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| To: | "[email protected]" <[email protected]> |
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| Subject: | Re: LF: Re: RE: Field Strength Measurements |
| From: | Chris Osborn <[email protected]> |
| Date: | Sat, 9 Apr 2016 08:20:27 +0000 (UTC) |
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Dear Rik, Many thanks again for the most useful explanation. Some of my past career was spent in an electrical calibration lab. so I cannot get used to the scatter of readings I get with field strength measurements - HI ! My calculated value of 0.38 watts ERP was obviously an average of several readings and they ranged from about 0.25 - 0.5W Your deduction of my aerial current is interesting. My system comrises the TX, a step down (voltage) transformer, variometer and aerial. The whole system is tuned by means of a 'scopematch' unit and with 56W output the aerial current, allowing for the step up current transformation is 1.8A My next step (once it stops raining) will be to take some measurements on higher ground as suggested. Ian G4GIR (Bedford) and I have been doing some comparison tests on MF. Using the G3YXM grabber we transmit simultaneously using the same RF power and then compare signals on the grabber's graphic display. Results for the two of us are similar and with outputs of 56W we indicate at the -60 dB level. Once we get an accurate measurement of our EIRP we could use that grabber as a calibrated monitor. 73 Chris G3XIZ On Friday, 8 April 2016, 11:30, Rik Strobbe <[email protected]> wrote: If you know the antenna current (I) you can calculated the EIRP
= G*I^2*Rrad where G = the antenna gain (referred to an isotropic
antenna).
The theoretical gain of a short vertical monopole is 3 (4.77dBi), but this assumes
a perfect ground and no obstructions. A non perfect ground will affect the antenna pattern and gain.
Whereas the max. takeoff angle over perfect ground is 0° it will rise over non perfect ground.
In addition the gain will slightly drop.
With a "real ground" of 20mS/m and Er= 5 the max. takeoff angle rises 17°
and the antenna gain drops to 4.2dBi (at 17°). At a low angle of 1° the gain has dropped to -4.3dBi, thus 8.5dB below
the max. gain.
The fact that maximum gain is not at 0° takeoff makes is difficult to measure
E(I)RP at ground level (cfr. Alans suggestion to
go for higher grounds, if possible).
Just out of curiosity a prediction of the antenna current based on the numbers above: with an ERP
of 0.38W (= EIRP of 0.62W) and a gain
of 4.2dBi the antenna current should be close to 1A, assuming
a takeoff angle of 0°. Now taking into account a 8.5dB low reading (due
to the higher takeoff angle) the real antenna current should be 2.6 A.
Van: [email protected] [[email protected]] namens Chris Osborn [[email protected]]
Verzonden: vrijdag 8 april 2016 11:36 Aan: [email protected] Onderwerp: Re: LF: Re: RE: Field Strength Measurements Many thanks for the rapid replies gentlemen; they are very much appreciated.
Rik, thanks for the explanation of the two formulae and I've inserted that ERP formula into my spreadsheet now - excellent !
Alan, I take measurements in open fields to the south of Biggleswade and at about 1 - 1.5 km distance from the transmitter.
Noted about the advantages of higher ground measurement but we're fairly flat around here - HI !
I'll look at the O/S map and see what I can do.
Transmitting 56 watts my ERP was calculated as being 0.38 watts which just seemed rather low.
My aerial is a 40 m long inverted 'L' at 9m mean height with a reasonable ground system.
Making measurments of G4GIR's transmissions with his 260 watts at 16 km returned a value of 0.88 watts ERP
73
Chris G3XIZ
On Friday, 8 April 2016, 0:24, Alan Melia <[email protected]> wrote:
Chris two other points
1. How far from the aerial are you taking the measurement .....you need to be at least 300 to 500 metres away
2 Because of ground loss problems even the best (fully in the clear) amateur short aerials measure around 5 to 6 dB low.(that is a 136kHz observation.) Often a better reading is taken say a mile away on slightly elevated ground
to the aerial site. Jim Moritz did tests in a field adjacent to the Puckeridge Decca mast, with a 10m high inverted L.
Alan
G3NYK
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