|VK6AAL Windom Antenna for 80m/40m/20m/15m|
Balun 4:1 Unbalanced to Balanced
Approx 200 ohm Feed Point Resistance to 50 ohm Coax
Overall Length.... = 468 divided by the frequency, multiply by .195072 (= metres)
160 thru 10 Mtrs
Long Section; 468/1.8 MHz = 260 x .69 x .195072 = 34.995 metres.
Short Section; 468/1.8 MHz = 260 x .31 x .195072 = 15.722 metres.
80 thru 10 Mtrs
Long Section; 468/3.6 MHz = 130 x .69 x .195072 = 17.497 metres
Short Section; 468/3.6 MHz = 130 x .31 x .195072 = 7.861 metres
VK6AAL - Windom
80/40m Long Side 24.61 m Short Side 13.73 m Total 38.34 m
Why my 80/40 antenna was non standard lengths to fit it on the block I had to bend some of both ends at 90 degrees down and they came very close to the trees used to suspend them. This required some readjustments of the lengths to over come the the detuning this caused.
Before you rush out to erect a Windom after you have read all the glowing marketing reports there are several considerations.
What SWR can you tolerate on each Band?
While this is not a major issue on SSB an SWR of 2:1 can be quite accepable even if not optimum it won't cause you major issues.
When it comes to SSTV this can a real issue as SWR in essence means Reflected Power that will be dissapated as HEAT in your finals, that are already running at 100% duty cycle.
Now before you say why am using a Windom for 80m if this is the case there is a very simple way around this issue and it is NOT an Antenna Tuner as this just makes a Bad radiator into a Tolerable Load.
The answer is in making the Windom easily tunable or Optimised for each band that you intend to use it on.
Sounds simple well after several days of hauling the antenna up and down the tower on a halyard I learned several important lessons about Windom's and a few tricks that you can have for Free!
All the material you read on this antenna design fails to explain one of the simple yet very important facts that makes resonating them easy or hard.
Once you have done your calculations as supplied at the top of the page and inserted your 4:1 Balun and it is in place. Now if you wish it to operate on 2 bands lets say 80m + 40m check with a calculator if they are exact multiples at the centre frequency you wish to operate.
Example you may wish to use 3.630 Mhz and 7.055 Mhz
As you can see 3.630 x 2 = 7.26 Mhz
Now here is where the books and reality differ they say just centre it roughly for All Bands
Reality you now have something that does not really resonate on any band and is a 2:1 SWR every where as it is a poor match hence a really great Heat Generator that hears well and radiates average.
Those are not the results you are looking for to create good quality pictures at the other end.
The solution is so simple I can't believe it was not in any of the books....
Firstly don't mess with the Long Leg of the Windom the calculations will get you close enough.
99% of the tuning will be by optimising the Short Leg - all you are looking for is an approx 200 ohm impedance point for the balun to match the feed line. This will occur with all the complex computer models at close to the calculated lengths.
Now at short end of the Windom solder a 3mm Banana Plug Socket. Now solder a 1.5 metre length of wire into a banana plug.
Tune the short end it just like you would a whip antenna, optimise it for the shortest length in my case it was 7.055 Perfect match and resonant.
Then once you have made the antenna for that length make a tuning stub for 3.630 Mhz. Now you have a perfect antenna for both bands all you need to do is plug the extension in when you QSY to 80m.
If you have the antenna on a halyard use 250lb fishing lines at the ends and use plastic milk bottles with water in them as counter weights if you have some trees it only takes a few seconds to drop the antenna at the centre and the ends will lower and plug in the stub - if you are on SSB don't bother just use the tuner.
I have found if I just pull the short end down with the fishing everything else goes up then when released it all settles back down correctly as the halyard line is just long enough to allow the weight at the tower to touch the ground.
Now you have a Perfectly Resonant and matched Antenna on 40m + 80m that fits on a House Block.
By the way you can drop the ends of the antenna vertically with little noticable loss of signal if your yard is a bit small on the long leg.
Now here is a MUST even with a 4:1 Balun place a 6 -10 turn RF Choke out of RG58 under the Balun as you are at a high voltage point in the antenna the feed line tends to radiate and becomes a problem if it moves close to the tower in high winds.
I use a 1 metre pole to move the antenna away from the tower at the top and must take care which way the 20m beam faces to reduce coupling when using the Windom.
Good DX and see you on 80m and 40m soon.