Cormac, EI4HQ wrote:
>
> I've an 80m vertical that
I want to use as my transmit antenna for
> topband.
>
It's about 20.5m high.
>
> For reasons of weather
survivial and because of the antenna construction
> my
>
plan is to base load for 160m with all the inefficiences/compromises that
> flow from that decision. However, I've been wondering about
something;
>
> The antenna is supported by three
non-conducting guys. The guys are
> connected to the antenna at
13.5m above ground. They are angled downwards
> at
>
about 45 degrees to support points on the ground. This leaves 7m of the
> antenna above the guy support point.
>
> 1.
Could the guys (replaced with conducting material of course and
>
insulted before connection to ground anchors) be used to provide some
> kind
> of capacitance loading on 160m i.e. to provide a
capacitance "skirt" as
> opposed to a capacitance
"hat" and thereby reducing the size of coil
> needed
> at the base for 160m?
> 2. Would this be any more
efficient than a well made high Q coil at the
> base?
>
3. Has anyone done this? How did you deal with practicalities like
> taking
> the guys out of the circuit when the antenna is
being used on 80m?
>
> --
> Regards
>
Cormac (EI4HQ)
Cormac, et.al.,
I would suggest
leaving the 80 meter antenna as described and not using the guy lines
as loading wires for 160 meters...
I belive one is better off making
a heavy duty 80 meter trap for the top of your vertical and then adding
one or more top loading wires to bring the antenna to resonance on 160
meters. The trap will add about 5 meters/16 feet of inductive
loading, so a single top loading wire would be only about 14-15
meters long... 3 or more toploading wires would probably be about 9
meters long each, assuming they are nearly horizontal or slope down at
less than 30 degrees. Steeper sloping means longer top loading wires
to get the effective length(s).
Otherwise base load the
vertical as you describe and consider two coils in series to get the 50
ohm match at the antenna.... one in series with the antenna to get the
loading and one across the feedpoint to ground to bring the impedance up
to 50 ohms... Alternately, resonante the antenna at about 1800
KHz or lower and put a shunt capacitor across the feedpoint to ground
to get the 50 ohm match up in the band.
I like the two coil
approach as it is a short curcuit for lightening.
YMMV 73 and HNY 2001
George K8GG
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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