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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