The Table Topper Meter

Compact, low-noise—and effective.

Richard Q. Marris G2BZQ 35 Kingswood House Farnham Road Slough SL2 IDA England UK

Man\ amateurs and SWLs 11 nd it very difficult, if not next to impossible, to erect an effective 160m antenna. First, there arc those in "no antennas here" zones. Others do not have the space to erect an effective conventional antenna. And anv who live

¡■p in an urban environment will probably have to exist w ith a high noise level anyway, which mas be so high that the band is impossible to use.

At my QTH I do not have the space to erect one. Even with a short loaded wire, the noise level is intolerable.

Noise at these frequencies is either atmospheric or manmade. Both types are picked up by the antenna, as is QRM Atmospheric noises are radio weaves, produced by natural causes, of irregular waveform and usually very short, repetitive duration. They cover a wide range of frequencies. and the noise level increases as the operating fiequencj decreases, litis noise ma\ be directional or non-directionaL depending on the cause an extreme case being nearby electric siorms,

Man made noise seems to be getting progressively worse as the years go by. Ii can be produced by inside sources such as thermostats: dimmer switches;

TVs: computers and other electronic devices; and, of course, ihc main power supply—house wiring.

Fxternaily, you can also pick up many of these noise sources from neighbors (especially in apartment complexes); external power supply cables: road traffic; neon signs; and so on.

Fortunately, much of all this noise interference is directional, and can be eliminated or greatly reduced by using a directional antenna such ¿is a well-designed kxip.

Coming hack to the 160m band after some years1 absence, i decided to design a narrowband, narrow beanrwidlh, small transmitting and receiving loop to specifically combat these noise problems on 160dl

Small, tuned-frame loops can he cither solenoid-wound ''box' types, or "spiral"-wound loops. With symmetrical matching/ feeding, they should produce the well-known "figure eight" polar diagram radiation pattern, giving a theoretical zero signal null at 90 degrees to the line between the TX to RX signal path.

The box loop is the most convenient to construct, but unfortunately signals cannot be completely eliminated at 90

degrees. However, (he more difficult to design and construct spiral loop can eliminate all signals at 90 degrees to the TX-RX signal path. Also, it does not need direct earthing/groundings

The Table Ibpper 160 loop configuration

First, 1 constructed a spiral octagonal loop. This was tuned wnh a variable capacitor and successfully loaded with a low-power 160m CW TX. On a good RX, hoth European and trans-Atlantic signals came through when conditions were OK. However, living in south central England, 1 found that North American stations were received on the forward lobe, while European signals were recoverable on the reciprocal or opposite lobe. The nulling at 90 degrees effectively eliminated all signals. Noise levels were dramatically reduced.

I ihen conducted experiments with alternate shapes of spiral loops, using the same amount of wire turns and alternate methods of feeding/matching- At each stage, I did comparisons with the original octagonal spiral loop, the object being to (I) increase signal strength in and out and \ 2) if possible, reduce the size of

73 Amateur Radio Today * November 1997 27

300 pf Hgfetottoge cia c«

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300 pf Hgfetottoge

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Fig. 1Aah*p schematic*

lhe rear lobe w hile increasing that of the forward. This would reduce the reciprocal hearing QRW (on receive) and increase the signal strength on receive and transmit. Atmospheric noise would also be further reduced,

The final TT160 loop is shown in Figs. 1 and 2. í found that by reducing the loop's width dimensions and increasing its height, the signal strength significantly increased when compared with the original octagonal spiral assembly. The excellent nulling at 90 degrees was increased, loo, Experiments also indicated that by adopting an asymmetrical feed, as in Fig, 1, tlie forward lobe could be increased and the reciprocal, decreased.

The end result was most satisfactory when the loop was pointed toward North America, where excellent W and VE signals could he heard, although lhe reciprocal European signals were greatK reduced. Rotating the loop through 180 degrees meant the opposite happened. Furthermore, the new shape and feed method gave ihc loop a narrower beamwidth which, with the loop's usual narrow bandwidth, reduced manmade noise and atmospheric noise to acceptable levels. The new radiation pattern was similar to Fig, 4b.

Loop construction

Refer to Figs, I and 2 lor the final loop assembly, just 30 inches wide and 54-3/4 inches in overall height, including the base mounting chassis. This is a size which can casih be accommodated 28 73 Amateur Radio Today • November 1997

on a tablctop and then stowed away when not required.

The TT160 consists of six spiralwound turns of PVC-covered wire (24/0.2 mm) w ith an OD of 2*05 mm and rated at 6 A. Any 6 A-minimum PVC-covered wire would suffice. The turns are supported h\ six-way terminal blocks, cut from \ 2-way standard ones (Radio Shack™ #274-679). It is important that the loop turns are wound count ere lock wise si Lining at the outside and fed progressive!\ through the terminal block holes. The inner w ire end goes to a three-quarter-inch standoff insulator (Fig. 2), which ensures that the wire end drops down to the VC (variable capacitor) with a half-inch clearance awaj from the loop turns. 1 he loop frame is made from well-seasoned hardwood as shown in Fig. 2.

The baseboard is \2 \ L) x 3/4-inch timber, onto which is mounted the simple chassis. This is a piccc of 8 x 4 x I/2-inch timber faced with single-sided copperclad Fiberglas™ hoard (tt \ 4 inches), with Lhe copper surface upwards, ft is fronted by an identical board to form the panel. The copper surfaces oi both boards should be scam-soldered together At the rear of the baseboard is mounted a timber vertical loop frame support 13 inches long by one and three-quarter inches in diameter (sec Fi^i- 2h The edge of this should be planed off to a small flat surface to allow the loop frame to be screwed to it as shown.

The two-gang by 500 pF-per-section variable capacitor (CIA and B) is mounted on the from panel. This VC should be of the larger, rigid, well-spaced, receiver type, with ceramic-mounted stators. which could well be salvaged from an old tube receiver. I used a Jackson type "L"\ In series with

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