[amsat-bb] Re: Preamp mounting?

Edward Cole kl7uw at acsalaska.net
Sun Apr 18 00:46:41 PDT 2010


At 04:42 PM 4/17/2010, Greg D. wrote:

>Hi folks,
>So, a follow-up to my previous message.  All this discussion about 
>EME got me thinking...  We're always taught to put your preamp as 
>close to the feed point as possible, but as with everything, there 
>are tradeoffs.  Where should the Preamp be mounted?
>1 - on the antenna boom, at the feed point (or attached to the CP relay)
>2 - on the rotor cross arm
>3 - below the rotor, where the flex transitions to the main feed
>I'm thinking that #1 is a problem.  Yes, it's the shortest run, but 
>you've got the weight of the preamp
>at the far end of the antenna, unbalancing things.  Depending on the 
>antenna, it might be right in the middle of a bunch of elements, so 
>putting a metal box there might upset the antenna's performance or 
>impedance.   It's would also be subjected
>to the strong RF fields of the antenna during transmit.

Best place on the antenna is just behind the reflector 
element.  Weight should not be that bad.  Maybe a 100# 1Kw PA, but 
not a little preamp.  As one goes up in frequency it becomes more 
important to mount the preamp as close to the feed as possible.  It 
because of the higher loss in feedline as you go into the 
microwaves.  For 144 and 432, a few feet of LMR-400 will not degrade 
things too much.

>#2 is probably the best; fairly short coax run on the antenna side, 
>and somewhat outside of the RF field.

That is practical and what a lot of us do even on eme.  Run low-loss 
coax between antenna and preamp.

>#3 (what I have) is most convenient, and has one less set of 
>connectors to go bad, and one less wire to snake around the 
>rotor.  Weather proofing is easier when the object being proofed 
>can't move, and mounting the preamp on the tower, for me at least, 
>is a lot easier.  And, it's not THAT much more coax to the antenna.

I have to do it that way because I have an 18-foot long feedline 
coming from each of my antenna in my 2m eme array that are combined 
at the middle on top of the elevation crossboom.  Then a ten-foot 
coax jumper around the two axis of rotation to a hoffman box that 
contains my preamp and six coax relays (three are between the antenna 
and my preamp).  That is too much stuff to mount closer.  The result 
is that my 2m-eme preamp has 28-feet of coax to a single 
antenna.  That adds close to 0.5 dB loss ahead of the preamp (and 
adds 0.5 dB to the NF of the preamp).

73, Ed - KL7UW, WD2XSH/45
BP40IQ   500 KHz - 10-GHz   www.kl7uw.com
EME: 144-600w, 432-100w, 1296-60w, 3400-fall 2010
DUBUS Magazine USA Rep dubususa at hotmail.com

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