Topband: Dual band shunt-feeding tower on 160/80

[Herb Schoenbohm]

The interaction of a grounded steel tower, I have found, can be 
minimized somewhat by having the end of the 80 meter antenna more than 
25' away from the structure and supported with non conducting material 
like braided nylon or Phylistran and having the angle of declivity at 
least 45 degrees or even less.  There may also be some useful 
directivity in the direction of the slope caused by the tower acting as 
an a periodic reflector.  Another way of of feed the 80 meter section is 
at the bottom to ground by making the wire 90 feet of a 3/8 wave 
vertical sloper and fed to a ground system consisting of some radials 
and a series capacitor to remove the inductive reactance. The feed will 
be close to 60 ohms for a coax feed at that point.  Doing this would 
remove feed line interaction with both 160 and 80. Additionally it might 
be possible to bring the 80 mete bottom back to the 160 feed point and 
be able to have a common coax for both the 160 meter match (if it also 
is just a series capacitor to a shunt).  I am somewhat reluctant to 
advise something I have never tired this way...but the 90' bottom fed 
wire sloped off a 90 foot tower with the 25 foot insulated extension 
works well in the past for me with it's own coax feed.

73,

Herb Schoenbohm, KV4FZ .





On 8/26/2013 12:49 PM, N1BUG wrote:
>> I've done exactly what Herb suggested: hung a sloper for 80M off the
>> side of my 160M shunt-fed tower.  Neither antenna knows the other is 
>> there
>> and both work pretty well - meaning I'm satisfied with what I get out of
>> them.
>
> Interesting. I have tried verticals, slopers, inverted V's and other 
> 80 meter antennas on or near my 160 meter tower with little success. 
> The impedance of any such antenna seems to be severely altered by the 
> nearby tower. I suspect it depends on the electrical length of the 
> tower. Mine is close to an electrical half wave on 80 - 100 feet of 
> Rohn 25 with a 7 element 6 meter yagi, approximately  30 foot boom 
> sitting at 103 feet.
>
> At one point in time the 160 meter shunt feed could be made to provide 
> an excellent match on 80 simply by changing the series capacitance. 
> However, after I cut down several nearby trees that no longer works.
>
> 73,
> Paul N1BUG
>
> _________________
> Topband Reflector