[amsat-bb] Re: Using Preamps In Shack

Edward R. Cole kl7uw at acsalaska.net
Sun Mar 13 23:54:59 PDT 2011


The main advantage of a preamp is increasing your sensitivity by 
lowering the noise figure of your receiving system.  You lose some of 
that  by locating the preamp in the shack.  Any coax loss between the 
preamp and antenna adds directly to the overall NF:  80-foot of 
LMR-400 is 1.2 dB loss and added to a 0.5 dB NF preamp nets you a 
system NF = 1.2+0.5 = 1.7 dB.

But it will still increase the sensitivity of most radios that have 
NF in the 6 to 12 dB range.

Recently I lost one of my eme preamps on my tower and used a preamp 
at the shack.  My normal system NF = 0.76 dB.  In the shack I had 
0.25 + 1.7 dB cable loss = 1.95 dB NF.  That lowered my sensitivity 
by about 5 dB but I still received eme signals.

I would suggest you make a comparison for yourself.  Try the preamp 
in the shack and try it near the antenna and see how much that 
affects your receiving ability.  It is not hard to enclose a preamp 
inside a plastic box for outside use (a small sandwich box works).

73, Ed - KL7UW

At 08:36 PM 3/13/2011, Paul Delaney - K6HR wrote:

>Does anyone have their preamps in the shack as opposed to mast mount?
>Any major disadvantage to having the preamps in the shack?
>I just acquired two AR2 SPxxxVDG preamps and understand they are not
>weatherproof and would need an enclosure to mount near antennas, which for
>the time being is not possible. Any thoughts would be appreciated.
>Paul Delaney - K6HR
>paul.hamradio at verizon.net
>Sent via AMSAT-BB at amsat.org. Opinions expressed are those of the author.
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73, Ed - KL7UW, WD2XSH/45
BP40IQ   500 KHz - 10-GHz   www.kl7uw.com
EME: 144-1.4kw, 432-100w, 1296-testing*, 3400-winter?
DUBUS Magazine USA Rep dubususa at hotmail.com

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