[amsat-bb] (no subject)

Zach Metzinger zmetzing at pobox.com
Fri Feb 17 21:02:30 UTC 2017

On 02/17/17 14:17, Frank Staffa Jr. via AMSAT-BB wrote:
> Hello everyone. I just purchased a M2 leopack antenna system and I
> was reading about different coax cables to use for low loss. Since I
> am mounting the antenna only about 10 feet off the ground, 50 foot
> will be enough to go into the shack. I wanted to purchase RG8x but
> read that the best way to go is with LMR400. What I dislike about it
> is that its much wider and very stiff, and will make it difficult
> around bends. I also have the Az/alt rotator which I will have to
> loop each antenna to have enough for turns. Any other suggestions out
> there for an alternative to LMR-400?Thanks Frank KB2CWN

Hello Frank,

You could always consider a LNA at the antennas, which sets your system
noise figure early on in the receive chain and allows the use of RG-8X
or similar. It's best to buy a weatherproof LNA and mount it right at
the antenna/rotator with a short section of high-quality, low-loss coax
(or direct connect). You'll want to run DC up the coax to power the LNA,
if possible. Make sure your LNA switches itself out of the way when you
remove power!

Of course, you'll lose power as heat on transmit with the higher-loss
cable, but I wouldn't that is much of a problem with today's radio
output levels.


Remember that you only need enough LNA gain to offset cable losses and
your receiver's noise figure (NF). More than that risks overloading the
front-end of your receiver and causing undesirable non-linear operation
(IMD). A good place to start is about 13-15 dB of gain, if you need a
concrete number.

That's my view on it, and I'm definitely no expert. It works for me.
YMMV. :-)

--- Zach

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