XBM VLF blog

Here, G3XIZ is coming in well today on 8.976662kHz since he started TXing about 45 minutes ago. His signal is around 20dB over noise iin 424uHz bandwidth. I have been unable to monitor for a week because I was on holiday without radio gear.

March 19th

There's been an interesting discussion on the LF reflector about how effective (compared with a smallish Marconi vertical) a TX wire loop antenna might be on 8.97kHz. The efficiency will be very low indeed, but then so is it with a Marconi because of ground losses and losses in the HUGE loading coil that is required. I was assuming that a loop 10m x 10m (or similar) could be erected in most small gardens like mine.

These were Rik's comments:

Roger,

as Jim calculated running 100W in a 10 x 10 m loop will give about 0.5uW ERP (is you use 4 x 1.5mm wire in parallel instead of a single 3mm wire in order to avoid skinn effect losses). Using more parallel wires of a coax cable might pump up the ERP to 1 or 2uW.

Looks pretty poor, but will a vertical antenna of a similar size do better ?

At 9kHz the radiation resistance of a 10m high + 10m topload vertical 75uOhm.

The antenna capacitance is 110pF, a reactance of 161kOhm. What means that you will need a loading coil of 2.84H (yep Henry). Apart from the fact that it will cost a lot on copper wire the coil losses will be high. You will need a Q of 160 to reduce the losses to 1kOhm. In addition for such a small antenna you can excpect several 100 Ohm ground loss, so let's assume a total loss of 1500 Ohm. 100W TX power will result in about 0.25 A antenna current and an ERP of about 8uW.

That's 6 to 10dB better than the loop, but instead of some cheap cap's you will need a monster coil. I ran it one on the online coil calculators and it came up to a 2.5m high and 1.8m diameter coil with amost 10km of 1.5mm Cu wire (weight 150kg). And running 0.25A into the antenna will result in 40kV !

So, despite the vertical could be 10dB better than the loop, the loop seems much more easy (and cheap) to build.

It might be easier and cheaper to get the extra 10dB by usung mor wire in the loop and pump up the power.

73, Rik ON7YD - OR7T

Roger,

the antenna reactance about 2.5 Ohm, so the antenna voltage (and thus capacitor voltage) will be less than 100V.

I assume that polypropylene caps that work fine on 137 and 500 will also be OK at 9kHz.

Farnell sells 1uF/275Vac at 0.44 Euro (10 QTY) and 0.1uF/305Vac at 0.32 Euro (10 QTY), so for less than 10 Euro you should be able to tune the antenna in 0.1uF steps. As the antenna Q is rather low (2.5/0.1 = 25) a 0.1uF step should be OK for a first try.

73, Rik ON7YD - OR7T

Jim M0BMU did some sums and his calculations also suggested the idea had some merit.

Dear Roger, LF Group,

Just wondering if anyone has done the maths to work out what sort of ERP

could be expected at 8.97kHz with, say, 100W to a smallish loop antenna in

the garden?

A 10m x 10m, 100m^2 loop of "thickish" 3mm dia solid wire would have a resistance at 9kHz of roughly 0.1ohm. With 100W available, 32A antenna current should be possible, assuming negligible tuning capacitor losses. Inductance would be of the order of 40uH. A tuning capacitor of roughly 8uF would be needed.

The radiation resistance of an electrically small loop is:

320 * pi^4 * A^2 / (lambda)^4, where A = area, lambda = wavelength

for 100m^2 at 9kHz, Rrad is about 250 pico-ohms (!)

The ERP is then 1.8 * I^2 * Rrad, about 0.45uW

So pretty low, but with a bit bigger loop and a bit more power, it would seem to be competitive with small verticals of a similar size. This is perhaps mainly because of the serious losses present in loading coils that people have been able to make for verticals, combined with high voltage limitations of fairly short wire antennas, and high environmental losses of various types also due to high electric fields. The voltage in this example would only be about 70V. So might be worth trying for "back garden" experiments (assuming your antenna masts can support thick enought wire!), although I think it would not be competitive for bigger balloon/kite supported vertical antennas.

Cheers, Jim Moritz

73 de M0BMU

March 17/18th 2011

DJ8WX came through last night quite well again on 8.970022kHz. I am now using SL with a split screen .usr file supplied by G4WGT which allows me to monitor both DJ8WX and G3XIZ at slightly different frequencies close to 8.97kHz. G3XIZ started to appear from around 0600 18.3.11 and was stronger later.The annotated screen shot on the left shows both signals well. Chris was peaking around 5-7dB over noise and Uwe probably 10dB over noise around 0100z. Both detected in 424uHz bandwidth on Spectrum Lab here in JO02dg near Cambridge UK. Antenna was the vertical wire 80m square loop used on 500/136kHz TX into the MPF102 preamp via a 3C39 step-up transformer.

March 16th 2011

A word of caution to those of us looking for very weak signals around 8-9kHz.....DJ8WX was copied here overnight without any doubt and my grabber was clearly able to see his close-down around 0700 today. The frequency and the trace timing corresponded perfectly. Likewise with G3XIZ and DK7FC/P some weeks ago.This evening I was looking very carefully again at the Spectrum Lab screen from the last 24 hours in 424uHz BW and readjusting audio gain ranges, screen colour saturation and contrast. Out of the noise appeared another very very faint line, not at 8.970022 but at 8.96998kHz. I've attached the screen shot here (I have overlaid it with the time for clarity). My immediate reaction was this must be Ossi OE5ODL. Then I checked his grabber and saw he was not operational overnight last night! So, what looked like a trace on his frequency cannot be him. It must instead be some artefact of SL or something else.We have to be very careful when seeing traces at specific frequencies: it may be an indication of a given station, but without some modulation or turning on/off of the carrier a doubt remains.

Sadly I don't think I did see Ossi today after all, but I shall keep looking