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Balancing dipole leg lengths


Richard Chism 2022/07/02 12:10

I have made a dedicated 75 meter half-wave dipole.  The leg lenghts are not identical (don't ask please) but I would like to adjust them to be equal without taking the whole thing down.  I can adjust each leg, so I can tune for frequency, but I need to be able to determine which leg is longer.  I suspect that I can use a Smith chart to do this, but it will be trial and error for me.  Can anyone tell me how I can use my nanaVNA to balance my dipole legs?  Thanks.  73 de AI7JN  Rick

Siegfried Jackstien 2022/07/03 12:20

Can you reach feedpoint??

You could wind a 1 to 49 transformer and feed each leg as an endfed halfwave
(just to check its resonance/length)

If you tune both halves to the same resonance spot.. They should be electrical
the same length (mechanical they may still differ a bit if the legs are
influenced by surroundings)

Hth

Dg9bfc sigi



Am 02.07.2022 21:10 schrieb Richard Chism <rchism@weedranch.com>:

> I have made a dedicated 75 meter half-wave dipole. The leg lenghts are not
identical (don't ask please) but I would like to adjust them to be equal
without taking the whole thing down. I can adjust each leg, so I can tune for
frequency, but I need to be able to determine which leg is longer. I suspect
that I can use a Smith chart to do this, but it will be trial and error for
me. Can anyone tell me how I can use my nanaVNA to balance my dipole legs?
Thanks. 73 de AI7JN Rick



_._,_._,_

* * *

Michael Brun 2022/07/03 04:13

Good Day Rick.

I would expect using the TDR functionality of NanoVNA is answer.    There are a number of YouTube videos available where folks demonstrate the necessary steps to configure it.

In your case, I would expect you will need a special adapter that allows you to connect one leg of your dipole at a time to your nano.   Once the unit gives you readings that reasonably represent the expected approx length of one leg, you would switch over to the other leg and contrast difference.  When you observe that one leg shows its major reflection earlier than the other, you will have identified the shorter one.

I would be inclined to perform the tests directly at the dipole feed-point and avoid the effects of coaxial cable variables.  This should also make it easier to switch between the two legs.

Hope this helps.

KE8PLM

Ken Sejkora 2022/07/03 07:35

Hi Rick,

Let me start out by stating that I am *NOT* an expert with the nanoVNA. I’m just downloading some of my thoughts.

Unless you are measuring each leg ‘independently’ at the center insulator of your dipole, I’m guessing it will be a challenge. How are you feeding the antenna – ladder line, coax, with or without a balun/unun/matching transformer, etc.? Those would obviously introduce some electrical characteristics you’d have to work through. That’s why I think you’d have to measure each leg independently at the center insulator, with the feedline disconnected. I don’t think you’d have any success measuring each leg ‘through’ the feedline.

Other things that might complicate the measurements would be if one leg is higher above the ground than the other, if it runs closer to tress than the other leg, if it runs closer to buildings/utility lines/??? than the other, etc. Any of those *might* result in the leg appearing longer or shorter electrically than it really is physically.

I’m not saying it can’t be done, but just pointing out some things that come to mind that could potentially complicate your measurements.

Good luck and have a great weekend. 73

Ken -- WBØOCV

From: Richard Chism
Sent: Sunday, July 3, 2022 12:30 AM
To: NanoVNAV2@groups.io
Subject: [nanovnav2] Balancing dipole leg lengths

I have made a dedicated 75 meter half-wave dipole.  The leg lenghts are not identical (don't ask please) but I would like to adjust them to be equal without taking the whole thing down.  I can adjust each leg, so I can tune for frequency, but I need to be able to determine which leg is longer.  I suspect that I can use a Smith chart to do this, but it will be trial and error for me.  Can anyone tell me how I can use my nanaVNA to balance my dipole legs?  Thanks.  73 de AI7JN  Rick

Richard Chism 2022/07/03 07:24

This is exactly what I would like to do, but how can I make a measurement with only one connection?  Doesn't the nanoVNA need both the inner and outer conductors of the port connected to something?  I see how this would work for measuring a coax, but I can't find anything online for measuring a wire unless I can access both ends.  How would such an adapter as you describe be configured?

My thoughts were to measure the impedance of the antenna, then shorten one leg and see if the impedance increased or decreased.  If it decreased, it would mean that I had shortened the longer leg, and vice-versa.  Does that seem correct?

Another issue is that I am using a 1:1 50 ohm cube balun to connect my coax to the antenna.  What will this do to my measurements?  Would I have to tie the ends of the dipole dirfectly to a connector?  It would be much easier if I could make my measurements through the balun.

Anne Ranch 2022/07/03 10:59

Comment
Assuming you have 1/2 wavelength long device and it has a fundamental
resonant frequency .
The feed point impedance varies depending on location of such feed pont.
The resonance does not change with location of the feed point.

As pointed out - the "legs " surroundings have bearing on the feed point
impedance too.

Hence In practice it would be (more) prudent to match the impedance
instead of adjusting the symmetry of the length.

Brian Machesney 2022/07/03 12:00

Rick,

What is the goal of adjusting the dipole leg lengths? Current balance?
Pattern cleanup?

Brian K1LI

On Sun, Jul 3, 2022 at 12:30 AM Richard Chism <rchism@weedranch.com> wrote:

Richard Chism 2022/07/04 07:16

I just want the optimum performance from the antenna, and since I am using a 50 ohm balun I assume the best match would be when the legs are equal length.

Brian Machesney 2022/07/04 12:37

Antenna modeling software shows that the antenna performance will change
imperceptibly as a result of the leg length changes you propose. For
example, shortening the length of one leg of a 133-ft dipole (which
resonates at 3.6-MHz in free space) by 6-ft reduces the gain by 0.04-dB,
while increasing the resonant frequency to 3.78-MHz.

Brian K1LI

On Mon, Jul 4, 2022 at 12:03 PM Richard Chism <rchism@weedranch.com> wrote:

Edward Newman 2022/07/04 18:08

Hi Richard-
here's some advice from an old antenna engineer- you may already know all this but I hope it may prove useful.
I assume you are discussing a half-wave dipole fed at the middle.  As has been discussed, it is difficult to make a good measurement of an end-fed quarter wave because you don't know what the VNA sees as the counterpoise for the wire.  I suggest you not worry about the dipole balance, and I assume you cut the two halves to be about the same. If not, read on.

The impedance match of any antenna is not very important if it is below 2:1 SWR, because the power lost is about 1/2 dB, which is virtually undetectable in communications.  My only exception to this is that some modern transceivers want to see 1.5:1 SWR for full power out.  If you are getting 1.5 or less, you really don't need to do better.
If you need to do better, here's how the wire affects the impedance.  For the half-wave dipole, the resistance will be around 50 to 100 ohms at the center.  If you move the feedpoint off center, the resistance will increase slowly until you get near the ends, and at the end you will have a resistance of thousands of ohms.  So if your feedpoint is off by a few percent, it will make no practical difference.
The reactance is largely determined by the overall length of the dipole, so trimming either end to get the reactance to near zero will work, even if the two halves are unequal.
An example of this is the off-center fed dipole, where the two halves are unbalanced by 50% or so.  The resistance is higher, but the resonant length is about the same.
We engineers often have to decide what is good enough.
73  Ed  W2EMN

Richard Chism 2022/07/04 11:27

Ed,

That is VERY good to hear.  Now all I have to do is take a couple of
feet off of the leg that I suspect is longer in order to optimize it for
the frequency range I use most often, and if I'm wrong it sounds like it
won't matter.  Yes, it is a center fed (approximately) half wave dipole
cut for 75 meters, so the percentage difference won't be much.  Maybe 4%
at most. Thank you.  That simplifies everything.

73 de Rick AI7JN

On 7/4/2022 11:08 AM, djed1@aol.com wrote:

gary miller 2022/07/04 11:32

A dipole at resonance will have a resistance of about 73 ohms. If the length imbalance is small compared to the length of the dipole the change in performance will be small compared to the balanced length dipole.

Siegfried Jackstien 2022/07/04 21:52

Performance does not change... You can feed a halfwave wire anywhere along its
length..

A good balun will keep the antenna well isolated from the cable...

So it will just work... Even if you feed at 48/52%split and not exact on 50
/50

Dg9bfc sigi



Am 04.07.2022 16:16 schrieb Richard Chism <rchism@weedranch.com>:

> I just want the optimum performance from the antenna, and since I am using a
50 ohm balun I assume the best match would be when the legs are equal length.



_._,_._,_

* * *

Chris Keladis 2022/07/05 09:27

I'd vote for using the TDR function.

Since this is copper wire, set the velocity-factor to around 98%, but you
only need to disconnect the feedpoint end you don't need to touch the far
end (to measure at least).

Check both sides, then figure out the length you need to shorten by,
shorten, re-measure, and both legs should be the same.

There are good videos on YouTube on using the TDR functionality, I'd
recommend the video by Alan w2aew (albeit for coax).



73s,

Chris.

On Tue, 5 Jul 2022, 2:03 am Richard Chism, <rchism@weedranch.com> wrote:

Chris Keladis 2022/07/05 09:35

...and answering in local morning just after waking up isn't always the
best idea :)

Yes you would need a current return.



Cheers,

Chris.

On Tue, 5 Jul 2022, 9:27 am Chris Keladis, <ckeladis@gmail.com> wrote:

Siegfried Jackstien 2022/07/05 12:39

A test transformer 1 to 49... Then endfed each leg (should be similar in
resonance on 40m)...that way very few is needed for a current return..(a bit
of capacitive coupling... Or a very short wire)

But overall you will not see any difference if you shorten one end.. Or both..

Sidenote.. Not cut the wire... Fold over and secure with a cable tie

Dg9bfc sigi



Am 05.07.2022 01:35 schrieb Chris Keladis <ckeladis@gmail.com>:

> ...and answering in local morning just after waking up isn't always the best
idea :)

>

>
>

>

> Yes you would need a current return.

>

>
>

>

>
>

>

>
>

>

> Cheers,

>

>
>

>

> Chris.

>

>
>

>

> On Tue, 5 Jul 2022, 9:27 am Chris Keladis,
<[ckeladis@gmail.com](mailto:ckeladis@gmail.com)> wrote:
>

>

>> I'd vote for using the TDR function.

>>

>>
>

>>

>> Since this is copper wire, set the velocity-factor to around 98%, but you
only need to disconnect the feedpoint end you don't need to touch the far end
(to measure at least).

>>

>>
>

>>

>> Check both sides, then figure out the length you need to shorten by,
shorten, re-measure, and both legs should be the same.

>>

>>
>

>>

>> There are good videos on YouTube on using the TDR functionality, I'd
recommend the video by Alan w2aew (albeit for coax).

>>

>>
>

>>

>>
>

>>

>>
>

>>

>> 73s,

>>

>>
>

>>

>> Chris.

>>

>>
>

>>

>> On Tue, 5 Jul 2022, 2:03 am Richard Chism,
<[rchism@weedranch.com](mailto:rchism@weedranch.com)> wrote:
>

>>

>>> I just want the optimum performance from the antenna, and since I am using
a 50 ohm balun I assume the best match would be when the legs are equal
length.



_._,_._,_

* * *

Richard Chism 2022/07/05 11:30

Thanks everyone for all the good advice.  I had used plenty of extra wire.  It was resonant at 3.750 so I took over 4 feet from what I thought was the longer leg and wrapped back more in case I needed to adjust the other way.  It was then resonant at 3.90.  I will probably take a few inches from the other leg, but it is working great!  Although I cut it for 75 meters, on 30 meters I talked  to a Russian about 35 miles east of Moscow using 100 watts and a cheap tuner.  I also heard clearly, but couldn't reach, a guy ferom Lithuania and another from Germany.  I am on the southern Oregon coast so that was ,pretty darned good!

I still don't understand how I can measure a wire using the TDR function.  A coax, yes, but for a wire do I just connect it to the center conductor of port 0 and leave nothing connected to the shield?

Anne Ranch 2022/07/05 14:34

"Not cut the wire... Fold over and secure with a cable tie"

just to be picky again

should this need

*"assuming the wire is bare" *

Richard Chism 2022/07/06 06:10

I had plenty of wire.  I still have well over a foot wrapped back on each leg.  Thanks for the advice,.  I really do pay close attention to responses.  :)

Rebel Thompson 2022/07/06 12:10

Just for fun, I took and old set of rabbit ears and wired it with some RG58. And mounted it on a not-too-high wooden pole in the back yard. I extended the sections to make a balanced 100 MHz dipole and verified for a baseline. From there I proceeded to vary the angle and feed point [changing relative leg lengths]. With the NanoVNA, you can make some rapid real time observations on how those adjustments affect performance.

Dirk 2022/07/09 23:40

>
> I still don't understand how I can measure a wire using the TDR function. 
> A coax, yes, but for a wire do I just connect it to the center conductor
> of port 0 and leave nothing connected to the shield?

Exactly. You can measure resonance frequency directly and adjust length.
https://m.youtube.com/watch?v=WEdDAKJlriQ

W0LEV 2022/07/10 16:19

With all the "commotion" on this thread, why not just drop the darn set of
wires and take care of the problem on the ground with mechanical rulers and
physical measurements????

Dave - WØLEV

On Sun, Jul 10, 2022 at 3:30 PM Dirk <dottensm@gmail.com> wrote:

> I still don't understand how I can measure a wire using the TDR function.
> A coax, yes, but for a wire do I just connect it to the center conductor of
> port 0 and leave nothing connected to the shield?
>
> Exactly. You can measure resonance frequency directly and adjust length.
> https://m.youtube.com/watch?v=WEdDAKJlriQ
>
>
>

--
*Dave - WØLEV*
*Just Let Darwin Work*

Richard Chism 2022/07/10 13:52

I am an elderly disabled veteran.  I needed a lot of ,help getting them up and I cannot take them down, measure them, and put them back up without a lot of additionmal help.  Each leg is 60 feet long and in the trees.

N2MS 2022/07/11 06:58

You can feed a dipole off center. What is the SWR at the transmitter side of the coax?

Mike N2MS

Nels Nelsen 2022/07/12 11:48

Commotion? I have only seen an orderly path to understanding.
I have not seen any reason to complain.

On Sun, Jul 10, 2022 at 10:27 AM W0LEV <davearea51a@gmail.com> wrote:

> With all the "commotion" on this thread, why not just drop the darn set of
> wires and take care of the problem on the ground with mechanical rulers and
> physical measurements????
>
> Dave - WØLEV
>
> On Sun, Jul 10, 2022 at 3:30 PM Dirk <dottensm@gmail.com> wrote:
>
>> I still don't understand how I can measure a wire using the TDR
>> function. A coax, yes, but for a wire do I just connect it to the center
>> conductor of port 0 and leave nothing connected to the shield?
>>
>> Exactly. You can measure resonance frequency directly and adjust length.
>> https://m.youtube.com/watch?v=WEdDAKJlriQ
>>
>>
>
> --
> *Dave - WØLEV*
> *Just Let Darwin Work*
>
>
>
>

--




n_n

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