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Vertical half-wave aerials for 4m

by Ross Wilkinson G0WJR (now G6GVI)

With the recent growth in activity of 4m FM, I have seen a number of requests for designs for vertical aerials for the band. This article describes three variants of a half-wave design which I have used successfully over the last few years.

Basic half-wave design

For vertical use, it is much more mechanically convenient to end-feed an antenna: this will allow it to be mounted higher, and avoids distortion of its omnidirectional polar pattern by the nearby mast. The disadvantage is that the end-feed point is high-impedance, and so requires a high-Q (i.e. narrow bandwidth) circuit to match it to 50-ohm feeder. However, as the FM section of the 4m band is such a narrow allocation (only some 0.4% of its frequency), this narrow bandwidth matching is no disadvantage (as it would be on say 6m, where an aerial may be required to operate over a 4% frequency range).

The matching circuit which I have used consists simply of a parallel-tuned circuit, resonated with a variable capacitor, and with the inductor tapped close to its "earthy" end to provide the low-impedance feed. The only difficulty in aligning this is setting the resonance correctly - it is not easy to find this using just a 4m transmitter and VSWR meter (but it is possible - I've done it this way several times). However, if you have a wider-tuning dip-meter or antenna analyser, this makes life easier.
The design of the coil will depend on the diameter of the former, and which variable capacitor is used (I have used a Philips "beehive" trimmer in most of mine). Two different coil designs are shown in the pictures below.
Once the resonsance is tuned to the required frequency, the tapping point should then be adjusted for lowest VSWR (you may have to re-tweak the resonance after moving the tap).

A simple parallel-tuned matching circuit used with half-wave end-fed antennas. This plot shows the VSWR against frequency for an end-fed half-wave, illustrating the narrowband nature of the matching.

As with all unbalanced antennas, it is useful to fit some kind of balun, or counterpoise arrangement. At this frequency, a counterpoise wire is conveniently only 40" long, and sleeve or choke baluns are easily made.

Practical designs

My first prototype of this aerial was simply a 80" length of flexible wire, suspended from a convenient point, with the end-matching circuit fitted in a 35mm film caninster at the bottom. This is an eminently portable arrangement, which does indeed "fold to a convenient pocket size"! It can be suspended from a nearby tree while out portable, or from a roof-timber in the loft.

This half-wave portable aerial can be tuned to 4m or 6m, and fitted with an appropriate half-wave wire. Detail of the tuned matching network, built into a 35mm film canister. The capacitor is a 'beehive' type. Detail of another matching network, built around a fibre-glass kite spar which supports the antenna wire.

The second version of this design was a more robust self-supporting form for permanent outdoor use. I found that my local kite shop sells 8ft-long white fibre-glass rods about 9mm diameter, with a hole up the middle. The radiating element comprises 21 SWG wire, which is threaded through the centre of tube for the top 80", and then out through a hole in the side. At this point, the wire is coiled around the tube for ten turns, to form the inductor in the matching circuit. All that needs to be added is a resonating capacitor, and the co-ax feed can then be tapped in. The feed point is protected from the weather using another 35mm film canister, with rubber grommets to allow the tube and feeder out. The wire is anchored at the top of the tube with a plastic cap, which also prevents rain getting in.
Best of all, there's still another foot or so of tube remaining, which can be used to attach the aerial to the mast, and as it's so light in weight, I found that a couple of heavy-duty cable-ties will suffice for this! The completed aerial is quite "whippy", but this does not seem to impair its performance, and the prototype has survived on the roof of the G3KAC club station for several years now.

The 4m vertical is inconspicuous on the top of the mast, above the 23cm and 4m beams and 3cm dish.

I completed the third version of this design in November 2002. This is a portable self-supporting version for mobile use, and was built from a number of old aluminium tent-poles. The design was similar to that above, but the metal radiating section ends in a plastic tube, around which the matching circuit was built. It lives in the boot of my car, and can be erected for operation within a couple of minutes of parking up. You can see it in operation on my home page.

The mobile antenna breaks into three sections for easy stowage, and the matching section is protected by a plastic cover. The protruding coil of wire is a choke balun. Close-up of the matching circuit. The inductor is wound around the plastic tube, and covered with white PVC tape.

I hope that these examples show that this effective 4m vertical aerial can be made to suit the operator's requirements from whatever materials are available.

Alternative design

I've recently tried an alternative half-wave design, based on the old favourite Slim Jim devised by Fred Judd G2BCX. This consists of a folded half-wave radiator, matched with a quarter-wave shorted stub. In order to fit in my loft, I had to fold the quarter-wave section so that it would lie horizontally, but if you have 10ft of "free space" then the familiar form of the original may be preserved.

A prototype 4m Slim Jim installed in a loft. The other aerial in the background is a horizontal 2m HB9CV.


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