By 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 long-wave, medium-wave and the lower short-wave
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 an easy 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 ground-wave signals and low angle skywave signals
exceptionally well. Distant medium-wave skywave signals will punch through
local electrical noise & NZ station flak significantly better than
on a loop, short long-wire or vertical. The difference is even greater
on distant, weak NZ ground-wave 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 in parallel
to your receiver.
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. Long-wave 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 long-wave 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. You can write to me for
any additional information, problems or just to let me know your results.
My address is P O Box 3118, Onekawa, Hawkes Bay Mail Centre, New Zealand.
Home phone number 06 836 7078 or work, 025 661 793. I am yet to try a shallow,
buried Radial Wire for permanent use.
Editor's Footnote: The WWV Solar Indices are also available
by subscribing to an e-mail list which send out the data every three hours.
To join, send an e-mail to majordomo@sec.noaa.gov and put the word - subscribe
- in the subject line.
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