40M and you will have it all.   I'd rather have two bands with super antennas than 6 bands with mediocre coverage.  It's more fun when we focus our energies.

More:

1) For longevity, always support the center of a dipole and keep the legs loose for wind.

2) For best pattern, try to keep the dipole flat and straight, textbook looking.  Feedline run at right angles as far as practical.

3) If a choice between an inverted Vee and  a dipole with the ends higher than the center, always choose the higher ends. This will reduce  feedline to dipole leg coupling and interaction.  Level is always best, however.

T

After a LOT of experiments over the years, for what I do, I find a dipole for 80 and one for 40 to work well and be trouble free, and they take plenty of power and nothing gets hot.
No tuner, no balun, no rfi in the shack, water and snow does not change anything.

New rope in the tree's every 5 to 7 years is all it takes.

Yep. Use steel cable and you won't even to need to replace any rope.

Coax is for girls.

I'm not sure any of it has to do with the thing being 4-legged or with the Murch tuner. Same thing went on with the MFJ. It does match, but there is apparently lot of inefficiency in the balun causing the heating, whether it is the Murch's internal balun or the external one 1 FT away. The marketing babble does not matter either, only the instructions which were posted. The only place in the instructions not really followed was the ground radials/plane was not put down, because I do not have the material.

Going back to the earlier portion of this, there is a reason why the balun is unhappy, and it must have to do with the match being at some extreme of the thing's ability.

Why don't I use the MFJ analyzer and sweep that antenna, right there at the end of the line behind the racks, and see if that yields some interesting information. My only gripe with the MFJ is that it has a limited range of impedances it can display. This can be overcome one way or another, as it was found that in general the 4:1 balun can be used to extend the MFJ's range, that is, when the MFJ says 100 Ohms, the actual load is 400. etc. or close within reason.

The system has never been swept so now would be a good time to find the R and X of it which might make it clearer why the problem is happening. Perhaps this weekend, Saturday or Sunday, it can be done.

In my somewhat uneducated mind regarding theory, I would shudder at the thought of using back-to-back baluns in an antenna system.
Murch balun....then the balun outside, etc etc.

I am not familiar with that tuner...Does it have  balanced line terminals?
Is this your tuner? ut2000A?

http://www.k7jrl.com/pub/manuals/murch/ut-2000a/UT2000A%20schematic.jpg
Look like this??

http://www.eham.net/reviews/detail/1531

Fred

It's a horizontal antenna. Why would radials be needed?

The instructions say so. Is that wrong?

No, it looks different, has a meter on the front and some HV terminals on the back. I think it is the B model.

http://forums.qrz.com/showthread.php?332952-MURCH-Electronics-UT-2000B-Transmatch-Antenna-Tuner

OK it has balanced line terminals for OWL.

My installation for my tuner and OWL was about 15 feet from the shack. Built a shelf on the basement wall and ran the OWL to a HUGE knife switch to select my Dentron tuner or the famous K1JJ tuner.
And from either tuner I can connect the 50 ohm coax to my radios. It may not be convenient to have the tuner at the operating position, but it was a necessary.
Running OWL between floor joists and into the shack with more wires and fluorescent lighting was a disaster. So, that's why the tuner is mounted on the wall and coax used.
150 feet of 6 inch spaced OWL going to a flat top dipole 240 feet long and 60 feet high.
Fred

Wrong is in the eye of the beholder. Adding a radial or radials will do little to improve the signal. So I would see it as wasted effort at the very least. The antenna is horizontal and balanced. Radials are not needed and won't improve the signal.

Here are a couple of pictures.

Fred

The MFJ is normalized for 50 ohm systems. It has limited use outside of that range as stated.

One can make an estimate of impedance seen at the tuner's output by looking at length of the doublet and feeders.

The weak point had been identified as the BalUn. Why not concentrate on that?

Once again, the 4:1 Balun is the wrong choice for your system on 80 meters and a 1:1 would be better as you already have tried. If the 1:1 still shows heating consider switching to a more robust 1:1 BalUn. The ferrite donut over cable ones should work fb or even a coaxial choke BalUn. Try that if you have some spare RG-8 spec cable. the drawbacks of the coaxial choke BalUn are size on 3.7 mhz and lack of good performance on all of H.F. Would make for a good way to prove your system without spending any more money if the cable is already available.

The same folks who wrote the advertising copy must have written the instructions too.  
Radials might help cool off R.F. in the shack when the antenna and feedline are improperly installed. The market is amateur radio after all.

Sorry
rushing always looks bad!!

Same knife switch here. I thought you said it was huge!

I have the same fan..... I use it to push the wood heat around the basement. Weather it wants to or not.


klc

The fan was used to push out fumes from painting the floor.
And the knife switch looked pretty big to me
Fred

I use the same kind of switch for the antenna, but there is a much bigger one on the wall.

Attached is the measurement. The resonances on 40M and others are off frequency. Unknown why, nothing's been modified from the original, but 40M has always worked, ans so has 20M.

This said, I can tune 7160 just fine with the 4:1 balun.

The fed line was re-measured, turns out it is 89 feet.

I looked at your results with the MFJ.  Doesn't look too good to me.  I see a near zero resistance, very high reactance and sky high SWR at the frequencies in the bands.  What is the point of this antenna??  Isn't there any other type antenna that you could set up??

The MFJ is probably seeing the parameters at the balun more so than what is really happening with the antenna itself.

Fred

The near-zero resistance bothers me too. And it is near-zero, because the MFJ has a 1-ohm resolution and no more.

How can it be the balun, if the resistance goes up to 75 Ohms at the resonant points such as @5Mhz?

The antenna was supposed to be just a simple antenna, a fan dipole. There was no evidence of it being tampered with, and nothing to suggest it was weird in any way.

There is a lot of this that makes no sense, like why it works nicely on 7160 and 14Mhz with very little balun heating.

I can go look at this again, and float the MFJ, attaching the ladder to its hot and chassis, unbalanced connector.

Try some other tests like disconnecting the ladder from the balun secondary.
If the balun is rated for 160-10M, and there is no load at all on the secondary, what should R read?

Isn't it like an audio transformer with a DCR and an inductance, so that it just transforms impedance?

An ohm meter will say zero, because it is a winding.
It might be a question for the MFJ manual.

Months ago, the wind blew the center insulator around and made a short but some jiggling on the rope, which has become slack, fixed that. I can see up there with binoculars.

I can not put up another antenna (dipole) myself, because I will not climb the tower.
I do not have the safety equipment to do it and have never done it, so I consider it too big a safety risk as a beginner to do it alone even if I had the safety gear.

One question I have is: will changing the length of the ladder feed change the resonant frequency of the antenna system?

I tend to say no, and say that it will change the impedance (because of transformation or length in degrees) seen by the transmitter (tuner), but don't see how it would move the resonances one way or the other.

If there was a short at the center insulator, I think your first step should be lowering the antenna to the ground for a thorough inspection regarding its mechanical and electrical integrity.

I'm no expert on this topic, so take what I say with a grain of salt:

   At resonance, and antenna has a resistive feedpoint impedance. That impedance could be very low to very high, where neither feedpoint impedance matches the transmission line impedance. Case in point, a 80 meter center fed dipole fed with coax on 40m...The coax at 50/75 ohm will not match the 1/2 wave elements that will be high impedance. The feedline SWR will be > 10:1, and will appear to vary a lot by varying the coax length; a 1/4 wavelength of coax (counting the VF) might appear to match the antenna, but the losses in DB will be huge. So varying the feedline might appear to move the resonance of the antenna when what is is doing is transforming the complex impedance at the transmitter. If the coax shield was radiating due to common mode current, then yes changing the length should move the resonance(s).


  Here are some experts from an old thread. This was back when AMFONE had a full quorum of heavyweights, such as Walt W2DU:

http://amfone.net/Amforum/index.php?topic=21940.0

I think posts 1 and 2 are the most interesting, whereas many of the other posts that follow take exception to post #1.

 For what it is worth, I use an Array Solutions OCF Dipole:

http://arraysolutions.com/Products/ocf_dipole.htm#top%20of%20page

They have a 2KW and 5KW CCS rated product FS.

This simple antenna used with a Heathkit 2060A antenna tuner gives me 80-6 meter coverage. To keep losses down, I use LMR-400 coax. This antenna does suffer from what Tom K1JJ describes in post #2 at the above link; deep nulls on the higher bands. On 20-10M it works like a house afire inline with the antenna with 1 or more nulls broadside. This is most evident on 20M. Ironically an OCF 80 antenna is not resonant  on 15m, and yet that is one of my best bands where I can get out very well with low power.

Jim
Wd5JKO

The feedline is electrically part of the antenna system but hopefully does not radiate. Changing the length will change the impedance whenever the line is not flat. Flat meaning the load impedance at the antenna end of the line is equal to the line's surge impedance.

Measuring impedance in the shack includes the line's transformation. Changing the line changes the system impedance.

That chart will not be very accurate when departing greatly from a 50 ohm non-reactive load with the instrument being used.

Good SWR does not equal maximum power transfer. If the MFJ is reading near 50 ohms with a 4:1 BalUn might play but it is the wrong choice. It will divide that 50 ohms by 4 and present a difficult load to a high pass T tuner. Those like higher resistive impedances.

This comment is not intended to contradict the other comments that were posted regarding the effect of the feedline on the "resonant frequency of the antenna system". It is intended only to add to those comments.

A resonant frequency of the antenna system (i.e. including the antenna and the feedline) could be interpreted to mean: a frequency at which the impedance looking into the feedline... from the transmitter end of the feedline... is purely resistive (no reactive part).

If that is the intended meaning of "a resonant frequency of the antenna system"... then for any given antenna (not including the feedline), and for any given frequency, there will be a length of feedline (less than 1/2 wavelength long at the frequency in question) that results in a purely resistive impedance looking into the feedline at that frequency.

The value of this purely resistive impedance is, in general, not equal to the characteristic impedance of the feedline, or 50 ohms, or any other such value. It could be a very low value (like 0 ohms in the case of a feedline that is connected to an open circuit or a short circuit, or a purely reactive load). It could be a value that is much higher than the characteristic impedance of the feedline (like infinity, in the case of a feedline that is connected to an open circuit or a short circuit, or a purely reactive load).

The advantage, if any, of choosing the length of the feedline to result in a purely resistive input impedance when connected to a particular antenna, and measured at a particular frequency... is that a purely resistive impedance can be converted to a different purely resistive impedance with a properly designed transformer (e.g. a balun, if you also want to convert from a balanced circuit to an unbalanced circuit).

Stu

I think the bottom line is as we change frequency, the antenna impedance itself varies all over the place. We need a matching device that can also vary.   A fixed 4:1 balun does not vary and will be the wrong value most of the time.

So we can either limit our frequency excursions (use a single band coax fed dipole) - or use a matching device that also varies. (antenna tuner with open line feeders)

The other gimmicks (traps, matching baluns, stubs, relays, etc.) have disadvantages resulting in losses, complexity, inconvenience and other problems for little gain.

Go with simple, proven, efficient methods;  a single band coax fed dipole, fan dipoles fed with one coax  -  or open wire with a good link coupled tuner.

T

Opcom

You wrote:

"The near-zero resistance bothers me too. And it is near-zero, because the MFJ has a 1-ohm resolution and no more.

How can it be the balun, if the resistance goes up to 75 Ohms at the resonant points such as @5Mhz?

Isn't it like an audio transformer with a DCR and an inductance, so that it just transforms impedance?"


Actually, the simplest (still approximate) equivalent circuit model of a real balun would be as follows

1. Start with an ideal transformer

2. Add an inductor across the input side. This corresponds to the "magnetizing inductance" of the transformer.

3. Add a capacitor across the input side. This corresponds to the capacitance between the turns of the windings, + other stray capacitance.

4. Add a series inductor to one of the two wires that feed the input winding. This corresponds to the "leakage inductance" of the transformer.

It should be easy to see that even with nothing connected to the output winding, the impedance looking into the balun would be purely reactive (neglecting winding resistance and core saturation effects). As an aside... the combination of: the equivalent circuit capacitance across the input winding, and the leakage inductance, will form a series tuned circuit at some frequency (perhaps not a frequency of interest)... that would look like (neglecting winding resistance and core saturation effects) 0 + j0 ohms from the perspective of the signal source feeding the input winding.

Likewise, with a mostly reactive load across the output side of the balun, the impedance looking into the input side of the balun would be mostly reactive... unless the reactance of the load was being fortuitously cancelled by the reactances associated with the balun.

Therefore, if ... over some range of frequencies... the transmission line were transforming the (high) input impedance of the antenna into a lower, mostly reactive impedance (i.e. with moderate amounts of reactance and very low resistance)... you would see the measurement results you posted when measuring R, X, and SWR in that range of frequencies.

A more complete equivalent circuit would be more complex than the above, and would also include the nonlinear effects of the core material. I.e. high fields in the core material will cause the core material to saturate... and this will result in the core getting very hot. When the balun is looking into an impedance that it was not designed to look into... core saturation can result at input power levels that at much lower than the specified power rating of the balun. This is why many types of baluns and other components containing high permeability magnetic materials (e.g. isolators) have their acceptable power levels specified in the context of what the devices are connected to.

Stu

Core saturation is a good point. That is possibly what's happening because I am always using 200W or more due to the nature of the TX.

The same 1:1 current balun used for the plot is the same 2.5KW size as the 4:1. It looks like there was no reason to use a 4:1.

It also looks like there will be no real need to move the tuner to the shack wall in order to minimize the amount of ladder line in the building because it does not radiate much and 89 Ft is one of the "good" lengths for 80M when using a dipole which is a 1/2 wave total on 80M, or close to it.

Maybe it is a good time to continue the balanced tuner project. A 1:1 current balun goes before the tuner and therefore ought not be involved in ugly stuff on the output end.
A link coupled tuner may be possible, depends on the parts lying around.

Either that or pull the antenna down and replace it with something else. That's not a good option right now either.

I just ordered "Reflections III" from the CQ book store. It supposedly has every page from I and II, plus some more. It's clear I have to study a lot more on this so that all of the comments make sense.

looking here,
http://www.scribd.com/doc/104793934/Reflections-III-Transmission-Lines-Antennas-Ham-Radio-Arrl
why is this a free download? Is this a scam or a pirated copy?
I am not logging into download anything, no way, but it bothers me as the book is too new to be free.

Opcom
et. al.

Reviewing one of your earlier posts (quoted below), and rereading your original post:

You mentioned (in your original post), that the whole antenna (not just the feedline) is constructed of ladder line. Therefore, each of the four sections of the fan is made of ladder line.

There is nothing fundamentally problematic about that, provided that: either the two wires in each of the four antenna leg sections of ladder line are shorted together at both ends, or the two wires in each of the four antenna leg sections of ladder line are shorted together at the end where each of the four antenna leg sections come together.

I think there could be problems if any one of the four antenna leg sections had its two wires connected together at the far end (opposite where the antenna leg sections come together), but did not have the two wires of the associated ladder line shorted together at the end where the antenna leg sections come together.

If the connection of the two wires in any one (or more) of the four antenna leg sections broke apart (at the end where the four antenna leg sections come together)... that would make a mess of the behavior of the antenna... including the feed point impedance on 80m.

Stu

Changing the length of the feedline will change the resonance of the system (the wires in the sky and the feed line). You have to look at the entire thing as a system (and if you have a tuner, you need to include it too). Here's an example. I have a dipole that is 148 feet long (74 feet either side of center). Just the wires will be resonant at about 3.16 MHz and odd harmonics thereof (9.48, 15.8 MHz, etc) . If I feed these wires with about 90 feet of the typical ladder line, the system will be resonant at about 3.8 and 7.2 MHz.

I just noticed that, in one of his above posts, Opcom provided a reference to a product description / specification sheet from the manufacturer of this antenna:

Tekrad_Mark_V_antenna_system_small.pdf
 

Reading the information there, I deduced the following:

1. The four legs of the fan are, in fact, each made of ladder line

2. The write up says: "The feedpoint impedance of the antenna is a nominal 450 ohms balanced, and is fed by a 450ohm balanced transmission line"

This makes me suspect (there is not enough information the manufacturer's write-up to be sure) that the intended antenna consists of two folded dipoles, connected together at the feed point location in some manner (probably in series at the feed point location; but maybe not).

This would imply that the integrity of the connections (far ends of each leg shorted, center ends of all four legs properly connected in a folded dipole configuration of some sort) is critical, and suggests (to me) that the center connection arrangement in this particular antenna (as installed, maybe after shaking the antenna to clear a short*) is not correct.

*You wrote:

"Months ago, the wind blew the center insulator around and made a short but some jiggling on the rope, which has become slack, fixed that. I can see up there with binoculars."


Stu

Stu
I will have to say that you are a very thorough investigator/troubleshooter.
OPCOM didn't say that the fan dipole was a ladder line of sorts.

I think an old problem came back to his antenna in the last quote.
hmmmm
Fred

Fred

Thank you!

Stu

Stu, that is very good analysis. It is totally made of ladder line, all the same kind.

IIRC the far ends of each leg are not shorted.
I don't think it is a folded dipole but I will look with the binoculars just to check.
The connections on the center section are very rugged, all wrapped several times around and soldered extremely well. They should not be easily damaged, but there was that previous incident.

I never used 80M much, and the issue of heating up the balun only showed up during long winded turns at the RT. Now I want to use it more.

It is like this old post, save the width being 1" not an erroneous 2".
http://amfone.net/Amforum/index.php?topic=26545.msg255887#msg255887
Worked fine with a tuner+balun  then but it was 100W SSB only, so not a good comparison as there is not measurement info from then.

This is the post where the antenna was discovered twisted around and shorted:
http://amfone.net/Amforum/index.php?topic=35465.msg273168#msg273168

and in about my second post I noted:
"R was 100 Ohms at 7160KC was what it was when working."
That is how it had been, before. Now on 7160, it's not tuned at all and R= about 0. No feedlines or anything was changed in between.

===

Stu's remarks helped me recall that there was resonance before at about 7160, and the R coming out the coaxial connector was 100 Ohms (I forget which balun was in use, maybe the 1:1). Anyway a decent R at 7160, which is not being had now. So there is an antenna problem. I have to go try to see it. I hope that during the time when it was shorted and I was pushing a lot of power into it, there was not some kind of arcing between wires maybe burning one of them. I also remember there are alligator clips just at the inside wall of the shack and they ought be checked. Tomorrow with the binoculars will tell. BTW holding them still enough while zoomed into that thing 55FT away is not simple. Wish me luck in finding the trouble!

---

I have grown suspicious of any solder joints that live outside. If your center feed for that antenna was with common everyday solder; then I would guess that might be where your trouble is lurking.
I think Silver Solder is a better approach for a longer lasting connection.
It might look good, and it might be worthwhile to lower that center point down and re-do it.
I hope you designed a maintenance feature into your antenna so, you can inspect and repair when trouble comes around.
Fred

If the entire thing was made from ladder line and was supposed to play on multiple bands it may have multiple doublet lengths hidden within the insulation.

One or more ends may have welded itself to something at QRO.

Opcom,

Some of the guys have mentioned the good value of feeding a dipole with coax.  This works very well as a single band tool (excluding the 40 meter dipole that can work on 15 meters).

Multi-band operations should use open wire feeders with the correct antenna coupler design.  The link antenna coupler is the tuner of choice for open wire feeders.

Link provides mechanical and electrical balance, tuning convenience in the shack, eliminates components that can break down under low and high power applications plus it affords one to add shunt reactance (an extra capacitor or inductor between the left and right sides of the ladder line) in the shack to add or subtract electrical antenna system length if needed.

Link also allows one to current feed and voltage feed their antenna system.

Link is simple to operate and once all the bands have been tuned and the tuner settings written down it is fairly fast to tune.

Coax = single band and works very well.
Open wire feeders = multi-band = link antenna coupler.

73,
Chuck

I have to get the tower guy over. He might take a look for a trade of something.

It can't be lowered without undoing all 4 ends because the tower guys are in the way.

There is also 30FT up, a stand off and I can't tell if a wire is touching the metal post or not.
Whatever happened in the storm the pulley rope is in a wrong position and 6-9" of one side of the fan looks barely crooked or odd way up top.

I did try the MFJ without a 1:1 balun. Got good "R" in various places and the same resonances, so the 'zero' is probably an artifact of the balun. note the 1:1 balun has never been overheated, and I have tested the MFJ with a resistor, and the balun also with the same resistor.

I better forget about this until I can get someone to go up, and Jacob over to help us.

Regardless of what is a right or wrong way, coax, matchers, etc, the thing worked before, and someone has to go up and check it first.
Otherwise changing any of those other things makes no sense.

OK OPCOM
we were thinking that you were able to do some hands-on troubleshooting to your system and get you back on the air
Fred

All of that I could do, was done and I appreciate all of the advice.

It's a tower climb now. The man that has done the tower work before, and might do it for a swap, is recuperating from a broken rib. Oddly after the miles and miles of towers all over the world he has climbed without being harmed, it was a freak accident that he fell out of bed and cracked a rib. I pray for his recovery, and not just because I need this fixed. The guy is some 75 years old and in better health than I. He deserves to get well soon!

That gives you time to fabricate a new doublet and feedline to replace the existing unknown :)

Put the antenna on a pulley system so that you can raise and lower it to inspect, repair, etc.

I went through my old antenna book, and couldn't think of any reasons that you'd be heating up a balun with an antenna of this sort. It's a "plain vanilla" 80 meter dipole from what we know, so there should not be enough reactance to heat a balun like that.

After wracking my brain for a couple of hours, I took the liberty of asking Mike Gruber, W1MG, to run some numbers on the computer at the ARRL lab. Mike was very helpful, and provided these results:

Frequency	3.885 MHz
---------------	------------------------------
Zl	70 Ohms
Feedline length	89 feet
Feedline Z	450 ohms
Zc (Complex)	122.4 -j332
Zc (Polar)	354 ohms @ -69.8 degrees

... which show, as we've all figured already, that there's nothing remarkable about the system in theory. Mike agreed with everyone on the board: something's wrong, and it's time to look further. He suggested lengthening the feedline (the 1/2 wavelength point is at about 115 feet for 450-ohm ladder line on 3.885), and I agree that would be a good first step.

I also suggest some other tests:

• Check the grounds on the tuner and transmitter: you might have a circulation path.
• Borrow or make a new 1:1 balun, and rerun the curves you published before.
• Examine the antenna as much as possible from the ground, and look for signs of arcing, charring, or other damage.

HTH.,

73,

Bill, W1AC

conclusion http://amfone.net/Amforum/index.php/topic,40863

Here is 2 cents worth.

The least lost and greatest in tuning flexibily is achieved with home brew ladder line feeding a balanced antenna.  The antenna tuner should be the link antenna coupler.  As one changes bands so does the current distribution on the line and antenna.  Thus, the balanced mechanical and electrical properties of the link antenna tuner can accomodate these changes.  A heating "balun" know today does not exist and thus no heat loss.

The most efficient multi-band system is the Link Antenna Tuned system.

Good Luck!
73,
Chuck

That's what i have ALWAYS done for a dipole. They are always prone to trouble and mother nature
Fred

Here is 2 cents worth:

K1JJ suggestion of using independent antennas will work.

AB2EZ, W3GL, K1KW and others have excellent suggestions.

The bottom line is baluns have fixed resistive requirements to work at optimum.  They cannot adjust for reactance which is present in a multiband antenna (some reactance too with mono-band antenna but not bad). 

Thus, to cure the challenge the link antenna tuner is the design for such a multi-band antenna.  Use series tune for low impedance at the tuner and parallel tune when impedance is high at the tuner.

The is no balun to fry.  Many adjust a link coupler to preset dial settings to obtain high efficieny and multi-band use.

Good luck!
73
Chuck