As a ham radio enthusiast, I take great pride in designing, building and testing my own antennas. I’m not an expert but I’ve had success at VHF, UHF frequencies and I wanted to try longer range communications possible on HF bands. A full blown HF tower 60 ft. in the air is not practical at my humble suburban abode. Nevertheless, a row of trees in the backyard provided space to explore the capabilities of a homemade antenna.
The antenna described here is commonly known as a Half-Wave Off Center Fed (OCF) Dipole or it’s cousin the Windom antenna. It is designed to resonate on a fundamental frequency and its even harmonics, in this case the 40, 20 and 10m amateur radio HF bands. The form is similar to other dipole antennas i.e. long wires suspended horizontally with a balanced electrical feed. Except for the ferrite toroid cores, all the components are commonly available at your local hardware store.
Referring to my ARRL Amateur Radio Handbook, for #14 insulated Cu wire, a half-wave length for 40m (7MHz) was 68 feet according to the formula below.
Length[ft.] = 468/(Vf * Freq) = 468/(0.98*7) = 68 ft.
where Vf is velocity factor (0.98 here) and Freq = Frequency in MHz
I determined that erecting a horizontal 68 ft. antenna was feasible in my backyard. There is much debate about feedpoint location but I chose 2 nominal segment lengths of 23 and 46 feet for a 1/3 to 2/3 split. A major compromise to performance would be that the maximum height of the antenna is limited to about 20ft. as shown in diagram below,
Feed point impedance is a function of elevation above ground so I wanted to simulate my site’s configuration using EZNEC demo, a free EM field simulator program. The SWR results below indicate a minimum at 7.21 MHz (middle of 40m band). Here, Z = 61 ohms @ 0.61 deg. (close to ideal).
Indeed 3 resonant frequencies are shown, the fundamental, 2nd and 4th harmonics. While SWR is not perfectly 1:1 at all 3 frequencies I took the EZNEC results as a 1st order guide. The real physical antenna would be optimized on-site by manual pruning. This situation illustrates the purpose of an Antenna Tuner but alas, not in my budget.
More EZNEC results are shown below. Max. field angle is about 19 degrees, pretty close to 15 degrees, considered ideal.
The Azimuth radiation pattern is not perfectly circular but much more omni-directional than a dipole in free space. I suspect part of the omni-directionality is due to the vertical vector components of the inverted-V configuration.
Guanella 4:1 Dual Core Current Balun
The balun transforms the 50 Ohm coax feedline impedance to 200 Ohm at the dipole offset feed point. It also transforms the unbalanced coax to a balanced feed. This 4:1 current balun is implemented using 2 pcs. 1.4×2.4″ ferrite cores of type 61 material. This is the most expensive component of the antenna. Using these large size ferrite cores and #14 Cu wire give the antenna a transmit power capability of 1.5KW I estimate. This is the legal limit for any Amateur broadcaster. The circuit diagram is shown below.
Note that the copper wire windings are in opposite directions for transformer 1-2-3-4 versus 5-6-7-8. Fourteen windings were used for each. Using different colored insulated wire helps avoid winding mistakes.
Below is a brief Bill of Materials:
- ferrite toroid core, 2 pcs. P/N FT-240-61, ebay.com $20 each
- SO-239 bulkhead mount female UHF connector, 1 piece, ebay.com, $2
- 100ft. #14 stranded THHN insulated copper wire, Home Depot, $22
- Plastic electrical utility box, 1 piece, Home Depot, $12
- Strain relief hardware, 2 sets, Home Deport, $4
- Miscellaneous 8-32 eye-hook hardware, $5, Home Depot
Total cost: $85
Assembly is very straight forward. I used a long table and tape measure to measure wire lengths. A magic marker was used to mark-off each foot with longer marks for each 10 ft. and a double mark for the theoretical ideal length. An extra 1 ft. was added for strain relief and 1 or 2 ft. for length margin. Two wire ends are soldered to tabs and connect to separate through-hole assemblies.
After erecting the antenna and connecting to my transmitter, I characterized SWR values using the built-in meter by sweeping amateur frequencies across 3 bands. Typical values are shown below. To my delight SWR range was reasonably low across all three bands.
Again, SWR being slightly high and variable is a good reason to use an Antenna Tuner. But this minor mismatch is good enough for my transmitter. I wish I could characterize SWR over a wider frequency range, but something like a VNA or Antenna Analyzer is way beyond my means.
I am able to hear SSB voice stations from Alaska to Costa Rica to Hawaii to Pennsylvania and make voice contacts about 2,000 miles away. Low-power digital modes (e.g.FT8, JT65) using just 30W of power yielded QSOs as far as 5,000 miles distance.
Due to physical area constraints at my home I was trapped in VHF/UHF world for too long. But I found a way to construct this HF antenna and have enjoyed continent-wide communications ever since.
Although this antenna’s performance is highly compromised at my site, it was still one of the most successful and gratifying amateur radio projects I’ve completed. With no antenna analysis capability, I had faith that employing good design and construction technique would get me close and that would be good enough. If you’ve been holding off getting on the HF bands because building a grand ideal antenna is not feasible or too expensive, I encourage you to try such a low cost solution .
by KF7GAX, 73