On 5/27/21 9:30 AM, Kevin Zembower via TowerTalk wrote:
Hello, all,
I'm trying to build my first Off-Center Fed Dipole for 40-10M. I'm
following the directions at
If you're looking for multiband, then a "fan dipole" is probably a
better bet - it's symmetrical, just needs a choke at the feed, etc.
The basic design is a 40m dipole, a 20m dipole, and a 10m dipole in
parallel, with 6-12" between the wires. (Commercial product is something
like an Alpha-Delta DX-CC). the 40m will also resonate on 15m.
Adjustment is a bit fiddly, because there is interaction among the
dipoles. What I do is "get it close" and then use an autotuner - the
mismatch if it's slightly off resonance isn't huge, so the loss in your
transmission line won't be huge.
Rig up a choke with a couple 31 mix 2.4" cores per Jim's choke cookbook.
A single choke will have trouble covering the whole 2 octave band,
although give it a shot, it could work ok - I have a box of cores, so I
just do more chokes because it's easy.
Easy to build. Works great as an inverted V on a single support.
If you don't have a tuner, make sure your support is on a pulley you can
hoist and lower. Get your measuring tool of choice out, cut the wires a
bit long, and do the "fold back or crumple" the wires to shorten it
iteratively until it matches where you need it to match. With
something like a NanoVNA it's easy to see where the resonances are
(although you can't get the whole 5-30 MHz band at sufficient resolution
with the NanoVNA 100 points) and a few tries will tell you how much
crumpling you need to do. There is a *lot* of interaction between the
wires.
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|