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Date: Wed, 29 Aug 2007 17:02:54 +0100 (BST)
From: John Pumford-Green GM4SLV <gm4slv@sighthound.demon.co.uk>
To: rsgb_lf_group@blacksheep.org
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Subject: LF: 500 Field strength / ERP
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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