Topband: AM BC towers

[Telegrapher9 at aol.com]

Merv
sounds like top band heaven to me. Tell me if I'm wrong, but I assume the 
towers are 1/4 wavelength tall, base fed at 870 kHz. I can run a "real" tower 
model later when I get the dimensions from you. 

Using a "fat" conductor, at 1.83 MHz the base impedance is 614 -j575 ohms. 
One way to feed it is with an L-network. A series L of 20.4 uH and a shunt 
capacitor to ground (antenna to ground) of  283 pF will do it. To provide a DC path 
to ground at the tower base a 200 uH inductor can be shunted from the antenna 
to ground. In this case the shunt capacitor value becomes 323 pF.

Here are the stresses of the inductors and the capacitor when running 1500 
watts:

20.4 uH: 5.4 amps RMS
200 uH: 0.6 amps RMS
323 pF cap: 5 amps and 1300 volts RMS

Easy-to-wind inductors are still good for continuous key down at the legal 
limit. A capacitor plate spacing of 1/8" inch provides a 2.5:1 margin between 
operating conditions and arcing. I can run sims to see if this is enough to 
avoid arcing if the tuner is tuned at 1500 watts. A vacuum variable would be even 
better.

This looks pretty easy on the components. Inductors made of ordinary #14 
house wire on PVC can give Q's of 300 and can take continuous key down in this 
situation. If you need winding dimensions I can provide them. If you send some 
info on the lightning protection (arcing horn, etc.) an L-network can be 
designed to withstand the stress of a direct lightning hit.
What to do with the other tower? Later on it would be great to feed it and 
make this thing directional. For now, grounding the base seems OK. It barely 
affects the input impedance of the fed tower. The pattern is distorted by j2 dB. 
It still beats a 90 degree vertical in all directions at low take-off angles. 

I will be interested to see what others suggest, especially those with AM BC 
experience. 

     Dave WX7G