kenny@m.cs.uiuc.edu (08/22/89)
I recently picked up a surplus GaAsFET antenna preamp for use with the
NOAA APT transmissions on 137.5 MHz. My problem is now to integrate
it into the rest of the system.
It is documented to use a 12V (plus or minus 4) center positive power
supply up the feedline. My receiver doesn't provide preamp power, and
so I've got to find some way to get this into the line (The
alternative, of bringing it into the amp, means more wiring to the
antenna, something I want to avoid).
Do any of you electronics wizards see anything wrong with a device
like this: ?
##############################################
# #
Ant in -=------------------------------+--- .01 uF --=- Rcvr out
# | #
# 6.8 uH #
# choke #
# | #
##############################)|(#############
# | 1000 pF FT #
# | #
# 50 ohm #
# | #
+12V -=-- Switch --+--------+--------+--1K--+ #
# | + | | A #
# 1000 uF .01 uF LED #
# | - | | K #
#############*########*###############*#######
The device would be built in an aluminum box with the two SO-239 jacks
for the RF placed as close together as possible, and short leads on
the cap and choke in the upper section. The 12V would be suppplied by
a small wall transformer. There would be an RF shield (of PC board
material, or beer can aluminum, or some such) between the two sections
of the box. Except for the electrolytic, I'd be using ceramic caps
throughout -- I might replace the .01 in the RF section with a 1000 pF
silver mica if I have one lying around.
Another question I have is about the care and feeding of the GaAsFET.
The literature on the preamp warns that the transistor will get very
unhappy if the drain goes negative with respect to the gate, and that
this can happen on the positive swing of strong local RF signals if
there is a low-impedance path to ground from the center conductor and
preamp power is not supplied. Does this mean that the preamp must be
powered up continuously, or else disconnected to avoid a 50 ohm return
path to ground through the (powered-down) receiver? The gate of the
GaAsFET is `protected' by a pair of Schottky diodes.
| / o Kevin Kenny (217) 333-5821
|< /) | | | |/\ Illini Space Development Society o , o ,
| \ X_ \/ | | | P.O. Box 2255 40 07 N 88 13 W
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