Topband: 80/160 trap vertical
k6xt at arrl.net
Fri Feb 1 09:42:46 EST 2008
I use exactly this system. It consists of 20 ft of R45, insulated at 15
ft. Inside the R45 is R25 to about 75 ft. Extending out the top of the
R25 is a 2-3/8 aluminum pipe, adjusted to resonance on the low end of
80, about 3550 in my case. Currently there are 9 elevated radials 1/4
wave on 80 connected at 15 ft, sloping to about 10 ft. This ant is full
size 1/4 wave on 80.
Connected at the top of the R25 at about 65 ft AGL is a pair of traps
with short wires sloping down at maybe 30 degrees tied off to convenient
trees. These wires serve to cancel the Xc on 160, and to lower the
resistance nearly in half, from just under 200 ohms measured to just
under 100 ohms measured. The active antenna coupler does the rest.
Experiments with the loading wires could maybe produce a better SWR but
the wx here is currently not conducive to such tests. I chose to stop at
X=zero. This ant is 1/8 wave on 160. The loading is more like 2/3 of
full height rather than top loading, for ease of assembly. I tried a few
radials 1/4 wave on 160 but saw no change in measured feed Z and no
change in performance. As others have reported, the shorter radials seem
to work fine.
Two wires were chosen because EZNEC showed a pattern skew with only one
wire which is not manifest with two. More than 2 did not appreciably
change anything in EZNEC but would have added mechanical complexity.
The traps consist of a pair of toroids wound to resonate on 80 with
about one foot of RG8 used as a capacitor (open at one end). I made the
caps a bit long and trimmed the open end to get where I wanted.
EZNEC shows, and empirical evidence tends to confirm, that the 160
hardware has negligible effect on 80. There was no change in feed Z
measured on my Autek VA1.
Voila! Two bands, one stick! Only 1/8 wave on 160, it compares at least
favorably to the 1/4 wave 140 ft tower with 4 el 40 top load at K0YW
about 10 miles away.
If you're still interested I can send you the EZNEC file.
Originally I had grand visions of relay switching the loading wires or
some other byzantine scheme to get this ant on the two bands. A big
Thanks to N6ND for suggesting I try traps.
> I would be interesting in hearing any stories about trap verticals
used for covering 80 and 160.
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