137kHz QRSS3 test

One of the aims for this week is to carry out that 137kHz QRSS3 test with my earth-electrode "antenna" in the ground with my FT817, E-field mag-mount on the car roof and SpectrumView software on my iPhone. To do this needs the coax fitting to my mag-mount. I must have "stolen" it for some reason.

Before going out in the car I can check things locally.

This software allows about 2 minutes of audio to be recorded as a spectrogram view. I hope to do this by just placing the iPhone next to the FT817's speaker. The photo of the SpectrumView screen is attached.

See  https://oxfordwaveresearch.com/products/spectrumviewapp/   .

E-field probes (EFPs)

These are tiny receive only antennas that can work extremely effectively at VLF, LF and MF where external noise is usually the dominant factor. I guess as local external noise is increasingly the dominating factor, these will be as effective as a "big" antenna at even higher frequencies. At LF they can be only a few centimetres long, but be as effective as antennas hundreds of metre long. 

EFP on a mag mount
for 2200m RX

As the signal decreases, so does the external noise! Overall, nothing is lost. There is no advantage by using a large antenna at low frequencies for receive only. For TX efficiency, a large antenna and good earth help! A popular EFP design is that PA0RDT (Google it). I have made versions for LF and VLF with a tuned peak as shown in the circuit.

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

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 .

VLF E-field probe?

 

This photo was taken some years ago. I think it shows my VLF E-field probe.  It is just possible it was the VLF loop pre-amp as they looked very similar.

137kHz tests with an E-field probe mobile

Before my stroke, I did some mobile tests on 137kHz QRSS QRP using an EFP with an FT817 RX. The EFP was mounted on a mag mount.  Results totally exceeded my expectations. After I reached 24km, I gave up. This was with a very low ERP (uWs).

See 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 .

136kHz E-field probe found

With a fresh battery, I think this will still work. It was re-found yesterday. 

If it does, it means I can do some local tests with my beacon into the earth-electrode "antenna" in the ground. I already have a local RX test site in mind.

UPDATE 1540z:  A short range test in the house shows the EFP still works. 

See https://sites.google.com/site/g3xbmqr

p3/antennas/137efp

136kHz experiments

Before moving to the current QTH about 7.5 years ago, I did some tests on 136kHz QRSS. I used the E-field probe for 136kHz on a mag-mount on the car and drove around to see where the beacon could be copied. The beacon produced 8W into 50 ohms, but the beacon ERP was only low milliwatts or even less. The antenna was the earth-electrode "antenna" in the ground. After successfully receiving the beacon on the far side of Cambridge (20+ km?), I just stopped as it was obviously working well both on TX and RX. The E-field probe is shown in the photo.

Other than a brief test with the earth-electrode "antenna" on RX, I have never yet tried 136kHz at this QTH.

My old site (links may not work as this old site is not maintained) has a description of the E-field probe RX antenna and other tests at 136/7kHz.

See https://sites.google.com/site/g3xbmqrp/Home/136k .

See also http://g3xbm-qrp.blogspot.com/2013/01/137khz-e-field-probe-mag-mount.html for more on the LF EFP.

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 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

137kHz E-field probe

Several years ago I made this mag-mounted 137kHz e-field probe to see how far away I could detect my QRPP 137kHz beacon. At about 24km I just stopped!

Both the EFP and the 137kHz beacon clearly worked.

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

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 .

E-field probe

My mini-whip E-field probe has arrived from PA0RDT. It will be some time before this RX antenna can be erected as I depend on help these days. It is tiny.

E-field probe RX antenna

Years ago, I would have built my own, but these days my fine motor skills are not as good as they once were. So I have bought a PA0RDT miniwhip RX antenna. It should arrive next week.

Suitably mounted in the garden, if should be quite an effective LF and MF RX antenna. This "season" I want to concentrate on 137 and 472kHz RX. I might also try top-band.

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

136kHz last night

There is no doubt that I have to improve my RX system on LF and MF.

Last night I spotted the same few stations on 136kHz WSPR and OPERA, giving reports way down on others spotting the same stations.

Next season I shall definitely have to try a mini-whip E-field probe which, although tiny, work really well on 136kHz and 472kHz RX as long as they are positioned to minimise noise pick-up. Usually this means as far away as possible from houses and in the clear. I know I can do better.

Up to now, I have just been using my earth-electrode "antenna" in the ground. It works, but is directional and probably 10-20dB down on an optimised E-field probe.