Receive Loop Antenna for 500kHz

I have good receive results on 500kHz, using my 40m Doublet with feeder strapped and no ATU I have copied the USA WSPR stations when only one other UK station was Spotting them also.
My TX antenna works well for RX too, yet with much discussion and some good results being reported using Indoor Loops I decided to try one.
I find one major problem when used indoors, despite the fact that the Loop can be oriented to reduce the pick up of interference from my Computer Monitor in the same room, the QRM from the Neighbour's Panasonic Plasma TV is severe and makes the band completely un-usable.

I hated the whole concept of Plasma TV's before one moved nearby. Despite the fact that I have many Panasonic Goods, I now despise that company for producing such poor equipment. In most cases of Plasma QRM it is Panasonic to blame.

In this wonderful enlightened age, the 42 inch Plasma TV is used in conjunction with a Dummy in the mouth of pre school children to rear and educate them with ranting American Cartoons on the Cartoon Channel. When the bleary eyed child decides he must sleep, the family continue to watch until the early hours. Therefore the QRM can seem to be 24/7 at times.

Other than that the Loop performance is impressive. I designed mine with 8 turns of 48 strand Litz wire spaced 10mm on a wooden frame.


It is tuned with a salvaged 3 Gang 500pF tuning capacitor. Coupling is by means of a toroid, just visible on the bottom mid turn in the picture below.


On a short test before the Loop was optimised, when the Child Rearer was off, I Spotted PA0A on WSPR in the daytime.

Some preliminary results on 500kHz with Litz Loading Coil

It is early days yet and only one station that is known to receive me, G7NKS has been active.
G7NKS is equidistant from G3ZJO and G3XBM and at reciprocal angles.


This is useful in theory. Previously I had compared the only available reports from signals not received at precisely the same time, this one is from 06:16 this morning thanks to WSPR net.

2009-12-12 06:16 G3ZJO 0.503810 -24 0 IO92ng 0.05 G7NKS IO92ub 46 120
2009-12-12 06:16 G3XBM 0.503865 -6 0 JO02dg 0.001 G7NKS IO92ub 46 240

Indicated Power levels should be disregarded G3XBM quotes an Estimated Radiated Power, I quote a QRPp Input, neither can be taken as exact. G3XBM runs 5Watts of RF, I run 160mW, this is 15dB less. The signal at G7NKS shows 18dB difference. I shall work more on my Antenna to see if I can eliminate the 3dB now apparent.

Litz wire for 500kHz Loading Coils

After designing my 'pump' adjustable Ferrite coil for 500kHz I found a reference by Mitchell Lee to a Ferrite push rod arrangement. This really is quite similar to my set up.

The same write up mentions that Litz wire is good for an extra 6dB in some instances.

Litz now being rare, I wound a coil with 'home brew' Litz. Actually three enamelled copper wires twisted into a trifila winding, providing three times the surface area of an equivalent size single
wire.
Some of the coils I tried were less efficient, due I think to poor power transfer with link coupling, despite the match looking good. This time I used a tap coupling. The large coil is some 680uH and the small Ferrite rod adjustable one is 50uH when air cored.

The large coil is wound on a genuine Tupperware container a further larger container is inverted over the coil as a protective weather cover.

All coil formers are fitted with screw threads so that they are interchangeable for tests.

I adjust tuning for maximum RF field, sensed on a whip or maximum current on a small QRP meter.

Using a field strength meter it is immediately apparent when a good match is achieved.



The QRP current meter has a single turn on the centre wire, the current into a 50 Ohm load is indicated by the mid scale reading.