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Re: New Preamp



----- Original Message -----
From: "Edward Cole" <kl7uw@acsalaska.net>
To: <amsat-bb@amsat.org>
Sent: Thursday, September 20, 2007 8:44 AM
Subject: [amsat-bb] Re: New Preamp

> Wonder what your opinion is of the mfg-1801 low-power transmitting GasFet?
> This is what I am using for eme with fabulous results.
>
> Although I stated earlier having no RF interference that is probably
> not completely accurate.
> But it seems this preamp is very durable and not particularly
> susceptible to RF overload.  It was measured at 0.15 dB NF with 16.8
> dB gain at CSVHFS in 1999.
>
> With four 10-element yagis I am talking with 2m mobile and even a
> couple HT's over 70-miles from me.  That is on FM simplex.
>
>
> 73,
> Ed - KL7UW

Hi Ed, KL7UW

Following the designe of WA5VJB and WD5AGO described in references
1 and 2 of my previous message and the EME newsletter december
1990 from K2UYH I obtained a few power GaAsFET MGF-1801 and
MGF-2116 from KY7B and at 145 MHz preamplifiers I got NF=0.22 dB
with G= 25 dB and input IP3= + 1 dBm as measured during official
measurements at the eighth  Experimentation Symposium Orvieto Italy

My three prototype preamplifiers are fully described in italian with all
mechanical drawings in Radio Rivista 1/1994 page 20 to 25 , Radio Rivista
9/1994 page 20 to 24 and Radio Rivista 1/1995 page 36 to 40

Extremely low noise figure, very high gain and high level third-order
intercept point can be all obtained at 144 MHz using GaAsFET's designed
for power amplifiers at 10 GHz

This very interesting and unexpected effect was at first casually discovered
by amateurs in 1990 when WB5GKL needed a high dinamic range preamplifier
for a commercial application near 150 Mhz and he hoped that power FET
would handle the strong local signals.

Using 10 GHz power FET's WB5GKL obtained at VHF kind of like building
an HF 160 meters receiver front-end using a 4CX250 and the same results were
obtained and described in the already mentioned references 1 and 2 by WA5VJB
and WD5AGO respectively.

Probably the larger gate of power FET's devices for 10 GHz just work better
at VHF because the power FET's have 3000 times as much gate area than
the small signal FET's and so a very, very low internally generated noise,
but what's really happening must be still carefully investigated because
power FET's got poor noise figure at 432 MHz  and it looks like the present
FET's designs are going to be usefull for just 144 and 220 MHz

I am very happy with the performance of my above 145 MHz preamplifiers
but the only problem is that they are a very long coaxial resonant line
about 45 cm long so that for real life satellite work at 145 MHz I prefere
to use a Norton type preamplifier which is smaller and lightweight.

Best 73" de

i8CVS Domenico





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