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Short Wave Magazine

August 1995

Dick Ganderton

Kiwa Medium Wave Loop Antenna

Over the years there have been many different designs published to enable readers to construct their own loop, usually from easily obtained materials. But not everyone wants to build their own loop. For them, the new m.w. loop antenna by Kiwa Electronics offers a solution. This is a magnificent piece of gear worthy of a place in the Tate Gallery! Hand crafted in black coloured materials antenna comprises the main former, 325mm diameter, carrying the main loop winding as well as the two regenerative feedback windings. This loop is pivoted on horizontal bearings within another former in turn pivoted in the vertical axis on a substantial base. All the bearings are Teflon and require no maintenance. The main loop is tilted via a gearbox and is calibrated in degrees. To indicate the alignment of the loop in the horizontal direction, a liquid filled compass is fitted to the top of the antenna.

A box is fitted to the inside of the main loop former and this has the pre-amplifier and regeneration oscillator inside with a Local/DX switch. This reduces the pre-amplifier's gain by 25dB by providing negative feedback through the part of the antenna coil used for regeneration, overiding the positive feedback used for selectivity enhancement. The switch can be used to prevent overload of the receiver input or to give a wider bndwidth.

A lead, just over 2m long, connects the loop to the control box. This contains all the controls, attenuator and the power supply. A 12V a.c. supply is needed to power the control box and this is connected via the coaxial power socket. Two screw terminals on the rear of the unit can be used to connect either a 12V battery of d.c. power supply. A series diode provides reverse polarity protection. Also provided on the rear panel were two 50 ohm SO-239 coaxial sockets. These are fed from separate amplifiers in the control box so that two receivers can be connected to the antenna.

Positioning the Loop

I used the Kiwa loop with a Lowe HF-150 Stack and also with my Eddystone 940. It took some time to find the best position fo the loop - the instruction manual suggests that it is positioned well away large metallic objects, flourescent lamps, computers and televisions, as well as the receiver if it is not well screened. In the end I balanced in on top of the HF-150 Stack! It is not very sensible to put it too high up as then it is not possible to read the compass - even on top of the stack I needed to climb up onto a stool to see it comfortably!

The instruction manual offers helpful advice on operating the antenna and nulling out stations. It also explains how the loop can be used as a direction finding antenna. Unusally, a full circuit diagram is provided.

The antenna is first tuned to the frequency of the signal being received. Course and fine tuning controls are provided for this. These are used to peak the signal using the receiver's S-meter, with the regeneration control set to a minimum. The attenuator control sets the level of signal presented to the receiver and is set to prevent overloading.

Regeneration

The interesting control, however, is the regeneration. This provides positive and balanced feedback into seprate feedback windings either side of the main loop windings. Increasing the regeneration increases the positive feedback raising the signal level and decreasing the bandwidth of the antenna. The manual suggests a 70 - 75% reduction in bandwidth with an increase in gain of over 15dB. I found the effect of this control to be very striking - just before the point of where positive feedback sent the receiver into oscillation the signal seemed to 'leap out' of the background.

However, I found the controls for the regeneration and tuning to be too coarse. A very delicated touch was needed to adjust the regeneration control for optimum performance.

The loop can not only be turned on its vertical axis to either null out or maximise a station, the loop can also be tilted about the horizontal. This offers further scope for optimising the signals. It also means that there are many adjustments to be made and I found that it took a little practice to optimise the settings of all the controls.

As an example of what can be achieved with patience, I looked for a frequency with a storng station and a weaker one about 90 degrees to each other. I found this, at 3:30pm on a Sunday afternoon, on 675kHz with the Dutch station at Lopok and the French one at Marseilles. The French station was much weaker than the Dutch one, but by careful adjustment of the loop I was able to completely null out Lopok and listen to Marseilles. According to the compass this occured with the loop set to 60 degrees and tilted at +30 degrees.

Impressive

At a retail price of 349 pounds plus P&P the Kiwa loop is probly too expensive for most readers. However , it really does seem to work and is a most impressive adornment to the shack. My thanks to Lowe Electronics for the loan of the loop as well as the HF-150 Stack.

Dick Ganderton G8VFH
Editor Short Wave Magazine

Kiwa Electronics

503 7th. Ave. N.E.
Kasson, MN 55944 USA
507.634.6134
kiwa@wolfenet.com


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