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Message-ID: <000f01c6ebe5$53712540$dd248351@w4o8m9>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <000801c6eba9$5caeae70$e6a4c593@RD40002> <012301c6ebb8$3e4d4900$21c428c3@captbrian>
Date: Mon, 9 Oct 2006 21:56:00 +0100
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Subject: LF: Re: erp ierp - bluff
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Status: O

Dear Bryan, LF Group,

> If a man from Ofcom turns up here and says "demonstrate that you are not
> exceeding the 1w ERP specified in your licence"  how would I theoretically
> dispose the dipole to which ERP refers . Assuming the rx antenna is a
dipole
> it would make a big difference how I oriented the Tx dipole to compare
with
> the inverted L and plastic laundry basket former-loading coil.
>

The ERP definition compares the field strength produced by the real antenna
to that of a half-wave dipole in free space. It is assumed the field
strength is measured in the direction the antennas produce their maximum
radiation. So theoretically, you would dispose the reference dipole with its
feed point where the feeder is connected to your TX antenna. It would be
convenient (although not neccessary) to position the antennas so that the
maxima of the radiation patterns of the two antennas coincided, and the
polarisation was the same. For this, the reference dipole would be vertical.
Since it is a dipole in free space, you would also have to remove the Earth
somehow, which would be awkward. The dipole would also have to be cooled to
absolute zero, in order to make it "ideally" resistance free.

Luckily, the Ofcom man won't ask you to do this, because he just needs to
know the field strength that the dipole would produce if it was there, which
can be calculated from a simple formula:- E = 7*sqrt(P)/d, where E is the
electric field strength, P is the power fed into the dipole, and d is the
distance from the dipole. This can be turned round to tell him what the ERP
is if he measures the field strength d metres from your antenna:-
P(erp)=(Ed)^2 /50. It's up to him to sort out the polarisation, direction,
etc. of his measuring system to suit the signal.

> Is there some way of showing and measuring distant FS without putting an
> antenna there?

In principle at least, you could use a bolometer to determine the field
strength (or the power density anyway), i.e. place some absorbing object in
the path of the radiation and measure how much its temperature rises.
Actually doing this would be fraught with practical problems, however. There
might be other ways of doing it which I can't think of at the moment.

Certainly the FS will be different if the tx dipole is
> broadside on or end on .
>
Yes, but the convention is that ERP has to be measured in the direction of
maximum radiation

> [ ps enjoyed the talk on Saturday - apology if closing my eyes to
> concentrate on loop  bandwith gave impression of dropping off ;-)  ]
>
Your impression really had me fooled - very impressive! Especially your
impersonation of snoring :-)

Cheers, Jim Moritz
73 de M0BMU

(BTW, it is EIRP, not ierp)


> G3GVB/ Bryan
>
>
> bandwidth gave the impression of sleep in the front row ;-)  ]
> ----- Original Message -----
> From: "james moritz" <j.r.moritz@herts.ac.uk>
> To: <rsgb_lf_group@blacksheep.org>
> Sent: 09 October 2006 14:46
> Subject: LF: Re: erp ierp VY1JA
>
>
> Dear Bryan, LF Group,
>
> ERP is defined by the field strength at a point distant from the TX. Field
> strength is a measure of how much signal is present at a particular point
in
> space. Of course, if you place a receiving antenna at that point, the
signal
> level at the antenna terminals will be reduced if its polarisation does
not
> correspond to that of the signal. But the field strength itself does not
> depend on the receiving antenna, or indeed the presence of any antenna at
> all.
>
> In practice, to measure field strength, you usually have to use some kind
of
> calibrated antenna to convert the radio wave into an electrical signal,
with
> a known relationship between field strength and antenna output signal
level.
> It is up to you to ensure that any effect of mismatched polarisation (or
> directional pattern, mismatch or any other kind of gain or loss)is taken
> into account.
>
> Cheers, Jim Moritz
> 73 de M0BMU
>
> -----Original Message-----
> From: owner-rsgb_lf_group@blacksheep.org
> [mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of captbrian
> Sent: 08 October 2006 20:01
> To: rsgb_lf_group@blacksheep.org
> Subject: Re: LF: Re: erp ierp VY1JA
>
> Well I always thought  there was a large loss depending on the extent to
> which polarisation differed between emitter and recvr.?
>
>
>
>
>
>
>