Topband: Detuning TX Antenna

[Tom W8JI]

>
> The switchbox is a homemade relay box located at the base of the vertical 
> about 100 feet from my shack.  One relay adds in a loading coil at the 
> base of the 60'  vertical antenna for 160 meters.  When the shack switch 
> is turned off, the loading coil is disconnected and the antenna is then 
> tuned for 80 meters - the default position.  I don't need to detune on 80 
> since the re-radiated noise on that band is minimal.  Another relay  can 
> add/subtract  additional inductance to move the resonant point around 
> somewhat on 160 and from 80 to 75.
>
> I built an "on/off" switch on my desk powered by a 12 v power supply that 
> controls the power to the relays at the base of the antenna.
>
> I had planned to add another  12v relay which would just control the 
> voltage from the shack powers supply and it would be controlled by the 
> TVSU.  The TVSU would be used to control a relay that would simply turn 
> the power from the shack supply to the desk on/off switch off and on.   My 
> plan is to wire the new 12v relay so when I hit the key, the TVSU will 
> result (ultimately) in the relay at the base of the antenna controlling 
> the 160 meter loading coil being activated -adding loading coil so the 
> antenna would be resonant on 160 meters.  When I release the key, the 
> loading coil would be disconnected and the antenna detuned (back to the 
> 75/80 default position).  The TVSU would make sure the timing is correct
>

The problem is the speed of the unknown relay types and how well they take 
repeated cycling. Every relay might add anywhere from 2 to 30 mS of time 
delay in switching response. This is especially true if something loads the 
relay coil, like a low impedance or a diode, on relay release. Just adding a 
diode across a relay coil can slow a relay's release time significantly.

I wasn't expecting a homebrew answer, so I still have no idea what type of 
relay you have. I don't know relay speed and reliability. I guess the normal 
mode is closed on transmit and open on receive.

73 Tom