Topband: Flags and loops in restricted space

n4is n4is at
Mon Dec 31 08:19:11 EST 2007


<< The TX antenna is located about 50-75 meter away from the RX antennas and
I am not sure if I would gain anything by detuning it. What is the simplest
way to find out? >> 

Any 1/2 wave length you have max reradiation and close to 1/4 you have it at
minimum, or odd 1/4 wave length.

A 1/4 vertical antenna when connected "with" the ground, become resonant,
the ground plane complete it. The solution is to disconnect the vertical
element from the ground and let it isolated form the ground. It becomes 1/2
wave on half of the frequency without the ground.

My tower is grounded so the solution was to create high impedance between
the tower near the base and the ground. I used a 3 wires skirt grounded at
the top and open at the bottom and it works like a 1/4 wave coaxial stub.

The way to know if you need to detune your TX antenna is disconnecting   the
feed line and listening with the RX antenna to check any noise level change.
In my case I can see 2 S units reduction in noise level using a EWE or my
Waller Flag when I disconnect the cable from the skirt.

This could be quite complex, I use a fast vacuum relay and a sequencer to
protect it. Also I use a speed up circuit to switch it faster.

Rotor cable and any other cable should be choked and connect to ground by
.1or .01 uF capacitor. Ii noticed an increase of noise when I was connecting
the rotor cable to it box. 

I have more information on my web page

You can see the detuning relay and two plots from the BWF, with and without
the skirt (grounding it with that yellow jumper cable). The RX antenna has
it pattern almost destroyed by the vertical antenna interaction.

The capacitor is because the length of the skirt is short and resonate
around 1900 KHz so it is necessary to tuned it down adding a capacitor to
ground, it is like you tune a 1/4 wave coax stub.


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