Repeater antenna tuning — A sad but interesting find

Hi everyone! A few days ago, I was told that the antenna meant for use with the new repeater had a few tuning issues. They just couldn’t get the antenna SWR for the 2 meter band to be lower than 2. Feeling ambitious enough, I decided to try tuning the antenna with my friend, Yong Quan.

This is a picture of the antenna.










So, we laid it down on the floor and started to move sections of the antenna up and down to¬† try to tune it. Here’s the original VSWR plot before we started.








Doesn’t look too good for the 2 meter band, ain’t it? We moved sections of the antenna up and down, and even removed some of the radials of the antenna to try to isolate the problem. We removed the feed point from the bottom half of the antenna, leaving only the top part of the antenna connected to the feed point, like this:







This is the VSWR curve now:



We got a SWR of 2.6! That still isn’t good, even though it decreased. We suspected something was wrong, and we removed the top section of the antenna from the feedpoint, leaving only the feedpoint.


This is the set-up:











And this is the SWR curve:








The SWR curve looks just about the same! And do you realize something? The
lowest dips in SWR are frequencies off our target frequencies! In fact,
we found that the antenna has a fixed resonant frequency around 133 –
135 MHz, which is around 10 MHz off the 2 meter amateur radio band
allocated to us! What in the world is going on here?!?

We decided to unscrew the cover of the feed point. This is what we got:











Yup, a tuned LC circuit. Apparently, they tried to make the antenna resonant only at a specific set of frequencies using this tuned LC circuit. However they are off by quite a lot.

Well… I feel for the person who bought this antenna. It cost more than SGD $400. This was a sad, but yet interesting and informative find.

Anyway, thanks for reading my article!

Joyce (9V1AN)