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
Have fun with it.
ON SIX METRES
David Macnaughton VK2BA
Copyright © David
tuning the halo
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.
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.
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!
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.
Radiation pattern of 500mm HALO antenna
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.
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.
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!
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.
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
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
tuning must be "right on the nose" or the performance will be poor
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.
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.
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.)
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.
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.