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Author Topic: Did I mess up my antenna? - cut the ladder line..  (Read 40055 times)
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Patrick J. / KD5OEI
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« on: April 16, 2014, 02:03:29 AM »

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.
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« Reply #1 on: April 16, 2014, 06:38:59 AM »

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
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« Reply #2 on: April 16, 2014, 09:18:59 AM »

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
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« Reply #3 on: April 16, 2014, 10:58:47 AM »

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.
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« Reply #4 on: April 16, 2014, 04:52:08 PM »

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  Roll Eyes...

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






 
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« Reply #5 on: April 16, 2014, 06:57:08 PM »

-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?

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« Reply #6 on: April 16, 2014, 07:15:17 PM »

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
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« Reply #7 on: April 17, 2014, 01:59:09 PM »

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




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« Reply #8 on: April 17, 2014, 05:02:44 PM »


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?   
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« Reply #9 on: April 17, 2014, 08:41:05 PM »

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.
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« Reply #10 on: April 17, 2014, 10:10:46 PM »


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?    

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.


* Murch 2000A-and-B Manual.pdf (497.16 KB - downloaded 1010 times.)
* Tekrad_Mark_V_antenna_system_small.pdf (3013.98 KB - downloaded 245 times.)
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« Reply #11 on: April 17, 2014, 11:47:44 PM »

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.

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« Reply #12 on: April 18, 2014, 02:18:03 AM »

Sounds like the usual antenna snake oil ...

Grant NQ5T
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« Reply #13 on: April 18, 2014, 07:41:05 AM »

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?
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« Reply #14 on: April 18, 2014, 10:19:24 AM »

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. 
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« Reply #15 on: April 18, 2014, 01:09:30 PM »

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.
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« Reply #16 on: April 18, 2014, 02:24:05 PM »


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.

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« Reply #17 on: April 18, 2014, 02:29:04 PM »

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  Cool)...

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... Roll Eyes   Grin
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« Reply #18 on: April 18, 2014, 04:04:19 PM »

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.



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« Reply #19 on: April 18, 2014, 06:29:43 PM »

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
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« Reply #20 on: April 18, 2014, 08:51:33 PM »

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.
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« Reply #21 on: April 18, 2014, 08:59:46 PM »

Yep. Use steel cable and you won't even to need to replace any rope.
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« Reply #22 on: April 18, 2014, 10:25:21 PM »


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

Coax is for girls.
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« Reply #23 on: April 18, 2014, 11:49:05 PM »

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.
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« Reply #24 on: April 19, 2014, 07:50:57 AM »

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
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