----- Original Message -----
From: "James Moritz" <[email protected]>
To: <[email protected]>
Sent: Friday, February 15, 2008 1:06 PM
Subject: LF: Re: NOV UPDATE
> Dear Mal, Graham, LF Group,
>
> G3KEV wrote:
> > As a rough guide to get started, assume the average amateur antenna is
1%
> > efficient which is doubtful then you would need 100w fed to the antenna.
> > Since a more realistic efficiency figure might be 0.5% or less you would
> > need 200w.
>
> This is incorrect. The ERP could be calculated as:
>
> P(erp) = (TX power) x (Antenna efficiency) x (Antenna directivity)
We all know about eirp and dipoles.
>
> The directivity of a small vertical antenna over the dipole reference is a
> factor of 1.8 (2.6dB), so 100W into such an antenna with 1% efficiency
would
> give 1.8W ERP.
>
> >The average pa efficiency is about 50% therefore you would need
> > anything between 200 and 400watts dc input to get into the ball park and
> > generate 1w erp.
>
> For the class D or E PA stages many of us are using, 70% - 90% PA
efficiency
> would be more realistic.
>
The majority are using LINEAR PA'S at 50% efficiency for their appliance
type modes and only a few of us using cw have class D or E pa's.
> > A large 1/4 wave inv L antenna might only be 5% efficient if you are
> lucky
> > at this freq, because the vertical part would normally only be a few
> metres
> > high with a very long horizontal part.
> > The antenna environment then needs consideration, buildings, trees,
> hedges,
> > other antenna wires in the vicinity.
>
> The field strength and impedance measurements I did on my own antenna at
my
> home QTH, basically an inverted L about 10m high and 40m long, gave an
> efficiency of about 0.6% at 500k, so I need around 90W TX to get 1W ERP.
> Increasing the height in the centre of the span to 14m roughly doubled the
> efficiency, which would reduce the power requirement to 45W. The
> environmental effects, giving rise to increased loss resistance and
reduced
> radiation resistance due to screening effects on the antenna by nearby
> objects, are quite large. The same 10m high antenna in an open field had
> about 3.6% efficiency, due to reduced environmental losses and screening,
so
> would only need 15W TX out for 1W ERP. The much bigger antennas at G3KEV
> should certainly have greater efficiency than my antennas, so would
probably
> only need a few watts from the transmitter to achieve 1W ERP - I guess the
> heaters of Mal's TT22s will be drawing more power than the anodes!
That WOULD be true if I was using the TT22 tx, which I am not.
>
> >Your 1w erp might effectively be
> > reduced to micro watts.
> > Check the near and far fields.
>
> > I recently removed a long wire rx antenna running near my 500 khz
antenna
> > and gained 2 db.
> > The above info is a rough guide because every radio amateur installation
> is
> > different, the antenna efficiency is hard to determine, hence the erp.
>
> The antenna efficiency can't realistically be determined without field
> strength measurements - if these are not available, a better approach to
> estimating ERP is to calculate the radiation resistance of the antenna
from
> its dimensions (see formulas in RSGB handbook, LF today, ON7YD's antenna
web
> pages, etc.) Then the ERP can be estimated by measuring the antenna
current:
>
> P(erp) = 1.8 x Rrad x (Iant)^2
>
> This method inherently includes the effect of loss resistance due to
> environmental effects, loading coil, etc. but does not include the
radiation
> resistance reduction due to the environment - but it is still a much
better
> estimate than trying to guess what the antenna efficiency will be.
>
Most of what you say is theoritical and guesswork, in reality you could be
at least 10 db out and that makes a big difference.
Your figures in the past on 137 khz varied between 6 and 10 db by your own
admission.
Amateur radio design and applications on MF and LF are not an exact science.
Mal/G3KEV
> Cheers, Jim Moritz
> 73 de M0BMU
>
>
>
>
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