The perfect feedline
for wire antennas
By John Doty
take very much wire to pick up an adequate signal for anything
but the crudest shortwave receiver. The difference between a mediocre
antenna system and a great antenna system isn't the antenna itself:
it's the way you feed signals from the antenna to the receiver.
The real trick with a shortwave receiving antenna system is to
keep your receiver from picking up noise from all the electrical
and electronic gadgets you and your neighbors have.
Any unshielded conductor in your antenna/ground system is capable
of picking up noise: the antenna, the "lead-in" wire, the ground
wire, etc. Even the widely recommended cold water pipe ground
can pick up noise if it runs a significant distance before it
Symptoms of this problem include buzzing noises, especially at
lower frequencies, clicks as appliances are turned on or off,
and whines from motorized devices. Sometimes the problem can be
reduced by running the radio from batteries.
The solution is to keep the antenna as far as possible from houses,
power lines, and telephone lines, and to use a shielded (coaxial)
transmission line to connect it to the receiver. To get this to
work well, two problems must be avoided: noise currents on the
shield must be kept away from the antenna, and, if you want to
listen to a wide range of frequencies, the cable must be coupled
to the antenna in a non-resonant way.
You can keep noise currents away from the antenna by giving them
a path to ground near the house, giving antenna currents a path
to ground away from the house, and burying the the coaxial cable
from the house to the antenna. Resonance can be avoided by coupling
the antenna to the coaxial cable with a transformer.
My antenna and feed system are built with television antenna system
components and other common hardware. These parts are inexpensive
and easily obtainable in most places.
The transformer is built around a toroid extracted from a TV "matching
transformer". If you're a pack rat like me, you have a few in
your basement: you typically get one with every TV or VCR (or
you can buy one). Pop the plastic case off and snip the wires
from the toroid (it looks either like a tiny donut, or a pair
of tiny donuts stuck together). The transformer windings should
be made with thin wire: I use #32 magnet wire. The primary is
30 turns while the secondary is 10 turns. For a one-hole toroid,
count each passage of the wire down through the hole as one turn.
For a two-holer, each turn is a passage of the wire down through
the right hole and up through the left.
the transformer in an aluminum "minibox" with a "chassis mount"
F connector for the coax cable and a "binding post" or other insulated
terminal for the antenna. Ground one end of each winding to the
aluminum box. Solder the ungrounded end of the primary to the
antenna terminal, and solder the ungrounded end of the secondary
to the center conductor of the coax connector.
Alternatively, you could use a commercial transformer. I recommend
the ICE Model 180 "Beverage/Longwire Matching Unit". Unlike the
MLB folks, ICE seems to actually understand what their product
does. In addition to the inherent electrostatic protection offered
by the transformer, this unit also has a gas discharge tube and
a blocking capacitor to further reduce the danger of electrostatic
discharge. It's very solidly constructed. Finally, at $32 it's
less expensive than either the Palomar ($40) or RF Systems ($60)
MLB's. You can order direct from ICE at 1-800-423-2666.
[The usual disclaimer: I have no interest in ICE, I'm just
a satisfied customer (I have three of these transformers: thanks
go to Richard Steck for alerting me to the existence of this product).]
a ground stake into the earth where you want the base of your
antenna to be (well away from the house). Mount the transformer
box on the ground stake: its case should make good contact with
the metal stake. Drive another ground stake into the earth near
the place where you intend for the cable to enter the house. Mount
a TV antenna "grounding block" (just a piece of metal with two
F connectors on it) to the stake by the house. One easy way to
attach hardware to the ground stakes is to use hose clamps.
Take a piece of 75 ohm coaxial cable with two F connectors on
it (I use pre-made cable assemblies), connect one end to the transformer
box, the other end to the grounding block. Bury the rest of the
cable. Finally, attach a second piece of 75 ohm coax to the other
connector on the grounding block and run it into the house. Use
waterproof tape to seal the outdoor connector junctions.
Attach one end of your antenna to the antenna terminal on the
transformer box and hoist the other end up a tree or other support(s)
(don't use the house as a support: you want to keep the antenna
away from the house). My antenna is 16 meters of #18 insulated
wire in an "inverted L" configuration supported by two trees.
If your receiver has a coaxial input connector, you may need an
adapter to make the connection; in any case, the center wire of
the coaxial cable should attach to the "antenna" connection, and
the outer shield should attach to the "ground" connection.
Multiple grounds and transformer coupling of the antenna should
reduce the danger posed by lightning or other electrostatic discharge,
but don't press your luck: disconnect the coax from the receiver
when you're not using it.
works, in more detail:
Coaxial cable carries waves in two modes: an "outer" or "common"
mode, in which the current flows on the shield and the return
current flows through the ground or other nearby conductors, and
an "inner" or "differential" mode in which the current flows on
the inner conductor and the return current flows on the shield.
Theoretically, outside electromagnetic fields excite only the
common mode. A properly designed receiver is sensitive only to
the differential mode, so if household noise pickup is confined
to the common mode, the receiver won't respond to it.
The "characteristic impedance" of the differential mode is the
number you'll see in the catalog or on the cable: 75 ohms for
TV antenna coax. The characteristic impedance of the common mode
depends on the distance of the line from the conductor or conductors
carrying the return current: it varies from tens of ohms for a
cable on or under the ground to hundreds of ohms for a cable separated
from other conductors.
A wire antenna can be approximately characterized as a single
wire transmission line. A single wire line has only a common mode:
for #18 wire 30 feet above ground, the characteristic impedance
is about 620 ohms. For heights above a few feet the characteristic
impedance depends very little on the height.
impedances of two directly coupled lines match, waves can
move from one line to the other without reflection. In case of
a mismatch, reflections will occur: the magnitude of the reflected
wave increases as the ratio of the impedances moves away from
1. A large reflection, of course, implies a small transmission.
Reflections can be avoided by coupling through a transformer whose
turns ratio is the square root of the impedance ratio.
The basic difficulty with coupling a wire antenna to a coaxial
line is that the antenna's characteristic impedance is a poor
match to the differential mode of the line. Furthermore, unless
the line is very close to the ground, the common mode of the line
is a good match to the antenna. There is thus a tendency for the
line to pick up common mode noise and deliver it efficiently to
the antenna. The antenna can then deliver the noise back to the
line's differential mode.
Some antenna systems exploit the mismatch between the antenna's
characteristic impedance and the line's characteristic impedance
to resonate the antenna. If the reflection at the antenna/line
junction is in the correct phase, the reflection will add to the
signal current in the antenna, boosting its efficiency. While
this is desirable in many cases, it is undesirable for a shortwave
listening antenna. Most shortwave receivers will overload on the
signals presented by a resonant antenna, and resonance enhances
the signal over a narrow range of frequencies at the expense of
other frequencies. It's generally better to listen with an antenna
system that is moderately efficient over a wide frequency range.
antenna system, grounding the shield of the line at the ground
stakes short circuits the common mode. The stake at the base of
the antenna gives the antenna current a path to ground (while
the transformer directs the energy behind that current into the
coax). Burying the cable prevents any common mode pickup outside
the house, and also attenuates any common mode currents that escape
the short circuits (soil is a very effective absorber of RF energy
at close range). Common mode waves excited on the antenna by incoming
signals pass, with little reflection, through the transformer
into differential mode waves in the coax.
A major source of "power line buzz" is common mode RF currents
from the AC line passed to the receiver through its AC power cord.
These currents are normally bypassed to chassis ground inside
the receiver. They thus flow out of the receiver via the ground
terminal. With an unshielded antenna feedline and a wire ground,
the ground wire is a part of the antenna system: these noise currents
are thus picked up by the receiver. On the other hand, with a
well grounded coaxial feed these currents make common mode waves
on the coax that flow to ground without exciting the receiver.
A few years ago, I put up a conventional random wire antenna without
a coaxial feed. I was disappointed that, while it increased signal
levels over the whip antenna of my Sony ICF-2001, it increased
the noise level almost as much. I then set up the antenna system
described above; in my small yard, the base of the antenna was
only 12 meters from the house. Nevertheless, the improvement was
substantial: the noise level was greatly reduced. This past year
I moved to a place with a roomier yard; with the base of the antenna
now 28 meters away I can rarely identify any noise from the house.
The total improvement over the whip is dramatic. A few nights
ago, as a test, I did a quick scan of the 60 meter band with the
whip and with the external antenna system: with the whip I could
only hear one broadcaster, unintelligibly faintly, plus a couple
of utes and a noisy WWV signal. With the external antenna system
I could hear about ten Central/South American domestic broadcast
stations at listenable levels. WWV sounded like it was next door.
I have also tried the antenna system with other receivers ranging
from a 1930's "chairside" to a Drake R8. I've seen basically similar
results with all.