Topband: Transmit antenna de-tuning relay.

[Richard (Rick) Karlquist]

On 3/23/2019 10:57 AM, Herbert Schoenbohm wrote:
> My tower is a Rohn 45 90 footer and there must be some way of detuning it
> via the skirt adjustment means to make it non-resonant at 1825 kHz  This is
> similar to electrical towers being detuned near a direction AM station so
> their pattern is FCC compliant.   In some cases, the power companies used
> some drop wires to get the electrical pole to resonate outside the AM
> frequencies range. But we are talking about a pattern shift here and not
> reradiated noise.  There must be a way other than moving the RX antennas
> 250 feet away from the tower.
> 
> Herb, KV4FZ.'
> 

We should differentiate between "de-tuning" and "anti-tuning".  Simply
moving the resonant frequency away from 1825 is better than leaving it
at 1825.  However, it may not be sufficient to fix the noise problem.
What you really want to do is make the tower anti-resonant (that is,
parallel resonant) at 1825 kHz.  This is easily done by adding a sloping
drop wire that is around 150 feet long with the bottom end insulated
from ground.  When the length is correct, you will measure a very low
impedance from the end of the wire to ground.  This indicates that the
tower plus the wire is a half wave.  Now there will be a very high 
driving impedance at the bottom of the tower which will not draw much RF 
current.

The connection of the drop wire to the top of the tower should be
opened on transmit with a relay.

Another interesting thing you can do as an alternative to the
cage feed is to run coax up the tower to the top and connect
the shield to the tower and the center conductor to the top wire.
On receive you still short the wire to the tower.

You would need to model whatever you are planning to do to verify
that these off the cuff suggestions will work for you in your
situation.

Regarding your RX antenna, you can always arrange for it to have
a null in the direction of the TX tower.  You can get a pretty
sharp null with one of those phasing boxes along with a "noise
antenna".  I have operated SO2R in 160 meter contests by simply
having a very deep null in the RX antenna toward the TX antenna.
The TX to RX spacing was about 1000 ft, in compliance with the
1000 foot circle rule.

73
Rick N6RK