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Author Topic: Did I mess up my antenna? - cut the ladder line..  (Read 40058 times)
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Steve - K4HX
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« Reply #25 on: April 19, 2014, 10:37:21 AM »

It's a horizontal antenna. Why would radials be needed?
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« Reply #26 on: April 20, 2014, 03:01:33 AM »

The instructions say so. Is that wrong?

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« Reply #27 on: April 20, 2014, 03:02:30 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


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


* murch 2000b.JPG (46.96 KB, 640x480 - viewed 786 times.)
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flintstone mop
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« Reply #28 on: April 20, 2014, 08:03:26 AM »

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
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Fred KC4MOP
Steve - K4HX
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« Reply #29 on: April 20, 2014, 09:38:52 AM »

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.

The instructions say so. Is that wrong?


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flintstone mop
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« Reply #30 on: April 20, 2014, 09:50:27 AM »

Here are a couple of pictures.

Fred


* DSC00005.jpg (141.97 KB, 1108x831 - viewed 891 times.)

* DSC00006.jpg (128.78 KB, 1108x831 - viewed 668 times.)
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Fred KC4MOP
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« Reply #31 on: April 20, 2014, 09:53:07 AM »

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.
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flintstone mop
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« Reply #32 on: April 20, 2014, 09:55:21 AM »

Sorry
rushing always looks bad!!


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Fred KC4MOP
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« Reply #33 on: April 20, 2014, 10:54:32 AM »

Here are a couple of pictures.

Fred

Same knife switch here. I thought you said it was huge!
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« Reply #34 on: April 20, 2014, 07:12:02 PM »

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


klc
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« Reply #35 on: April 20, 2014, 07:58:37 PM »

The fan was used to push out fumes from painting the floor.
And the knife switch looked pretty big to me
Fred
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« Reply #36 on: April 20, 2014, 09:36:18 PM »

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.


* measurement_20140420.png (95.88 KB, 1273x564 - viewed 561 times.)
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« Reply #37 on: April 20, 2014, 10:29:25 PM »

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
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« Reply #38 on: April 21, 2014, 02:31:49 AM »

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.
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« Reply #39 on: April 21, 2014, 02:57:55 AM »

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.

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« Reply #40 on: April 21, 2014, 03:50:04 AM »


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.


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.
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« Reply #41 on: April 21, 2014, 07:24:17 AM »

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

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


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« Reply #42 on: April 21, 2014, 08:26:42 AM »

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.



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.

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« Reply #43 on: April 21, 2014, 11:16:27 AM »

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.



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
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« Reply #44 on: April 21, 2014, 12:03:48 PM »

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
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"Season's Greetings" looks okay to me...


« Reply #45 on: April 21, 2014, 12:15:40 PM »

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

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« Reply #46 on: April 21, 2014, 01:08:40 PM »

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.
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« Reply #47 on: April 21, 2014, 01:13:08 PM »

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.
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"Season's Greetings" looks okay to me...


« Reply #48 on: April 21, 2014, 02:10:53 PM »

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

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 #49 on: April 21, 2014, 02:56:24 PM »

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.
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