Low Noise Antenna Connection
John Doty
Hi! I'm new to this news stuff, but I've been listening to shortwave
on and off for about 3/4 of my life. I see a lot of discussion of
antennas in this forum, so I thought I'd put in my two cents.
It doesn't 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.
The Problem:
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 goes
underground.
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:
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.
Construction:
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.
Mount 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.
Drive 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.
How 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.
If the 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.
In my 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.
Performance:
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 no longer 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 1930's consoles to a Sony ICF-SW55. I've seen basically similar
results with all.
Rob Winters
: This past year
: I moved to a place with a roomier yard; with the base of the antenna
: now 28 meters away I can no longer identify any noise from the
: house.
Other than the obvious down-side of this arrangement (the cost of
the coaxial cable), is there any significant signal loss with this
arrangement? What is the practical distance limit, if any?
My "city" prospects are pretty hopeless: an RF-noisy townhouse with
few antenna possibilities. However, I do have a great "country"
location on weekends. The only impediment thus far has been that
my co-owner didn't see an Alpha-Beta sloper DX-SWL as an attractive
addition to the bucolic setting (go figure :-). I think I could
get away with said antenna if it was off in the woods. So, how far
off into the woods can it reasonably be before its performance is
impacted by an overly long lead-in? And what are the implications
(other than the trenching) of burying coax cable? Do I need a special
kind of cable to stand up to that sort of treatment?
/// Rob
Steve Byan
Winters) wrote:
> Other than the obvious down-side of this arrangement (the cost of
> the coaxial cable), is there any significant signal loss with this
> arrangement? What is the practical distance limit, if any?
For RG-8 or RG-213 coax cable (the 0.405-inch diameter stuff) at SWL
frequencies, loss is about 1 dB per 100 foot or less. 300 feet would be
half an S-unit. Half an S-unit is probably just noticeable.
> [...] And what are the implications
> (other than the trenching) of burying coax cable? Do I need a special
> kind of cable to stand up to that sort of treatment?
There are at least two kinds of damage to look out for: water and rodents.
Water in the coax will increase the signal loss significantly; be sure to
weatherproof the coax connections well. The other problem is rodent damage,
either by chewing into the cable or just by tearing open the outer jacket,
which allows moisture to enter the cable.
You can buy special cable which will self-seal minor punctures in the
jacket. You sometimes see the RG-213 type advertised as "buryable" cable,
but I don't think RG-213 has the self-sealing jacket.
I've heard of people protecting coax by putting it inside a length of
garden-hose before burying it. This sounds to me like an excellent idea.
Myself, I just buried plain old RG-8, with the expectation that I'll dig it
up and replace it in a few years.
Steve Byan internet: st...@hicomb.hi.com
Hitachi Computer Products (America), Inc.
1601 Trapelo Road phone: (617) 890-0444
Waltham, MA 02154 FAX: (617) 890-4998