OCF-Dipole 60/30m

Off-Center-fed Dipole

Multiband antennas almost never cover 60m and 30m because the frequencies are odd. So i was looking for a solution to this. I heard about DL1JWD’s excellent software suite and the program „multi resonance finder“ as a part of it. When you run the demo version of this program and you enter „1 feedpoint“ as a parameter it calculates almost exactly the measurements for a ZS6BKW. It proves mathematically, what ZS6BKW found when he modeled and constructed this antenna. But the ZS6BKW provides no good efficiency on 60 and 30m, so I used the calculator to find a solution. First, i tried a center fed dipole, but no joy. So i had the software try 25 feedpoints, here’s the result. Hits #3 and #4 – a 23.8m long dipole, fed 28% (6.66m) from one end with 5m CQ553 looked very promising.

7 hits for a ocf dipole fed with CQ553 wireman cable, SWR below 1.2 on both bands. Very nice. Will this antenna work? My dummy load has perfect match on ALL bands, but is a lousy antenna.
The suite features another program, the Doppelzepprechner -double zepp or doublet calculator. It calculates the efficiency for wire dipoles, so i entered the parameters for my project, 23.8m overall length for the dipole , about 5m wireman CQ553 attached 6,66m (28%) from one end of the dipole. No tuner in use, a W1JR balun between coax and wireman. Here’s the result.

WOW! A windom-ish antenna but without a ferrite transformer, no tuner necessary, just like the ZS6BKW, but fed off center? 95% efficiency on both bands? I had to try this! Well, built that and it worked!

A common mode choke (W1JR, DG0SA or similar) between the wireman and the coax is a must. Failing this, the coax becomes part of the antenna and the good match your antenna analyzer measured when you attached it directly to the wireman cable will be f***ed up. Keep that in mind. If you need to shorten the antenna when you fine tune it, calculate the overall length to be cut off. You already know how that works, right? While it is simple to shorten a center fed dipole – you just cut half the calculated length equaly off of both sides, it’s different with an off center fed dipole. Assuming you need to shorten your antenna about 5,5cm , you shorten the shorter side 0.28*5.5cm = 1.54cm and the longer side (1-0.28)*5.5 =3,96cm.

In a formula:
o*X for the shorter side
(1-o)*X for the longer side,
where o represents the offset factor „OCF“ in the above Multi Resonanz Finder picture while X represents the overall length to be cut off.

Pretty good on 60m, a little long on 30m, but no problem for any radio. With a bit of tweaking …….
When you hear me on 60m or 30m, it’s this antenna @12m agl at my home qth , or from my /p qth, with an identical antenna, but 5m agl. Reports are always good.

The MultiResonanzFinder demo version does not allow shorter than 5m or longer than 13m ladder line AND is restricted to wireman CQ553, but if you run the full version of this nice software, you can enter different max. or min. lengths and even alter the properties , so you can calculate with , say, 600Ohm ladder line. This will find you an antenna for 60/30/15m with a max. swr about 1.7 , no problem for any built-in tuner. So, with this OCF-dipole, you can work on all bands a ZS6BKW can’t handle. Yeah, ok, 160m……

P.S. : use the Doppelzepprechner to check YOUR antenna. You might find out why it does not shine on some bands, despite a perfect match…