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Dipole antenna with Low Z0 feed point Impedance

 

Using a VNA to measure dipole antenna feed point characteristics has been discussed. I believe this is a little different twist.

I have an attic mounted 1/2 wave dipole antenna with a 20-25 ohm Z0 at the feed point. I expected this because of the 12 foot attic mounting height, non-linear non-symmetric shape, near by structures, poor ground effect, other constraints, etc. Having confirmed this, how to best match this dipole's Z0 of 25 Ohms to a 50 ohm coax line is now the challenge. Would a 1:2 UnUn be appropriate? 1:2 BalUn? If so, what would this UnUn / BalUn look like? I have seen 2:1 commercial units available. The input and output connectors are for 2:1. I have not seen one with the connections setup for 1:2. If this is a viable are there any recommended commercial sources for it? I would then use the VNA to verify results.

Thoughts and suggestions?

Thank you.
Bill WA2WIO
 

Greetings.

I had a 40 meter dipole at 12' above ground with fair ground conductivity
and had a feed point Z of 22 ohms and very little reactive component. I
went to Balun Designs and bought a balun that was 25 to 50 (or 1:2 Z match)
and it fixed the problem. Worked very well at full legal limit for power,
also. Balun Designs is my recommendation.

Best Regards,

Michael L Robinson, KC0TA

“In the beginning of a change the Patriot is a scarce man, and brave, and
hated and scorned. When his cause succeeds, the timid join him, for then it
costs nothing to be a Patriot.” ― Mark Twain

When Tyranny becomes Law, Revolution becomes Duty!






On Fri, Nov 8, 2024 at 11:54 AM Bill WA2WIO via groups.io <sla=
slalighting.com@groups.io> wrote:

Using a VNA to measure dipole antenna feed point characteristics has been
discussed. I believe this is a little different twist.

I have an attic mounted 1/2 wave dipole antenna with a 20-25 ohm Z0 at the
feed point. I expected this because of the 12 foot attic mounting height,
non-linear non-symmetric shape, near by structures, poor ground effect,
other constraints, etc. Having confirmed this, how to best match this
dipole's Z0 of 25 Ohms to a 50 ohm coax line is now the challenge. Would a
1:2 UnUn be appropriate? 1:2 BalUn? If so, what would this UnUn / BalUn
look like? I have seen 2:1 commercial units available. The input and output
connectors are for 2:1. I have not seen one with the connections setup for
1:2. If this is a viable are there any recommended commercial sources for
it? I would then use the VNA to verify results.

Thoughts and suggestions?

Thank you.
Bill WA2WIO
 

Bill,
Single band or multi?
Because the 1:2 transformer (also use a ferrite choke) only matches the feed point essentially on one band (there will likely be some higher-order sorta-matches in there, too), I always just put up the attic loop or dipole and put an LDG RT-100 (or similar) at the feed point. Then, DC to Daylight.
Interesting question, though. I want to know the answer as well. :)
Regards,
Kirk, NT0Z

My book, "Stealth Amateur Radio," is now available from www.stealthamateur.com and on the Amazon Kindle (soon)

On Friday, November 8, 2024 at 11:54:24 AM CST, Bill WA2WIO <sla@...> wrote:

Using a VNA to measure dipole antenna feed point characteristics has been discussed. I believe this is a little different twist.

I have an attic mounted 1/2 wave dipole antenna with a 20-25 ohm Z0 at the feed point. I expected this because of the 12 foot attic mounting height, non-linear non-symmetric shape, near by structures, poor ground effect, other constraints, etc. Having confirmed this, how to best match this dipole's Z0 of 25 Ohms to a 50 ohm coax line is now the challenge. Would a 1:2 UnUn be appropriate? 1:2 BalUn? If so, what would this UnUn / BalUn look like? I have seen 2:1 commercial units available. The input and output connectors are for 2:1. I have not seen one with the connections setup for 1:2. If this is a viable are there any recommended commercial sources for it? I would then use the VNA to verify results.

Thoughts and suggestions?

Thank you.
Bill WA2WIO
 

40m and 20m. Anything else is a gift.



Right now, system is: 1/2 wave Dp antenna to about 40ft of RG58 coax to 15T coax wound CMC on a 4" PVC form to ATU10 to rig (SDR QRP for now, eventual 200w).



Without ATU SWR 5-6. With ATU SWR 1.2-2.5.



Also, S4-5 noise level.



LDG is cool but expensive (may be worth it). I am using the ATU at the xmtr to keep the rig happy. I am using the VNA looking to make the antenna itself work better.

I found these products:



1:2 1-56MHz 200W Balun HAM Shortwave Antenna Balun 50 ohm to 100 ohm Unbalanced Balance

SKU: 1741737 (looking into this one)



https://www.elecbee.com/en-31311-12-1-56MHz-200W-Balun-HAM-Shortwave-Antenna-Balun-50-ohm-to-100-ohm-Unbalanced-Balance?srsltid=AfmBOoqsPq--uC-nDdi7QNC6zy_Gm-Hg-Cn8e60oDuW0OGRoqPtNCsw2





https://www.balundesigns.com/1-2-balun/

Model 1213 - 1:2 Balun (25 to 50 ohms) 3kW

This is a unique design by Dr. Jerry Sevick (W2FMI) that combines a 25 ohm balun with a 1:2 unun for final transformation to 50 ohms. Used anywhere there is a need to transform 25 ohms balanced to 50 ohms unbalanced.



Big price difference (again, might be worth it).



· Would I be correct that a 2:1 and a 1:2 are very different animals? You can't just turn around a 2:1??



Thoughts?



Bill WA2WIO

-----Original Message-----
From: nanovna-users@groups.io [mailto:nanovna-users@groups.io] On Behalf Of Kirk Kleinschmidt, NT0Z via groups.io
Sent: Friday, November 8, 2024 7:08 PM
To: nanovna-users@groups.io
Subject: Re: [nanovna-users] Dipole antenna with Low Z0 feed point Impedance



Bill,

Single band or multi?

Because the 1:2 transformer (also use a ferrite choke) only matches the feed point essentially on one band (there will likely be some higher-order sorta-matches in there, too), I always just put up the attic loop or dipole and put an LDG RT-100 (or similar) at the feed point. Then, DC to Daylight.

Interesting question, though. I want to know the answer as well. :)

Regards,

Kirk, NT0Z



My book, "Stealth Amateur Radio," is now available from www.stealthamateur.com and on the Amazon Kindle (soon)



On Friday, November 8, 2024 at 11:54:24 AM CST, Bill WA2WIO <sla@...> wrote:



Using a VNA to measure dipole antenna feed point characteristics has been discussed. I believe this is a little different twist.



I have an attic mounted 1/2 wave dipole antenna with a 20-25 ohm Z0 at the feed point. I expected this because of the 12 foot attic mounting height, non-linear non-symmetric shape, near by structures, poor ground effect, other constraints, etc. Having confirmed this, how to best match this dipole's Z0 of 25 Ohms to a 50 ohm coax line is now the challenge. Would a 1:2 UnUn be appropriate? 1:2 BalUn? If so, what would this UnUn / BalUn look like? I have seen 2:1 commercial units available. The input and output connectors are for 2:1. I have not seen one with the connections setup for 1:2. If this is a viable are there any recommended commercial sources for it? I would then use the VNA to verify results.



Thoughts and suggestions?



Thank you.

Bill WA2WIO
 

I use 1:2 transformers on all bands in my stackmatches similar to this one:
https://www.ebay.de/itm/266831127121?srsltid=AfmBOooImSZ0y4ti-tVjjI7S-aKbKLn1DY4_1VuzjulCvvymePiyeIPa
I use it also on my 40m yagi with its 27 ohm impedance.
On the high bands I use just 5 trifilar windings with the stackmatch.

73
Peter, DJ7WW




-----Original-Nachricht-----
Betreff: Re: [nanovna-users] Dipole antenna with Low Z0 feed point Impedance
Datum: 2024-11-09T01:08:27+0100
Von: "Kirk Kleinschmidt, NT0Z via groups.io" <sohosources@...>
An: "nanovna-users@groups.io" <nanovna-users@groups.io>

Bill,
Single band or multi?
Because the 1:2 transformer (also use a ferrite choke) only matches the feed point essentially on one band (there will likely be some higher-order sorta-matches in there, too), I always just put up the attic loop or dipole and put an LDG RT-100 (or similar) at the feed point. Then, DC to Daylight.
Interesting question, though. I want to know the answer as well. :)
Regards,
Kirk, NT0Z

My book, "Stealth Amateur Radio," is now available from www.stealthamateur.com and on the Amazon Kindle (soon)

On Friday, November 8, 2024 at 11:54:24 AM CST, Bill WA2WIO <sla@...> wrote:

Using a VNA to measure dipole antenna feed point characteristics has been discussed. I believe this is a little different twist.

I have an attic mounted 1/2 wave dipole antenna with a 20-25 ohm Z0 at the feed point. I expected this because of the 12 foot attic mounting height, non-linear non-symmetric shape, near by structures, poor ground effect, other constraints, etc. Having confirmed this, how to best match this dipole's Z0 of 25 Ohms to a 50 ohm coax line is now the challenge. Would a 1:2 UnUn be appropriate? 1:2 BalUn? If so, what would this UnUn / BalUn look like? I have seen 2:1 commercial units available. The input and output connectors are for 2:1. I have not seen one with the connections setup for 1:2. If this is a viable are there any recommended commercial sources for it? I would then use the VNA to verify results.

Thoughts and suggestions?

Thank you.
Bill WA2WIO












 

One thought would be to use a 1/4 wavelength of RG59 for example at the frequency your dipole is cut for. I did not notice that reference in your original message. This should perform the proper transition from a 50 Ohm feedline to the 25 Ohme impedance at your dipole center.
DaveR KB7GP
 

On Sat, Nov 9, 2024 at 08:06 AM, Bill WA2WIO wrote:

You can't just turn around a 2:1??
Back to your question: no, you can't just turn around a 2:1 balun - that is, unless you re-wire it. If it is in a box that is easy to get to the internals, you may be able to flip the coil around - but you need to wire it properly such that it stays being a balun, i.e. the shield terminal from your coax connection has to be on the leg of the transformer that is not directly connected to one of the balanced terminals.

The other 1:2 balun you found is a very nicely done design, but made for 3kW it is expensive. You don't need 3kW: a 100W version of that same balun should be much more reasonably priced - or you could make your own on a FT240-43 core.
 

Dave,

You're onto something, but to match 25 ohms to 50 ohms you need 1/4
wavelength (electrical length) of 37 ohm (not 75 ohm) coax.
This is easily achieved with two 75 ohm coaxes in parallel.
I've used this many times and works great!

A word to the wise: if you use foam dielectric coax the velocity
factor may be wildly different than the published value

Good luck!

73
Frank
W3LP

----- Original Message -----
From: "Dave Robinson via groups.io" <kb7gp@...>
To: nanovna-users@groups.io
Sent: Saturday, November 9, 2024 5:33:30 PM
Subject: Re: [nanovna-users] Dipole antenna with Low Z0 feed point Impedance

One thought would be to use a 1/4 wavelength of RG59 for example at the frequency your dipole is cut for. I did not notice that reference in your original message. This should perform the proper transition from a 50 Ohm feedline to the 25 Ohme impedance at your dipole center.
DaveR KB7GP
 

I have a MUCH MUCH less messy solution.

Is the dipole (center fed) 130 ft (80m) or 66 ft (40m) or 33 ft (20m)?
Center feds will work on the 3rd harmonic, so a 40m can also tune to 15m.

So…. try this … for $1.

Put a small inductor (say 5 turns of wire 1 inch diam) across the dipole where it attached to the coax.
If you have a 1:1 balun, leave it in place.

This will RAISE the input impedance (just like the method to couple a mobile whip to 50 ohm coax.
You MAY have to then shorten the wire a bit at each end ( try 1 ft at first) to get it back to the resonant freq you want.

I often do this on a very low 80m dipole (130 ft) when it is low to the ground.
Impedance at 20 ft above gnd is often about 20 ohms.

But if you try to change the impedance by changing the LENGTH of your coax, the SWR will not change.
You will only move the impedance around the Smith Chart, so you may get a higher impedance than 50 ohms,
but the SWR will still not change.

If on 80m you may need more inductance across the dipole input…. so try 10 turns of wire, 1 inch diameter.
Eventually the inductance in parallel with the dipole will raise the impedance to where the 50 ohm coax will match a bit better.

Let us know if that works.
Messing with 1:2 baluns is kind of silly and expensive when a simple inductor across the input will work.

de k3eui. Barry
 

Using a NanoVNA or a MFJ 259 either is the source of the RF... whereas using a transmitter then a short length of coax to a SWR bridge, changes the length of the source of RF to the antenna..

Larry W8LM
ARRL Life Member Licensed 59 years
 

k3eui. Barry;

Dipole is center fed 1/2 wave 66ft #18 stranded insulated in a rt angle horizontal V. The shape is necessitated by the complicated and crowded attic construction. Interesting idea. I need to read up on it. Thank you.

Kirk, NT0Z: I bought a copy of your book. "Stealth Amateur Radio" some time ago. Great resource. I can see I am not alone in my quest. Thank you

Bill WA2WIO

-----Original Message-----
From: nanovna-users@groups.io [mailto:nanovna-users@groups.io] On Behalf Of Barry K3EUI via groups.io
Sent: Sunday, November 10, 2024 7:10 PM
To: nanovna-users@groups.io
Subject: Re: [nanovna-users] Dipole antenna with Low Z0 feed point Impedance

I have a MUCH MUCH less messy solution.

Is the dipole (center fed) 130 ft (80m) or 66 ft (40m) or 33 ft (20m)?
Center feds will work on the 3rd harmonic, so a 40m can also tune to 15m.

So…. try this … for $1.

Put a small inductor (say 5 turns of wire 1 inch diam) across the dipole where it attached to the coax.
If you have a 1:1 balun, leave it in place.

This will RAISE the input impedance (just like the method to couple a mobile whip to 50 ohm coax.
You MAY have to then shorten the wire a bit at each end ( try 1 ft at first) to get it back to the resonant freq you want.

I often do this on a very low 80m dipole (130 ft) when it is low to the ground.
Impedance at 20 ft above gnd is often about 20 ohms.

But if you try to change the impedance by changing the LENGTH of your coax, the SWR will not change.
You will only move the impedance around the Smith Chart, so you may get a higher impedance than 50 ohms,
but the SWR will still not change.

If on 80m you may need more inductance across the dipole input…. so try 10 turns of wire, 1 inch diameter.
Eventually the inductance in parallel with the dipole will raise the impedance to where the 50 ohm coax will match a bit better.

Let us know if that works.
Messing with 1:2 baluns is kind of silly and expensive when a simple inductor across the input will work.

de k3eui. Barry
 

To Bill WA2WIO:
You received several good suggestions on how to transform 25 ohms to 50 ohms. Here are a few more ideas that use only coax.

Method 1:
Connect a 70.0 degree length of 50-ohm coax to the antenna feedpoint. Follow this with a 129 degree length of 75-ohm coax. Now you have 50 ohms. At 7.15 MHz this could be 17.3 ft of RG-58A/U (Belden 8259) and 32.2 ft of RG-59/U (Belden 8241).

Method 2:
Connect a 143 degree length of 50-ohm coax to the antenna feedpoint. At the other end connect a shorted stub that's 56.3 degrees long made from 50-ohm coax. The impedance at the junction will be 50 ohms. At 7.15 MHz the first section could be 35.4 ft of RG-58A/U (Belden 8259). The shorted stub could be 13.9 ft of the same.

Method 3:
Connect a 34.5 degree length of 50-ohm coax to the antenna feedpoint. At the other end connect an open stub 34.7 degrees long made from 50-ohm coax. The impedance at the junction will be 50 ohms. At 7.15 MHz the first section could be 8.54 ft of RG-58A/U (Belden 8259). The open stub could be 8.58 ft of the same.

Notes:
1. 360 degrees equals 1 wavelength.

2. The following factors primarily determine the actual physical length of coax for each of the methods listed: the start and end impedance, target frequency, coax characteristic impedance, and coax velocity factor.

3. All these methods have less than 0.6 dB loss at the target frequency using the coax types noted. Method 3 is the lowest at 0.2 dB.

4. Although these are not broadband methods per se, they yield about 170 KHz of bandwidth below SWR 2 which is about the same bandwidth you'd get with a broadband solution. Most of that is due to the amount of change in feedpoint impedance with frequency from the antenna itself.

5. It probably goes without saying that after the 50-ohm impedance has been achieved, any additional length of 50-ohm coax can be added as needed to get to the rig.

6. All results were solved graphically in SimNEC. It took longer to annotate the attached screenshots and write this reply than it took to create the SimNEC models and get the results. Once you learn SimNEC you can graphically create impedance transformations like this (and more) very fast with no math although it greatly helps to understand the underlying principles.

Dave NU8A

Method 1 -- 25 to 50 with 50 and 75 ohm coax.png
Method 2 -- 25 to 50 with 50 ohm coax and shorted stub.png
Method 3 -- 25 to 50 with 50 ohm coax and open stub.png

 

Dave forgot to model what is likely to be the broadest bandwidth solution
using coaxial cable: Two 90 degree lengths of 75 ohm coax in parallel

73
Frank
W3LPL

----- Original Message -----
From: "DP via groups.io" <dpoinsett@...>
To: nanovna-users@groups.io
Sent: Monday, November 11, 2024 10:20:35 AM
Subject: Re: [nanovna-users] Dipole antenna with Low Z0 feed point Impedance

To Bill WA2WIO:
You received several good suggestions on how to transform 25 ohms to 50 ohms. Here are a few more ideas that use only coax.

Method 1:
Connect a 70.0 degree length of 50-ohm coax to the antenna feedpoint. Follow this with a 129 degree length of 75-ohm coax. Now you have 50 ohms. At 7.15 MHz this could be 17.3 ft of RG-58A/U (Belden 8259) and 32.2 ft of RG-59/U (Belden 8241).

Method 2:
Connect a 143 degree length of 50-ohm coax to the antenna feedpoint. At the other end connect a shorted stub that's 56.3 degrees long made from 50-ohm coax. The impedance at the junction will be 50 ohms. At 7.15 MHz the first section could be 35.4 ft of RG-58A/U (Belden 8259). The shorted stub could be 13.9 ft of the same.

Method 3:
Connect a 34.5 degree length of 50-ohm coax to the antenna feedpoint. At the other end connect an open stub 34.7 degrees long made from 50-ohm coax. The impedance at the junction will be 50 ohms. At 7.15 MHz the first section could be 8.54 ft of RG-58A/U (Belden 8259). The open stub could be 8.58 ft of the same.

Notes:
1. 360 degrees equals 1 wavelength.

2. The following factors primarily determine the actual physical length of coax for each of the methods listed: the start and end impedance, target frequency, coax characteristic impedance, and coax velocity factor.

3. All these methods have less than 0.6 dB loss at the target frequency using the coax types noted. Method 3 is the lowest at 0.2 dB.

4. Although these are not broadband methods per se, they yield about 170 KHz of bandwidth below SWR 2 which is about the same bandwidth you'd get with a broadband solution. Most of that is due to the amount of change in feedpoint impedance with frequency from the antenna itself.

5. It probably goes without saying that after the 50-ohm impedance has been achieved, any additional length of 50-ohm coax can be added as needed to get to the rig.

6. All results were solved graphically in SimNEC. It took longer to annotate the attached screenshots and write this reply than it took to create the SimNEC models and get the results. Once you learn SimNEC you can graphically create impedance transformations like this (and more) very fast with no math although it greatly helps to understand the underlying principles.

Dave NU8A
 

Hi Frank,

I'm a huge fan of yours, and very grateful for your countless contributions that continue to elevate the hobby. Many many thanks! Yes, I counted your suggestion of two 1/4-wave sections of 75-ohm coax in parallel as one of the best. Regarding effective bandwidth, that method will also be dominated by that of the antenna.

Best 73s,
Dave NU8A

On Mon, Nov 11, 2024 at 01:26 PM, Frank Donovan wrote:

Dave forgot to model what is likely to be the broadest bandwidth solution
using coaxial cable: Two 90 degree lengths of 75 ohm coax in parallel

73
Frank
W3LPL
 

You have a resonant antenna with an SWR of 2:1 which is completely inconsequential at 40 and 20 meters. There is no functional reason to do anything to correct this; you will see no improvement in transmit or receive from doing so. I understand the intellectual and experimenter's challenge in achieving an SWR closer to 1:1 but your rig will work fine at that SWR and you achieve nothing other than satisfaction of doing it. Personally, I would spend my time and effort elsewhere where it might make a difference.

Warren Allgyer - WA8TOD
 

some rigs are not that happy with 1:2 SWR
dg9bfc sigi

Am 12.11.2024 16:10 schrieb "Warren Allgyer via groups.io" <allgyer@...>:




You have a resonant antenna with an SWR of 2:1 which is completely
inconsequential at 40 and 20 meters. There is no functional reason to do
anything to correct this; you will see no improvement in transmit or
receive from doing so. I understand the intellectual and experimenter's
challenge in achieving an SWR closer to 1:1 but your rig will work fine at
that SWR and you achieve nothing other than satisfaction of doing it.
Personally, I would spend my time and effort elsewhere where it might make
a difference.

Warren Allgyer - WA8TOD
 

Other than the 2:1 reading on the SWR meter what negative effect have you experienced. 55 years worth of rigs, tube and solid state, I have never experienced an ill effect from a 2:1.

Warren Allgyer - WA8TOD
 

Consider the case of a 100 foot long, 50 ohm coax feedline on 20 meters. And let's look at three different types of coax. Below are the losses for 100 feet with a VSWR of 1:1

RG58 - 1.55 dB
RG8X - 1.10 dB
LMR400 - 0.47 dB

If we assume a VSWR of 2:1 those losses become:

RG58 - 1.88 dB - increase of 0.33 dB
RG8X - 1.31 dB - increase of 0.21 dB
LMR400 - 0.58 dB - increase of 0.11 dB

A 2:1 balun will likely have more loss than the loss caused by the 2:1 VSWR.

Warren Allgyer - WA8TOD
 

1:2 would mean SWR 0.5
A SWR less then one is impossible.

73
Peter, DJ7WW


-----Original-Nachricht-----
Betreff: Re: [nanovna-users] Dipole antenna with Low Z0 feed point Impedance
Datum: 2024-11-12T16:48:32+0100
Von: "Siegfried Jackstien via groups.io" <siegfried.jackstien@...>
An: "nanovna-users@groups.io" <nanovna-users@groups.io>

some rigs are not that happy with 1:2 SWR
dg9bfc sigi

Am 12.11.2024 16:10 schrieb "Warren Allgyer via groups.io" <allgyer@...>:




You have a resonant antenna with an SWR of 2:1 which is completely
inconsequential at 40 and 20 meters. There is no functional reason to do
anything to correct this; you will see no improvement in transmit or
receive from doing so. I understand the intellectual and experimenter's
challenge in achieving an SWR closer to 1:1 but your rig will work fine at
that SWR and you achieve nothing other than satisfaction of doing it.
Personally, I would spend my time and effort elsewhere where it might make
a difference.

Warren Allgyer - WA8TOD











 

Dave, Frank;

Wow! Thank you for the detailed suggestions.

Question: How would tuned stubs accommodate multiple bands? 40m (1/2 wave nom 66ft)/20m (nom full wave), maybe even 15m (nom 3/4 wave)?

Also, what if the dipole is not perfectly balanced (probably far from it?) and Z0 is not purely resistive?

Where can I read up on the theory behind using tuned impedance matching stubs?

I think I may need to move a cot into the attic. It seems I will be spending some time up there! 😊

Best,
Bill WA2WIO

-----Original Message-----
From: nanovna-users@groups.io [mailto:nanovna-users@groups.io] On Behalf Of DP via groups.io
Sent: Monday, November 11, 2024 2:25 PM
To: nanovna-users@groups.io
Subject: Re: [nanovna-users] Dipole antenna with Low Z0 feed point Impedance

Hi Frank,

I'm a huge fan of yours, and very grateful for your countless contributions that continue to elevate the hobby. Many many thanks! Yes, I counted your suggestion of two 1/4-wave sections of 75-ohm coax in parallel as one of the best. Regarding effective bandwidth, that method will also be dominated by that of the antenna.

Best 73s,
Dave NU8A

On Mon, Nov 11, 2024 at 01:26 PM, Frank Donovan wrote:

Dave forgot to model what is likely to be the broadest bandwidth solution
using coaxial cable: Two 90 degree lengths of 75 ohm coax in parallel

73
Frank
W3LPL

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