On Thu, 09 Aug 2007 11:51:10 -0700, Bill Turner wrote:
>After many years of using trap dipoles and individual switched dipoles
>I recently put up a set of three parallel-connected dipoles
>(160/80/40) and was very pleasantly surprised how easily they were to
>tune.
Yes, they work quite well. Some things I've learned.
1) The dipole for the lowest frequency band exhibits the SWR bandwidth of a
single
dipole. The dipoles for the higher bands exhibit a considerably narrower
bandwidth. This is predicted by NEC, and is consistent what I measure.
2) 3-wire dipoles for 20/15/10 are a very good match to 50 ohm cable, even when
very high off the ground. Single dipoles are a better match to 75 ohm cable if
they are high, 50 ohm cable if they are low.
3) HyPower Antenna Co sells a very nice 3-band dipole for 160/80/40 that is a 2-
dipole fan for 80 and 40, with loading coils for 160. I've got two of these up
in
the air, one at home and one at W6BX. They work very well. Google to find them.
Although the SWR bandwidth is pretty narrow on 160, it works over the bottom
100
kHz with a decent tuner. On 80, it can be cut to work well with a tuner from
3500-
3900. This antenna also works pretty well on 30M, 17M, and 15M.
4) It's easy to build fan dipoles with inexpensive materials. I start with a
standard dipole connector. I build spacers by cutting UV-resistant 1/2-inch PVC
conduit into lengths of about 16 inches for 3-wires and 9 inches for 2 wires,
then
drill holes about 3/4 inch from each end for the wire to pass through. I use
spacers about 6 ft apart on an 80/40 dipole, beginning with one near the
center. I
use #10 or #12 solid copper for the longest wires, #14 for the shorter ones.
5) One good way to hold the spacers in place is with split-bolt connectors
designed for splicing grounding wires together. I then lash the connector to
the
spacer with a UV-resistant Ty-Wrap and tape it up with Scotch 88. Another good
method if you're using bare copper for one of the dipoles is to solder a loop
around the spacer to hold it in place. The other wire still needs a split-bold.
I
get the split-bolts from McMaster-Carr. I use the smallest ones they sell.
Bill continues:
>I would suggest parallel verticals, 1/4 wl on each band.
Yes, when I have time, I'm going to try that too. I can string a wire between
two
redwoods that will hold up a quarter-wave wire for 160. I hope to have that
going
in time for ARRL 160 this winter. I expect 80 to be narrower, like the
dipoles,
but the feedline is short enough it doesn't matter.
73,
Jim Brown K9YC
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