Radial Wire Antennas

Peter Chambers

Peter Chambers

Ever thought a Beverage antenna would be really great to try but you’ve lacked the room, umpteen hundred metres of wire, sufficient poles etc? Peter Chambers has an interesting alternative. Peter is an avid MW DXer and lives in the Hawkes Bay area of NZ (On the eastern coast of the North Island, near Napier, which is 240km South-West from the eastern-most point, East Cape). Peter is also and equally avid experimenter and has spent many hours fine-tuning his Radial Wire aerials, now he shares with us his secrets…

Why Radial Wires – Radial Wire aerials are a very simple and easy way of obtaining amazingly good radio reception on longwave, mediumwave and the lower shortwave bands. The Radial Wire aerial is simply a length of wire laid on the ground in a straight line. Connect the wire to the receiver as normal and connect an earth, the same as any longwire type aerial. The wire length & direction depends on the frequency and direction of the stations you wish to hear. This is not a practical aerial for your home location, not in a town or city anyway. If you are prepared to drive off to a good location with your radio and wires for an afternoon and/or evening of amazing listening from the relative comfort of your vehicle, then read on.

With suitable Radial Wires, a good receiver and a suitable location, you can look forward to receiving mediumwave stations from the Americas & sometimes further afield, e.g. you can get good reception of the North American Expanded Band stations (1610-1700kHz). The Radial Wires also pick up all manner of low-power SW stations in the tropical bands. You may gain some idea of the effectiveness of this system by experimenting at home. Lay 10-15 metres of wire in a straight line down your driveway or back yard. Operate your receiver from the car battery (e.g. via the cigarette lighter socket). Attach the Radial Wire to your aerial terminal, find a weak but copiable MW station. Now raise the whole wire length off the ground to around head height. They station should drop very significantly in strength or vanish entirely. This will give you an idea of how effective the Radial Wire aerial is on low frequencies compared to short above-ground wires.

Wire Lengths – Whilst a 100 metre wire is the ideal length for receiving DX signals in the 1600kHz – 1700kHz band, I have received Caribbean Beacon 1610, KXOL 1660, KDIA 1640, KGXL 1650 and WRNC 1670, as well as MW stations from Hawaii, North and South America with only a 40 metre Radial Wire. A 70 metre wire has much better pick-up on these frequencies though a 100 metre has more still. For 1600 – 1700kHz, exceeding 100 metres sees a slow DROP-OFF in pickup and even less pickup at 170 metres long. Lower frequency medium-wave and long-wave pickup may increase with wires over 100 metres, though a 100 metre wire has no shortage of pick-up right to the bottom of the MW band. For tropical band SW pickup, 20 – 40 metres works really well, the 70 metre and 100 metre wires showing at best only slight advantage over the shorter wires so a 100 metre wire is a good choice to work both the MW and tropical bands.

Earthing – The chassis of your car will act as an adequate earth for the receiver. The Negative of the cigarette lighter socket is the easiest connection point. For lower medium-wave and long-wave, the car chassis is not good enough for full pickup so if your location is noise-free enough, zig-zag a 40 metre wire length over a 4 metre x 4 metre square of ground and lay it to one side or behind the Radial Wire, not under or over it. Then connect one end to your receiver’s earth terminal The other option is to drive an earth-stake into the ground only recovering the stake afterwards may be interesting! Do not risk flattening your car battery with the receiver, a separate battery is a good idea.

Impedance Matching – A 100 metre Radial Wire below 2mHz has a surprisingly high impedance when run over dry high resistance ground, i.e. river bed shingle, stop banks, hillsides and beach sand above the high tide line, 500 ohms to 1000 ohms being the range. Radial Wires run over wet, low resistance ground have impedances more in the order of 200 ohms. If your receiver has a 500 ohm or greater High Impedance (often referred to as ‘Hi-Z’) terminal, then use it for Radial Wires. If you only have a 50 ohm Low Impedance (Low ‘Z’) terminal (often a large SO-259 socket), you will need a 9:1 or 16:1 balun, if you don’t you could lose half your signal. A good signal lift can be achieved by running two 100 metre Radial Wires in parallel, 1 metre apart, connecting both together at the receiver. Using the parallel wires will halve the impedance mismatch, and 4 parallel wires will also add even more gain and further reduce the mis-match. This can be significant on the X-band North American signals if your location is electrically quiet enough.) Baluns are easier, Paul Ormandy makes and sells 9:1 matching baluns, see his ad in the DX Times “Market Square”). Over wet ground the 200 ohm impedance matches to 50 ohm inputs pretty well. Precise matching gives no significant pickup improvement. Reactance of the Radial Wire is minimal and series tuning produces no signal lift on any frequency. Above 2mHz impedance drops closer to 50 ohms though on some frequencies it rises back to 500 ohms. This could be wavelength dependent but I can make no sense of the frequency versus impedance results so far. A 15 to 20 metre Radial Wire matches into 50 ohms from 4 – 6 mHz brilliantly producing very good pickup.

Low Angle Pickup – Radial Wire aerials receive groundwave signals and low angle skywave signals exceptionally well. Distant mediumwave skywave signals will punch through local electrical noise & NZ station flak significantly better than on a loop, short longwire or vertical. The difference is even greater on distant, weak NZ groundwave signals.

Directional Characteristics – The directional characteristics of ground wires are remarkable for the short wire lengths involved. A front to back ratio significantly better than an unterminated longwire, loop antenna or dipole. Rejection of side-on signals is also quite good and compares with a 200 metre terminated Beverage or longwire. And with the latter directionality drops off considerably as the frequency lowers on MW… this does not happen with Radial Wires. The directional pattern you get on 1650kHz is exactly the same on 600kHz and even 210kHz. Amazingly, a 70 metre Radial Wire is only a little less directional than the 100 metre wire and a 40 metre wire only slightly less than the 70 metre wire. The same occurs at lower SW frequencies too, a wire at 140º pulls in signals from Alvorada 2460, Educadora 2380 (recently inactive), R Provincias harmonic 2540 etc, far better than a 110º or 70º wire. The sharp directional patterns can be too tight, so try two 40 metre wires at right angles to each other, connected into your receiver together.

DXing Locations – If you’ve gone to the effort of getting your receiver working on 12 volts, rounded up some wire and are looking for a location to set up your Radial Wires, then do some advance thinking about where you will drive to. If you are keen to receive X-band North American stations or low powered tropical band stations, then location is everything. Look at your area map and find out the areas to avoid, e.g. local MW transmitters, airport or marine beacons etc. If you have two or more 2 to 5kW transmitters at one site, then you need to get at least 10km away and preferably as far as practical. With 50kW transmitters get at least 25km away, and at least 10km away from any GPS transmitters on 1817kHz. Longwave beacons (often found near airports) are fairly clean transmissions and usually don’t cause much trouble however a strong signal will intermodulate with local MW transmissions so they too, should be avoided. One can operate successfully within a couple of kilometers from a longwave beacon if it’s the only big signal hammering into the receiver.

Next consideration is power lines. Rule of thumb is 200 metres minimum away from 230v/11kV power lines, 1km from 33/66kV lines & 5km from 110kV lines. Power lines radiate less noise inland away from the sea because the lower air-borne salt level means the insulators stay cleaner.

Obviously you need 100 metres of traversible terrain in the desired direction. My Hawkes Bay Oceanside Beach fits all the criteria. It is very steep hillside, about a 60º slope downwards, just negotiable without breaking my neck. This steep downgrade of the wires does not appear to reduces signal pick-up or directivity. To give you an idea of electrical noise travel, my Radial Wire can pick up TVI from suburban Napier, 2km away from the nearest house or power line, at a level that degrades X-band reception. In intensive sheep and cattle farm country, another noise-source to be wary of are electric fences. You will soon notice the “tick, tick, tick” noise on MW and tropical band signals.

East Coast beaches are brilliant for receiving the Americas. All electrical noise sources are behind you and thus attenuated by the directionality of the Radial Wires, as are the majority of NZ MW stations. The high conductivity sea enhances the strength of incoming low angle skywave signals and all the better if dry hill country or cliffs are behind you. You know you’ve found a great location when you attach a 100 metre Radial Wire to your radio and hear no increase in background noise from 1600kHz to 7mHz and only natural static. My Ocean Beach location is that good, a 45 minute scenic drive from Napier City, a wonderful view out to sea and up and down the coast, right from where I park the van.

In the middle of the North and South Islands and on the West Coast, look for places on the Eastern side of high hills and mountains. The Stratford Plateau car park up on Mt. Egmont in Taranaki would be a great spot with a wide open horizon to the East and high enough above farmland to escape the electric fence ticks and far enough away from all power lines and transmitters. It can be a bit cool, damp and breezy at times so take precautions.

Swampy low land can still produce good reception if noise levels are low. I have listened with the Radial Wire running over slushy, muddy ground with 10 metres of wire actually under 300 mm under water and pickup was excellent! Run the wire over the top of thick, wet, low vegetation and don’t bother running wires under heavy forest or native bush floor as pickup will be very poor.

Receivers and Connections – The 100 metre Radial Wire has mighty pickup from longwave right through to shortwave. Most receivers will be driven into overload if your location is too close to transmitters. A pre-selector or wave-trap can allow you to operate closer to transmitters than I recommend in the location notes. Many receivers are significantly less sensitive on MW than 2mHz and above, this will prove annoying if you find a good location. A pre-amp can fix this and the risk of over-loading the receiver. Most receivers can be modified to restore full sensitivity on MW. Modern receivers like the Icom IC-R70, R/71 and Drake R8A actually radiate electrical noise on LW, MW & SW and in a quiet location the Radial Wire will pick up this noise to the detriment of weak signals. Connecting headphones, a tape recorder or speaker cables only increases this unwanted radiation. The cure is to use a metre or so of coax between the receiver and ground wire. I run coax to a small switch board. A balun can be mounted on a board with 2 or 3 Radial Wires terminated via strip-connectors screwed onto the base, with a short, strong copper tag poking out of the other end of the strip connectors and another couple of strip-connectors terminating the receiver coax, and short alligator clip leads to connect either directly to the balun or bypass it and connect directly onto the Radial Wire tags. Set the board up away from the dash if your vehicle has a digital display of any sort (e.g. clock).

The Wire and Handling Multiple Wires – Light, multi-stranded insulated wire is excellent. Black insulation is not easily seen on the ground and draws less curious passers-by. Multi-strand wire is pliable, doesn’t knot, kink or break internally (most of the time). I have had a Radial Wire running over a dirt access track with vehicles driving over the wore causing no damage. I carry 3 loose 100 metre wires in a plastic fish tub and tie the ends of the wires into holes at the rim of the tub. Tangles cannot then occur. One wire at a time pulls out easily with no trouble though you must start with the correct end of the top layer of wire (a different coloured wire for each aerial would be a bonus). The same process goes for retrieving the wires though in reverse. A board cut to neatly fit into the tub over the top of the wires will further prevent tangling if the bin bounces around when you’re driving, then you can add the rest of your gear to the tub, all ready to cart inside when you get home.

Directions, Bearings, Propagation – My favourite bearings are 40º magnetic for North America, 90º magnetic for northern South America and 120º magnetic for south-eastern South America. These 3 bearings cover 90% of your Eastern DX and don’t vary much throughout the country. The forward beam width on the 100 metre wire is +/- 20º anyway, about the same as a well-designed loop and much broader than Beverage wires. When propagation opens up across the Atlantic path, then Radial Wires can pick up MW Portuguese, Spanish and North African signals, and even stations deeper into Europe. The 90º bearing is probably the best for these.

My observations on propagation: To get the best results of your mileage and effort, having some idea of when propagation will be good helps. For MW North Americans & X-banders a high solar-flux figure (i.e. 200 or more), with low solar activity usually produces best results. Geomagnetic activity makes little difference. High solar activity, solar flares etc, usually mean that North Americans won’t come up to strength till well after sunset. Low solar activity can see North American signals up an hour before sunset, this period is great for receiving East Coast signals on the X-band as they propagate through earlier than West Coast stations. This applies through the Winter half of the year, (April – September). For the rest of the year, all areas of North America fade in together. For northern South America, lower solar flux figures are better, i.e. 150 or less, likewise low solar activity will see signals come in more than an hour before sunset. High solar activity will delay fade-in times. High geomagnetic activity can weaken or completely absorb signals to northern South America on low frequencies. For stations from Brazil, Argentina, and Uruguay which come in on the 120º bearing, solar flux needs to be under 100 and the lower the better, low solar activity is desirable and very low geomagnetic activity is essential. Even with all these factors in place there is no guarantee that propagation will actually happen. Seasonally Summer is best for North America and Winter for South America. Equinoxes are the best for propagation to all areas. The best source of solar activity information is WWV and WWVH on 2.5, 5, 10, 15 & 20mHz who alternately give solar information at 18 and 45 minutes past the hour. The solar flux figure given after 0000 UTC covers the previous 24 hours up to that time. The solar activity and geomagnetic activity comments are the average for the preceding 24 hours. The last geomagnetic index number, the K index, summarises geomagnetic activity in the preceding 3 hours. The actual strength of the 15mHz WWV transmission at our midday and early afternoon gives an indication of what the Sun is doing to the ionosphere between Colorado & NZ in real time. An unusually weak signal for the current flux value indicates that low frequencies will be late fading in, whereas good, strong signal indicates signals will fade in early. These guidelines apply true for about 70% of the time. As in fishing, you can never be completely certain of anything at a given moment. Mouthing or writing ideas usually guarantees that the opposite happens shortly after – such is life!

Conclusion – Radial Wire aerials allow you to go on a mini-DXpedition and hear some really excellent low frequency reception that very few could hear from a home situation. A pleasant scenic drive in the country, some minor exercise, a few hours great listening and home again when it gets too cold. Great enjoyment for a miniscule effort. Radial Wires will be beaten for raw signal pickup by long-wires and blitzed for pickup and directionality by Beverages. The Radial Wire will beat any longwire on signal-to-noise ratio in it’s pickup direction however, and unlike a loop, there is no need for re-tuning and rotating and you can sit in your vehicle. Best of all is the absolute ease of running the wires out, retrieving them and heading home. I hope this article inspires someone to give it a go.