Hairpin match VSWR problem

[Michael Hatzakis]

Good morning and happy Sunday.

Have a problem with a post-maintenance VSWR, wonder if I can ask the group's advise.  

I just pulled my VHF/UHF beams down to replace the Azi-Ele rotors and while the antennas were down, I replaced the jumper cables from the mast mount pre-amps with LMR-400 cables, and also replaced all the connectors, otherwise, everything else exactly the same.  

My pre-install my VSWR was 1.15/1.2 to 1 measured in the shack, on both 2m and 70cm, and now, it is 2.4 to 1 only on 2m, 70cm is still 1.2 or so to 1 (yes, the install took longer than we thought, so I did not check at the feed point).   The only thing I can think I did differently is that my jumper cable now sits much closer to, what I believe is called a "hairpin match".  On 70cm the hairpin match is quite small and is no-where near the cable.   I tried to loop the coax a little away from the match, it is still a lot closer than before and concerned the metal of the shield is interacting with the match.  Otherwise, I cannot think of what else had changed to impact my SWR, the antenna is the same, the jumper cables roughly same length, any other ideas on what might be causing my high VSWR, see photos ?

Michael K3MH

[Roger - W7TZ]

Hi Michael,

I think you're on the right track. I would space the cable 2 to 3 inches off the entire length of the boom. Recheck you connector at the antenna.

Good luck.

73, Roger

W7TZ
CN83ia

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[Michael Hatzakis]

Roger,

Thank you for weighing in.  So you think that other metals near a hairpin match will change its impedance characteristics?   

The VNA plot certainly reflects incorrect matching rather than out of tune, see attached.  

Michael K3MH CN87

[Lance Collister, W7GJ]

Hi Michael,

Your recent VSWR readings may just be a result of having lower loss in the coax,
giving you a more accurate reading. If you have a hairpin match (presumably to create
a 50 ohm balanced impedance at the driven element) you will need some sort of 1:1
balun at the feed point to change the 50 ohm balanced feed to match the 50 ohm
unbalanced line. I know if you are using RF chokes at the feed point, they need to be
mounted away from the boom. Once you are back to the regular feedline, that can be
secured to the boom, as long as you are sure not to pinch it or kink it.

Some info on how I made and mounted the RF Choke on my 6m yagi is here:

https://www.bigskyspaces.com/w7gj/EF0609GJ%20by%20YU7EF.pdf

And a closeup photo showing how it was mounted is shown in the attached photo.

GL and VY 73, Lance

> IMG_5055.jpegIMG_5054.jpeg

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[Rod Johnson]

Michael,

  From what I can see from the pictures, I think you have what appears to be a 'T-match'.  Maybe I am not seeing it correctly. 

   A 'T-match is a balanced feed, and I see nothing to transform the unbalanced coax to a balanced feed.  Most commonly, a balun is used.

     Rod Johnson

Issaquah, WA

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[Michael Hatzakis Jr MD]

Rod,

That's interesting because this is an antenna array thats been up for 20 years, and for 15 years, it was working extremely well through the satellites, and I only stopped using it because the elevation rotor shorted out.  I don't know if I still have the documentation from it, I probably don't, but I bought it new and put it up myself and it was working extremely well for many years.  I suppose I could redesign it, but then I'm entering into a domain that I am not as familiar with

Michael  K3MH

On Sep 7, 2025, at 4:33 PM, Rod Johnson <we...@aol.com> wrote:

Michael,

  From what I can see from the pictures, I think you have what appears to be a 'T-match'.  Maybe I am not seeing it correctly. 

   A 'T-match is a balanced feed, and I see nothing to transform the unbalanced coax to a balanced feed.  Most commonly, a balun is used.

     Rod Johnson

Issaquah, WA

On Sunday, September 7, 2025 at 02:31:06 PM PDT, Roger - W7TZ <ai7rog...@gmail.com> wrote:

Hi Michael,

I think you're on the right track. I would space the cable 2 to 3 inches off the entire length of the boom. Recheck you connector at the antenna.

Good luck.

73, Roger

W7TZ
CN83ia

On Sun, Sep 7, 2025 at 12:57 PM Michael Hatzakis <ma...@hatzakis.net> wrote:

Good morning and happy Sunday.

Have a problem with a post-maintenance VSWR, wonder if I can ask the group's advise.  

I just pulled my VHF/UHF beams down to replace the Azi-Ele rotors and while the antennas were down, I replaced the jumper cables from the mast mount pre-amps with LMR-400 cables, and also replaced all the connectors, otherwise, everything else exactly the same.  

My pre-install my VSWR was 1.15/1.2 to 1 measured in the shack, on both 2m and 70cm, and now, it is 2.4 to 1 only on 2m, 70cm is still 1.2 or so to 1 (yes, the install took longer than we thought, so I did not check at the feed point).   The only thing I can think I did differently is that my jumper cable now sits much closer to, what I believe is called a "hairpin match".  On 70cm the hairpin match is quite small and is no-where near the cable.   I tried to loop the coax a little away from the match, it is still a lot closer than before and concerned the metal of the shield is interacting with the match.  Otherwise, I cannot think of what else had changed to impact my SWR, the antenna is the same, the jumper cables roughly same length, any other ideas on what might be causing my high VSWR, see photos ?

Michael K3MH

<IMG_5055.jpeg><IMG_5054.jpeg>

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<IMG_5055.jpeg><IMG_5054.jpeg>

[Michael Hatzakis]

Roger,

Thank you for weighing in.  So you think that other metals near a hairpin match will change its impedance characteristics?   

The VNA plot certainly reflects incorrect matching rather than out of tune, see attached

Michael K3MH CN87