A few years ago I switched from centre fed to off centre fed (OCF) antennas. If the 80m OCF is fed with a 4:1 balun it produces a capable and flexible antenna which will cover all HF except 30m and some of 15m at better than 3:1 SWR, it is also a capable, though not ideal, antenna on 6m. In addition to its low SWR experience indicates it is a very capable radiator on all bands.

Conventionally OCFs have a balun, which is fed with co-ax, located at the feed-point and two arms of appropriate length attached to the balun. I use a 12m tall Spiderbeam (posh squid pole) to support my bush antennas and whilst it is a robust support it is not sufficiently strong to support a balun at the top. As a quarter wave OCF is nominally 200R at its fundamental frequency it occurred to me that feeding the two arms with 300R TV ribbon cable for the length of the Spiderbeam and putting the balun at its base may be a solution... and it seems to work very well.

Of course once one tunes away from the 80m fundamental the antenna impedance changes significantly and the TV ribbon is no longer a near match nevertheless the antenna continues to work well right across HF. There is some argument about the polarity of signals produced and/or received by an OCF but I suspect it operates in both vertical and horizontal and this is part of the reason for its efficiency - however this is guesswork on my part.

It is a complex antenna in that the currents are not balanced in either the arms or the TV ribbon and adding the TV ribbon required shortening the arms from their calculated length for a conventional OCF to achieve a good SWR so it's clear the TV ribbon is "antenna-ing" in some fashion. One of these days I'll model it.

The antenna comprises:
Two arms, one of 25m, the other 12m
10m of 300R TV ribbon
A 4:1 balun
RG58 to the radio

Invariably, when I erect the antenna in the bush, the arms are in inverted V configuration, I try to get the ends as high as possible although this is often only 2m or 3m - it doesn't seem to make that much difference. Additionally, the longer arm frequently passes across a tree branch again this doesn't seem to affect performance.

The TV ribbon connects to the arms at the apex and drops down 10m where it connect to the balun. RG58 then connects the balun to the radio.

The 'hybrid' balun combines a 4:1 Guanella balun preceded by a 1:1 common mode isolation balun. The OCF, by its nature, produces unbalanced currents on the co-ax sheath and the 1:1 helps reduce them. The balun shown below was kindly made by my good friend Richard (VK3TXD) and utilises two ferrite cores for each balun. Using two cores increases power handling and this balun should be good for better than 100W continuous SSB although it's unlikely to see more than 50W from me.

None of the dimensions (arms or TV ribbon) are critical but they do interact and my experiments suggests the above are around optimum for an 80m fundamental. I have used this antenna all across Victoria and the ground (mineralisation) doesn't seem to make much difference. The antenna runs a SWR better than 2:1 across all HF excepting 10MHz and some of 21MHz. A basic antenna tuner will easily produce 1:1 but is not essential.







Page last updated August 2018