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Re: Mast mtg preamp question



In a message dated 98-02-03 02:21:50 EST, you write:

> I haven't mast mounted my preamps before and I was thinking about doing it 
> this time.  What is the  practice? I was thinking about a short piece of
coax 
> to the preamp from the antenna located a couple feed away from the  antenna 
> feedpoint.   Are they ever connected directly to the antenna at the
feedpoint?
>   I was also thinking of mounting a plastic box under the antennas and 
> putting the preamps in there to keep them out of the weather. 

Hi Allen,

My "rule of thumb" for mounting preamps is to keep the coax loss between the
antenna feedpoint and the input to the preamp to less than the noise floor of
the preamp. On my current system, I use SSB preamps on 2M and 70cm. Their
noise floor is <.8 dB. I mounted them below my rotors on the tower for easy
access (they have adjustable gain). The coax I use between the preamps and
antenna feedpoint is AirCom Plus. At the time I installed my system, this was
the only coax that had low enough losses and was able to withstand being
twisted by both my AZ/EL rotors without deforming and thereby increasing
losses. My calculated loss on 2M is .2 dB and on 70cm .45dB. These loss
figures equate to about 15' of coax each. Every year I take down my system and
check it over carefully, including measuring the loss of the coax jumpers. It
pays to be extra careful in this area. Remember, once you've lost that signal
between the antenna and preamp, you can never recover it!

Good Luck!
73,
Mike
N1JEZ



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