As featured January 2018, Practical Wireless
Author: Richard Constantine

Richard Constantine G3UGF describes a simple winter project that could be ideal for those forthcoming spring days when you fancy going out portable again. It even uses that champagne cork that you may have left over from Christmas!

Owning a small rescue dog, that suffered from separation anxiety meant foreign DXpeditions and 5-star hotels were off the holiday list. On the upside, dog friendly, country cottages are usually located in electrically quiet locations, ideal for relaxing radio holidays.

Brochure pictures frequently look ideal, with topband-length gardens, conveniently located trees and fences. However, personal experience includes discovering phone and overhead power lines across a garden in Northumbria and a high voltage, pole-mounted line transformer, ‘Photo Shopped’ away, in Devon. I could go on… 

A recent sortie into remote Somerset was most carefully chosen. It offered a newly converted property with post and rail fence, suitable for securing vertical masts. There was also an adjoining paddock plus convenient distant tree. Having packed a 10m telescopic mast/vertical antenna, an inverted-vee dipole plus a 40m longwire antenna for 160/80m, the reality proved be to very different. The allocated cottage was stunning but the stout fence posts turned out to be a newly built drystone wall, offering no anchor point at all. Completely ‘left-field’, the ideal longwire-length paddock was out of bounds − it was the owner’s helipad!

A Solution
I learned to cope with these sorts of eventualities a long time ago, discovering a solution almost by accident.

Not wishing to repeat a previous, less than family friendly experience of roadside operating from a cold, dark car; a damaged photo tripod I’d been hoarding for several years seemed to offer a simple, low-cost solution (and low-cost tripods abound on eBay and in charity shops).

Removing the broken centre post winder column revealed a hollow tube, too small for a large diameter mast but ideal for coaxial cable to pass through. The addition of several ferrite beads, anchored both ends with tie wraps, immediately provided a neat line isolator, perfect for dealing with temporary antenna systems. The RF chokes added weight and strain to the antenna base connections, so some sort of strain relief was needed. The answer was to use a wine cork at the bottom of the tube, shaved down to fit, with a push-fit, coax-sized hole drilled through it plus a tie-wrap inside the tube. 

Owning numerous HF mobile antennas, with different bases and threads, made and collected over many years, the choice of antenna base was also proving to be an issue. This was finally and easily solved by fitting an SO-239 chassis socket using self-tapping screws, plus a SIRO™ magnetic mount adapter, purchased online. This flexible solution allows for both HF and VHF/UHF antennas with either PL-259 or 3/8th UNF threaded studding, to be used and changed at will.

Dual- or tri-band VHF/UHF mobile antennas, terminated with PL-259 plugs, fit directly on to the socket and a short earth tag and link wire connected to the original post locking wheel forms an excellent groundplane to the 1.6m tripod legs – ideal for the 6m, 2m and 70cm bands.

For HF antennas it’s more practical to attach a ground wire and substantial spike to the same locking wheel. Because ground conditions cannot always be relied upon, I much prefer to deploy counterpoise wires rather than use an earth. Green Kevlar wire is ideal. It’s flexible, unobtrusive and easy to roll up and transport. 

Also fixed to the SO-239 socket is a short, heatshrink-covered lead and wander-type socket. Each counterpoise wire is terminated with an RS Components stacking plug, allowing more to be easily added, as required, 

Left unsecured they simply pull apart in the event of anyone tripping on a wire. This quick-release feature prevents the whole array from being dragged, upended or damaged. 

Counterpoise Wires
Differences of opinion abound concerning counterpoise wires. Do they need to be resonant lengths, 5% longer, on the ground, off the ground or what? Over the years I’ve tried them all, in a variety of locations. In practice, it often depends on the type of ground you are dealing with and the capacitance generated between the wire and the ground itself. Following sage advice from the late and respected antenna guru Les Moxon G6XN (see reference) that radials do not need to be resonant, I now use a variety of different lengths, typically close to amateur band lengths − 20, 10, 7.5 and 5m long. These deploy either as a 360° fan or are sometimes bent to the available space. On poor, dry, stony ground or decking, off the ground at a sloping angle appears to work better. (In some situations, wires can also be used independently, as HF sloper type antennas, in their own right.) 

Adjusting the position and height of the wires is often a simple way to bring the feed impedance into auto-tuner range but, to be sure, I never leave home without my trusty antenna analyser. For extra safety I use bright orange tent pegs as remote anchor points.

Anchoring the Tripod
On breezy days, with an HF antenna fitted, the tripod can become unstable, depending on the whip I use. Finding a way to anchor the tripod was paramount and easily solved. Small holes in the lower legs allow metal tent pegs to pass through each leg at a 90° angle, holding the tripod firmly in place. Some higher-end camera tripods incorporate retractable spikes − look out for those. A neat trick for decking, hard ground or breezy conditions is the use of a draw string bag (wash-bag) filled with local stones or sand, carabiner-clipped to the centre support, problem solved. 

Mobile antennas are very much a passion with me and something I’ve studied and played with for many years. Base, centre, top, mono/multiband and helically loaded − I have them all. Small/compromise antennas such as mobiles need all the help they can get. Centre-loaded, helicals and combinations are among my favourites. 

With the mounting system described, it’s possible and practical to add screw-together extender rods. However, it’s important that any band-resonating coil does not become located more than halfway up the now lengthened antenna. Beyond this point, the RF current vital for best radiation falls away sharply. To prevent this happening, a longer whip needs to be used above the coil, to compensate. You may also need to attach light, nylon tent guy line to the lower mast section, just below the coil, to keep everything stable.

While there’s never any real substitute for metal in the sky, in terms of performance, this simple back-up plan has rescued many holiday operations and something I wouldn’t now leave home without.

Reference
HF antennas for all locations. Moxon. L, RSGB, ISBN 1-872307-15-1.

Richard Constantine G3UGF
E-mail: [email protected]