[amsat-bb] 1/4-wave 70cm Stub Fixes RX Desense

["Tom Smith" <cwdxer@xxxxxxxxxxxxxx>]

A few weeks ago I posted a question about desensing my downconverter when I
transmit on 70cm.  I want to thank all of you who sent suggestions!  The
problem has been solved.

I am using a 20 inch (51cm) offset dish and UEK-3000 downconverter.  The
problem was a strong (4 S-unit) raspy white noise when I transmitted on 435
MHz.

First, I evaluated the power to the downconverter.  I provide 12 VDC to the
power connector, instead of through the center conductor.  Wrapping the
power wires around ferrite cores (type 43 material) did not help.

Disconnecting the 2401 MHz antenna input eliminated the noise completely.
Thus, the offending signal is likely to be coming in through that cable.

I connect the dish to the downconverter by using a short cable with BNC
connectors.  An adapter, BNC to Type-N, is at the downconverter input.  Yes,
I know BNC connectors are taboo at 2401 MHz, but that's another subject...

I made a stub of RG-58 coax (Belden 8262, velocity factor 0.66) with a BNC
connector on one end, the other end was open.  Attached this at the antenna
input to the downconverter by using a BNC T-adapter.

The theoretical length of a 1/4 wave stub at AO-40 uplink passband center
is:

(12 inches/ft * 246 * 0.66) / 435.675 MHz = 4.47 inches (11.4 cm)

I initially made the stub longer, about 6.5 inches (16.5 cm).  Then measured
the noise, and cut the stub, until the noise disappeared.

When finished, the stub length was 4.94 inches (12.5 cm), measured from the
end of the BNC pin to the open end of the coax.  I did not continue to
reduce the length once I had eliminated the noise.  Applied black electrical
tape to both ends to weatherproof.

Tom Smith KB6A
DM13au, Anaheim, California, USA
Amsat # 18079
cwdxer@concentric.net


----
Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
Not an AMSAT member? Join now to support the amateur satellite program!
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org