Re: LF: ERP calculation (revised)

[Markus Vester <markusvester@aol.com>]

Dear Roger,

 

thanks for sharing your results!

 

The directional dependence should be a simple cosine law, so going from 45° to 0° would give you another 3 dB, or 3.8 uW ERP. Thus your total radiated power was 2.0 uW (EMRP). At 62 km, this gives -14 dBuV/m, which should indeed be well readable in QRSS 3 under quiet conditions.

 

Taking a loss resistance of 60 ohms, 4 watts would have given you an antenna current of 0.26 A. The radiation resistance is then
 Rrad = EMRP / Iq^2 = 30 microohms.
A standard formula for loops is
 Rrrad = 31171 ohm * A^2 / lambda^4,
resulting in an effective loop area
 A = 155m^2.

Note this is using the radiation resistance for a loop in free space, as the effect of ground is already included in the earth antenna picture. For an above-ground loop with a mirror image beneath it, radiation resistance would be doubled.

 

Best wishes,
Markus (DF6NM)

 

-----Ursprüngliche Mitteilung-----
Von: Roger Lapthorn <rogerlapthorn@gmail.com>
An: rsgb_lf_group@blacksheep.org
Verschickt: Mi., 1. Sept. 2010, 13:41
Thema: LF: ERP calculation (revised)

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).

Next stage is to try this arrangement for a few more days using QRSS3 and WSPR before swapping to a full "in the air" loop and repeating these tests.

Great fun and I'm leaning as I go, which is the whole point of ham radio.

73s
Roger G3XBM

--
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G3XBM   GQRP 1678    ISWL G11088