Re: LF: Re: Re: E-Field Probe calibration question

[Andy Talbot <andy.g4jnt@googlemail.com>]

On the basis of two measurements out in the field - the results suggests the voltage being picked up is slightly more than the effective length as stated, but resukts for such a quick first approxiomation are in teh right region:

 

At two /P monitoring sites at distances of 3.5 and 14km, the measured power was calculated back to a field strength using the quoted antenna factor, then this E field was related to ERP using the standard equation.

 

At 3.5km the ERP worked out at around 200mW which isn't too far from what I thought it was from Rloss / Rrad calculations.   BUT, the Rx was surrounded by trees and a power cable close by, so could reasonably be expected to be lower than it should be.

 

The second site was in the middle of flat land, at the top of a Hill   (Lane End Down for those microwavers on this reflector), where there were virtually no local screening.    ERP calculated back to 900mW.  This result is too high, suggesting the E-Field probe antenna is picking up more signal than it ought to.    I need to make some more measurements further afield, and to compare with a loop before coming to any conclusion - but an afternoon pleasently spent getting a measurement in the right region at least.

Andy
www.g4jnt.com

On 1 April 2010 13:05, Mike McAlevey <mcalevey@xtra.co.nz> wrote:

Hi Andy.
I would go along with Alan's thinking. It's about the probe to reference voltage. I am not a scientist or "enjuneir" either but with a one square metre untuned loop you can't go far wrong.

Mike
----- Original Message ----- From: "Alan Melia" <alan.melia@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Friday, April 02, 2010 12:44 AM
Subject: LF: Re: E-Field Probe calibration question

Hi Andy maybe I dont read it right, but if that is an E-field probe it is
not a "dipole" The input capacitance senses the voltage at two points on the
wavefront. The ground plane is, I believe, irrelevant. You just need another
capacitance separated from the "probe" to reference to voltage to. See
Renato's balanced probe (www.vlf.it)  In your case this is the car.....but
it could be the size of a dinky toy.The area of the wavefront intercepted by
the probe or the ground reference does not affect the terminal
voltage.....as I see it.................of course I might just be wrong cos
i am really a scientist not an enjuneir!!
:-))
Alan

----- Original Message ----- From: "Andy Talbot" <andy.g4jnt@googlemail.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Thursday, April 01, 2010 12:02 PM
Subject: LF: E-Field Probe calibration question

I have a calibrated E Field antenna (Procomm AAC-1) with a quoted
antenna-factor of 0.15V into 50 ohms when placed in a field of 1V/m,

This is mounted in the centre of a car roof and fed via a bulkhead

connector

to a calibrated measuring receiver inside the vehicle, which we can assume
is a Faraday cage at LF,  S0 my question is :    How is the effective

height

of the antenna changed?    It was 0.15m - the specification said so - but
this would have been over an infinite groundplane.  Do I regard the body

of

the car as being the other half of a dipole?  In which case, at 1.5m it

adds

20dB to the received signal and makes a mockery of calibrated measurement

of

the E-Field value..

Is it worth persuing this route, or better to stick to untuned loops for
calibrated E-Field measurements at LF/MF?


Andy
www.g4jnt.com