[NLRS] preamps - response

[jim.k0mhc]

Dave, in addition to the good advice you've already received there is 
another configuration where a receive pre-amp may make a noticeable 
improvement. That is when you are using external "bricks" to boost your 
transmit power. Often, the relatively poor internal relays used to switch 
the RF around the power amplifier introduce significant loss in the receive 
path. Bricks usually have an internal receive pre-amp option available to 
help make up the receive signal loss but, often these are of marginal design 
(poor NF and too much gain).

In these configurations your options are to by-pass the brick externally 
with (expensive) low loss RF relays or introduce a good receive pre-amp at 
the antenna to:
1) set the overall system NF
2) make up the transmission line losses,
3) AND make up the internal receive path losses within the brick.

I've found that by-passing the brick's internal pre-amps (turning them Off- 
if an option or hardwiring around them) and then using a good external 
pre-amp mounted on the mast near the antenna is necessary for configurations 
from 432 MHz on up; depending on your overall transmission line loss.
    - At 222 MHz it depends on the details of your configuration 
(transmission line loss, brick receive path loss and transverter front end 
gain & NF).
    - At 50 & 144 MHz external pre-amps are usually not beneficial for 
terrestrial operation.

Yes, spend your hard earned, after tax dollars first on good transmission 
line (e.g., LMR-600, etc.) which benefits both the transmit and receive 
path. Then examine your brick's receive path to determine what your losses 
and potential gains are.

Pre-amp Unanticipated Consequences
As has been mentioned, pre-amps can be difficult for many reasons:
1) Overall transmit handling power capability (hot-keyed vs. hard-keyed)
2) Receive NF (considering internal RF relay and filter losses) - 
diminishing returns set in for normal terrestrial operation
3) Overload from nearby out-of-band signals
4) Overload from nearby in-band signals
5) Potential for zapping the GASFET due to improper sequencing
6) Potential for zapping the GASFET due to wind/rain static discharge 
(particularly while roving during very dry or rain storms!)
7) Potential for zapping the GASFET due to nearby RF fields (particularly 
while roving with close antenna spacing!)
8) Potential for introducing significant distortion in the antenna pattern 
due to proximity to the antenna (when mast mounted)
9) Service access (when mast mounted)
10) DC power distribution


Happy Holidays!

73, Jim
K0MHC


Come to the Central States VHF Society 40th
Conference in Minneapolis on July 27-30, 2006!
Info at www.csvhfs.org

----- Original Message ----- 
From: "Dave Aho - N9TTX" <n9ttx at yahoo.com>
To: "NLRS" <nlrs at mailman.qth.net>; "Steve Bluem" <ka9omy at arrl.net>; "Jay" 
<jpacker99 at yahoo.com>; "Charlies Midwest Monsters" <cvvhf at cvvhf.org>; "Bill 
Noble" <wgn at chibardun.net>
Sent: Saturday, December 17, 2005 11:00 PM
Subject: [NLRS] preamps


>
>
> A quick question:
>
>  I know the "purists will say to get a better setup
> etc.... but for the portable work/antenna
> swappping/etc... I tend to do, this is ideal for what
> I am doing at this time.  I just would like to know if
> the numbers are fairly decent.
>
>  222 preamp  RF(to 100W) or remote switchable, in
> shack, not mast mount, GAsFET with .6 db or better
> noise figure. 10-25db RX gain.
> under $150
>
>    Let me know on the thoughts.
>
>              Dave...N9TTX
>
> I modulate, therefore I am!
>               ...73, and all that jazz,
>
>
>                 Dave...N9TTX
>
>
> Always remember:  "It could be worse.....it could be raining."
>                             Marty Feldman from "Young Frankenstein"
>
>
>
> Come to the Central States VHF Society 40th
> Conference in Minneapolis on July 27-30, 2006!
> Info at www.csvhfs.org
>
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