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Author Topic: Go-Kit mounted UHF/VHF Ant. SWR problems  (Read 3501 times)
W4CPM
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Posts: 9


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« on: July 09, 2012, 12:46:20 PM »

So, I have built myself a go-kit and am having a problem with high SWR on it's 2M/70CM antenna.  Here are the specs:

Radio: Icom 7000
Feedline: 18" of LMR400 with Amphenol Silver Plated PL-259s
Mount: directly on a 2" UHF barrel fed through Go-kit's top
Antenna: Diamond CR270 2M/70CM

SWR readings (per an MFJ HF/VHF/UHF antenna analyzer):
420MHz 2.1 SWR
lowest point on 70CM band - 437MHz at 1.5 SWR
450MHz 2.3 SWR

On 2M the SWR is 6.3 at both ends of the band (after manually tuning the antenna for lowest SWR) with not much better in between.

Most of the time this station will not have access to an earth ground, so I have a common "chassis ground" for the antenna tuner and the radio.  When I first encountered SWR problems I guessed that grounding was the issue, so I added an 8 inch bit of antenna line connecting it to the chassis ground (as can be seen in the picture).  This seems to have little effect.  If I touch the antenna while checking the SWR on in the 2M band it drops from 6.3 SWR to about 4.2 SWR.

Here is a picture:

https://picasaweb.google.com/lh/photo/L4FLZHzRACNVw8-U_iKQu041P_D1qefOOjjBLk3hM7c?feat=directlink


What do I need to do to fix this?
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WB6BYU
Member

Posts: 13287




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« Reply #1 on: July 09, 2012, 01:23:42 PM »

The CR-270 is a mobile antenna that relies on a ground plane RIGHT AT the mount
for good performance.  A few inches of wire doesn't work as a ground on 2m.

There are two approaches you can take.  The first is to use a metal case and/or
line the inside of the case with sheet metal to make an adequate ground plane.
The other is to add 1/4 wave radials inside the case where the antenna connects:
bend them to follow the contour of the case and do NOT connect anything to the
far end - try to keep the ends spaced away from other equipment and metal
objects as far as possible.
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