LF: Re: Re: NOV UPDATE

["Graham" <g8fzk@g8fzk.fsnet.co.uk>]

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" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
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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