Saturday, 12 December 2009

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.

Friday, 11 December 2009

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
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.

Saturday, 5 December 2009

That tiny signal on 500Khz

The next thing needed to get a signal on the 500KHz band is an Antenna. I removed several in my collection which I don't use much now, a WX Satellite Quadrahelix, a 10 Turn 23cm Helix, a 2.4GHz Dish and a 30m wire Dipole, in order to tidy up and dedicate a 6m mast to holding one end of my top span for 500KHz.

Even by Amateur standards my T or off centre Inverted L is small, a 6m Vertical copper wire is loaded by 10m of Horizontal. Increasing the number of wires in the Top Load would make an improvement to the signal but not to my state of embarrassment re my crowded Antenna Farm (Patch). No I think it has to stay as it is.

At the far end another 6m mast is used to support the Top Load and my 40m Doublet. In the photo the centre feed point can be seen and some high visibility beads on the wire which have all crowded toward the centre.

I measured the Antenna capacitance as 132 pF, an inductor of 751 uH is needed to tune this to resonance. This needs to be variable, Variometers are often used for this, I believe they are a bit of a lossy solution so I devised a method of 'dipping' a Ferrite Rod into the top of the coil rather like a bicycle pump arrangement. The test set up is shown here.

This turned out to be surprisingly efficient in some test arrangements but not good enough. One problem is the small wire diameter which adds losses. The other, mechanical, I mounted it too close to ground thus adding loading.

I have a large Racal glass fibre coil former, it's built like a tank and fully waterproof which has never really done an efficient job on HF. I had already re-wound it starting with 16g enamelled copper wire stepping down to 20g at the end this is 250 uH. I added a 4 turn coupling coil and SO239 Socket.

Instead of a screw in whip as is intended I added a threaded bush to my 'ferrite pump' coil to enable this to be screwed on top. This adjustable coil 41mm dia. wound using 1mm wire is 230 uH with no ferrite.

With Ferrite it can be adjusted up to as much as 900 uH. The 'pump' set up if fully waterproofed using plumbing goods.

It is a patch antenna, (cabbage patch) the Purple Sprouting Broccoli will get taller and will be gone next season.
I wonder if the closest plant does cause some loss, we will have to eat our way out of that one in the spring.

I really am that restricted for space, I shall continue to try for improvement but I think there is not a lot left to achieve judging from signal comparisons with other stations who run 5 watts.

I have definitely improved on my 40m Doublet with strapped feeders by 3dB using this antenna.

My signal has been spotted by 4 stations the most distant being G0KTN at 145Km

More results and comparisons another time.

Friday, 4 December 2009

500kHz QRPp

I decided to try QRPp on 500KHz. I have run my Low Power MEPT's with WSPR on HF and VHF / UHF with good results.
I then developed the XORgan which uses 2 Crystals and an XOR mixer to produce WSPR code at Audio frequency for PC Less and thus lower power consumption WSPR.

The XORgan led me to try the Hypersonic XORgan using the same method to directly generate WSPR, QRSS and Hellschriber at a very very high Audio frequency :-) 500KHz in fact.

I added a one chip Buffer / PA stage so that in just 2 chips I have a complete LF Transmitter with an RF Otput of 160mW. Existing stations on 500kHz were geting good results using 5Watts from their TX's and WSPR. I reconned I could get usable results with my 160 mW (15dB less power) so that was the basis for my Experimental Permit.

Quite rightly some advice said that on LF if you have a little Antenna Loss it is no great problem as you can just use a little more power to compensate. On the RX side there is little need for a super efficient Antenna. At my QTH I dont have the possibility for a very good Antenna so I have to make the small set up I have as efficient as possible. With QRPp there is nothing available to waste.
Up to now results have been quite encouraging. Antenna tests continue, more on that and the results later.