E-field probes (EFP)

These are very small RX antennas that can be very effective at VLF, LF and MF.  As you move HF, they get less effective. The main issue is that at lower frequencies external noise dominates.  

There are several designs around. Important factors are dynamic range (they can be hit with some very strong signals) and positioning away from buildings to minimise noise pick-up.

See https://sites.google.com/view/g3xbm4/home/antennas/e-field-probe-efp .

E-field probe antennas

 

At VLF, LF and MF, external noise usually is the limiting factor.  Making the RX antenna bigger also increases the noise picked up, meaning that tiny RX antennas can be as effective as huge ones. This is the case with E-field probe antennas, that can be tiny.

Many years ago I made one on a mag-mount for 136 kHz so I could drive around trying to pick up my QRSS 6uW beacon. There are several designs around, some which are untuned. Much to my surprise, I could easily pick up my little beacon at 24km.

As the frequency is increased, external noise ceases to dominate and they become less effective.

You may find one useful for LF or MF bands.

See https://sites.google.com/view/g3xbm4/home/antennas/137khz-efp-rx-antenna .

E-Field probe RX antenna

This tiny antenna is very effective on VLF, LF and MF. Versions are available which can be bought ready made or as kits.

See https://sites.google.com/view/g3xbm4/home/antennas/e-field-probe-efp .

See also https://sites.google.com/view/g3xbm4/home/antennas/137khz-efp-rx-antenna .

Mini-whip RX active antennas

These antennas can be very effective on VLF, LF and MF. They are extremely small but work every bit as well as big antennas. They are the best way to listen on these lower bands.

They are sometimes known as E-field probes (EFP). They work because external noise dominates. This noise decreases as the wanted signal decreases so actual S/N is just as good with a small antenna. This is not the case when the RX noise figure dominates. Often they are optimised for lower frequencies. With active antennas it is important to have good dynamic ranges.

See https://sites.google.com/view/g3xbm4/home/antennas/e-field-probe-efp . 

UPDATE 1253z:  Now 137kHz EFP on new site - see https://sites.google.com/view/g3xbm4/home/antennas/137khz-efp-rx-antenna .

E-field probes

There is a common myth that VLF, LF and MF antennas have to be big. Whilst this is true for radiated TX antennas, this is absolutely not true on RX. 

In the past I have used E-field probes very successfully on 136kHz RX and good ones can be quite as good (or better) than large antennas and can work from VLF to 30MHz despite being very tiny. Typical ones are just a few cms long.

One of mine, used taped to the window, is shown in the photo. Normally they are placed outdoors away from noise sources. They work because as antennas get smaller the signal level falls, but so does the noise. At low frequencies external noise usually dominates.

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

E-field probe RX antennas

It is total myth that VLF Rx antennas have to be big. They do not!! Very effective VLF, LF and MF RX antennas can be very small indeed. Some years ago I tried an e-field probe antenna on 136 and 472kHz with excellent results.

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

E-field probes (EFP)

It is some years since I have tried these antennas. They are RX only and can be very effective at VLF, LF and MF. Some use them on RX up to 30MHz. They are very small.

Positioning can be important to minimise man-made noise pick-up. Normally they are placed as far as possible from houses and electrical noise sources.

In the past I would have made my own but as my fine motor skills are not what they once were I have bought one from PA0RDT.

Next winter, I may erect this high up in a nearby tree and have a serious go at 137kHz and 472kHz RX. It would probably be effective on 1800kHz too.

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

LF and MF RX

Over a year ago I bought an E-field probe from PA0RDT. It has still to be erected!

In the meantime I have been busy on 472kHz WSPR using my earth-electrode "antenna" in the ground using 10mW ERP.

My idea was to concentrate on 136 and 472kHz RX with the EFP high up in a tree. As ladder work would be needed, and I am no good on ladders these days, this may have to wait.

The tree I want to put the EFP up at the top of is the silver birch, which has still to shed its leaves. See photo. It is furthermost from all nearby houses.

See  https://sites.google.com/site/g3xbmqrp3/mflf/winter

Maybe THIS year?

Looking back over 2018 I see I failed in at least one project aim!  I actually bought the e-field probe, but it has still to be erected. Maybe next season?

See https://sites.google.com/site/g3xbmqrp3/mflf/winter

E-field probe RX antennas

These are RX only antennas, but they can be remarkably successful on VLF, LF and MF. Years ago I made my own, but these days building and erecting is much harder.

A while back I bought an EFP from Roelof PA0RDT, although I have not got it erected yet. Roelof gets good results with his, although they need careful sighting to avoid noise pickup. If you are interested in the lower bands RX only, these antennas may be your answer.

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

137kHz E-field probe on the car mag-mount

Many years ago, I tried looking for my tiny 137kHz beacon with a small E-field probe mag-mounted on the car. After seeing the signal quite well on the far side of Cambridge at about 24km, I stopped the test. As you can see, the EFP used readily available parts and was very simple.

At some point I should rebuild this and try from the QTH we moved to almost 5 years ago, maybe using the earth-electrode "antenna" on TX. I have felt too unwell since my stroke.

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

E-field probe RX antennas

It is a common misconception that big antennas are needed for MF, LF and VLF reception. This not true!! Some very small antennas work very well indeed. One of these is the E-field probe (EFP) which makes an excellent antenna on receive. Years ago, I made one and was very impressed. Siting is important with placement as far from houses as possible recommended. EFP antennas are available from several sources, although you can make your own.

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

Mini-whip E-field probe

There is little doubt that I shall have to try one of these on LF and MF and possibly other bands next autumn. I have the time from now until then to find the best spot to mount one. I may buy one from PA0RDT, although I have had success in the past with my own.

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

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 .

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 on 8.97kHz VLF

Route taken from A to F (about 4km)

Today I did a fascinating experiment on 8.977kHz VLF using my 5W earth mode transmitter and a mag-mounted E-field probe and laptop running Spectran in the car. Basically I did a "drive around" test to see where the signal could and could not be copied.

Signal strength on 4km run between 2 villages

A continuous carrier was transmitted and I continuously monitored the received signal in the car. The drive was from the middle of the next village (Swaffham Prior), out through to the main road, then back along the main road to Burwell, around part of the village and then back home to the TX location. The signal was visible in Swaffham Prior at 5-10dB S/N, then disappears and returns on approaching Burwell where it is up to 40dB/S/N in 0.18Hz bandwidth. Within Burwell it is almost solid copy. The red timing ticks are every 30 seconds.

What I am detecting (I think) is the local E-field from the VLF signal in the ground, no doubt aided by local utilities. What puzzles me is why there is NO copy in between the 2 villages when there are, I think, pipes and cables in the road.

In the coming days this test is worth repeating locally in other directions and further afield. Fascinating to think a 5W VLF signal injected into the ground can be copied on a 19 inch whip on the car roof like this.

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.

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.

QRSS3 tests with QRP on 137.685kHz.

This afternoon I started my tests on the 137kHz band again. With my 8W (RF out from the unit - ERP will be uW level) crystal controlled beacon sending QRSS3 feeding the 20m spaced earth electrodes, currently under 8cm of snow, I ventured forth in the car complete with an E-field probe built into the bottom of a mag-mount antenna on the car roof. The E-field probe whip is about 12-15cm long only. It looks like a small UHF quarter wave, rather than an antenna for 2200m! The TX frequency is currently around 137.685kHz and this QRSS3 beacon will remain on overnight. I am hoping that stations out to around 100km will take a look for it.

QRP earth-electrode antenna signal at 8.2km on 137kHz band

Well, results have been spectacular with some excellent signals around 8.2km away in the line of the earth-electrodes. Orthogonally, the signal was not copyable at all at a similar range. This is classic loop directionality again.

/M RX signal at 6km orthogonal to TX antenna.

The receiver consists of the 10-15cm long E-field probe feeding the FT817 with IPO button pressed (improves sensitivity when ON at 137kHz) feeding the PC running Spectran at 0.34Hz resolution.

I do find it unbelievable that such a TX "antenna" buried under the snow with NOTHING in the air at all and with earth rods just 20m apart can put out such a decent signal on the 2200m band.