Dear Jim, David,

 

I have been successfully using a variant of the second method, which works by comparing received voltages of a strong distant transmitter (eg. DCF39) from the transmit antenna under test, versus a small nonresonant loop of known geometrical area. The induced loop voltage is

 

 Uloop = H * omega * aloop = E * aloop * 2 pi / lambda.

 

The effective height of the TX antenna then is

 

 htx = Utx / E = Utx / Uloop * aloop * 2 pi / lambda,

 

and the radiation resistance

 

 Rrad = 1579 ohm * (htx / lambda)**2

 

One advantage is that electrical near field shielding effects (eg. by trees) are actually included in the voltage ratio, because the reference measurement is based on magnetic field and thus hardly affected by moderately conducting objects in the vicinity. As the effective height is only weakly frequency dependent, the measurement can be done at a different frequency, depending on signal availibility.

 

Absolute receiver calibration is not required for the voltage ratio, but you may have to apply a correction for loading of the tx antenna source impedance by the receiver impedance:

 

 Utx = Utx_loaded * (Rant + Rrec) / Rrec

 

I also found that I had to detune or disconnect the big antenna during the loop measurement.

 

Best regards,
Markus (DF6NM)


-----Ursprüngliche Mitteilung-----
Von: James Moritz <[email protected]>
An: [email protected]
Verschickt: Mi., 7. Okt. 2009, 21:57
Thema: Re: LF: Re Propagation report

Dear David, LF Group, 
 
There are basically two ways of determining ERP, briefly: 
 
Method 1 is to calculate the radiation resistance (Rrad) of your antenna, which can be done using various text-book formulae, or using antenna simulation software like EZNEC or similar. Then you measure the RF current flowing in the antenna (Iant). The ERP is then Iant^2 x Rrad x 1.8 (1.8 is the directivity of electrically small vertical or loop antennas, which in practice is what all amateur 136kHz antennas are). 
 
Method 2 is to measure the field strength some km from the antenna. The ERP is then (E x d)^2 /49, where E is the field strength, d distance in metres. This requires a calibrated antenna and measuring receiver, so is more complicated to do. 
 
Method 1 is simpler to do, but yields ERP results that tend to be lower than those obtained by method 2. This is thought to be due to various combined environmental effects on the antenna, which are not accounted for in the simple calculation. So to get a more accurate and definitive figure, you need to do method 2, but as far as compliance with licence conditions is concerned, method 1 is a conservative way of checking. 
 
Good luck in your efforts! 
 
Cheers, Jim Moritz 
73 de M0BMU 
 
----- Original Message ----- but From: "David Weinreich" <[email protected]> 
To: <[email protected]> 
Sent: Wednesday, October 07, 2009 8:17 PM 
Subject: Re: LF: Re Propagation report 
 
Peter, 
 
As an interested observer from the other side of the pond, how do you determine that you are close to the ERP limit?? I'd be interested in knowing as I am trying to do some pro-bono work for the ARRL on getting use of 137 kHz here in the USA. 
 
Tnx & 73, 
 
David Weinreich 
WA2VUJ/3