Hello Chris,
a lazy-L of 9m by 40m should have a radiation resistance
(Rrad) of +/- 0.25 Ohm.
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.
I simulated your antenna using Mmanagal:
Over perfect ground the max. takeoff angle is 0° and the antenna gain is 4.7dBi.
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.
73, Rik ON7YD - OR7T
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
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
----- Original Message -----
Sent: Thursday, April 07, 2016 11:24 PM
Subject: LF: RE: Field Strength Measurements
Hello Chris,
(1) will give you the EMRP (Effective Monopole Radiated Power)
(2) will give you the EIRP (Effective Isotropic Radiated Power)
The formula P = 0.0203252(Ed)^2 will give you the ERP (Effective Radiated Power)
73, Rik ON7YD - OR7T
LF,
I've recently resurrected my old RF field strength meter as designed by PA0SE and published in the LF Experimenters' Handbook.
I've modifed it to measure signals in the MF band and have calibrated it with a home made Helmholtz coil arrangement.
This question may have been raised before but PA0SE gives the formula for deriving the transmitter power as
P = 0.0111 (Ed)^2 . . . (1)
where E is the electrical field strength and d the distance
I've read in other places that the power from an isotropic radiator is
P = (Ed)^2 / 30
i.e. 0.0333 (Ed)^2 . . . . (2)
My calculated radiated power seems rather low so I'm wondering whether I should be using formula (2)
Any clarification would be most welcome.
73
Chris G3XIZ
