Date: Thu, 08 Oct 2009 12:22:22 +0200
To: "rsgb_lf_group@blacksheep.org" <rsgb_lf_group@blacksheep.org>
From: Rik Strobbe <rik.strobbe@fys.kuleuven.be>
In-Reply-To: <F1FBC7A69D7E409DA52460C6547DC64B@JimPC>
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Subject: Re: LF:  Re Propagation report
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Jim,

maybe a method 3:
Measure the field strength of a strong TX (eg. DCF39 for 137kHz). 
This can be done using a small calibrated RX as describe by PA0SE.
Based on the you can calculate the antenna voltage that a lossless 
isotropic antenna should deliver to its termination resistance. Let's 
call this U1.
Match the antenna properly to the frequency of teh strong TX and 
terminate it properly.
Now measure the antenna voltage delivered by the strong TX. Let's call this U2.
Based on the ratio of U1/U2 the "efficiency "(in dBi) of the antenna 
system can be calculated, and thus for a given output power you know 
the E(I)RP.

73, Rik  ON7YD - OR7T


At 21:57 7/10/2009, you wrote:
>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" <David.Weinreich@globalstar.com>
>To: <rsgb_lf_group@blacksheep.org>
>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