Hi Eric
Any wire close to 1/2 wave or a 1/4 grounded need to be detuned. The way to
do it is to split them in 1/4 wave segments. Even two 1/4 wires grounded in
one point become 1/2 if not properly grounded, a low RX dipoles can kill any
RX antenna too. Elevated radials also need to be individually disconnected
from the ground and from each other.
There are two ways to do that,
1- Using a brutal force choke >5000 ohms, this works well for reception, you
can use it on rotor cables, DC supply, and RX antenna wires, but it does not
fit well for TX.
2- Using a relay and physically disconnect the wire from the coax center, in
the case of a dipole or sloper it is necessary to open wires, the center and
the ground.
It's not about to change the impedance of the feed line. You need to remove
any connection with the wire and keep the capacitance very low.
I recommend a vacuum relay because it needs to be fast and handle high
voltage. A Russian W2W can do a good job. When you do that it is very
important to choke the relay feed line, so you need to use a choke.
Several friends have reported 2s a 4 s unit's reduction on EWE's and Flags
noise floor when them disconnect the wires on Inverted V, Inverted L, sloper
etc. When I detune my 1/4 full size vertical I can notice 2s units reduction
on the RX noise floor using my Waller Flags.
You have a good sequencer inside the MicrokeyerII and a good front-end saver
to protect the RX port and avoid Hot-key on the TX relay. Those will help a
lot.
Regards
JCarlos
N4IS
-----Original Message-----
From: topband-bounces@contesting.com [mailto:topband-bounces@contesting.com]
On Behalf Of Eric Rosenberg
Sent: Wednesday, March 10, 2010 12:16 AM
To: Topband
Subject: Topband: Help Needed Decoupling the TX Antenna
My noisy urban lot is small (40x140), and the Pennants are awfully close
to the TX antenna. (inv-l's on 160 and 80, 1/2 square on 40).
I need to decouple the TX antenna when listening on the Pennants, but am
not quite sure how to do it.
I'm using a MicrokeyerII for the radio and a KD9SV DXpedition
preamp/antenna switch/front-end saver for the RX antennas.
Once I get this straightened out, I'll be sure to work all of you who
heard me but I didn't hear in the CQ 160 and ARRL DX contest.
Any help is appreciated!
73,
Eric W3DQ
Washington, DC ("Roger, Roger DEE CEE!")
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