Greg,

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?

Choices:

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@hotmail.com ======================================