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Re: New Preamp



----- Original Message -----
From: "Gary Memory" <gmemory@tks-net.com>
To: "'Edward Cole'" <kl7uw@acsalaska.net>; "'i8cvs'"
<domenico.i8cvs@tin.it>; "'Amsat-BB'" <amsat-bb@amsat.org>
Sent: Wednesday, September 19, 2007 7:12 PM
Subject: RE: [amsat-bb] Re: New Preamp
>
> Adding a preamp in my case is a total disaster.  I could probably put a
> diode in my coax line and then charge batteries with the resulting DC.
>
> 73!
>
> Gary, N7BRJ/DA1BRJ

Hi Gary, N7BRJ/DA1BRJ

Your case is similar to my situation in the bay of Naples and our problem
cannot be solved using commercially made GaAsFET preamplifiers because
they have a very low 1 dB compression point and a very low IP3 intercept
point.

Buy the way very rapid advances have been made in the last twenty years
in the development of semiconductors.It seemed that at first the field
effect transistor (FET) and then GaAsFET were getting more and more popular
over bipolar transistor, especially with respect to the noise figure from
VHF to UHF

However the modern bipolar transistor is still of importance in receive
technology,especially since a well know type of "Noisless Feedback Circuit"
developed by Dr. David Norton allow them to be made more linear and
exhibits less noise than a FET and greater IP and dynamic range
than a GaAsFET

Since desensitization and overload are becoming a serious problem
particularly in 2 meters as more and more amateurs started using high power
linears and large directive antennas, the Norton circuit seems to be a
modern solid state solution to the designe goals in those case like our case
when, for 2 meters satellite communications, an extremely low noise figure
less than 1.5 dB is not strictly necessary but, a very high IP and
compression performance may be more useful.

On today crowed 2 meters amateur band and the high incidence of very near
kilowatt transmitters, strong signal performance (1 dB compression and IP3)
in preamplifiers is usually much more important than sensitivity.

For the above mentione reason I have homebrewed a Norton preamplifier
for 2 meters using a BFG-195 + BFG-134 that exibit the following
measured performance:
Noise Figure NF= 1.4 dB
Gain G= 20.5 dB
1 dB compression point = + 28 dBm = 631 mW of output power and
since +28 dBm - 20.5 dB = +7.5 dBm = 5.6 mW it means that 5.6 mW
applied to the input of this preamplifier are necessary to compress the
gain of only 1 dB
Third Order Intercept Point IP3 = +22.5 dBm input

For 70 cm I have homebrewed a Norton preamplifier with a AT-42085
+ AT42085 that exibit the following measured performance:
Noise Figure NF= 1.3 dB
Gain G=19 dB
1 dB compression point = + 26 dBm output power
IP3  = +22 dBm input

For those who are interested to duplicate the above Norton preamplifiers
as well some special type of power GaAsFET preamplifiers having low
NF with high intercept point and dinamic range here are some useful
references:

1) High Dinamic Range Transistor Amplifier using Lossless Feedback
     by David Norton Microwave Journal May 1976

2) A new type of  Preamplifier for 145 and 435 MHz Receivers by M.
    Martin DJ7VY VHF Communications 1/1978

3) High IP - LNA for 432 MHz by R. Bertelsmeier DJ9BV DUBUS
     N° 2 /1992

4) Proceedings of the 24 th Conference of the Central State VHF Society
    ARRL Catalog Nr 3266 "Power GaAsFET Preamps, by Ken Britain
    WA5VJB pages 6-8

5) Proceedings of the 25 th Conference of the Central States VHF Society,
    ARRL Catalog Nr 3614 144 MHz GaAsFET Preamplifier by Tommy
    Anderson WD5AGO, page 35-36

NOTE:
Only a simple noise generator made with a diode and described in any
Handbook is necessary to adjust the above preamplifiers for the lowest NF

If the ENR of the noise generator is not calibrated the value of the
obtained noise figure will be unknow but it will be the minimum possible
obtainable for that device in that preamplifier.

73" de

i8CVS Domenico







































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