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365 pf Capacitors

Tune your antenna for better DX

by Roy A. Walton
(adapted by Bruce Carter)

Almost 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".

Normally, 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 big problem.
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.

The Antenna 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).

A long-wire 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.

Bringing 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.

The proper 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 compound.

The lightning 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.

Terminal "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.

Drawing pi couplerClick for large image


Figure 1. The coupler is particularly useful in matching erratic (sometimes high, sometimes low - as you tune various bands) impedance of a single wire antenna.

C1: 250 pF variable capacitor

C2: 365 pF variable capacitor

J1, J2: RCA phono jack

L1: 112 turns on 1 3/4" diameter, 4" long form, 18 guage wire, tapped as described

S1: 1 pole, 12 position rotary switch

S2: spst switch

Building the Coupler
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 about $6.00.

The author 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).

Start 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 operation.

Wind tightly 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.

Wind four 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 protruding.

Now cut 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.

The outer 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.

Using the coupler
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).

The coupler 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.

The dial settings should be logged to simplify re-tuning.

Editorial notes

I have several comments about this excellent article. Also, in the years since this article was published, several things have changed:

Unfortunately, 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.

And note that the prices listed have changed during the years.

Coil forms 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, however.

(This article originally appeared in Popular Electronics, March, 1968, pp 53-55)

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