Tune your antenna
for better DX
(adapted by Bruce Carter)
any receiver covering 540 kHz to 1600 kHz is suitable for
broadcast band DX'ing, and there are a lot of them.
But the antenna is another matter.
If you are new to the hobby, or if you have been having trouble
DXing those really difficult stations, you may find the answer
to your reception problems in a properly constructed long-wire
antenna - and the "Pi-section Coupler".
the best receiving antenna is considered to be a resonant
antenna - one that resonates at a particular frequency. The length
of such a BCB antenna would range between 865 and 290 feet - much
too long for the space most listeners have available. That's the
The solutions is to build the long-wire "Pi-section Coupler" and
get the maximum signal transfer out of the antenna you have erected.
If you use
a "Pi-Section Coupler", your antenna can be as short as 30 feet
or as long as 100 feet. The combination can be made to work efficiently
at all frequencies between 500 and 6800 kHz.
Construction of a long-wire antenna is simple. If you just
remember and adhere to the following rules of safety. Never construct
your antenna so that it could fall on power lines or they on it.
Always use a lightning arrestor; this not only can save you equipment
but may improve the signal, since it drains static electricity
from the antenna. Never use wire or metal cable to support the
antenna (1/4" nylon line is your best bet).
antenna receives best from the directions perpendicular to its
sides. Erect the antenna N-S to receive E-W. It should be at least
30' long and made of multi-strand copper antenna wire supported
a minimum of 15 feet off the ground. A long-wire antenna
works best 60 feet above electrical ground (40 to 50 feet above
land surface). Egg-type insulators should be used to physically
connect the antenna to the supports. To prolong the life of the
antenna, coat all solder connections with plastic rubber.
the signal from the antenna to the receiver calls for the
use of insulated copper wire of 16 to 18 guage. It should be wrapped
(to afford mechanical strength) and soldered to the end of the
antenna at the insulator closest to the receiver.
feed line entrance into a house is through a wall using an "All
weather wall feed thru bushing" or a similar tubing. Of you can't
drill holes in the house, try dirlling a 1/4" hole in a window
sash, inserting a 1/4" o/d 1" length of bakelight tubing into
the hole, and sealing the hole with a non-conducting caulking
arrestor should be wired into the feed line according to the manufacturer's
directions, which are packed with the arrestor. Lightning arrestors
can be bought for as little as 59 cents.
"G" or "ground" on the receiver should be connected to a good
ground. A good ground is a cold water pipe (never use hot water
pipes, gas pipes, or the telephone company's ground). Clean the
cold water pipe with emory cloth at the point where you wish to
place a "ground clamp" (clamps cost approximately 50 cents). Secure
the clamp tightly and affix a length of 16-guage insulated wire
sufficiently long to go to "ground" on the receiver. Then coat
the ground clamp connections with rubber glue.
for large image
1. The coupler is particularly useful in matching erratic
(sometimes high, sometimes low - as you tune various bands)
impedance of a single wire antenna.
250 pF variable capacitor
365 pF variable capacitor
J2: RCA phono jack
112 turns on 1 3/4" diameter, 4" long form, 18 guage wire,
tapped as described
1 pole, 12 position rotary switch
If you have never built electronic gear before - don't despair
- you can build this coupler. Not only will you greatly increase
the incoming signal, but you will also gain experience in apparatus
construction. The only prerequisites are reading, soldering, and
assembled his coupler in a clear box. In selecting a box, make
sure it is large enough to accept the components. Mark and drill
mounting and rotor shaft holes in the box for C1, C2, S1, S2,
J1, and J2 (see figure above).
making up the coil by drilling two holes in the coil form
to pass the coil wire. Insert wire through these holes, leaving
4" extra wire protruding (which will be used for hookup later).
The holes are intended to hold the wire secure during the winding
two turns of coil wire and drill a hole adjacent to the second
turn to press-fit a brass nail, used as a lug. Scrape the wire
adjacent to the lug hole (this is lug #1), and solder to the coil.
more turns, drill another adjacent lug hole (this is lug #2),
and solder. Proceed in this manner, winding the specified number
of turns as shown in Figure 1 until the coil is complete. Secure
the coil winding through two more holes, leaving 4" extra wire
11 5" lengths of hookup wire and strip 1/4" insulation from one
end of each. Solder them to the terminals of SW1, leaving one
terminal bare. The bare terminal is #0. The terminal next to #0
is #1, the one next to it is #2, and so on, around the switch.
Remember that lug #1 is on the coil end with only 2 turns. Cut
he wires soldered to the switch terminals so that they will just
reach the corresponding lugs when the switch is one inch from
the coil. Strip 1/4" of insulation from these wires and solder
them to their corresponding lugs on L1.
conductors of J1 and J2 are "ground," as are the terminals
associated with the rotor plates in the capacitors. Solder the
necessary wires to the proper points to connect the remaining
components. Then connect the receiver to J1 and the antenna to
J2 using a coax cable.
To operate the coupler, set C1 and C2 at the half-open position
and rotate S1 until the signal is strongest. Adjust C1 and C2
to peak the signal to maximum (while adjusting C1, switch S2 on
and off to find its best setting; C2 will have no appreciable
effect at some frequencies).
is basically an attempt to effect a more efficient transfer
of signal energy from the random length of antenna to the receiver.
At some frequencies the coupler will seemingly have no effect,
which means the antenna and receiver are matched as closely as
possible. At other - or most - frequencies the coupler will have
a very decided and noticeable effect. Capacitor C2 whould be switched
in and out of the circuit when the coupler seems to have the least
effect - especially at lower frequencies.
settings should be logged to simplify re-tuning.
I have several
comments about this excellent article. Also, in the years since
this article was published, several things have changed:
the number of coil turns on the original schematic was obscured
incompetant Xeroxing. I had enough information to make a pretty
good guess in most cases.
that the prices listed have changed during the years.
such as the 1 3/4 inch form described here have disappeared along
with electronics hobbyist shops. An length of empty tubing from
Christmas wrapping paper might be found that is 1 3/4 inches in
diameter. It will not be very strong, but may suffice. A far superior
core can be made from a length of PVC plumbing pipe (but what
do you do with the other 7 1/2 feet?). Some changes in assembly
technique might be required, because I don't think it can stand
up to soldering temperatures.
It may be
next to impossible to get a 250 pF variable capacitor. Discarded
two section tuning capacitors can be used, the larger section
is 365 pF. The extra capacitance will make no difference, as you
can just use 250 pF of the tuning range.
The AM band
expansion to 1700 kHz should make no difference - as the coupler
is usable to frequencies of 6800 kHz.
A good grounding
clamp is a 1" hose clamp from an automotive parts store.
A cheap version
of this coupler can be constructed using a discarded ferrite bar
antenna and two tuning capacitors. The coil is tuned by moving
the ferrite bar in and out. The tuning range will not be as great,
article originally appeared in Popular Electronics, March, 1968,