A Reversible Parallel Slot Antenna
by David Billheimer W1GQL

In the Second ARRL Antenna Compendium, Russell Prack K5RP wrote an article entitled "Magnetic Radiators -- Low Profile Paired Verticals for HF". The article caught my attention a couple of years back and I decided to build one for the low end of 20 Meters. I was pleased and surprised at the results. I now had an antenna with vertical polarization that did not require a large system of radials. It also did not have to be way up in the air. In fact it worked quite well just a few feet off the ground. It compared very favorably with the ZL-Special antenna I had used for years. Actually it seemed better at DX than the ZL which I should not have been surprised at, since it does have a radiation pattern which is better at low take-off angles.

When using it, the only thing that I didn't particularly like was the fact that it was bi-directional. I live on the coast of Maine where Europeans are very easy to work. I speak Russian and love to work Russians in their own language both using cyrillic CW and a cyrillic font in Digipan which I use for PSK31. When I used the ZL Special, aimed at northern Europe, I rarely even heard stateside signals. With K5RP's antenna I was often bothered by qrm from the states.

This spring, I decided to play around with the EZNEC antenna modelling program that I have used for several years and see if I could come up with an antenna like K5RP's MU (multiple U), but with some gain in one direction and a usable front-to-back ratio. I read Lew Gordon's article in the Fourth ARRL Antenna Compendium "The Double Magnetic Slot Antenna for 80 Meters" where he tells of playing with some ideas suggested by John Brosnahan W0UN about improving the slot antenna. I started calling my antenna a "slot" after reading his article for convenience, realizing that it was really a pair of verticals fed in phase.

Lew doubled the slot in length and got some gain, but his antenna was still bi-directional. I decided to put two slots parallel to each other at about 1/8 wavelength spacing (that is what my ZL had) and see what happened. Since I had modelled my original slot a couple years ago, it was easy with EZNEC to copy the wires and put an identical slot beside the original.

I will never forget my delight at seeing that the pair of identical parallel slots did exactly what I wanted them to do! I had 7.17 dbi gain in one direction and -8.45 dbi in the other. Maximum gain was at 19 degrees and I had usable 2.05 gain at 5 degrees! I was excited. (I was to learn as I continued playing with the model, which I did for weeks, that I had a problem in my model with wires crossing and was getting readings about 1.25 dbi high)

My original slot was cut for 14.070 the 20 meter PSK31 frequency. The new double parallel slots resonated at 13.960. At 14.070 the new double slot had an SWR of about 3:1. At the resonant frequency I had some gain, but the pattern was much more bi-directional. I was not discouraged and continued to play with all sorts of parameters.

I found that the spacing between the two slots was not really too critical. In the end I settled on a spacing of 10 feet to be about optimum. A 14 foot spreader was about the longest I could imagine putting up in the air. That let me use 2 feet between the loops inside each slot and 10 feet between the slots. Modeling the slot higher in the air helped bring down the SWR, but going above about 16 feet for the bottom wires did not help as much as it would cause me problems actually putting the antenna up.

I found that, as I expected, if I made the second, non-fed slot slightly bigger it would improve the performance somewhat, just as a reflector about 5 percent longer than the driven element in a yagi does. It did help if I lengthened the long wires in the non-fed slot. It actually did not seem to be much different if I lengthened just one of them.

One day it occurred to me that I could easily make the array reversible if I could have a feeder going to each slot. I feared that having a feeder on the non-fed slot would interfere, however. What I discovered was that, if I made the feeder an odd multiple of an electrical 1/4 wave and left the far end open, I could have a second feeder with no detrimental results. Once I knew this, I decided I wanted to leave both slots identical so that I could reverse the pattern. I did find that if I made the non-fed feeder about a foot and a half longer it did not have much negative effect and did improve gain just slightly and definitely improved the front-to-back ratio. It seems to have the same effect as slightly increasing the size of the non-fed loop. I ended up with 36 foot feeders running to my remote antenna switch relay box.

In the end, after much playing around with the model in EZNEC, I settled on slots that resonated about 14.16 when they were alone and about 14.05 when put together. I got about 7 dbi of gain in the favored direction and a very nice SWR curve centered on 14.05, with SWR going up to 2:1 at 14.00 and 14.10. Since I only operate 20 meter CW and PSK31, this is a fine antenna for me. It obviously is not a multi-band antenna, nor does it cover more than the low end of 20. But where it works, It really works! In PSK31, on the Digipan waterfall, I can often make entirely different stations appear by simply turning the selector on my remote antenna relay control box. Signals that are strong when the array is oriented NE disappear completely when I switch it to SW and new signals appear. Exciting!

There are compromises involved. To get the kind of pattern I wanted I had to make the antenna SWR minimum lower in the band than I planned to operate at 14.070. What happens, if I interpret things correctly, is that the non-fed slot, even though it is identical in size to the driven slot, appears to be bigger because it is resonant at a lower frequency than I am using. If this is not correct I hope someone will correct me.

I put the antenna up in the same spot I had had the single slot. I bought a box of 25 electric fence insulators at a local hardware store for under seven dollars and used them at all 16 bends in the two slots. I ran the number 14 bare wire through the nail hole in the insulators and twisted some 8 inch lengths of wire around the wire on both sides of the insulators to hold them in place. This arrangement let me make final adjustments to the element lengths easily since I could untwist the 8 inch wires and slide the long wires through the insulators. Each insulator as a depression running around it where the electric fence wire is normally tied on. I twisted another short length of wire (I used insulated #10 this time) around the depression and poked the other end through a hole in my 14 foot spreaders and bent them on the far side so they would not pull out.

I made the long wires each a few inches longer than the model indicated and then when I had the antenna in the air I checked the resonance with my Autek RF-1 antenna meter. I was delighted to find an SWR of 1:1 at 13.93 mhz. I figured out how much I needed to shorten each long wire with a little math, after I remeasured each long wire. and with those two measurements in mind, I shortened the long wires, some more, some less to make them the same and bring the resonance up to where I wanted it. I got it to 14.02 and left it there.

At that point I did not have enough coax on hand to run my feeders to both slots, but I went ahead and put the antenna on the air aimed NE. That way I could compare it with my ZL which was still up a ways away. They compared very favorably. The ZL has higher gain but at a considerably higher take-off angle so it usually did a slightly better job into England and Spain. But when it came to working Asiatic Russians the Double Reversible Slot showed its stuff. I was working beyond the Urals easily and they were calling me one after the other. (It helps when they see my cyrillic characters come up on their screens I think!)

The real test came when I got addtional coax and was actually able to reverse the slots. The first qso I had in the reverse SW direction was Chile. I also was hearing west coast PSK31 stations that I had not heard before. I could switch my remote antenna relay and they would disappear and Europeans would take their place. Hard to believe I had done it! A nice way to celebrate being in my 50th year of ham radio.

If you are interested in this antenna, please email me at W1GQL@midcoast.com

diagram (15K) This is the pattern at the high end of the usable range. The pattern at the low end shows less gain and worse front-to-back. PATTERN (5K) pattern2 (6K) SWR (4K)

This is how I handled the 16 bends in the two slots

slotinsulator (88K)

I was eager to get the antenna up and used lumber I had around. You can see that one of my spreaders has bowed badly and will have to be replaced.

slot2 (383K)

Here is a shot of the ends with the crossing wires with short lengths of 3/4 pvc installed to make sure the wires don't touch where they cross. That's my ZL-Special antenna you see in the background.

slot1 (239K)