Topband: 80m quarter wave on 160m... can support guys be used to provide capacitance loading?
billsstuff at gotsky.com
Thu Jan 27 10:21:37 PST 2011
This is a bit late to the subject but I have a 160/80 vertical with 3
dual-use wires as both top guys and loading elements. Seeking a better
signal on 160, my 80 meter Inverted L (an old TT posting) was converted
by extending the telescoping Al tubes (6') vertical portion to 17.5
meters and replacing the horizontal-ish upper wire with 3 loading wires
that are 17.2 meters long coming down from the top at about 45 degrees
off vertical.They are supported at 3 equally spaced angles around the
center vertical and act as top guys.
One DX benefit of the symmetrical top loading wires is there is a null
in the radiation pattern straight up.Non-conducting guys at two lower
levels are used as well.The original six 80m elevated radials were 17m
long at 3m height.Two of these in opposite directions were extended to
39.9m for 160 and elevated more to about 5m.The radials begin at the
base at 0.5m then up at a 45deg angle to the first radial support at 5m.
The MFJ-based raw impedance at 1.825 MHz is ~ 8-j12 which can be pretty
well matched with a "hairpin" inductance coil of about XL of +j18 ohms
across the input, giving SWR<3 over about 40 kHz.Getting a good match
required fiddling with the radial and top loading wire lengths to fit
with the limited raw impedance range of hairpin matching. On 80 meters
CW the resistance is about 100 ohms and the raw large inductive
reactance is tuning out with a series capacitor.Currently manual
switching between 160 and 80 at the base is required to select the
appropriate simple matching network.On 160 this antenna seems somewhat
better than average collecting 30+ new countries this season so far,
including several of the stronger Europeans from here on the west coast
on good nights.
It still seems okay on 80 but the input resistance rises noticably with
frequency so it is a poor match on 75.With more fiddling that probably
could be improved.
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