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Re: Have Distortion Problems-Can't Work FO-29

----- Original Message -----
From: "Lee McLamb" <lee-fl@cfl.rr.com>
To: <amsat-bb@amsat.org>
Sent: Thursday, July 07, 2005 2:19 PM
Subject: RE: [amsat-bb] Have Distortion Problems-Can't Work FO-29

> Another problem which I have here locally is that without a good filter
> ahead of the preamp my preamp gets overloaded and mixes the 2m second
> harmonic with the local TV56 audio subcarrier.
> 727.750 - (145.950 x 2) = 435.850
> The mixing of the TV56 FM signal with my LSB uplink sounded like VERY
> distorted SSB audio.  It wasn't until I accidentally left both rigs in
> FM during one pass and actually heard the TV audio that I realized what
> was happening.
> 73,
> Lee-KU4OS

Hi Lee, KU4OS

I have the same problems here and I use a passband filter ahead of my
home-made 70 cm preamplifier but we must be carefull with filters because
the input of a properly "noise figure" mached preamp does not look like
50 ohm and it can have an input VSWR as high as 8 or 9.
This high VSWR (looking from the antenna into the receiver ) normally has no
detrimental effect on receiver performance. Hovewer if a large multipole
filter is placed between the antenna and the preamp, problems can develope.
Such filters are designed to load into 50 ohm impedance but in this case the
preamp does not present 50 ohm load and the filter's insertion loss and
passband characteristics can be significantly altered.
This is why I use only a simple single pole filter in front of my 70 cm
preamp as recommended by his designer K3PGP early in 1977 and well know in
the 432 MHz EME circles because such filter should not be readjusted once it
is in place.
In addition the K3PGP filter supply GaAsFET protection because it consists
of a low Q halfwave transmission line with two diodes at the peak voltage
point and has an insertion loss of only 0.062 dB at 432 MHz or 435 MHz
depending on the frequency in wich it is tuned and provides the bonus of
adequate selectivity to -3dB points at approximately 412 and 466 MHz to
clean up the garbage at most locations.
The diodes are connected back to back in antiparallel at the center of the
halfwave transmission line i.e. at the peak voltage point so that if
accidentally RF power enters from the TX end of the line than the diodes
start to conduct and the center of the filter goes short circuit.
In this condition the halfwave transmission line looks like two 1/4 wave
lines with the common end shorted by the diodes so that the TX end and
the RX end of the lines are showing very high impedance and with 400 watt
into the filter less than 80 mW was measured at the output.
Contruction of the filter is very simple and can be described.
Get a brass square  tubing 1" x 1" ( 25.4 mm) and 6" long (152.4 mm)
Prepare two 1" x 1" square plates from brass-sheet and for both drill a
2 mm hole in the center of it.
One side of the square tubing is large 1" x 6" and it must receive 3
longitudinal holes.
A center hole diameter 15/64 (6 mm) is to receive an air variable capacitor
type MAV03A10 Microelectronics or similar from Johanson 0.8 to 10 pF
and Q > 5000 at 100 Mhz
Another hole must be centered  1" (25.4 mm) from one end of the square
tube to receive a connector and the diameter of this hole depends if a BNC
or N connector is used for it.
Similarly another hole must be drilled 1" (25.4 mm) from the other end of
the square tubing.
Instead to use screws to fasten the flange of connectors it is best to tin
solder the flange to the square tube at this point.
Now slip the air capacitor inside the square tube and screw up the nut
outside the 15/64 (6 mm) hole.
Get two diodes 1N914's or better of the HP 5082-2800 series and solder
the anode of one diode and the cathode of the other diode to the capacitor
Working inside the square tube is difficult and solder the other
ends of the diodes to ground inside of the tube is not easy so that it is
best to drill two small holes in the wall of the square tube and solder the
terminals of the diodes to ground simmetrically outside.
Now get a # 12 AWG (2 mm) silver plated copper wire at least 8" (20 cm)
Slip it into the tube and solder the wire to the capacitor stud and to both
the central pins of the input and output connectors.
Now carefully slip the copper wire in to the central hole of one of the
1" x 1" square plate and position the plate to fit the end of the square
Solder the plate with tin along four sides of the square tube including the
wire in the center.
Do the same to close the other end of the square tube and solder the wire
to the center of the other square plate.
Both connectors can be used for IN and OUT and they can be interchanged.
Since the connectors are simmetrically soldered to the center conductor
respective to the ground than the impedance of the load connected to one
port is seen unchanged to the other port at resonance.
To tune the filter is enough to place it between the antenna and the
preamplifier and adjust the capacitor for maximum output.
If an Automatic Noise Figure Meter is available connect the noise head
to one port of the filter and the preamplifier to the other port and adjust
the capacitor for the lowest Noise Figure.
Since the insertion loss of the filter is less than 0.1 dB depending on the
quality of used connectors than the overall NF of the system with and
without filter should be very small.
The drawing of this filter was publisched by Al, K2UYH in to the
"432 EME NEWS  April 1977 and if the above description is not completely
clear I can email the above drawing to everyone is interested to it.

Enjoy !

73" de

i8CVS Domenico
Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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