David Macnaughton and his Fascinating World of Amateur Radio

50MHz HALO ANTENNA FOR MOBILE

CONTENTS

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 This article by David Macnaughton, VK2BA was originally published in Amateur Radio Action magazine Volume No.7, No.2 (on sale 19th June 1984)


 I have reworked the original halo manuscript for the Internet because I have had inquiries regarding the antenna, and the published article is now very hard to obtain.

The authors note in Year 2000: This antenna was designed for operation on 52.05MHz. We are all on 50MHz now and that is presumably the frequency on which a modern Six Metre halo would be designed to operate. Today, I would suggest that a Six Metre halo be centred on 50.125 MHz so that it can work efficiently in the DX segment of the band but will still be effective on 50.150 MHz and above. It may be better to make the loop 600mm diameter instead of 500mm to account for the lower frequency. Please read my additional notes at the end.

Have fun with it. 


THE HALO ON SIX METRES

David Macnaughton VK2BA

Copyright David Macnaughton 2000

David VK2BA tuning his halo

David VK2BA tuning the halo

Back in the crystal locked A.M. days on two metres a remarkable little antenna was used extensively by the author for mobile work. It was horizontally polarised, was substantially omni-directional and produced very little mobile flutter unlike many other mobile antennas. It was a mini-halo, made out of sixteen gauge, silver plated copper wire and was just 200mm (8") in diameter.

Recently it was decided to go mobile on Six Metres SSB  and once again I turned to the halo. Would it be a successful on Six as it was on Two? It was planned to construct two halos . . . one to experiment with construction methods and to fiddle with, and the second to be the final product. To date the final product has not eventuated. The rough one took two hours to construct and is still in use and performing remarkably well.In the five months since it was made, in November 1983, it has worked into Japan, New Zealand, New Caledonia, Norfolk Is. and all Australian states except VK1, using an FT680R with 10 watts out. It is also used daily talking to stations throughout Sydney while driving to and from work.

It was decided to pass on the basic information at this stage so others can build one also. Perhaps the final production will have to wait until the prototype hits a branch!

The HALO on the mounting bracket above the mudguard

 THE HALO THEORY

Essentially the halo is a dipole, wrapped around in a circle until its ends almost meet. Capacity is then added between the ends of the dipole to lower the resonant frequency of the loop. Alternatively, with added capacity the loop can be made smaller for a given frequency. We really have made a single turn parallel resonant circuit. The loop is then gamma matched to the coaxial feedline. The halo does have, however, a serious disadvantage. Because it is a very high Q tuned circuit it has a narrow bandwidth as can be seen from the plot of Frequency / VSWR in fig 1.   I like to operate within plus or minus 25khz of the centre frequency. It is easily put off by a knock or even drops of water hanging on it unless certain precautions are taken with construction.

Plot of Frequency/VSWR. Note the narrow bandwidth

Fig 1: Plot of Frequency/VSWR. Note the narrow bandwidth.

Radiation pattern of 500mm HALO antenna.

Fig. 2: Radiation pattern of 500mm HALO antenna

 

THE MECHANICAL CONSTRUCTION

I don't propose that this be an article on how to duplicate my antenna. Instead I will describe what I did and make a few suggestions and leave you to experiment and make improvements.

Keep in mind the fact that it is a high Q device and there is a very high impedance across the capacitively loaded ends. This means that very high RF voltages will develop across the ends even from a ten watt transmitter. It is quite possible to get an RF burn off the ends or draw a little arc to the lead of a pencil. This also means that any capacitor that you use to tune the loop must be of the highest quality. Fortunately only a few picofarads are needed and so it is easy to make a suitable capacitor out of brass plate or even metal from a Milo tin! A suggested method of making a suitable capacitor is shown in Fig. 3. I modified an old ceramic insulated air spaced variable capacitor for the prototype halo, most of the plates being removed leaving about 3mm spacing. This is adequate for ten watts but larger spacing would be needed for higher powers. The capacitor is housed inside a small plastic box available from Tandy stores. This keeps water off the plates that would otherwise detune the loop right out of the band. It also provides support for the ends of the loop.

NOTE by author in Year 2000: I have added a section at the end of the article in which I will pass on some of my later findings regarding making the halo work with an 80 watt transmitter. It is not as easy as it may appear and I even had an occasion when the tuning box CAUGHT FIRE!

My loop is made out of brass tubing of 4mm outside diameter. This is usually available from large hobby stores for an exorbitant price. Overall you will need a length of 1.66 metres (1.52 metres for the lop and 140mm for the gamma arm) but you may have to purchase two lengths and join them by sliding telescopic tubing into each end and soldering. NOTE by author in Year 2000: If you are making a 600mm diameter loop for 50.125 MHz as suggested in the comment at the start, then you will need slightly more brass tubing than indicated above. Alternatively, a brass screw of the right diameter can have its head removed before sliding into the ends to be joined and soldering. Again, a brass screw should be soldered into each of the other ends and these pass through holes in the plastic box and are secured by nuts on the inside. These screws also enable the mounting of the tuning capacitor plates.

I used 12.5mm wooden dowel for the upright and supporting crossarm. Fibreglass rod would be better if you can obtain it. A 12.5mm copper water pipe "T" was used to mount the crossarm at a right angle to the upright. Yes, it whips around in the wind but it has survived speeds of up to 110km/hour and has had several brushes with trees and hasn't broken yet.

Suggested construction of tuning box

Fig. 3: Suggested construction of tuning box

The gamma match was made from a 140mm length of the same brass tubing and was spaced away from the main loop by 14mm centre to centre. The matching arm was mounted in two places. At the coaxial feed end it was supported by a small coaxial feedthrough insulator. This handy little is hard to describe and harder to obtain! If you can't locate one then make up a little mounting insulator out of fibreglass. The other end is held by the series capacitor which is simply soldered across from the matching arm to the main loop. One would think it would be affected by moisture but my capacitor did not fail. Possibly some form of waterproofing would not go astray. I used a 27pf old style mica capacitor. I suggest that you experiment with different types of capacitors and values here.

Construction of Gamma Match

Fig. 4: Construction of gamma match

 

THE TUNING

The tuning must be "right on the nose" or the performance will be poor indeed.

Start the tuning procedure by mounting the halo well away from metal objects and measure the resonant frequency of the loop with a grid dip meter (or the solid state equivalent). Couple the dip oscillator to the loop near the gamma arm ie at the low impedance end opposite the tuning box. If the frequency is too high then increase the capacity across the ends of the loop and vice versa. Once you have the loop resonant then apply some RF with a VSWR meter in the line and again adjust the loop for resonance. You should see a sharp dip in the VSWR as you sweep the transmitter across the resonant frequency. Use the minimum power necessary at this stage to avoid damage to the transceiver output stage and also to avoid upsetting other band users. Now adjust the position and value of the gamma capacitor while ensuring that the loop has not gone out of resonance. You should see a VSWR of 1.1 to 1 when correctly tuned. It's all a bit fiddly but when you have tuned one halo and gained the "feel" for it you should be able to tune your next halo much quicker. You will find that mounting it on the car will throw it off tune but a quick adjustment of the tuning capacitor should pull it back to where it was. Seal the top of the box by smearing silicone grease on the lid before tightening the screws.

You will need to keep a VSWR meter in line and observe the reflected power frequently. The meter I use is of the single meter variety with a switch to select forward or reflected power readings. I leave it in the reverse power position and wind the sensitivity well up. As soon as I notice the reading getting a little high I retune the halo. Rain will cause the frequency of the loop to drop and the VSWR to rise but usually not far enough to worry about. I find it necessary to tune my loop about once a week. It only takes a moment and it really pays dividends.

Local contacts often comment on the small amount of mobile flutter produced by the halo and I never cease to be surprised by the way it "gets out" of valleys and areas one would normally expect to loose the other party. Interstate QSO's on sporadic E are often well over S9 and the comment frequently made is that my signal does not sound mobile (except for road noises that I really don't want to get rid of anyway. I think a little road noise adds atmosphere.)

It may take your family some time  to get used to people staring at the halo. I really don't understand why so many people find it amusing. Years ago I had a three element two metre beam permanently mounted on my car but people didn't seem to take as much interest in that as they do now with the smaller halo. I don't mind people showing interest but when they point and burst out in laughter, that really bugs me.

I hope that you get as much fun out of Six Metre mobile as I do. I find it a refreshing change from repeaters and much more of a challenge.

 

The VK2BA Six Metre HALO on the vehicle

 

A YEAR 2000 HALO UPDATE

            Having "revisited" the old article in the process of putting it on the web, I have thought of a few points that eventuated after the article was published in 1984. Since there has been a flurry of mobile halo construction in the last year or so then other halo experimenters may well find these extra comments useful.

            I did eventually make up the second halo and it was 600mm diameter (the unit described was 500mm diameter). It did not look much bigger, but less capacity was required to tune it. This made the step up to high power somewhat easier. The first time I pushed 80 watts into it was quite a shock. The tuning capacitor flashed over and I was unable to find any commercial unit that could handle the many thousands of volts that were developing across the "hot" end. I decided to make a capacitor but It was difficult to find a material that did not "cook" in this intense RF field. One evening as I was tuning up prior to a journey to the country the next morning, I was sitting in the car running carrier and observing the reflected power. It kept creeping up and up. I wondered what the problem was and when I got out of the car to have a look I saw that the plastic "Tandy" box had actually caught fire. Needless to say that was the end of it and I had to fabricate something else in a hurry to tune my loop. Most plastics simply melted. Even nylon screws melted. Materials that didn't melt were too lossy and killed off the "Q", making the performance poor. PTFE (Teflon) was the only material that I could find that handled the power without even becoming warm. and so I made up a variable capacitor using teflon sandwiched between plates of metal, similar to the suggested tuning box in the old article. That was when I realised just how good PTFE was as an RF dielectric, and being unaffected by ultra violet light as well, it is great for all antenna projects.

            The halo that I finally ended up with worked better than the early one because of the low losses in the PTFE tuning dielectric. The early unit would not have handled the high power. The ten watts or so were insufficient to show the losses up. So there is a suggestion - run high power, even if only as a test. If the insulator cooks or melts, then you have a lossy halo and you can do much better.

            The only other point that I want to mention is that many people have made up halos that have not performed as well as expected. This is nearly always a result of either high losses at the tuning end (as just discussed) or lack of satisfactory tuning. A halo is extremely high Q and must be tuned "right on the nose". When operating well it will have a narrow bandwidth. A broad bandwidth indicates high tuning losses. If one sweeps a transmitter across the passband of a well tuned halo, the reflected power reading will sky-rocket either side of resonance, as little as 50kHz away, while at resonance it will show 1 to 1. If your halo is not exhibiting this characteristic, it is not working as well as it can.

            A really good article on mini-halo antennas was published in Amateur Radio magazine, July 1965. It was reprinted from an earlier article by E. Postans, G4AC which was printed in "The Short Wave Magazine", January 1965. I think it was that article that caused me to build my 1966 series of 200mm diameter two metre halos.

David Macnaughton