Operating on the 137kHz (2200m) band - simply

With description and schematic of a simple QRP transverter with just 6 active parts

Click on any image to enlarge it

This page is more about my progression on 137kHz rather than an expert's guide. If you are like me and want to experiment and find out how to make improvements then keep checking back to see any progress I make. My ethos has always been "keep it simple", so if I can make something simple work on 137kHz you'll find it here. My real challenge will be getting an effective 137kHz antenna in my small garden without upsetting the neighbours and XYL. I succeeded with a 5m long antenna on 500kHz, so some results are certainly possible, especially with modes like WSPR and QRSS.

Jan 25 2013
For the last few days I have been running my 8W (RF out, not ERP) QRSS3 beacon on 137.766kHz into the 20m spaced earth-electrode "antenna" and waiting for reports. Yesterday I got comprehensive reports from G4FEV (56.7km) and G3XDV (61km) who both copied my signal very well.

I also did some field measurements to establish the ERP of the beacon in the best directions of the "loop in the ground" by comparing the field strength against that of DCF39, the German commercial station just outside the band. Using a well known formula, it is possible to work back and get an ERP figure. Here is how:
  1. Travel 2km from the home QTH with E-field probe, FT817 and laptop PC running Spectran.
  2. Choose a location in the best direction for the "loop in the ground" so the measured signal strength is close to the maximum possible i.e. not off the sides of the loop.
  3. Set up FT817 so the RF gain is adjustable and turned well back and the AGC is inactive.
  4. Tune in my LF beacon and adjust the RF gain to the lowest possible detection level.
  5. Measure the S/N of my signal with Spectran and note reading.
  6. Retune to DCF39 (138.830kHz) and, without adjusting the RF gain at all, measure the S/N with Spectran.
  7. Repeat these measurements 5 times.
  8. Calculate the difference in dB between my signal and DCF39 (in my case 41.3dB)
  9. Using the assumption that DCF39 has a field strength of 1mV/m, work out my own field strength (in my case 9uV/m)
  10. Using the formula ERP = (E^2 * d^2)/49 work out the ERP.
Answer? My measured ERP on 137.766kHz is 6.6uW

  1. If DCF39 is not approx 1mV/m then the ERP needs to be adjusted up or down proportionally.
  2. Measurement error is +/- 2dB.
  3. The ERP is the figure measured in close to the best direction. Off the sides of the loop the ERP will be much lower.
  1. Stations able to detect the QRSS3 signal at any distance are doing well as the signal is very weak indeed.
  2. Using my proposed WSPR transverter at 32W (a reasonable target output) would give me 6dB more ERP at around 25uW.

Jan 22 2013
Today saw my return to 137kHz with the testing of an 8W CQ/QRSS beacon transmitter used in conjunction with my 20m spaced earth-electrode antenna in the garden. Also tested was a new E-field probe attached to a mag-mount with 15cm whip mounted on the car roof. This EFP has proved to be excellent and allows me to do local (up to around 15km) drive-around tests to check coverage when coupled into my FT817 and a laptop PC running Spectran.

Details of the E-field probe is on my blog at http://g3xbm-qrp.blogspot.co.uk/2013/01/137khz-e-field-probe-mag-mount.html .


Sept 18th 2012
It is over a year since I last transmitted on the band. I am working on a new transverter for 472kHz and hope to adapt a version of this for 137kHz by changing the crystal and output network only. As part of the work in preparation for 472kHz I am either going to re-erect my loop (with thicker wire diameter) or a Marconi vertical. Not decided yet.

Watch this space......

Sept 8th 2011

Recently I have been updating my kit for 137kHz. I have now rebuilt the QRSS3/30 beacon to produce around 23W and the transverter used for WSPR has been improved to give 24W from the PA. So far only the beacon has been tested on air but reports from G4AYT (Whitstable) and M0BMU (Tring) today confirm the signal ERP is better. I estimate the ERP as being 0.2mW currently using my 80sq m loop in the garden. The photo below shows the latest version of the beacon TX. Some while back the original beacon was copied in Holland, so with the increased ERP further DX is possible.

The image below is of the latest version of the transverter which produces 24W from the (very efficient) PA. The heatsink hardly gets warm at all yet the 50 ohm load is VERY hot!

Feb 1st 2011

Bert PA1B has done some interesting "reverse analysis" of the WSPR reports I've given people. Working back from these you can estimate the lowest levels (in mW) that stations could have used and still been copyable with me. This also gives a good guide to the likelihood of these stations receiving my weak 50uW ERP signal. It is clear that M0BMU should be able to copy me signal frequently, which he does, but that I'd need a lot more ERP to be able to get reports from the PA3DGO, PA0A, DF6NM  or SM6BHZ.

Yet again, this sort of analysis really shows the value of WSPR when making station improvements and comparing results. It is not perfect as there are other variables than just the TX station's ERP and the quietness of the LF receiving station (occasionally large signals nearby can screw up WSPR decodes for example), but overall it gives a clear indication of the work that remains to be done to get consistent results and reports from further afield.

My thanks to Bert for running this analysis. See Bert's QRPp website at http://a29.veron.nl/pa1b.htm.

Jan 30th 2011
Yesterday I combined the RX and TX parts of my 137kHz QRP transverter for the first time and all is working although a few dB in RX sensitivity is being lost in the ultra-simple RX-TX switching method. Despite this, I'm copying DL, SM (951km with 50mW ERP) , PA and G stations on WSPR. My best DX on transmit is still 148km.  
Next I need to improve the antenna efficiency as power from the IRF510 FET PA is 8W, but the ERP from the vertical wire loop is just 50uW. Even so, WSPR reports suggest that 100km + should be reliably possible even with the present set-up. With another 6dB or so ERP I could be getting WSPR and QRSS3 reports from continental stations out to around 300km.

My loop ATU uses a capacitance decade box to select a value of C to resonate the loop: with high-Q loops, the voltages (even with QRP) can be very large in the loop and one really needs 500-1000V capacitors that can take the voltage and heat. At the moment the ATU tuning is not always constant as the caps in the decade box are getting a bit stressed. So, the loop ATU needs improvement or I'll try a vertical Marconi again with much more wire in the air and a large loading coil of around 4-8mH.

The transverter circuit is now simpler still on TX with just 2 gain stages after the mixer. On RX I have a small tuned preamp to make up for the lack of sensitivity of the FT817 at 136/137kHz. This uses a grounded gate MPF102 FET with a tuned drain followed by an emitter follower.
Antenna switching between TX and RX is ultra-simple using the back-to-back diode method common in QRP circuits with no need for relays. I am pretty sure the SBL1 could be replaced with a simple transistor mixer followed by a low pass filter - something else to try.

The FT817 is used in split mode (TX on 10.136MHz but receiving directly on 136kHz. This simplifies the design.  In all, just 4 transistors and 2 FETs are needed for the whole transverter.  Please remember, this is a work in progress and be prepared to optimise if you copy it. There may be some benefit in putting a 5V regulator in the oscillator supply, especially if running very slow QRSS modes. Also, keep the crystal as far away as possible from the PA heatsink to minimise thermal drift.

The latest 137kHz QRP transverter schematic is shown on the right. Click on the image to get it full size.

Jan 14th 2011

Today I re-erected my vertical loop for 137/500kHz and was rewarded with a string of reports from Jim M0BMU who is 69km away near Tring when I was using around 50uW ERP. Reports of up to -22dB S/N suggest that >100km should be consistently possible given some WSPR activity in range from stations using decent receiving systems in quiet locations.

Jan 12th 2011

Disappointing results yesterday with a very short Marconi vertical tuned using a ferrite rod loading coil and variable capacitor. ERP was calculated as around 200uW, but in reality the ERP was around 10dB down on the vertical loop (based on WSPR reports locally), so the ERP was around 5uW only. Most of this can be put down to the coil losses and to tiny current actually flowing in the antenna part actually radiating. A much more efficient antenna for the band will require a BIG loading coil outside and more wire in the air with some top loading. I could erect a 6m vertical antenna with a sloping top capacity hat horizontal section with an overall vertical rise of 10-12m. Antenna improvement could increase ERP by >6dB. TX power could be raised by 6dB. Taken together this would make a difference, but it would not meet my QRP ethos.

My conclusions are that an ultra-simple QRP approach to 136kHz TX is just not possible if one is to achieve useful results consistently.

Jan 11th 2011

Today I did a recalculation of my 137kHz ERP using the loop antenna with my WSPR transverter. I have a small RF ammeter (ferrite toroid with 50:1 transformer) in series with the loop wire and I measured the current in the loop wire as 1.3A.

For 137kHz the ERP of a loop is = (I^2 * A^2) / (205 * 10^6)
where I = loop current and A = loop area (see "LF Today" RSGB book p101)

Putting in the numbers ERP = 1.3^2 * 80^2 (205 * 10^6)

So my ERP = 52uW on 136kHz.

A further 3dB could be obtained by thickening up the wire used in the loop taking the ERP up to around 100uW. Other improvements could come from increasing the TX power output from the present 5W.

Later today I'll work on a small ATU to resonate my 5m feeder to the 28MHz halo as a Marconi vertical tuned against all my grounds.  First I will work out the antenna current needed to beat the ERP of the loop. At least a Marconi will not be directional, which is a drawback of the loop.

October 25th 2010

A fairly easy way to increase my ERP on 137.5kHz WSPR is to up the supply voltage to the IRF510 PA. Using a 20-24V supply I am able to get another 3-4dB ERP judging by the reports from M0BMU and G3WCD this morning. This evening I'll give it a go with the higher supply to see if I can get any reports from further afield.

Sept 26th 2010 wet and miserable!

This is the provisional schematic of my 137kHz QRP transverter. I've still to add the RX preamp (and need to add an RF choke at 10MHz between the RF attenuator and the FET tank circuit) and some values shown may be wrong. I thought it was time I shared this here as people are starting to ask me questions about it.  In the next few days I hope to update it and add latest values everywhere.

137kHz transverter (provisional schematic
(click for full sized image)

Sept 21st 2010 dry and sunny

Real WSPR DX at last!!!

Just had a report from G3YXM who gave me -24dB S/N on 137.5kHz when I was running no more than 20uW ERP. He is now my 5th WSPR report on the band and the best DX by far at 148km. This is so thrilling! The transverter is just sat on my desk and the PA isn't even warm.  My expectations are exceeded now by a LONG way.

This is the WSPR screen showing my QRPp signal as received in Birmingham this evening at 148km. My signal is the one on 137.471kHz.

Since starting on 137kHz WSPR just over a week ago I have now been copied by 5 different stations so far, but Chris G3YXM is definitely the best DX thus far.

Sept 19th 2010

Lots of activity on 137kHz WSPR today, but sadly I only got only two unique reports today, again from G3XIZ and G3XVL.  With the ERP I'm running there's just not enough signal level for more distant stations to copy me. Already I've learned how far 10-20uW is copyable and this is not as far as I'd like!  100km is, I think, just possible on WSPR but only under favourable QRN/QRM conditions.  Although I can see relatively simple ways of gaining a few more dBs of ERP I have to decide how seriously I want to take 136kHz now: to get more reports at greater distance needs at least 10dB more ERP I think to make breakthroughs in range. This implies running at least 20W RF (6dB) and another 3-4dB in antenna improvements i.e. some significant changes will be needed. Do I want to do this?

Sept 18th 2010

WSPR success....

Chris G3XIZ has successfully received my QRPp WSPR beacon this morning on 137.5kHz. ERP is around 10-20uW from the class-E PA producing around 4-5W. The heatsink runs quite cool at this power, barely a few degrees above ambient thanks to the class-E configuration. Chris's most recent reports around lunchtime have been better with up to -20dB S/N. At this sort of level my signal should be copyable in the better directions (E-W loop orientation) out to considerably further, even before raising the ERP. Later in the day my first report on 137.5kHz WSPR from G3XVL near Ipswich, which is well off the best direction for my small TX loop antenna.

This brings my total number of WSPR reports on the band to 3 with the best DX on this mode now 61km.

Sept 17th 2010

My WSPR beacon is on this evening but, despite alerting people via the LF-reflector, GQRP Yahoo group, my blog and direct emails I can see only a couple of people looking for me so far. No reports seen yet via the WSPR database. To be honest, I'm disappointed with the activity level as potentially I could be copied out to greater than 100km with the current set-up.

This is the TX transverter breadboard currently. Note the T106-2 toroids and the TO-220 heatsink for the IRF510 FET

Sept 15th 2010

Well, I think I've discovered why my FETs have been dying during assembly: my trusty soldering iron earth was not earthed at all and I suspect that every (recent) attempt at soldering devices has been killing them in the process, probably by applying a large AC voltage between gate and source. I've now fixed the grounding problem and I'm happy to report that recent insertions have been fine. So, tomorrow I hope to get the QRSS3 beacon and WSPR beacon TX working and on-air.

Sept 8th 2010   dry

Well, I'm really on a roll now - blowing up FET PAs that is!  Out of the blue my IRF510 (my last working one here) decided to die on me in mid beaconing. I then replaced it with an IRF530 but this took 3.5A and appeared dead on insertion, despite careful handling. 2 others followed and appeared dead. I think I have a bad batch as I cannot see anything wrong with the circuit there were fitted in. I'm told that the IRF510 is rugged and not prone to static damage because of the high input and output capacitances. My next batch of 10 is due tomorrow along with heatsinks. Better luck next time?

Sept 7th 2010  dry

Today I made my first proper attempt at a QRP class E PA design for 136kHz. Although the values in the output circuit seemed to tie up well with theory I wasn't able to realise the expected high efficiencies, possibly because of the gate drive design. In fact I managed to blow up my last IRF510 because it got very hot and this meant aborting a WSPR test (at a sensible power level).  One resource recommended to me is http://www.alan.melia.btinternet.co.uk/classepa.htm which has a E class design spreadsheet. I also discovered today that 16mm diameter 3C90 toroids do NOT make good LF PA output bifilar transformers: they get very hot. The larger diameter 3C90s appear to be fine.  Overall, a frustrating day but I have learnt a bit by making mistakes. I've ordered some heat sinks (to replace the croc clips!) and 10 more IRF510s to play with.

Sept 6th 2010  rainy

This evening I had a good report and screenshot (see right) of my 137.675kHz QRSS3 beacon from Chris G3XIZ 48km away. This is my best report in terms of S/N and distance yet, with very clear images on the Argo screen.  It suggests that in this direction the signal might be just readable at 100km.  I was going to try WSPR on 137.5kHz but had problems with the PA so QRSS3 beaconed instead. ERP was less than 20uW.  Antenna was still the "loop in the air" matched now via a much larger (and not lossy) 3C90 transformer and the capacitance decade box.

Also had this interesting analysis regarding the difference in WSPR reports (over a 3km distance) between 136 and 500kHz from VK2XV:

G'day Roger,
I read with interest your observation of the behaviour of the "in the air" loop via WSPR.
"On 136kHz the best report was about -20dB S/N whereas on 500kHz it was +10dB. This is a difference of 30dB!!"
I am not surprised at the 30dB difference as the radiation resistance varies with the 4th power of frequency for the same dimensions giving 23dB difference (i.e. 10 log ((136/500)^4)).   In addition WSPR reports a S/N against in-band noise.    Atmospheric noise is inversely proportional to frequency and there is about 36dB difference (500kHz being quieter) between 2200M and 600M.    This difference is effectively reduced by man-made noise which is higher wrt to atmospheric noise at 500kHz compared to 136kHz.    This reduces the differnence in noise levels from 36dB to about 10dB to 15dB (depending on location).    Therefore I would expect about (23 + 10) = 33dB to about (23 + 15) = 38db difference between WSPR reporting for 2200M and 600M.
Your measurements are reasonably close to this - given there would be other differential effects such as ground loss.
Keep the interesting stuff coming !!!

73 Steve Olney - VK2XV

Many thanks for this Steve.

Sept 5th 2010 mild and dry

Just started up the WSPR beacon into the "in the air" loop antenna and running what must be between 10-20uW ERP. So far the only reports have been from my local friend G6ALB who is 3km away and he is giving me only -20 to -22dB S/N, which is not very good at this distance. ATU tuning seems less sharp that with my lash-up, so I need to see if the step-down transformer needs a different turns ratio. Otherwise, apart from rebuilding the ATU, nothing has changed. Adjusting the turns ratio a tiny bit has now got me -16dB S/N from G6ALB.

I've just discovered that the 3C90 cores (about 16mm diameter) I use in my ATU step-down transformer (top left in the picture below) is getting very hot, suggesting it is lossy. I have one much larger 3C90 core so will wind a step-down transformer on that instead in the morning and see if that is better. I'm surprised that at 5-6W into the transformer this gets hot.

This evening I've been running about 5W PA output (not ERP) WSPR from an IRF510 PA on both 136 and 500kHz WSPR and getting reports from G6ALB who is 3km away. In both cases I'm using the same ATU (just different capacitor settings) into the same loop antenna. On 136kHz the best report was about -20dB S/N whereas on 500kHz it was +10dB. This is a difference of 30dB!! For some reason, 136kHz seems a heck of a lot harder than 500kHz. The radiation resistance is considerably lower on 136kHz, so losses become more significant I guess, but the difference still seems to big.

Sept 4th 2010    mild and dry

This morning I tidied up the arrangement to match my "in the air" wire loop antenna currently being used on 136kHz and 500kHz. The loop looks like a few ohms resistive when tuned so I have a 4:1 turns ratio step-down transformer wound on a 3C90 core. The loop is tuned using an old decade capacitance box and this is fine tuned with the 365pF variable capacitor. At QRP power levels the rating of the capacitors in the decade box is quite adequate (100V?). However, if I was using QRO I'd be needing to use capacitors rated at greater than 400V, probably 1kV because of the high voltages in the loop. The ATU is tuned for a peak using the current meter which consists of a small ferrite toroid with a 1:50 turns ratio, a 470 secondary load connected to a diode detector. Measured current flowing in the loop is around 1A, suggesting the match is pretty good.

The piece of paper at the bottom shows the values needed to resonate the loop on both bands (15.7nF on 137kHz and 1.2nF on 500kHz). It is simple, but works fine at the QRP power levels I'm using (5W max from PAs on both bands). My report from M0BMU at 69km on 500kHz WSPR was -8/-9dB S/N which is pretty good. I'll be trying the loop and ATU with my QRSS3 and WSPR beacons on 137kHz on Sunday and during the following week.

Sept 2nd 2010  warm and dry

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 is 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. The tuning is done with a capacitance decade box with a 365pF variable capacitor to tune it "on the nose".

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.

Attracting attention is turning out to be a serious handicap 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.

Sept 1st 2010   warm and dry

Today I managed, I believe, for the first time to accurately measure the ERP of my QRPp system on 137kHz.This is the method used:
  • Using the E-field probe, FT817 (AGC off, gain backed off as far as possible and a 10dB pad between the EFP and the FT817) and Spectran I went to my usual test site 1.5km away from the QTH, 45 degrees off the main lobe of the TX loop/earth electrode antenna.
  • Measured the signal level of DCF39 on 138.83kHz
  • Measured the signal level of G3XBM on 137.675kHz
  • Repeated this three times to reduce errors.
  • Noted the difference in FS level.
Difference in signal level = 44dB . I feel pretty confident this is an accurate figure now and not effected by AGC and overload. Assuming DCF39 is 1mV/m here (info from Alan Melia)  then my FS at the test site is 6.4uV/m.  Using the formula ERP = (E*d)^2/49 where E = 6.4*10E-6 and d=1.5*10E3 gives an ERP = 1.9uW giving an antenna efficiency of -63dB using the earth electrode antenna with the elevated feed and 4W from the PA. 

The test site is about 45 degrees off the main line of fire of the antenna, so in the best direction it could be 10dB (?) stronger, i.e. 20uW ERP giving an antenna efficiency of -53dB in the best directions. Frankly I'm amazed that anyone can copy this signal at any distance, so full marks to G3XIZ (48km) and G3XDV (62km).

G3XDV sent me some more screenshots of my signal during this morning's test and on one image you can, looking carefully, make out the "G3X" part of my callsign. Far from a strong signal, but there nonetheless.

Aug 31st 2010

Today I changed the earth electrode antenna by raising the height of the connecting wires to form a (part) loop in the air as well as in the ground. The loop bid in the air has an area of >100 sq metres.  This should increase the effective loop area by a factor of at least 2, thereby improving the ERP. Later I'll repeat the tests with just the loop in the air only, so I have 3 different sets of results to compare. On 500kHz WSPR my signal with M0BMU (69km) was 6dB stronger than before with the earth electrode antenna. At the moment I am QRSS3 beaconing on 137.675kHz and seeking reports.

August 29/30th 2010

Well, it is no great DX at 3km away in the next village, but I've now received my first WSPR report on 136kHz from G6ALB. He gave me -21dB S/N when I was running 20uW ERP from the earth electrode antenna. G3XIZ (48km) is receiving the WSPR sequence but has not yet decoded it successfully. Unfortunately no further WSPR reports received over the long weekend.

August 28th 2010   Dry

Now WSPRing on 137.5kHz.....

Having got the TX part of the 136kHz transverter going today I am, for the first time, putting out a WSPR signal on 137.5kHz. Power from the PA is around 4W and the ERP from the earth electrode antenna is around 20uW. I am hopeful that Chris G3XIZ in Biggleswade may be able to decode the signal - he was able to receive the QRSS3 beacon earlier today at a just not readable level (would have been OK in QRSS10) - and possibly others not too far away. In the picture the transverting bit (from the 10MHz band) is on the right and the PA in the top left with low pass filter visible using the T106-2 toroid. Note the heatsink for the IRF510 - a couple of crock clips as this was all I could find in the junk box!

This is the signal Chris G3XIZ got from me today on QRSS3. This is the strongest yet on 136kHz (I was running around 20uW ERP) and my first correct report as he got "G3XBM JO02dg". This evening Chris was not even able to see my WSPR signal, which was a bit disappointing as the QRSS3 signal was quite clear.

On Sunday I will leave the WSPR beacon on during the morning. Next week I shall try to get a more efficient antenna erected, either the loop or a Marconi properly loaded and matched.

August 24th 2010 (again)

Wow - DX reception!!

Today I ran an earth electrode transmission test with Mike G3XDV in Welwyn Garden City 62km away. I put my QRSS3 8-10uW ERP 137.675kHz beacon on and Mike took regular screen shots through the day until mid afternoon. Although unable to copy the message in QRSS3, when using QRSS30 speed for reception the start and end of the message sequences (9 mins 7 seconds apart) could clearly be seen (see example screen shot left). He was able to precisely detect when the beacon was turned off at 2.45pm.

Mike estimates another 6-10dB should allow him to read the message properly. Signals were weaker later, possibly because of rain here altering the effective loop size within the ground.

This is the first time my QRPp signals have been received at any distance apart from my own local testing sessions. It has certainly encouraged me to get a bit more ERP and the transverter build is now progressing steadily. I hope to erect the "loop in the air", as opposed to the "loop in the ground" within days.

August 24th 2010   Dry after heavy overnight rain

Having properly tuned my loop to the centre of the 136kHz band - it was several kHz low - I went out to my local RX test site at the Devil's Dyke National Trust carpark 1.5km away from the home QTH to check signal levels using both the loop and the E-field probe antennas.Good signals were received on 137.675kHz QRSS3 using about 2W from the PA and an ERP believed to be about 8uW (accuracy +/- many dBs). As before, the TX antenna was the 20m spaced earth electrodes in the ground.

This test was after a night of very heavy rain and although this would improve the earth probe's contact with the soil it should, theoretically, reduce the loop area formed within the ground using an earth electrode antenna. If anything the loop current was slightly higher (expected) but the field strength was also good suggesting the loop area within the ground had not been significantly altered.

This is the Spectran display when using the 80cm loop to receive the signal.

August 22nd 2010   Dry after overnight rain

Today I attempted to calibrate my 136kHz 80cm receiving 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.  On earlier tests at the same location the signal level from my beacon has been 6dB stronger, so I suspect variations are as a result of moisture content in the soil between the earth electrodes.

August 16th 2010

The 20m spaced earth electrodes "antenna" has about 30m of wire coming back at 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,  the peak is 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. With an elevated TX loop with low AC resistance in the wire, then matching the loop will be critical.

August 15th 2010

Having simply tuned and matched my 5m vertical coax feed to the 28MHz halo (the halo is used as a top capacity hat) I'm now putting out the grand level of 8uW (+/-3dB) on 136.93kHz in QRSS3 from my 2W out beacon TX using this somewhat small Marconi antenna. Tuning was with 1.2mH wound on a small piece of ferrite in parallel with some fixed and variable capacitance. Matching was via a 3C90 toroid transformer. Although the power is still very low, I'm hopeful someone may now copy this signal.  I'd appreciate any screen shots and reports. I'll compare field strength levels in the local area (1-15km) against those obtained using the earth electrode antenna which got 14.9km a few weeks ago).

LATER: Field results were disappointing with the earth electrode antenna stronger. Back to the drawing board!

The next stage is a loop antenna that can be used on both 500kHz and 136kHz.

August 2nd 2010 - warm with light drizzle

This morning I did some more tests using my little 136.93kHz 2W QRSS3 beacon feeding into the 20m spaced earth electrodes. The TX end was unchanged, but this time on receive I used my 30t 80cm loop resonated with about 700pF and fed straight into the high impedance input of my E-field probe.

Results were as follows:
  • At 2.4km good copy by ear (12wpm CW would have been very solid)
  • At 8.6km good copy using Spectran
  • At 14.9km clear copy of my callsign and locator in QRSS3 with Spectran on the Gog Megog Hills near Cambridge (see screenshot)
All these locations are more or less in the line of the earth electrodes that run NE-SW from the home QTH
(JO02dg).  Another RX test some 11km to the south of the QTH  resulted in no visible trace, suggesting the TX earth electrodes are behaving as a loop antenna with some directionality. This was not the case when the same earth electrodes were used at 0.838kHz some weeks ago and when the mode was mainly conduction through buried utilities.

Remember, this is ultra simple stuff:
  • A 2W out crystal controlled QRP 136kHz beacon TX on a 5 x 5cm board (see picture)
  • An invisible ground "antenna" that cannot be seen, even when a few metres away from it.
  • No attempt whatsoever to match the TX to the earth electrodes.
  • A simple loop and FET impedance matching circuit and deaf FT817 at the receiver end.
It would be interesting to work out what the ERP must be. Can anyone estimate this please based on the QRSS3 trace on Spectran at just under 15km?  I was using 0.17Hz resolution and 11025Hz FFT sampling rate.

Interesting to speculate what would happen with such a system at 73kHz. Pity there is no allocation there today.

July 30th 2010 - dry after a few rainy days

Today I increased the power from my QRSS3 136.93kHz beacon to 2W out of the PA into the 20m spaced earth electrodes. Another "ride about" test locally with the deaf FT817 and E-field probe RX antenna gave the following results:

At 2.4km good copy on the Spectran display
At 4.6km clear copy on the Spectran display.
At 8.6km the signal was detectable, but weak (see picture attached from Spectran screen).

These reports are roughly "end on" to the electrodes and I've yet to try other directions such as orthogonal to them. No attempt was made to match the PA output to the impedance presented by the earth electrodes which are around 40-60 ohms below 10kHz; I've not measured it at 136kHz yet. Even with true QRP it looks like a range of at least 10km should be possible on QRSS3 (and probably WSPR) with a half decent receiver (better than my crude set-up) using just a 20m earth electrode TX antenna on 136kHz.  I'll leave the QRSS3 beacon running over the weekend on 136.93kHz from my QTH in Burwell, Cambs (JO02dg) and any reports and screen shots would be much appreciated.

Further tests will then be on hold now for about 10 days as my young grandson is coming to stay again. After he's gone back home I'll start on the Marconi vertical or a loop antenna, and of course on the full transverter.

July 24th 2010

A repeat of the test on July 19th gave less encouraging results today. Reasonable copy on QRSS3 at 1.5km but zero copy at the same  spot 4.6km away from the QTH (within 20m). Not sure what has changed as the earth electrode "antenna" is the same. Maybe the soil is slightly more damp?  My next stage (through August) is to make the beacon TX at least 5W out (IRF510) and repeat these tests.

After that I'll make a start on the full transverter to 10MHz so I can run WSPR both on RX and TX. The Marconi vertical I plan (in my head at least) will have a spiral top inductance, 5m vertical section, a 2mH coil (large diameter) a further run of wire to the last vertical down wire (2m long) with a 2-3mH variable tapped coil at the base. I'll have a couple of ground rods and some radials from the ground point This is about the best I can manage in the space I have.

July 19th 2010

Today I tried some more experiments with my 20m spaced earth electrode "antenna" and small (250mW from PA) QRP beacon TX on 136kHz. I switched over to QRSS3 and  used the FT817, E-field probe and PC with Spectran to see where copy stopped. 1.5km from home and solid copy on QRSS3 when holding the E-field probe box as high as possible above the car roof. Then I drove on to a spot 4.6km from home and copy was again achieved, albeit not as strongly.  I'm certainly getting a signal radiated beyond the near field.  I'd like to check "antenna" directionality by looking for signal levels with the QRSS3 beacon at all compass points some 4-5kms away from the QTH.

In the meantime, if anyone near Burwell, Cambs wants to look for the QRSS3 signal near 136.93kHz I'll leave it running until about 8pm tonight when I have to switch off as my grandson is sleeping in the shack bedroom. Any reports would be amazing.

July 18th 2010

Today I transmitted my first QRP signal on 136kHz using a small 250mW TX with 2N3904/2N3906 PA. Signal source was a 3560kHz crystal which was divided down by 26 using a clever little circuit based on a micro-controller that was sent to me by Francis F6HSI. For the keyer I used a K1EL keyer chip sending either 10wpm CW or QRSS3.  As I had no suitable toroids for the low pass filter on the output I used a small slab ferrite antenna which had some 50uH inductance wound on it.

Using my 20m spaced earth electrode "antenna" on TX and with a PA0RDT designed active antenna with a 19 inch whip antenna feeding into the (deaf) FT817 I could copy the 10wpm CW at 0.4km across the fields.