Topband: Trap for 160m/80m inverted L
k9ay at k9ay.com
Fri Dec 9 14:32:16 EST 2005
> After being licensed for almost 45 years now I'm thinking I'd like to
> try my hand at 160m at my new QTH. I'd like to start with an effective
> and simple to install transmitting antenna and am thinking an inverted L
> would fill the bill. Since I also have some desires on 80m it would be
> an advantage to build a wire trapped inverted L .... snip>
> Jerry N8BM
If you have a little more room, a dual inverted-L may be feasible -- which
needs no traps.
When you need to share a single radial system and a single high support on
both bands, it is possible to feed two inverted-L's at a common feedpoint,
as long as the vertical wires of the two antennas are separated by enough
distance to reduce coupling to manageable level -- 24 inches works fine.
The top wires should be pulled out in different directions.
Matching is simplest with a shunt coil from the feedpoint to ground -- make
sure it's a really good inductor. I used 1/4-inch aluminum refigeration
tubing for the coil on a DXpedition vertical I made a few years ago -- 7
turns, 3 inch diameter, with spacing between turns about 1-1/2 times the
tubing diameter. The shunt coil has the advantage of increased reactance at
80M, where the feepdoint impedance will be higher. You should be able to
get an acceptable VWSR simultaneously on both bands. An unun should also
work fine for matching, and there may be a transmission line section
matching scheme that works, too.
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