[NLRS] preamps

[Gerald]

On Sat, 2005-12-17 at 22:00 -0800, Dave Aho - N9TTX wrote:
> 
> A quick question:
> 
>   I know the "purists will say to get a better setup
> etc.... but for the portable work/antenna
> swapping/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, GaAsFET 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
> 
> 
The effective NF goes up directly with the feed line loss before the
preamp. The basic radio's crunch level goes down directly with the
preamp gain (less the feedline loss). Often the crunch of strong in and
out of band signals in the radio negates the preamp's benefit. That can
be very dependent on location. I've modified my 220 module in my FT-736
to improve its dynamics, but in sight of a channel 13 TV signal I still
need an external bandpass filter to hear anything but TV at 222.1 MHz. I
don't plan a preamp until I can develop one with a really good NF and
only 2 MHz RF input bandwidth to keep the strong TV signal from
crunching the preamp device.

On 2m, W5UN found long ago that a 0.1 dB NF preamp on his 2m array was
nearly useless because the low loss wide band preamp input circuit let
local TV and pagers clobber the preamp device while using a more complex
input circuit that only allowed 0.5 dB NF rejected those out of band
signals enough that the preamp wasn't clobbered by them and he could
hear signals off the moon better with the "poor" preamp just because of
that front end selectivity.

Yes the 736 VHF front ends need more gain so the AGC threshold is only
10 dB above the noise instead of 30 but they have a bad front end gain
distribution made worse by terminating the IF port of the double
balanced mixer with a crystal filter instead of a broad band
termination. In my 220 module I've slipped in a MAV-011 between the
mixer and the filter and it has made it sound like a real receiver
without the annoying condition of having the run the audio gain wide
open to hear the weak signals (and then suffer hearing damage when
someone 30 dB stronger is heard). I hope to document this some day with
measurements on the extent of the improvement in sensitivity and dynamic
range.
-- 
73, Jerry, K0CQ
Dr. Gerald N. Johnson, electrical engineer
All content copyright, Dr. Gerald N. Johnson