Graham.
With that small antenna you might be lucky if it is 0.1% efficient,
therefore to get 1w erp you need 1Kw less 2.6db ie 555 watts to the antenna
feed point.
If you are using a linear pa at 50% eff you would need a DC input of 1.110
Kwatts or a class D at 70% eff 793 watts.
This is a rough estimate, other factors need consideration as explained
before.
You probably know all about the subject anyway.
To measure the far field properly an airborne platform is necessary to get a
clear run and avoid obstructions like buildings and trees etc. This is the
procedure used professionally. If you have a friend in the RAF this might
help

mal/g3kev

----- Original Message -----
From: "Graham" <[email protected]>
To: <[email protected]>
Sent: Friday, February 15, 2008 9:47 PM
Subject: LF: Re: Re: NOV UPDATE


> James,
>
>
>
> That's a interesting, report on your 'actual' findings  , I run a inverted
> 'L' here as well, but alas not the 40 meter top , a little more like 40
feet
> ! , but the top is a two wire capacity section spaced some 4 feet apart
(no
> reason, that's how long the two conduit off cuts where) . From what I
could
> gather, I think I have managed round 75 mW with 25 watts feed to  the atu
at
> the base of the ae , this is based on comparisons with other stations,
range
> reports  and the erp calculators.
>
>
>
> In a linear situation  I doubt I could reach the power level required to
> attain 1 watt erp with such a short system, without replicating a small
> marine installation and even then it would be non-linear. A UK warship
would
> only run  200/300  watts on  MF !
>
>
>
>  Optimised  modelling of the system always dictates a much larger capacity
> section. In this situation increased wattage to the antenna looks to  be
the
> only way to  enhance the signal, but from experience I doubt it will
defeat
> the skip distance
>
>
>
> I'm quite amused  at concept of running wattage levels well in to three
> figures to what amounts to a self confessed  high gain system , 400 watts
is
> a quoted as the carrier level feeding the local MW multi channel
transmitter
> ! , perhaps half a gallon (us)  is  stretching the  plunger  a tad more
than
> really necessary ?
>
>
>
> G ..
>
>
>
>
>
> ----- 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)
> >
> > 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.
> >
> >> 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!
> >
> >>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.
> >
> > Cheers, Jim Moritz
> > 73 de M0BMU
> >
> >
> >
> >
> >
> > --
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2/14/2008
> > 18:35
> >
> >
>
>
>
>
>
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