Loop TX antennas at VLF

At first sight the idea of a 10m x 10m wire loop as a TX antenna at 8.97kHz VLF sounds a non-starter as a way of radiating any useful power. Having tossed the idea around with others though it appears that, although very inefficient, it would not be that much worse than a smallish Marconi vertical because these antennas need very large (and lossy) loading coils, height and good grounds to be effective. With 100W to such a small loop around 0.45uW could be radiated. Already G3XIZ has spanned 218km with just 2uW to his vertical, so for small gardens the VLF TX loop is worth a look.

The loop works well on 500kHz TX

Unique reports on 500kHz WSPR this evening

My vertical loop of 1mm diameter wire seems to work very well on 500khz WSPR with some excellent reports this evening, indeed some of these reports are better than I ever received (I think) on the short Marconi vertical last winter. Certainly there will be some directionality, which is the downside of this antenna. Results on 500kHz are SO much easier than on 136kHz where the lower radiation resistance and electrical size make it so much less efficient.

Loop antenna on 500kHz

My vertical loop antenna (about 80sq m area) made with 1mm diameter wire (tuned with a capacitance decade box) performs quite well on 500kHz judging by reports. Yesterday I lowered the bottom wire to run along the ground, increasing the loop area from about 70 to 80sq m.  Reports from M0LMH (223km) and M0BMU (69km) are pretty good with my 1mW ERP signal at M0BMU up to -5dB S/N on WSPR, which is as good as I've ever had and he's not in the best direction for the loop. Also, PA3EGO was up to -2dB S/N on receive. Results on 500kHz are much easier to obtain than on 137kHz although this is explainable by the much lower ERP (probably 20dB less than on 500kHz).

More loop experiments at 500 and 136kHz

Today I converted my earth electrode antenna with elevated feed wires into a full "in the air" loop for both 500kHz and 136kHz. As might be expected, tuning is now very sharp as there is no longer a 50 ohm earth resistance in circuit dampening the Q. At first I found matching and tuning impossible until I decided to match the loop with a step-down transformer (3.5:1 on a 3C90 core) and separately do the tuning. This works a treat and it is now a simple matter to tune the loop either for 500kHz or 136/137kHz.

First reports on 136kHz with the QRSS3 beacon suggest the signal is about 6dB stronger, which is a great result. The loop is wound with quite thin wire (1-2mm diameter), so a higher efficiency is possible with thicker wire, but with even sharper tuning.

LATER: Attracting any attention is turning out to be a serious issue with real QRP on 136kHz. Even with alerts on my blog, the LF-reflector and the GQRP Yahoo groups it's hard to get that many people taking a listen/look. With uW ERPs I'm beginning to think that I'm never going to be able to get much further than my present best DX of 62km.

136kHz RX loop calibration

Today I attempted to calibrate my 136kHz 80cm loop antenna by measuring the signal strength of DCF39 in Germany on 138.83kHz. I'm told that the field strength of this signal in daytime in this area is around 1mV/m. The level of DCF39 was -4.5dB on the Spectran screen with the settings I used. So, by extrapolation 0dB on the screen should be close to 1.7mV/m.

I then measured the signal level of my QRSS3 beacon on 137.675kHz  to see how much lower the level was at the same point about 1.5km away from the home QTH. My QRSS3 beacon was measuring -42dB approximately, corresponding to a field strength level of 12uV/m at 1.5km.

To work back to my ERP from the earth electrode antenna I used the formula ERP = (E * d)^2 /49 and this gives a result of 6.3uW and an antenna "loss" factor of 56dB (power in to RF out).

There are several sources of error:

• loop orientation +/-3dB
• loop Q/tuning +/-3dB
• measurement accuracy +/-2dB

So, the ERP could be as high as 40uW or as low as 1uW.

Resonating the earth electrode antenna on 136kHz

The 20m spaced earth electrodes "antenna" has about 30m of wire coming back ato low height to the feedpoint upstairs. I wondered if I could increase the current flowing into this structure by tuning out the small inductance presented by the wire. Using a small capacitance switching box I selected the value that gave most current with the 2W beacon TX. It was 100nF although because of the resistance of the ground return path the Q is low and the peak small. This suggests the inductance of the feed wire is only 13.5uH. I also tried matching the loop using a 3C90 transformer with various secondary taps but this made little difference. So, conclusion is that resonating the loop and matching it makes very little difference to the current (and ERP) because of the low Q.

Earth electrode loop effective area at 500kHz

From some measurements of my earth electrode "antenna" this evening and some estimates of the ERP calculated from received field strengths 69km away, I've received an estimate of my effective loop area including that part that is within the ground. This was calculated by Rik Strobbe OR7T as 70m^2 in total, suggesting some 20m^2 of the loop was within the ground. These were Rik's calculations:

"Assuming

- 50uW ERP

- antenna is traded as a loop (gain = -0.4dBd)

Due to the negative gain (-0.4dB versus dipole) a loop antenna must "radiate" 55uW to get 50uW ERP.

Since the antenna feeding current is 0.15A the radiation resistance is 2.44 milliOhm (R= P/I^2)

The radiation resistance of a small loop is : Ra = 320xPi^4*A^2/L^4

where A = loop area (m^2) and L = wavelength (m)

For 500kHz Ra = 5*10^(-7)*A^2 or A = 1416*sqrt(Ra)

If Ra = 0.00244 Ohm then effective the loop area is 70m^2"

0.5km on earth-mode with 4W

This evening I achieved my best results so far with 1kHz earth mode TXing on a 20m base electrode pair and receiving on an 80cm loop and listening by ear. Even this range I think can be bettered with yet more 50Hz rejection. With a PC at the receiving end I think 1km is certainly possible now.

Resonating the VLF receiver loop

Today I've been resonating my receiving loop to see if this improved things. Well, I tried it - no better. Then I improved the active high pass (50Hz) filter which got the mains hum down some more, removed an audio gain stage in the RX just leaving 2 common emitter stages. With this, 4W TX, and the loop resonated around 2kHz I did a further walk-about test. 0.42kms was the limit of range. S/N better and 50Hz hum not a problem even close to cables. The RX needs some more gain and narrower selectivity.