40 meter Off Centre Fed Dipole

Here we have my design for an Off Centre Fed Dipole. These are sometimes called Windom style antennas. This version will give us 40 meters, 20 meters, 15 meters, 10 meters and 6 meters without an ATU. Other bands are possible with an ATU.

This antenna requires a 4:1 Balun.  If you need to build one, I have  a tutorial here : http://www.m0zpk.co.uk/4-1-balun/ This enables us to match antennas with a 200 Ohm impedance to the 50 Ohm required for modern Radio equipment. It will also let us connect a balanced antenna to an unbalanced load. In other words, we can use Coax to connect our antenna. This would not be possible without the Balun. Balun is short for Balanced to Unbalanced transformer. The Off Centre Fed Dipole or OCFD as the name suggests is longer on one side than the other. This is what gives us the 200 Ohm impedance.
There are lots of different versions of this antenna around and some different configurations. The plain OCFD is connected straight to coax. A variation called the Carolina Windom has a vertical radiation element. This is achieved by adding an RF choke below the 4-1 Balun. You will be able to find endless dimensions and configurations. I tried a few and was never very happy. So I decided to design my own.

We need to work out the size of the radiating elements. For this I spent many hours doing research and computer modelling. The best i found was a ratio of 62 % to 38%.

We can make this antenna in any size we want but i use the 40 meter band a lot, so i decided to use that as my base. So now we need to work out the size of the Radiating elements. First of all, we need to work out the size of a half wave dipole, for the lowest band our antenna will cover, this will also depict the size.

This is simple, its the 40 meter band ! so half wave is 20 meters ! BUT ! we are going to do this properly.

To work out wavelength we divide the speed of light by the frequency :-

30071

This gives us around 42 meters. which we divide by 2 to get the size of one half wave. Giving us 21 meters. Why is it 42 meters and not 40 ? 40 is easier to remember. Most bands are like this, so it pays to work it out !

so we have our half wave size of 21 meters. Now we need to find 62 % and 38 %.

The simplest way to do this is 21 meters X 0.62 which gives us 13.02 meters. and 31 meters X 0.38 = 7.98 meters.

Multiplying our half wave by 0.62 and 0.38 will give us 62 % and 38 % respectively.

So we have our sizes ! One side is 13.02 meters and the other is 7.98 meters. You should cut them a little larger so we can tune the antenna later. If you don’t, you may find it impossible to tune. Antennas are affected by objects in their surroundings. They can add capacitance and inductance, changing the antennas resonant frequency.
If you want to use a different base frequency the process is the same. Its best to work sizes out based on the parts of the band you use most often but here is a rough table :-

OCFD

Completed Antenna
Below is our completed antenna. For the driven elements, I have used steel core, plastic coated washing line from a supermarket. They are very strong, waterproof and cost £2 ! I am a big fan of using anything I can get my hands on for my antennas. To use the copper wire would have cost nearly 20 times more. I have copper antennas and have never noticed any difference in performance. You can see that one side is longer than the other.
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Common mode choke

OCFD-CHOKE
One more little thing with this antenna, well actually i do this with all my hf antennas. Is the RF choke. You can see this hanging below the 4-1 Balun on the picture above. Its very simple, just 4 or 5 turns of coax about 6 inches diameter. These are fed through type 41 Ferrite toroid’s. I have used 4 on either side as can be seen in the picture above. Once in place, wrap the ferrite toroid’s in self amalgamating tape to protect them from the weather. Type 41 toroid’s are good for RFI between 5 Mhz and 200 Mhz.
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You should place this about 2 meters below the 4-1 Balun.  This prevents RF getting onto the outside of the coax and then into my Shack. Preventing lots of nasty RF problems. I always use an RF choke like this just before the feeder enters my building on all HF antennas.. In this case it has an added bonus. It allows the feeder above it to radiate. This gives us some high angle vertical radiation. Which combines with the horizontal radiation of the rest of the antenna. This can be seen in the following Radiation patterns. Plotted while I was designing this antenna.

OCFD-40M
40 meter Radiation pattern

OCFD-20M

20 Meter Radiation pattern

A consideration in How to mount the Antenna.

strain
Above the 4-1 Balun is a dog-bone insulator. It is used to take the strain of the antenna, Balun and coax. The Antenna elements are just wrapped around it once. This is then sealed with Self amalgamating tape. This becomes more necessary at larger wavelengths or for long term installation.

For long term installation, you should also wrap the pl259 in self amalgamating tape. To help support the coax and choke. This will also prevent water getting onto the joint.

Tuning

Tuning the antenna is simple. The antenna was created for the 40 meter band and above. So the measurements are based on a 40 meter 1/2 wave dipole. Tuning is carried out in the same way that you would tune a standard 40 meter dipole. Its best to do this with the antenna in the position that it will be used. This can be important, objects around the antenna can change it. They can add or reduce capacitance and inductance. Changing the resonant frequency and subsequently the SWR.

Take SWR readings at 7 Mhz,, 7.1 Mhz and 7.2 Mhz. ( more readings are better ) If all is well the SWR will be low across all of them. If you find the SWR at 7 Mhz higher than 7.2 Mhz then the antenna is too short. You will need to attach a small length of wire. This is the reason we build antennas slightly bigger than they should be. So we can tune them later. If the SWR at 7.2 Mhz is higher than 7 Mhz then the antenna is too long. You can snip short pieces off both ends of the antenna, until the SWR is good across the 40 meter band. A beter way is to fold short pieces back along the wire and cable tie them. This will enable the antenna to be retuned at a later date. Remember you have to do the same amount to both sides !

Once the SWR is good for the 40 meter band, you can test 20, 15, 10 and 6 Meters. you should find that the SWR on 15,10 and 6 will be low. 20 meters may be slightly higher but hopefully below 1.7.

If like me you like to use Data modes, you will probably use 20 meters a lot. A small amount of tuning adjustment can lower the SWR on 20 meters. If it is too high to use. Go slowly and carefully to make sure you don’t loose the other bands.

At my QTH i use this antenna in a flat top configuration, you have seen this above. It is my main HF antenna and i use all the HF bands with great results. I have also used it as a slopper and inverted V at field days. This antenna benefits from being rasied as high as you can get it. It has been surprisingly efficient. I have made lots of great contacts on it. I have recently made 300 mwatt contacts to Australlia and the United states using WSPR on this antenna. This can be seen below.

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