On Thu, 8 Dec 2005 21:44:07 +0000 (UTC), "Reg Edwards"
wrote:
The problem fundamentally revolves around the gain of short vertical
antennas, both transmitting and receiving,
Hi Reggie,
This is not what you proposed the first time through this exercise. As
your original was stated in terms of a field of 1V/M there is no need
to elaborate on about the radiator.
Numbers cannot be avoided. So let's keep them as simple as possible by
starting with the MF standard of 1 Kilowatt, radiated from a short
vertical antenna above a perfect ground. Actual antenna height and
frequency don't matter.
Then its mention at being both MF (MW?) standard, and being a short
vertical is in distinct conflict. Again, there is nothing inherent to
the problem short of demonstrable evidence in the field, which returns
us to the classic FCC ground wave charts for MF (MW?).
According to the text books, the field strength from 1 Kw at 1
kilometre = 300 millivolts, which (according to the text books) is
correctly calculated by my program.
Are you claiming your program computes field V/M or power delivered by
a field of V/M?
To calculate matched reciever input power from field strength
Here again is a new specification to your original query. Matched? I
would have suspected so, but being unmatched or being mismatched
brings a spectrum of answers that span to more than 1V/M down to
microV/M.
it is
necessary to state vertical antenna height, frequency and radiation
resistance. Again choosing simple values -
Antenna height = 1 metre.
Frequency = 20 MHz.
Calculated radiation resistance = 1.758 ohms.
Matched receiver input resistance is also 1.758 ohms.
Even more elaboration that goes beyond the original. I suppose for
the purposes of discussion it works, but it goes well beyond the
premise of classic MF (MW?) work.
According to requirements antenna height is short compared with a
wavelength. I am confident that radiation resistance is correct at 20
MHz for a 1 metre vertical.
When we have theoretical work supported by real field data in the MW
band, why take up a frequency outside that?
Antenna reactance is tuned out and disappears from the argument.
This is to be expected, but was never actually expressed anywhere.
So we have a simple circuit consisting of a generator with a resistive
load of the same value, both equal to 1.758 ohms.
The generator's Z is immaterial to the discussion if you have a
defined field.
According to the text books (as confirmed by Roy) the generator
voltage is 300 millivolts. (A 1 metre high antenna with a field
strength of 300 mV per metre.)
I have obtained comparable (though not exact) results for real (not
perfect) grounds to compare against actual (not theoretical) data.
Those not comparable have come from mismatched 1M high antennas
against various loads (already discussed above).
The power available to the receiver is therefore -
Pr = Square( 0.3/2 ) divided by 1.758 = 12.8 milliwatts.
Where'd the 2 of the ( 0.3/2 ) come from?
Which is the value calculated by my program
Nevertheless, I think my informant may be correct.
It is quite simple to fool experts. They are by far easier targets
than the naive who ask for the missing work (like this fool here who
wonders just what your correspondent offered to convince you).
Is it 12.8 milliwatts, or is it 3.2 milliwatts?
EZNEC computes it at EČ/R
73's
Richard Clark, KB7QHC