As an introduction to launch the new Liverpool Amateur Radio Club, ‘your new club’. I have devoted some time, in an attempt to encourage members to try their hand at making their own
aerial(s) which would not only save a huge amount of ‘spondoolies’ but would also help to enlighten and educate those of the rudiments about an aerial which is often lacking in the knowledge of many.
Particularly if the aerial has a design which allows it to be adjusted and tailored to suit any location where it is to be used. Further more, as we are ‘amateurs’ and not ‘professionals’ I always seek to
find a design which needs very little cost in its construction, and will fit into a small garden.
I am continuously reviewing developments in aerial design and having spent over fifty years in radio both professionally and as a radio amateur I came across the following : –
An adjustable coil loaded dipole aerial by Ken Maxted GM4JMU (Glasgow)
My first attraction was his call sign which reminded me of my old daytime QTH which was JMU. As I didn’t have a photo copier handy, to copy the original article from the Autumn 2016 Sprat
magazine, I spent an hour or two re-drawing the details and included some of my own comments as I thought
may be helpful.
To make up each side of the aerial, the original article suggests using SOTA-Beam green aerial wire which is PVC covered 2.2mm diam but most types of wire will work fine but it needs to be stranded type of wire to enable it to be flexible enough to be adjustable on the coil former. However it is not essential in order for the design to work.
The lengths used will be affected by the fact that the wire is covered with PVC. Each side of the aerial uses a total length of wire cut to 10.5 metres. Then measure from the centre Tee piece about 4 metres and dress the wire around the first peg on the coil former.
Wind twenty three turns over the length of the former, spacing the turns out evenly (in the original article the coil had only 21 turns actively wound in the coil assembly, but by its design the number
of turns can be adjusted easily) Then at the other end of the former dress the wire around the second peg, as shown in the diagram. Use some tie wraps to secure the wire from unravelling from the
pegs, but do not over-tighten them as they may be required to slip off for any adjustments. This is a slight departure from his original design which he suggested bunching up 21 turns in the
middle of the former, with a single turn to take up the end distances of the coil, but the coil ‘Q’ (Quality) Factor is compromised if this method is used.
One of the problems with any aerial which is loaded by a coil is the losses which the coil endures. By neatly spacing the turns reduces the losses to a minimum, this is because the capacity which
exists between each winding causes higher currents to exist in the coil causing higher losses. These losses can have a significant effect on an aerials behaviour. So in any coil loading it is always better to space the turns out as far as possible. After winding the coil continue a further 3 metres to create a total distance from the Tee piece of
about 7 metres. Any surplus wire is doubled back on itself when threaded through the end insulator. Finally the wire is bound using a tie wrap or similar to hold it over itself and around the insulator.
This type of aerial has the current maximised at the feed point so this is where it couples its energy into space, so that needs to be at a high point above ground. If only one support is available then the
so called ‘Inverted Vee’ arrangement can be used.
The original aerial uses 50 ohm co-ax as a feeder and not even using any balun. It will work that way and should present a low VSWR but it will be slightly quieter on receive if a current balun is
used at the Tee piece. This is because the aerial will be better balanced and therefore it will give better ‘common mode’ rejection to any local noise from TV’s or heating systems etc. A current balun
can be easily made by winding half a dozen turns of the co-ax through a ferrite ring.
The aerial as it stands will work well on 40 metres and also 15 metres and should provide a good match on most of the higher bands. The ability to be able to adjust the number of turns on the coil
and its position on the aerial length makes it very flexible to experiment for each band and even optimise it for your favourite bands. There are many coil loaded aerials out there but this makes it
easier to set up and adjust at any location. As stated it can be seen that a wire which will allow being flexed is desirable to avoid the wire from fracturing, if it is to be moved often. I haven’t yet made one myself as I am able to erect a bigger doublette aerial but I am contemplating making one of these aerials to use portable.
Another point worth mentioning is that the centre Tee feed could be fed using ribbon, either 300 ohm or 450 ohm type, or even ‘figure of eight’ twin lead. These are all truly balanced types of feeder
as opposed to co-axial feeder, but in the ribbon case would need a suitable matching network in the shack to make the aerial work efficiently. The twin lead being of a lower ohmic value (75?) may be
fed directly from the rig with a one to one balun. This method of feed would reduce the weight which a ferrite balun would add to the Tee piece if it is unsupported.
Enjoy the satisfaction of home brewing your own aerial and spend the money you save wisely.