Topband: High voltage and coupling from a vertical to adjacent coax

[K4SAV]

Kristinn Andersen wrote:

>I am working on a topband vertical, approx. 12m/36ft tall, with four
>top hat wires sloping downwards, approx. 6m/18ft long each.  Just
>about midway up this vertical I am contemplating mounting a dipole for
>the higher frequency bands, and running a coax from this dipole down
>along the vertical.
>
>
>  
>
That sounds like a very short vertical for top band.  It should resonate 
somewhere around 4.1 MHz.  The resistive part of the feedpoint impedance 
should be somewhere around 3 ohms on 160, which means it will take more 
than an inductor to match it. 

Adding a dipole at the halfway point and running the coax down the tower 
will cause the dipole to look like loading for the 160 antenna.  The 
coax will heavily couple to the tower, and since the dipole is attached 
to the coax, the net effect will be about the same as connecting the 
dipole to the tower, as far as the 160 meter antenna is concerned.   
Because that loading will take place at the half way point on the tower, 
it will lower the radiation resistance of the 160 meter antenna, 
increasing losses.

It is possible to remove a lot of coupling to the dipole by providing 
common mode chokes on the coax, both at the bottom of the tower and at 
the feedpoint of the dipole.  However your suspicion that there may be 
large voltages at the feedpoint of the vertical is correct.  Since you 
will have to match a load that is very low resistance and high 
reactance, the voltages present at the base will be very large indeed at 
1KW.  This voltage will also appear across the common mode choke, which 
may make it very difficult to fabricate a choke that will hold up to 
this voltage.  There may also be problems designing a matching network 
that can withstand this voltage as well (probably somewhere around 5500 
volts RMS).  Disconnecting the coax to the dipole won't change the 
coupling to the 160 meter antenna.

Consider a much longer wire for top loading the vertical if possible.

Jerry, K4SAV