Thursday, 12 June 2014

The 'Selby Shorty' Experimental HF Mobile Antenna

Ive been operating HF mobile for some time, using 'hamstick' type antennas. These are ok, but not the most efficient, or the smallest!

Now, generally, small isnt good at HF. And this is especially true when mobile. We want the biggest antenna we can safely use. But, invariably were talking loaded whips. If we can make the loading coil as efficient as possible, in other words nice and wide, airspaced turns, etc, then we can go some way to being more effective.

The ultimate goal of this experiment is to create a multiband HF mobile antenna, which will utilize an air spaced center loading coil, and a 'flying lead' system to select from tapping points on the coil to tune the antenna to different bands.

The first problem, is the air spaced loading coil. Such coils are commercially available, but unless your surname is Rockerfeller your unlikely to be in the market for them! I certainly cant afford the ones ive seen on sale! So, Its a case of, literally, rolling your own.

Some time ago, I constructed a former for just this task. Made from a section of 40mm diameter waste pipe, slit down its length, and with a length of hardwood strip inserted to 'open up' the slit to the original pipe diameter. Removal of the hardwood strip causes the former to contract slightly, allowing the coil to be slid off. This is all well and good, but of no use unless we can keep the coil in its form as it comes off! The trick here is to use some sort of spacer - in this case, lengths of 'flexible grommet strip' and liberal helpings of hot-melt glue.

The photos here show the construction of a smaller, thicker coil, using bare copper wire (ex mains flex) but demonstrate the principle by which the bigger coil was made.

Firstly, a strip of masking tape was wound onto the former, sticky side out. To this were stuck the cut lengths of grommet, with the slots facing outwards, equally around the former. I used three, but two or four could be used depending on application. The end of the wire to be wound is attached and secured by a bit of tape, and carefully wound around the former, each time resting into the slots on the grommet strips.


 Once all the wire is on (leaving sufficient 'tails' if required), a hot-melt glue gun is used to fill the grommet strip with glue. This is best done slowly and carefully, as the glue must completely fill the void and flow around the wire turns. Once the glue has set (ideally overnight!), the hardwood strip is pushed out of the former, allowing it to collapse slightly. The tape holding the tails is removed, and the coil slid from the former. If required, any bends, kinks etc in the wire should be straightened out first.


 With the former removed, the coil will look like that bellow. Push the masking tape gently inwards to separate and remove it.


And what you should be left with is a reasonably tidy, but at least functional air wound coil. The thicker the wire the sturdier it will be. The one below is for another antenna experiment.


So, anyway, onto the antenna! I had already made a 'mast' section from the cut down remains of an old hamstick. In order to put the metal ends on, a pair of hefty pliers/mold grips, a vice, and a blow lamp are needed! This one is particularly short, a mere 38cm long! I dont particularly expect good results from such a short mast section, but its enough to prove the concept.

The idea for this is in no way original. It is simply a variation on the well proven 'bug-catcher' design.
One problem with connecting a home made coil like this, is that in order to solder to the ends of the mast, the iron required is one from that mythical ham radio supplier that trades under the name 'B.F.O'! Not having a BFO soldering iron, I had to make use of the fact that previously this antenna section had been used for another trial, and already had enamel covered wire attached. Unfortunately, this wire, and that used to make the coil shown below, came form the same antenna supplier, who for reasons of saving a shekel somewhere used non-self fluxing enamel. This meant I had to laboriously and carefully scrape the enamel off with a knife.

The problem now was soldering, without melting the glue and causing the coil to unwind! This is easily solved by using a heatsink. In my case, a pair of locking forceps were clamped to the wire between the joint and the coil former! The wire tails of the coil were bent inwards and wrapped around the mast fibreglass section before being soldered. The solder joining the loop of each tail to itself and its connection to the antenna ends.


Not shown of course is the whip, just over 1m in length, that attaches to the top! All that remains now is to mount the antenna on the car, and run the analyzer over it. I have not really made much effort to get the number of turns right for any particular band, but am hoping it might be in the ballpark of 20m. Once ive analyzed it and know its resonant frequency, the coil can be reduced to make it work on a ham band.

It is purely an experiment though, I dont intend using such a short mast design mobile on 20m! Its more to prove the fact that I can manufacture my own, effective, air wound antenna coils. The fact that the enamel is hard to get off also makes this coil less suitable for the multiband design, where bare copper is preferable. That said, if it works ok on, say 15m or 12m, or 10m perhaps, then it might find its way into my mobile radio armoury!

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