Literature & Web Survey

Rather than reinventing the wheel, every antenna project should start with a quick literature study of similar published antenna designs.

Doing so for the multiband Windom or off-center-fed dipole antenna (see table below), leads to the stunning revelation that, over these many years, antenna dimensions and feed locations have not changed much from the original single-wire 500Ω design, whereas balun feed impedances have decreased to 300 or even 200Ω! This, of course, is reason enough to become very suspicous about published multiband Windom designs.

design	year	designed for Zin (Ω)	ltotal (m)	lshort (m)	offset (ltotal = 100%)	h (m) & configuration	bands (m)
W8GZ	1929	500	0.483λ	0.18λ	37.3%	single-wire-fed	single-band
VS1AA[1]	1937	500	41.00	13.60	33.2%	single-wire-fed	(80) 40 20 17 12 10
K4ABT	1949	200	40.5	13.4	33.1%	12.2 to 6.1	80 40 20 17 12 10
DJ2KY[2]	1971	300	42	14	33.3%
«FD4»[3]	1971	300	41.45	13.50	32.6%	≈ 8 inverted-V	(80) 40 20 17 12 10
«FD4»[4]	1971	300	inconsistent data in brochure†			12 to 8 & 3 asymmetric inverted-V	(80) 40 20 17 12 10
K8SYH	1970's	300	39.08	12.74	32.6%
I7SWX[5,6]	1988	300	41.0	13.5	32.9%
JA7KPI	1994	200	41.0	13.6	33.2%	11.0 inverted-V	80 40 20 17 10
K3MT	1997	450	42.06	12.65	30.1%	4.6 to 13.7 asymmetric inverted-V	(80) 40 20 17 15 12 10
ON4(B)AA	2002	300	41.00*	8.10	19.8%	13.5	(80) 40 20 15 12 10
ON4(B)AA	2003	200	41.00*	6.25	15.2%	13.5	80 40 20 17 10
W8JI	2006	300 or 200	41.8	8.35	20.0%		(80) 40 20 15 10
ON4AA	2007	200	40.66*	11.76	28.9%	16.75	80 40 30 20 15 10

Notes:
^†: End-insulator heights & separation do not correspond to angle at specified height.
^*: Antenna made of soft-PVC-insulated HO7 V-K 4 wire, described here.
(XX): VSWR ≤ 3÷1 only within a limited portion of the XXm-band.

As a matter of fact, I first made this table in 2002, when I held the call ON4BAA. It promptly triggered me to start using the power of computer modelling to study this poorly understood antenna. The outcome were two designs, a 300Ω and a 200Ω design. Both have feeding locations much closer to the antenna end than usual. This resulted in a much improved VWSR-performance. In 2006, Tom Rauch, W8JI, repeated these findings with a slightly longer bare copper-wire antenna.

However, two problems remained:

1. Although the OCF dipole has very broad band-segments, the VSWR-minima of either the 80m or the 40m-band can not be perfectly aligned to the respective band-centers. Also, designs that do better on 80m, lack 15m. Apparently, OCF dipole designs have so far been a compromise between these three bands. The renowned «FD4» antenna is not an exception; Have a look at the VSWR specification on page 13 of this brochure.
2. Furthermore, none of these designs offer 30m. This is a pitty, because most hams would put up a wire antenna especially for the lower bands and less for the higher bands where beam antennas are more predominant.

The next section will explain why OCF dipoles have these inherent limitations. Rest assured, both problems will be dealt with by the new center-loaded OCFD design already mentioned at the bottom of this table!