Using an LNA

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ZL2BMH
Posts: 26
Joined: Wed Jun 13, 2018 11:49 am

Using an LNA

Postby ZL2BMH » Wed Jan 23, 2019 3:21 am

Hello All.

From what I read and hear the closer to the weather antenna you put your LNA the better. Is this correct?
The reason I ask, I really would like to put my Turnstile or QHA on top of my amateur radio mast, which stands at 36ft. I am surrounded by some obstructions ,but 36ft would clear them.

On top of the mast I have a 5 element Yagi which allows me to put 500 watts out. If I had the LNA strapped to either WX antenna then in turn the wx antenna strapped to the mast probably about 2 feet above the Yagi would I fry the inner of the LNA even if it has an aluminium casing.

However if I joined the coax to either antenna ran the coax either into where my RSP1a is (obviously inside my shack--Bedroom!!!) a distance of 10 meters from the top of the mast, then attached it to the LNA, would this be okay or will I have another problem.

I could also run the 10 meters of coax to under the house, attach it to the LNA, then run another 5 meters to my RSP1a.

Some help on this would be greatly appreciated.
Regards,
Jim.

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g1hbe
Posts: 456
Joined: Sat Jan 17, 2015 3:28 pm
Location: Cheshire, UK

Re: Using an LNA

Postby g1hbe » Wed Jan 23, 2019 9:05 am

Hi Jim.
Pre-amps always give their best performance when connected close to the antenna. The further along the feeder to towards the RX, the less good they do.
It's easy to fry a pre-amp by putting too much RF on the input, and in your case there would be a very large amount present because of the proximity of the two antennas and the high power of your transmission. You may damage your RSP as well.
Moving the pre-amp along the coax won't help, as the two antennas are still co-sited and the input of the pre-amp will still see a massive overload.
AFAICS, the only solution is to keep the turnstile WELL away from your yagi. This will greatly reduce the amount of RF entering the pre-amp. It will still be a huge signal, but it is far less likely to do damage.
If you can take advantage of natural screening (trees, your house etc) between the two antennas it would be even better. Still a whopping signal, but better. Hope this helps.

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Andy

ZL2BMH
Posts: 26
Joined: Wed Jun 13, 2018 11:49 am

Re: Using an LNA

Postby ZL2BMH » Wed Jan 23, 2019 11:37 pm

An excellent reply.

One I expected but when the truth is told by another person somehow it suddenly makes more sense than yourself rolling and rolling it over in your own mind.

Thank you very much indeed. I really appreciated it.
Jim.

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