Anti-Jamming Antenna 1

I. Materials

1. One shortwave radio with rod antenna (A).

2. One (additional) rod antenna (or metal wire 2 cm shorter than the radio antenna) (B).

3. One non-conductive, dry board (C) big enough to fit the radio and metal sheets (about 30cm x 60cm) plywood or a similar material is OK.

4. Four cup hooks (D).

5. Two rubber bands or string (E).

6. Two sheets of tinfoil/aluminum foil (or other conductive metal sheet) (F). The sheets should be larger than the height x length of the radio (see photo).

7. One small wood block (about 4x4x2cm) (G).

8. Two small wires (15 cm each) long enough to connect the antenna to the metal sheets (H).

9. Two small screws (I) for connecting the wires to the two metal sheets.

II. Construction

1. Fasten the two sheets of tinfoil (F) onto the board (C), as shown in the picture. They should be at least 2 cm apart (they should not touch).

2. Install the four cup hooks (D) on each side of the tinfoil that you will put your radio (A) on (they will be used to hold the radio down when rotating the board for best reception).

3. Drill a hole in the small wood block (G) approximately the size of the bottom diameter of the antenna. Insert the additional antenna and wrap one of the wires (H) around the bottom of the antenna. Ensure the wire and antenna make a firm electrical connection. Connect the other end of the wire to the tinfoil the radio will be placed on with one of the small screws (I). Fasten the small wood block with antenna onto the board as shown on the picture (use glue/screws/ or nails). (You may use a wire to replace the additional antenna. Tape the wire to a wood dowel and mount the dowel to the wood block. Connect the wire antenna to the tinfoil (F).) D. Use the second screw(I) to fasten the second wire(H) onto the second sheet of the tinfoil (F) and connect the other end of the wire onto the radio's built in antenna.(tightly wrapping the wire around the radio's antenna is fine)

4. Place the radio on the tinfoil, as shown in the picture, and use the rubber bands or string (E) to fix the radio onto the board. At this point, both antennas should be vertical to the board. The radio's antenna should be extended fully while the second antenna should be slightly lower that that of the radio's. Your system is ready to use.

III. Operation

1. With the radio mounted in position on the board you should move to a relatively open area, near a window, on a balcony, or even outdoors.

2. Tune the radio to the desired frequency. Rotate the board to different angles until you find the best reception. Do not touch the tinfoil, radio, or antennas when rotating the board. Keep the board away from your body to limit the effects your body will have on the radio's reception. What you are doing by rotating the board is trying to block the interfering signal while keeping the desired signal strong.

3. If you cannot get a satisfactory reception, turn the radio's antenna to different angles and continue to rotate the system until you have the best reception. If one frequency doesn't work, change to a different frequency and repeat the above steps. Radio Free Asia broadcasts on a number of frequencies from various locations.

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Antenna 2

It counters jamming via sky wave, where the jammer propagates by ordinary HF propagation and counters ground wave jammers - noise transmitters located near the receiver, usually on the outskirts of the city being jammed.

Ground wave jamming is exactly similar to arcing power lines or other locally produced noise from appliances and the like - even from lightning.

The HLA
Villard's Noise Rejecting
Horizontal Loop Antenna

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Get a 2' x 2' base - plywood, cardboard, stiff foam plastic, even the back of a wall-hung picture.

Cut a 6" wide piece of aluminum foil. Lay it on the base, with its edge along the edge of the base. Note the 6 1/2" gap where the foil begins.

Fold the foil over itself as shown. Use tabs of scotch tape to hold the foil in place. Continue taping it down on three, and part of the fourth, side:

Cut a piece of newspaper 7" wide and 12" long. You can use saran wrap, waxed paper, or lots of other thin, insulated material for this - even a handkerchief! But thick insulation could prevent tuning the antenna to low HF frequencies.

Put the insulating sheet over the first part of the foil and tape it in place, per the drawing.

Leave the fifth flap of foil loose, so it can be lifted up when necessary. Cut off excess foil so it does not short circuit against the foil beneath the newspaper.

When using a battery-operated, pocket size shortwave receiver, set the antenna on a table, couch, bed, or on the floor. Keep it hoizontal.

Put the receiver on the foil as shown. Don't worry about electrical connection: the capacity between the receiver and the foil will do the trick nicely.

Pull the whip antenna out, and lay it on the other side of the foil loop. Use a small weight to make sure the whip contacts the foil.

Use a book to press the loose foil flap down against the newspaper. Pull excess foil up and back over the book. The book will be the antenna tuner.

Tune the antenna to the right frequency by sliding the book back and forth while keeping excess foil up and over the book.

To tune the antenna, set the receiver about the middle of the desired band, with the volume control fairly high. Tune for strongest signal, or strong increase in noise from the receiver.

If you use a table model receiver (or an amateur transceiver,) make a single turn loop of about #18 insulated hookup wire by taping the wire in place one inch inside the foil loop. Bring the wire away from the antenna on the middle of the side opposite the tuner. Don't connect this loop to the foil - it isn't necessary.

The pickup loop can be brought to the receiver by twisted pair or by RG-58 or similar coaxial cable.

CAUTION

THIS ANTENNA IS NOT DESIGNED FOR TRANSMITTING!!

Now here's the really neat part of the design - how to cancel noise. Put the antenna on some insulating support so it can be tipped up on one side or corner. A pillow serves very well for this support!

While listening to the interfering noise, tip the antenna a bit to reduce the noise. Many times, with just a minute or so of adjusting, the noise from power lines, nearby TV sets, etc. can be reduced 20 dB, and further noise canceling can be obtained with a bit more care.

Great! You now own the "HLA" antenna for noise cancelling. I finish with a simple schematic diagram showing what you built.

The newspaper and two foil flaps make a capacitor. The book allows you to vary the amount of overlap of foil, so you made a variable capacitor which is attached to the ends of a single turn loop, made of a very wide, flat conductor.

The receiver, sitting on the foil, has a good deal of capacity between its internal circuit board (or chassis) to one end of the loop. Its whip antenna is connected to the other end, so the voltage developed across the loop is injected into the receiver.

Tuning the loop causes the voltage at one frequency to be maximized. This causes the increase in signal strength.