137kHz E-field probe mag-mount RX antenna

Some years ago I did some tests (in the car) looking to see where I could copy my low uW ERP 137kHz signal from home. Results surprised me, even with my deaf FT817.  I gave up on the far side of Cambridge (approximately 24km) with S/N still respectable.

This is all that you need to receive on the LF bands.

See https://sites.google.com/site/g3xbmqrp3/antennas/137efp.

E-Field Probes

Some years ago (in the days when I was fit and well) I tried these compact receive only antennas on 137kHz and higher bands. They work on the principle that S/N matters more than absolute sensitivity, especially on lower bands when external noise is the limiting factor rather than rig noise figure. I recall using an EFP mag mount on the car and driving around picking up my QRSS3 signal on 137kHz at 24km (still a decent signal even with very very low ERP from my earth-electrode "antenna" in the ground.  Having great success, I gave up and came home! Just a small whip, very very low TX ERP and 137kHz.  These really work!

See https://sites.google.com/site/g3xbmqrp3/antennas/137efp .

472kHz compromise antennas

Looking at nearby G0VQH's list of WSPR stations successfully decoded on 472kHz WSPR it is clear Jenny is copying far more stations than me. Even with my preamp in circuit and maximum sensitivity I am simply not copying, at all, some stations. I think it is time I tried an E-field probe on this band on RX. It is a combination of antenna (in)efficiency and directivity I think that is stopping me copying more on MF now.  I'm not sure what Jenny is using on MF but it clearly works well.

136kHz QRSS3 test

Well, I have found most of the bits needed for my 136kHz QRSS3 test including the E-field probe (EFP) to mount on the car roof. I need to solder 1 short BNC to free end coax in the E-field probe and charge up a PP3 battery (for the EFP) and the FT817.  I also need to match the beacon to the earth electrode "antenna" at 136kHz.  I shall start copying the beacon in the lounge first before venturing out in the field.

Although I now have a preamp for the (deaf) FT817 thanks to a very very kind person, I'll not use this as I did not use one on the last test 2 years ago from the old QTH.

A few years ago this would have been so easy to do. Now it is a major exercise! Life is so much harder now since the stroke, but I remain determined to get better. Given time (maybe a lot more time) I hope things get back to more as they once were.

136kHz QRSS3

My big project for the week (this would have been trivial a few years ago) will be to rebuild my E-field probe for 136kHz and do a fairly local test with it on the car roof on QRSS3. I have the TX beacon, although it has not been used for over 2 years and never from this QTH. I think I have the RX bits somewhere but need to find these first!  For the TX antenna I am going to use my earth-electrodes which will need rematching for 136kHz.

136kHz QRSS3 field test maybe?

If I can manage it (health worries) I want to try a test with my 136kHz QRSS3 beacon and earth-electrode antenna from my current QTH. Last time, several years ago, from the old QTH, I managed to copy my signals quite well on the far side of Cambridge using an E-field probe on the car roof. This time my aims are far more modest: I just want to see if I can copy the signal at all in my usual carpark test site about 1.8km away. If I can copy the signal, I might ask others to take a look. It is so long since I used this kit I have forgotten what is inside the box!

My biggest challenge will be reassembling the RX kit as I am still very wobbly. 15-20 minutes and I am normally "done in", especially if this involves physical or mental effort. I wish so much I was fit and well again.

I know I have to be patient, but at the same time I need to get the most from my body as it is now. Although I still get tired, I seem to regain a more normal  state more quickly than I did, which is hopeful.

Although I am hopeful of a full recovery, given more time, I am beginning to think I shall always have some level of disability. I need to know my absolute limits. If this is the end of field work it will be a very sad day.

E-field probes

A very successful antenna for VLF, LF and MF receive only is an E-field probe. Ideally these should be mounted outside the house with some experiments to find the quietest spot. Size is not important and these are much smaller than many antennas for much higher frequencies.  The picture shows an example EFP. This is the complete antenna - no wires or loops etc are needed in addition. They can be made by just about anyone, so there is nothing stopping you having fun on 136,472 or even VLF RX.   Some people have built these inside a short length of uPVC pipe. The important thing is they are very small and work well on the lower bands.

I have also used EFPs as mag mounts on the car when looking for my signal on 136kHz QRSS3. These are very compact antennas. PA0RDT has created a good design that many people are using.

See https://sites.google.com/site/g3xbmqrp3/antennas/efp .

Analysis of the PA0RDT E-field probe RX antenna

Image of miniwhip on VK1OD's website

VK1OD has done an interesting analysis of the PA0RDT E-field probe miniwhip receiving antenna. This is very popular as an effective antenna from 8.9kHz right through to the HF bands. I have used my own versions on the VLF, LF and MF bands with great success, but I commend the PA0RDT design to you. His design uses readily available semiconductors to give excellent IP2 and IP3 figures. You may be surprised how something this tiny and simple can work so well if mounted away from the house on a small pole. Remember, you do NOT need big antennas to receive on the lower bands: it is S/N that matters not very low noise figures. VK1OD's analysis is somewhat harsh: in my view it works as a true E-field probe should, even at VLF.

Mobile 8.977kHz VLF loop tests started

Today I started to do my tests on 8.977kHz using my 5W earth mode transmitter at home but using a mobile loop antenna on the car connected to my PC via a tuned preamp. The idea is to be able to drive around and measure signal levels with Spectran software whilst actually on the move.

30t 80cm loop mounted behind the car

The loop was mounted behind the car in such a way that it would detect any ground propagated signals. The loop is about 10cm off the ground.

A drive test to Swaffham Bulbeck was carried out and signals were detected more or less continuously out to 3.5km from home before they disappeared in the noise. Bandwidth used was 0.18Hz with a continuous carrier. Although coverage was as I anticipated, signal levels were not as great as when the loop was actually laid directly on the ground. At one of my usual test sites 3.5km out in the fens there was no copy with this loop arrangement yet there was a decent signal copied with the loop on the ground a few days earlier. I need to do some direct comparisons between the loop on the ground, the loop mobile mounted 10cm above the ground and with the E-field probe on the car roof. Initial indications are that the difference between the EFP and the mobile loop is probably no more that 5-6dB.

Signal received with Spectran and the mobile loop

VLF earth-mode mystery deepens

This afternoon I did a larger coverage test with 5W  8.977kHz earth-mode, driving in several directions locally in the car with the roof mag-mounted E-field probe RX antenna and with a PC in the car monitoring the signal. I drove for several kilometres in different directions recording where the signal could, and could not, be copied. The best reception distance with the E-field probe was 3.2km.

The map shows the results. Yellow shows where I drove and red shows where there was signal present and recorded on Spectran. I have recordings of the whole trip which I will more carefully analyse later.

5W 8.977kHz earth-mode coverage using EFP RX antenna

The interesting thing is that the signal could be copied in 4 local villages (Burwell, Reach, Exning and Swaffham Prior) but there was almost zero coverage once outside of these villages. This is NOT the case when looking with a magnetic loop RX antenna, where the signal can be copied more extensively in the rural areas at even greater distances.

It would appear that the E-field signal needs to be strong above ground to be copied with the EFP and this only happens where there are buildings i.e. in the villages. It suggests I'm detecting the signal from cables or pipes in houses.In more rural areas the signal is weaker above ground and not detected, at least not with 5W TX.

8.97kHz earth-mode tests with mag-mounted E-field probe

Today I started some tests at 8.97kHz with my mag-mounted E-field probe (EFP) on the car roof. As usual, the transmitter was my 5W QRSS3 beacon feeding the 20m spaced earth-electrode antenna in the garden.

The logical starting point was to drive to my usual "strong" test site 1.6km from home and compare signal levels on the usual 80cm square RX loop with that on the EFP.  For reasons I have yet to understand the signal was NOT copied, even on the loop! What I did see was what looked like an FSK telemetry signal, possibly from overhead power lines close to 8.97kHz. I've never seen this before. Anyone know what it is?

8.97kHz 5W earth-mode signal at 0.4km on E-field probe

I then drove to my new QTH (due to be occupied in about 4-5 months time) which is 0.4km from the TX and sat in the car in the drive with the iPod Touch 4g running SpectrumView software connected to the EFP. The antenna was a short 19 inch whip. Signals were copied quite well (see photo showing "3" from my callsign). The bandwidth on SpectrumView cannot be narrowed enough to optimally receive QRSS3 and results would be better with the PC set to 0.34Hz bandwidth.

Tomorrow I'll have to find out why I couldn't see my earth-mode signal at the usual test site on either antenna. I'm also going to do some /M reception at 8.97kHz using the E-field probe and laptop with Spectran set to 0.34Hz or 0.17Hz bandwidth. With a continuous signal I'll be able to log the signal level as I drive around the village and nearby.

Mag-mount E-field probe on 8.97kHz

Just as an experiment, this afternoon I modified my mag-mounted FET drain tuned E-field probe to work on 8.97kHz. The last time I rigged an E-field probe on 8.97kHz and went looking for my earth-mode signal results were very disappointing wih just a couple of places within 2km radius where the 5W beacon was copied, but tomorrow morning I'll see how well (or not) it works. To tune the FET drain to 8.97kHz I just substituted the 137kHz tuned circuit with an 83mH Toko potted coil in parallel with 4n7. Tuning is reasonably sharp. Whether the car grounding will make a big difference we'll see in the morning.

137kHz E-field probe on my website

This afternoon I put a new page on my www.g3xbm.co.uk website describing the 137kHz E-field probe antenna used in my recent "drive around" tests in the local area out to 25km. You may recall I put my 6uW ERP QRSS3 beacon on and went out into the Cambridge countryside trying to see how strong it was in various locations.

The whole unit is fixed to a car mag-mount antenna and works very well indeed.