ERP solutions? - NOT amateur radio

Today I got an email advertising Microsoft's ERP solutions.

OK, I have been retired now for almost 12 years, but I have no idea at all what this is about!  Perhaps I am out of touch with modern acronyms used in industry. To me ERP stands for "effective radiated power".

Please would someone enlighten me, and some others?

UPDATE 1015z:  Enterprise Resource Planning. I Googled it!  Sounds a similar idea to throwing cushions to break ice at meetings - equally stupid.

472kHz ERPs

Something seems wrong to me in that the difference between reports I give and I get on 472kHz WSPR suggests either my ERP is higher than I think or the ERPs stated by some others is lower than they think.  I am pretty sure my ERP is about right give or take a few dB "experimental error".  I am able to measure antenna current, I know the actual antenna height and can calculate the effective height. If anything, I believe my ERP is over-stated as my losses are pretty high.  I suppose my reports of others may be a few dB down because of the poor-ish sensitivity of my FT817 at 472-479kHz?

UPDATE 1740z:  I have been on 472kHz WSPR for just over 1 hour now. Just the usual early evening reporters with some very decent reports from G3WCB (101km) and G8LCO (58km) amongst others.   As always, PA3ABK/2 (306km) is putting in a big signal again this evening from across the North Sea. 

UPDATE 1752z:  Just been spotted in Wales by GW0EZY (251km) at -25dB S/N.

ERPs on 472kHz

I am pretty confident of my "just less than 5mW ERP" on 472kHz (with the very short Marconi) and am a bit suspicious of the claimed ERPs of some stations,, which, judging by the reports exchanged, may be lower than some people think. I think a lot more stations are less than 500mW ERP than they think. Perhaps losses are much higher than they think?

ERP calculation on 472kHz

In an attempt to establish my ERP on 472kHz, I have crunched some numbers. I think it is error free, but no doubt someone will soon tell me!

With a 1:46 step up toroid, I am measuring the DC volts rectified across a 470 ohm secondary load resistor using a small 50uA meter in series with 240k ohms (120k twice) and a series silicon diode. This circuit is straight out of the "LF Today" book available from the RSGB bookshop.

DC current = 30uA
DC voltage across 470ohm secondary load  = 0.6 + (240 x 10E3 x 30 x 10E-6)  = 7.8V
DC voltage (rms) across 470 ohm = 7.8/1.414 = 5.52Vrms
I secondary = 5.52 * 10E3/470 = 11.7mA
I primary =  46 * Is = 46 * 11.7mA = 0.54A

Rrad = 160 * pi^2 * Heff^^2/lambda^^2 = 160 * 3.14 * 3.14 * 2 *2/(630 * 630) =0.0159 ohms

P(erp) = I^2 * R = 0.54* 0.54 * 0.0159 W = 4.6mW

My original guesstimate of 5mW ERP is not far out!

Clearly getting the antenna much higher would help enormously as ERP is proportional to Heff^^2. All one can really be sure of is the order of magnitude. I am running low milliwatts.

472kHz ERP over-estimates?

Looking at the reports I send and receive, I wonder if several stations are actually using a LOWER ERP than they think? From measurements I am pretty sure my ERP is 5-10mW with 5mW likely to be the closer figure. Some stations claim 0.5 to 1W ERP which is considerably more than me. Some stations ARE using 6-12dB more than me and to much better antennas but, judging by reports, I should be reporting them much stronger than I do. This could be because (1) my noise floor is worse, (2) my RX is not optimally sensitive, or (3) their ERP is lower than they think. In at least some cases (3) is the reason.

New seasonal DX record on 474.2kHz last night

Last night I received several spots from DL4RAJ at 990km,which is a record, so far this year, for my QRP WSPR signals on 474.2kHz. I am hopeful of better results yet.

My 10mW EIRP maybe be a bit over-estimated as I forgot to divide the measured voltage by 1.414 to convert to RMS from peak. This means my ERP was closer to 5mW, even lower than I had expected, even though I am still showing 10mW ERP on WSPRnet.. This proves, even more, that high power and fancy big antennas are NOT needed to have fun on 474.2kHz MF.

12 unique WSPR spots in last 24 hrs with QRPP on 474.2kHz

EIRP calculation on 472kHz band

So I plugged my measured figures into the equations to work out ERP on the 472kHz band with the very low height Marconi.  Measured antenna current is 0.64A and effective height about 1.6m.  The formula gives EIRP taking  into account the lack of directivity.

EIPR (472kHz) = I^^2 * Heffective^^2 /100  (source LF Today 3rd edition)
                          = 0.64*0.64*1.6*1.6 / 100 = 10.48mW

ERP is 1.8/2.52 times the above figure, so around 7.5mW.

My estimate of 10mW  ERP is a few dB too high, but not a bad estimate, considering experimental error.

As the ERP is proportional to current and effective height both squared, it is easy to see how ERP can be much improved by increased antenna vertical height, increasing the power, or doing both. The other thing is to reduce system losses.  An antenna/earth that looks like 300 ohms is probably lossy.

How much am I RADIATING at VLF?

As a matter of interest, this afternoon I worked out how much power I am actually radiating when carrying out my earth-mode tests. The main transmission mode is conduction through the soil/rocks and buried utilities, but an earth-electrode antenna will produce some very very small amount of radiation too.

The first thing is the effective area of the "loop in the ground" and based on a guestimate of 40 ohm metre soil resisitivity (could be somewhere between 10-100 ohm metres) my calculations give me an effective loop area of 600m sq at 8.97kHz - i.e. the signal current flows quite deeply into the ground.

The second figure is the current flowing in the loop (I) which I measure as 0.2A using a current transformer to sense the current.

Rrad = (31171 x Ae^2)/lambda^4 = 0.94 *10^-8

Lambda = 33km
Ae = 600 sq m
I = 0.2 amps

Rrad = 0.94 * 10^-8 ohms
ERP = I^2 x Rrad

So, plugging in the numbers:

ERP = 0.2 x 0.2 x 0.94 x 10^-8

ERP = 37nW

There may well be errors in my sums and in the assumptions made, but clearly 37nW is a tiny amount of radiated power and (almost) negligible. To get to a level where the radiated signal is detectable over 100km away, I would need to radiate around 4-8uW, i.e. several orders of magnitude more. Radiation resistance is proportional to the loop area squared so increasing the baseline by a factor of 10 increases the radiation resistance by 100 times. This could be helped with a much longer baseline (200m long rather than 20m) and increasing the power into the earth-electrode loop to 500W and elevating the loop part that feeds the far earth electrode with current. Such changes could result in a radiated power level of around 4-8uW based on the assumptions about soil/rock resistivity and skin depth. A 200m long piece of wire (e.g. along a field edge) and 500W of audio power are not that hard to envisage and a LOT easier than winding a huge loading coil and raising a kite supported antenna several hundreds of metres high.

137.766kHz ERP - now measured at 6.6uW

So, today I repeated my field measurements to establish the ERP of my 8W 137.766kHz beacon feeding my earth-electrode antenna. This time I did the measurement correctly. Here's how.

1. Travel 2km from the home QTH with E-field probe, FT817 and 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

Caveats:

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.

Conclusions:

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, for example as a reasonable target output, would give me 6dB more ERP at around 25uW.
3. Based on results with QRSS3, I should be able to be copied using WSPR-2 at up to 100km on ground wave. At night with sky wave, considerably further is possible. 
4. Using WSPR-15 and 32W my ground wave range should be up to around 200km, possibly a little more. 

The QRSS3 beacon is still running over the weekend and more reports would be welcomed

My ERP on 137.766kHz is.......

Thanks to G4FEV, I've just realised that I made a fundamental mistake measuring my field strength, forgetting to turn off the AGC when taking the measurements of my signal relative to DCF39. Whereas my initial measurements indicated the ERP was in the low mW region, in reality my signal is much, much weaker. For example, G4FEV reckons my signal is some 70-80dB weaker than DCF39 where he is, if not more! So, tomorrow I shall have to try the test again and see if I can get a sensible figure with AGC turned OFF.

G3XDV's screenshot of my QRSS3 LF signal this afternoon.

Mike G3XDV (61km) has just sent me a very comprehensive report with 178 screenshots of my QRPp signal over the last 30 hours. Copy was remarkably good considering everything. Again, I am amazed how well this works.

Checking my 137.766kHz ERP by measuring field strength

Well, today has been a bit of a disappointment: my QRP QRSS3 beacon has been running for over a day on 137.766kHz and, apart from my own highly successful reception tests out to around 8km (as far as I went in the best direction) not a single station has reported seeing it  ....yet.

Tomorrow, I am going to do one more experiment using the QRSS beacon: knowing the field strength of DCF39 on 138.830kHz, a commercial station in Germany, is around 1mV/m here in the south of the UK in daytime, I want to measure the S/N of this station about 5km from home using the mag-mounted E-field probe. Then, at the very same spot, I will measure the S/N of my beacon with the same bandwidth settings and RX kit.

Based on these two measurements, I should be able to work out the ERP of my beacon to an order of magnitude and probably to within +/-3dB. The field strength of my own signal can be worked out just by comparing the S/N with that of DCF39.  Knowing the field strength and the distance from the home QTH, I can work out the ERP needed to produce this.

Any guesses what ERP I am using? I'll tell you the answer tomorrow!

New ERP calculations on 472kHz with earth-electrode antenna in 8cm snow

With several repeatable ground wave reports on 472kHz WSPR using the earth-electrode antenna, it is time for a recalculation of my ERP based on a formula (for 500kHz) supplied some years ago by M0BMU.  The assumption (from M0BMU, that I am not at all sure is right) is that the noise floor is around 3uV/m in a 2500Hz (WSPR) bandwidth. So, based on the WSPR S/N ratio the field strength when using the earth-electrodes can be calculated.

• G3ZJO at 79km gets me at around -19dB giving my field strength around 355nV/m. 
• G3XDV at 61km he gets me around -12dB S/N giving my field strength around 810nV/m.  
• M0BMU at 69km he gets me around -30dB S/N giving a field strength around 100nV/m

ERP Calculations

• ERP using G3ZJO's results = (E*d)^2 / 49; with E = 355nV, d = 79km   ERP = 16uW
• ERP using G3XDV's results = (E*d)^2 / 49; with E = 810nV, d = 61km  ERP  = 49uW
• ERP using M0BMU's results =  (E*d)^2 / 49; with E =100nV, d = 69km  ERP = 1uW

Now, to me these figures seem far too low, so perhaps the noise level is much higher than 3uV/m. Certainly that is the case here where I have an S4- S8 noise floor due to SMPSUs and other noise sources. I think these numbers can be doubled based on ground wave losses but, even so, the figures appear to be extremely low.  My Marconi antenna is some 8dB (average) better than the earth-electrode antenna and I am sure the ERP is well above 1mW using this antenna.

Conclusions

I am totally puzzled: measuring antenna current on the Marconi I get ERPs that are an order of magnitude higher yet measured results with the earth-electrode antenna average just 8dB down on the Marconi. This suggests ERPs for the earth-electrode antenna in the low mW region, which I find much more believable.

Any thoughts please?

Ten WSPR reports now on 137.5kHz

This evening G3WCB (101km) reported my 137.5kHz WSPR signal for the first time bringing my total number of reports on this more difficult band up to 10. My ERP is currently around 500-600uW, although I hope to run the transverter PA from around 20V tomorrow which should increase the ERP by around 3dB.

These are the reporters so far:

G6ALB
G3WCD
M0FMT
G3XIZ (46km)
G3XVL (69km)
M0BMU (69km)
G3WCB (101km)
G3YXM (148km)
M0PPP (182km)
GW0EZY (251km)

What is my ERP currently on 137kHz WSPR?

Based on some measurements and calculations using the improved ATU (see photo) today, this is my calculation:

Measured antenna current in the TX loop = 2.13A
Area enclosed by the loop = 80 sq m
P = (I^2 x A^2)/(205 x 10^6) at 137kHz   (see p101 of "LF Today" book available from RSGB)
where P = ERP,  I = current in amps and A = loop area in sq m

P = (2.13 x 2.13 x 80 x 80)/(205 x 10^6) = 141uW

This is not a lot (but more than ever before) and this is before I start on the "ERP improvement" programme.

137/500kHz ERP improvements

Today I made a start on improving my ERP on 137kHz and 500kHz.

Firstly, I've bought a pile of Wima 1000V polypropylene capacitors from Farnell. With these I have now made a new ATU to match my TX loop (breadboard only ATU so far) which will allow me to run some 6-10dB more power into the loop without the ATU components overheating and failing. My old switched capacitor array was an old school capacitance decade box and this was marginal with tuning changing as caps heated up!

Secondly, I am going to change the MOSFET PA design to increase the 137kHz power from around 7W RF to about 30W. This should be worth around 6-8dB. I plan to do something similar on 500kHz too.

Finally, I've got some more wire to allow me to double the thickness of my TX loop. This will be done later and will be worth 2-3dB as long as I alter the 3C90 ATU (step down) matching transformer.

In all, these changes will give me up to 10-13dB more ERP and, with some luck will allow me to get more reports on QRSS and WSPR. It will take some weeks to implement these, but by late October my range should improve on both bands.

ERP with the loop on 136kHz

Today I did a recalculation of my 136kHz ERP using the loop antenna with my WSPR transverter. I have a small RF ammeter in series with the loop wire and I measured the current as 1.3A.

For 136kHz 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 will work on a small ATU to resonate my 5m feeder to the 28MHz halo as a Marconi vertical. 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.

500kHz ERP with the vertical loop antenna

Results on 500kHz WSPR with the vertical 80sq m area loop (made from 1mm diameter wire) are very good with signal reports from G3XIZ this evening of +4dB S/N. M0BMU has given me - 4dB S/N. This leads me to believe the ERP of the system is now somewhat higher than with the short Marconi vertical. Based on earlier calculations I think my ERP, in the best direction, must be about 2mW now, some 6dB more than it was.

ERP on 500kHz (part 2)

Yesterday I did a recalculation of the G3XBM ERP based on the classic formula and re-measurements of my antenna current. My antenna is a 5m long vertical with a spiral top hat of about 7 turns about 1.5m x 1.5m. Not being clear how to model the top hat I assumed effective heights of 5, 4 or 3m. The measured antenna current is 0.12A. My results are as follows:

Assumed effective height 5m     ERP = 1.53mW
Assumed effective height 4m     ERP = 0.98mW
Assumed effective height 3m     ERP = 0.55mW

So, I believe my 1mW ERP previously calculated is not a bad estimate, and it may indeed be lower than this figure.

Rik ON7YD (OR7T) has just sent me this email:

"I simulated the antenna based on your description (with MMANA) and the current through the vertical wire seems to be quite constant. So the tophat works well and the effective height will be more or less the physical height. The simulation gives a radiation resistance of 0.074 Ohm, so with 0.12 A a radiated power just over 1mW EMRP (1.75mW ERP). In the real world it will be some dB's less (as Jim stated recently), probably something like 0,5 to 1mW ERP.  Very impressive that it covers over 1200km."

ERP calculation on 500kHz

I've recalculated my ERP based on a better measurement of antenna current. I was using a dust iron core for my current transformer, so swapped this for an FT37 ferrite toroid today and got a lot more current measured now. My antenna current is actually around 0.12A. My antenna is 5m long with a small spiral top so the effective height must be <5m. Even putting in 5m into the classic formula I cannot make my ERP exceed 1mW. I make it more like 600uW. Interestingly this is 20dB more than the original estimate from M0BMU and since then I'd increased the ERP, I believe, by around 20dB which would also make it 600uW. Of course the margins of error must be large.

New 500kHz ERP calculation

Some weeks ago Jim Moritz M0BMU made an estimate of my 500kHz ERP based on received signal strength levels at his QTH 69kms away. As I've recently increased my ERP, I asked Jim if he would kindly do a recalculation. These are his latest estimates.

Well, repeating the original calculation, taking your "best" signal level as -12dB on a noise level estimated at 3uV/m in 2.5kHz, your received field strength at my QTH is 0.75uV/m. At a distance of 69km, and assuming only "geometric spreading" propagation losses (i.e. an inverse relation between FS and distance), your ERP is

ERP = (Ed)^2 / 49 , with E = 0.75u, d = 69000, ERP = 55uW

Then we have to consider what additional propagation losses exist; originally I suggested 3dB, which would increase the ERP to 110uW. However, looking at G4FGQ's grndwav4 program, and the ground resistivity around this area, the "ground type" in the program might be 6 - 8, making the ground wave propagation losses between 4.2 and 8.5dB. This would make the ERP between about 140uW and 390uW.

Obviously there is a lot of uncertainty in this kind of estimate - the two main things are the possible variability in noise levels between when I estimated the noise level and now, and the propagation losses. Both these could contribute several dB variation, so the ERP estimate should only be regarded as "order of magnitude" accuracy.

Still, your signal is certainly much stronger now - one of my nearer-term projects is to make some more accurate FS measurements by injecting a calibration WSPR signal into the loop antenna, which should give a more direct and accurate measurement of the signal levels.

Cheers, Jim Moritz
73 de M0BMU