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Message-ID: <F1FBC7A69D7E409DA52460C6547DC64B@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <57a24ca70910070818o2e4adde4k1c8daa318eb2eb30@mail.gmail.com> <006001ca4763$a69a1290$0301a8c0@mal769a60aa920> <002601ca4766$1ff29bb0$6c01a8c0@DELL4> <4ACCD7FA.5080501@ukonline.co.uk> <4ACCB110.8A4E.00FD.0@globalstar.com>
In-Reply-To: <4ACCB110.8A4E.00FD.0@globalstar.com>
Date: Wed, 7 Oct 2009 20:57:31 +0100
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Subject: Re: LF:  Re Propagation report
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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