[TowerTalk] OCFD: Should I be able to do better?

[Lux, Jim]

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.