Topband: 160m/80m duoband vertical question

[Richard Karlquist]

On 2013-11-05 06:27, Jim Garland wrote:
> My 160m vertical is 79ft high (wire running up a 26m Spiderbeam 
> fiberglass
> pole), with a capacity hat. Is it workable to put an 80m 1/4 wave wire 
> on
> the same mast join both antennas at the base, in the same way people 
> make
> multiband fan dipoles? The idea would be that when operationg on 80m, 
> the
> 160m antenna would present a high Z, and vice-versa when operating on 
> 160m.
> I don't have a feeling for how much interaction there would be between 
> the
> two wires, separated by only a couple of inches. Another option would 
> be to
> electrically isolate the wires and select one or the other with a 
> relay.  If
> possible, I'd like to avoid matching networks and complicated switching
> arrangements, because the antenna is 700 ft from my station. Any advice 
> is
> most welcome.
> 
> 73,
> 
> Jim W8ZR


I just rebuilt my 90 ft top loaded 160/80 vertical, also located 700 
feet from
my station.  It is made of irrigation pipe, but that detail
doesn't matter.  I put an insulator at the 60 foot point, because
the pipe is in 30 foot sections.  I put a relay across the
insulator that I can close to short out the insulator.  With
the relay energized I work 160m and when not energized I work 80m.
The stray capacitance on 80 is enough to tune the frequency of the
60 ft pipe down to the CW band.

The relay is a 30A "contactor".  I modified it to handle the very
high voltages on 80 meters as shown here:

QST, May 2009, Hints and Kinks, page 66

This relay has worked flawlessly for years, as opposed to
previous attempts with vacuum relays.  BTW, I have a string
of resistors across the relay so the top of the antenna is
always at DC ground.

The antenna is connected with 700 feet of homebrew open wire
line with 50/450 ohm baluns at each end.  The coil voltage
for the relay is sent over the OWL.

Except for the early problems with the vacuum relay failures,
this system has been very reliable.  At one time I had another
relay at 30 feet for 40 meters.  This is a little more involved
because I had to pass DC current past the 30 ft insulator to
energize the upper relay.

The antenna was on the air just in time to log K9W on 160m
and 75m this morning.

Rick N6RK