Build a Better Windom
Although the Windom off-center fed antenna has been in use for over sixty years, it has not been commonly used in recent years, at least in its original form.
Figure 1 shows the basic Windom design, recognizable by the location of the single wire feed point, which is 14% off center. This antenna requires either a pi-network or an inductor with a link matched to the output of the transmitter for proper coupling. Typical of many single wire feedlines, this version is often characterized by the presence of RF in the shack. This problem frequently can be dealt with by proper grounding of equipment and limiting the amount of feedline in the radio room.
RF A Side Effect
Figure 2 shows a modified Windom design. Although the half wavelength is still fed off center, in this case the feedline consists of two parallel conductors, normally either 300 or 450-ohm ribbon ladder line. In an effort to eliminate the feedline radiation of RF in the shack, a balun is connected to the twinlead and the system is coaxially fed. Obviously, the unbalanced coaxial feed can’t be properly matched to the antenna by the balun as the antenna is itself unbalanced. In this case, the balun is merely used as an isolation device.
Use a Choke Rather Than Balun
Louis Varney, G5RV, in the original design of the antenna bearing his call, tried a balun. He later commented that he had not fully understood the operation of his design and recommended the use of a coaxial choke, instead of a balun, to eliminate the radiation of RF from the coax cable feeding the antenna. Such a choke can be constructed readily by winding 10 turns of coax cable around a six-inch diameter form. This will “choke off” the flow of RF current which otherwise might have traveled along the outside of the coax shield, causing problem with distortion or feedback in electronic equipment and with sparks flying between metallic objects and the operator’s body. A second effect is the directivity if the antenna is affected in some degree, which is not necessarily bad.
Shack Construction
When I completed the construction of my new shack, I was fortunate to have three tall hardwood trees situated nearby which I felt would lend themselves nicely to supporting various antennas. Because of their spacing, I had room for an 80-meter dipole, but the feed point would have to be rather far from the shack, requiring a considerable amount of coaxial cable. A Windom, however, fit perfectly having a 70-ft. gum tree for a center support and equally tall oaks on each end.
Use RF to Advantage.
Antenna Advantages
An advantage of this antenna, by virtue of its size, is that the gain of the antenna reaches a 3-dBd on the 10-meter band. Additionally, the antenna is quieter on the lower frequency bands than a full-size vertical I also utilize. Using a MFJ tuner, the antenna tunes easily on all bands from 80 to 10 meters.
An understanding of the directional quality of this antenna clarifies the reason for the stated gain figures above. On 80/75, the antenna is a half a wavelength long and so, exhibits a simple “figure eight” pattern on each side of its axis. On 40 meters, being one wavelength long, the pattern resembles a cloverleaf, with two major lobes on either side of the axis. From that point, as the frequency increases, the direction of the lobes lie closer to the axis, e.g., 38 degrees on 20 meters, 30 and 25 degrees on ten. A series of minor lobes exist between the major lobes, but their configuration has not been calculated. The effect of the vertical section of ladderline seems to have the effect of “smoothing” the minor lobes together and filling in any existing voids or nulls. It must be realized that the directional analysis is theoretical, because I do not have an antenna test range, but experience with the antenna for years seems to confirm my calculations.
Take Some Precautions
A couple of simple precautions taken when assembling the antenna will considerably increase the longevity of the system. Under windy conditions, twinlead or ladderline exhibits a “ribbon effect”, which is a condition of mechanical resonance caused by the velocity of the wind and length of the line. This produces a rapid twisting of the twinlead back and forth, ultimately wringing the wire off at the ends.
Figure 4 shows a detail of the top portion of the ladder line and the center insulator. The vertical piece of PVC pipe holds the parallel conductor fast, yet, the portion without tape allows slight movement, so that the line flexes over a distance of several inches rather than one point. This should be repeated at the bottom of the ladderline where the coaxial balun is connected.
PVC Can Help
The ribbon effect can be lessened by taping a number of foot-long pieces of PVC pipe randomly along the length of the twinlead, at distances of six to ten feet. Again, the pipe should be taped in the middle and not at the ends.
Good Performer
I have been satisfied with the Windom antenna as described here. It is a good performer, it is cheap, and it frequently opens a new avenue of conversation in a QSO.
References
- ARRL Antenna Book
- ARRL Antenna Compendium
- Amateur Radio Handbook, ARRL, 1967
Originally posted on the AntennaX Online Magazine by John Somers, KC3YB
Last Updated : 6th March 2024