Hello LF,
I've been experimenting with a system to make field strength
measurements, with the goal of estimating ERP.
I found some information on characterising a single turn untuned loop,
in terms of its "antenna factor" - relating incident field strength to
terminated signal voltage - and set to with the pocket calculator and an
excel spreadsheet.
I know there is a design for a FSM out there that use a ferrite rod
antenna and requires calibration with a pair of Helmzholtz coils. I'm
too lazy to build both the RX/meter/antenna AND a calibration facility!
I've written a short note describing the maths behind my setup, which is
at http://www.sighthound.demon.co.uk/gm4slv/500kHzERP.pdf
I've made lots of measurements within 1.5km of my site - an uncluttered
rural area - and get reasonably consistent results. I now estimate that
I need 15W TX power to achieve 100mW ERP (55mW EMRP).
This morning I made measurements of field strength between 450m and 13km
(admittedly over 8km I was at the limit of sensitivity and needed to
switch in the RX pre-amp).
I find that out to the limit of my immediate locality, where the path
back to the TX antenna is unobstructed by hills, my ERP calculates out
to be 100mW (3 results = 98, 101, 100).
At 4.3km I was at the wrong side of a hill, and a small village, and the
ERP came out at only 57mW - field strength was lower than to be expected
given the distance - extra path loss....
At 8km I was higher up, but still further away and across more rocks and
hills, and ERP came out at 36mW.
At 10.7km I was back at sea level the wrong side of a large mass of rock
and across a sea loch (known as a "voe" in Shetland), ERP calculated to
28mW
These lower calculated ERPs are consistent with lossy ground of around
1mS/m over the path. Totally unscientific I'm sure.
I'm now happy to use readings within my immediate surroundings, out to
3km, and be confident that the results are reasonable.
For the record I'm now using no more than 15W transmitter power for all
operation.
Antenna is 11m vertical with 4 short sloping capacity wires of 3.5m
length (ie not much top loading).
Feed current is around 0.55A from 15W, ground loss resistance comes out
at around 50 ohms.
For 100mW ERP (55mW EMRP) and feed current of 0.55A I calculate Rrad =
0.18 ohms
This seems reasonable. The capacity hat is raising the Rrad from an
estimated 0.14 ohms for a bare 11m vertical to 0.18 - a gain of
20log(0.18/0.14) = 2.2dB
I'm impressed that I can detect the effects of excess ground losses over
the propagation path - which lead to E-field reducing at a rate greater
than than 1/D - once I get out of "line of sight" path conditions,
although they aren't "line of sight" in an optical sense but rather that
no large rock or other structures get in the way, just gently rolling
heathery peat/farm land.
I now at least hope I'm within a few dB of 100mW ERP as far as I can
tell.
Cheers,
John
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