So you want to build a multi band wire antenna

VK3XL

I have experimented with multi band wire antennas for about 15 years on and off and here is where I have ended up. The off center fed dipole or Windom antenna.

A true Windom antenna has a single wire feed, so a coax fed version of the Windom is more correctly described as a “Off center fed dipole”. I will describe how I made my Off center fed dipole and present some of the information I have collected of the internet which I hope explains a bit about how and why the antenna works as well as it does.

Firstly a bit of history as to how I arrived at this particular antenna in my quest for a wire antenna to cover as many HF bands as possible with a single antenna.

I have tried common feed dipoles, i.e. 80, 40 ,20 15, and 10 meter dipoles fed from a common feed point with 1 length of coax. The result was less than expected, with every added dipole to the original 80 meter dipole, the bandwidth of each band was reduced considerably, until the acceptable VSWR bandwidth on each band only allowed a small segment of each band to be below the 1.5:1 VSWR required by modern solid state rigs without the protection circuits reducing output power considerably taking effect.

Next I tried my hand at winding coax cable traps and found similar effects as the common feed dipole antenna. The more traps that you add the lower the bandwidth on each band.

Then I tried the conventional “lumped constant “ traps, you know the coil and capacitor type, of various designs, with similar effect.

Ok so it looks like we have to get away from coax fed antennas and build a tuner and feed the dipole with open wire line. Well this works very well, and I can recommend this as a good compromise if you either have a tuner or want to build one, but the drawback is the need to adjust the tuner for each band change, and the efficiency of the tuner may not be all that good across the whole HF spectrum. I wont go into details here , but you can search the net and find lots of info on tuner efficiency. Sufice to say it is quite difficult to design and build a tuner that has good efficiency across the entire HF spectrum without going to quite a bit of trouble. So being a lazy type I thought there had to be a better way.

I found an article on the internet about a neat little auto tuner and having recently purchased a IC706MK11G thought that it might be a good thing to have this auto tuner for portable operation, so a contacted the local dealer an purchased a LDG Z100 auto tuner. The Z100 tuner combined with the IC706 worked well with the open wire fed dipole and homebrew 4:1 balun at the shack end of the home brew 450 ohm open wire line, but I wanted to find an antenna design that was a bit more flexible than the open wire line fed 80 meter dipole. While on the LDG auto tuner forum someone asked the question “What antenna do you use with your auto tuner” and the majority of the replys were “windom” or “Carolina windom”, not knowing much about the windom antenna, I did some searching and found quite a bit of info on the net. I decided to give one a go and here is what I ended up with.

The antenna I made is a ½ wave dipole for 80 meters fed 13.5 meters from one end thru a home brew 4:1 Gunella style balun to 50 ohm coax back to the shack. A 6:1 Balun may work better than the 4:1 but this requires 2 toroids and a bit more work, remember I am a lazy type.

The of the off center feeding of the antenna is that the feed point impedance at 13.5 meters from one end (and 27 meters from the other end) will be between 200 and 400 ohms on all harmonically related HF bands to the ½ wave dipole frequency, in this case 80 meters.

Here is a VSWR plot of the antenna as originally installed without the use of a tuner

So as you can see the bands that show a relatively low VSWR are 80 meters 40 meters 20 meters 17 meters 12 meters and 10 meters. Although the minimum VSWR was still a bit high on most bands I had noticed that the Z100 tuner would not even try to tune on 15 meters and had heard that there may be a significant amount of RF on the outside of the coax on all bands.

So I added a choke balun to the coax just below the 4:1 gunella balun to see if this improved the ability of the Z100 to tune on 15 meters. The choke balun consists of 6 turns of coax about 100 mm diameter. Here is a VSWR plot of the antenna including the choke balun.

As you can see there was a significant improvement in the VSWR on all of the harmonically related bands except 80, which went up slightly, but still no joy on 15 meters. I didn’t expect too much, as all the literature I had read said that the antenna designed around a ½ wavelength on 80 would not work on 15 meters. Surprisingly the Z100 could now find a match on 15, but unfortunately the antenna does not radiate well at all on this band.

Here are some details and pix of the antenna components.

Here is a typical Gunella balun design. I used 14 bifilar turns of 1.25 mm enameled copper wire on a amidon T43 toroid purchased from TTS systems. The balun was installed in 2 X 50mm PVC pressure pipe end caps machined so that they socketed together. I.E machine a step on the outside diameter of the opened end of one end cap and machine a step on the inside diameter of the open end of the other end cap so that the 2 caps can be fitted together a bit like a medicine type capsule. A so239 coax connector was fitted to the bottom of the balun capsule and an eyebolt was fitter to the top so that it could be hoisted up the mast. Two stainless eyebolts were opened up to form rams horns to attach the antenna wires to, as well as 2 terminal post were installed to make the electrical connections between the antenna wires and the balun inside the PVC capsule.

Here is a pic of the balun container complete with stainless steel rams horns for supporting the antenna wire and screw terminals for making the necessary connection to the antenna wires. The “rams horns” were made by twisting open 2 stainless steel eye bolts so that there was enough gap to get the insulated wire used for the antenna thru the gap.

Unfortunately the image is a bit bright but I think you can see the 2 “rams horns”, the terminal posts and the eye bolt at the top of the balun housing that is used to support the antenna and feed line. My fingers hide the SO239 coax connector.

Well that is about it, so have a go at this antenna and I think you will find it a good compromise all band (minus 15 meters) antenna.

Mike VK3XL