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Tune Tram 1485



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1 minute ago, Luish19779 said:

Hi everyone. I am try to tune this antenna, but I am little lost on the chart. I want tone for GMRS. Right now the SWR is too high(4.5) if anyone help me to read the chart to get the right measurements. 


I would tune it to 465 MHZ, which means cutting it to the following lengths.

L1 needs to be trimmed to 15 3/8 inches
L2 needs to be trimmed to 9 13/16 inches

However, do not try and trim down to the specified length at once. (Trust me I know, I screwed this up once), Cut a little bit off at a time toward the desired length. After cutting L1 and L2 a little bit each time, re-attach them and measure the SWR again, and then cut again, etc.  You will slowly see the SWR starting to come down. The reason for doing this is to not accidentally cut it too short, in which case there is nothing you can do.

Side Note: You can get a really good SWR, DO NOT try to get a perfect 1 SWR. Is it possible? Yes. Is it easy? No. Most people consider any SWR lower than 2 as a " good" SWR. Me personally, I try to get it close to 1.5.

Also having said this, do not do it immediately! Some one else in this forum may have a better way than I know of. There is a wealth of knowledge here! 

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21 hours ago, Luish19779 said:

Thanks brother. I thought that it could be tuned by adjusting the screws and lowering or raising the elements L1 and L2

That may be possible to fine-tune once you get the lengths into the GMRS range.

Doing a frequency sweep with a UHF capable antenna analyzer (or VNA which tends to do sweeps automatically, rather than requiring one to slowly turn a knob) would show you where the SWR is lowest. From that frequency it may be possible to compute how much needs to be removed (it's simple for true dipoles, but when you have unbalanced legs or loading coils it falls apart somewhat https://www.hamuniverse.com/easydipole.html )


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