Topband: Tower Question (Long)

[W8AV at aol.com]

In a message dated 10/21/2003 11:49:32 AM Eastern Daylight Time, w0ah at a-1st.net writes:

>   Is there an easy
> way to isolate the cables going up the inside of the tower OR would it be
> easier to bypass the insulators so that the tower is 
> grounded- and then
> shunt feed it. 

  There is a way to isloate the coax and the rotator cables.  I do this on my 160 vertical since it is base insulated and my 20 and 40 meter arrays are mountes on the tower. You need to wind isolation coils for each coax cable.  Mine are harvested out of an old AM array sample system but you can roll your own pretty easily.  Wind about 20 turns of coax on about an 18" -24" diameter (you will need about 125' of coax for each coil with a 24" diameter).  My coils have the turns spaced about 1/2 inch apart. The shield of the coax on the tower side of the coil needs to be bonded to the tower and the shield on the other side of the coil is bonded to ground.  This provides somewhere around 85 microhenries of inductance (about 1.9 K Ohms of inductive reactance at 3.525 MHz).  For the rotator cable, I would a choke with about 50 turns of #12 wire for each wire of the choke.  The windings were stacked on top of each other and wound on a piece of 3" PVC pipe.  The ground wire for the rotator (#1 on a Hygain/CDE Rotator) is bonded to the tower of the tower side and to ground on the other side of the coil.  I then used 0.1 mf 5 kV bypass caps for the wires going to the pot in the rotator to ground to bypass the RF out of the indication circuit.  Install the caps on the tower side of the coil and also on the grond side of the coil. You need to do this with the Hygain rotators since you will get meter deflection on the rotator box from the RF in the tower and can also fry the zener in the meter circuit without this bypassing.  The other wires for the rotator need not be bypassed.

  With this configuration, I can transmit on 160 (in your case 80 meters) at the legal limit and not see any RF in the rotator box in the shack.  You also keep from shorting out the tower with this configuration.  One thing to keep in mind, install these isolation coils before you match the tower for 80 meters or you will end up having to touch up the match after you add them.  You will also have to touch up the match if you take any antennas and their associated coils off the tower at a future date.

  Hope this helps.

73.........de Goose, W8AV