Re: VLF: ERP calculation (was: dj8wx on 8970.022Hz)

[uwe-jannsen@kabelmail.de]

Hi Markus,

 

thanks for your instructions.

 

Ill try this substitute mothode one hot day in the month of august.

 

Thanks for the link to the US grabber.

 

vy 73

Uwe/dj8wx

 

 

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Von: rsgb_lf_group@blacksheep.org
Gesendet: 08.03.2011 20:33
An: rsgb_lf_group@blacksheep.org
Betreff: VLF: ERP calculation (was: dj8wx on 8970.022Hz)

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Dear Uwe, LF,

 

my usual method for ERP measurement is as follows:

 

1. Build a simple nonresonant loop of known area A, eg. 50 cm x 50 cm x 4 turns = 1 m^2. Do not connect a capacitor.

2. Using a selective voltmeter or soundcard, measure the voltage Vloop induced by a strong transmitter in the far field (eg. DHO). Note that the main Marconi antenna should be disconnected (ie. grounded via 1 megohm) during this measurement to decouple it from the loop.

3. Calculate the magnetic fieldstrength B (for DHO) using Faraday's induction law:

  B = Vloop / A / omega

4. Derive the undisturbed electric field from this transmitter:

  E = Zo * H = B * Zo / µo = B * c

5. Series resonate your Marconi for DHO, connect a high impedance voltmeter, and measure the open circuit voltage Vmarconi.

7. Derive the effective height of the Marconi:

  heff = Vmarconi / E

8. Calculate the radiation resistance at your transmit frequency:

  Rrad = 1579 ohm * (heff / lambda)^2

9. Connect your TX and loading coil, measure the antenna current I, either at the cold end of the coil or in the ground line, and calculate radiated power:

  EMRP = Rrad * I^2

10: If you want, you can convert radiated power to ERP referenced to a halfwave dipole:

  ERP = EMRP * 1.83

 

You do not have to absolutely calibrate the voltmeter because the result depends only the ratio between the loop and Marconi measurement. This is handy with the soundcard, as you will only need to read a dB difference.

 

The method also correctly includes the shunt effect of nearby objects (eg. trees) on the radiated power. It measures the disturbed electrical field, and compares it to a reference based on the undisturbed magnetic component.

 

Best 73,

Markus (DF6NM)

 

From: uwe-jannsen@kabelmail.de

Sent: Tuesday, March 08, 2011 7:27 PM

To: rsgb_lf_group@blacksheep.org

Subject: LF: VLF dj8wx on 8970.022Hz

Hi all,

 

Im doing some measurements trying to find out how low (high?) the ERP of my system is.

the test lead of the oscilloscope in use has a resistor of 10Mohm. The oscilloscope advise text says that I have to multiply the result by 10.

OK. I added two other 10Mohm  in series multiplyed the result by 1000 and got 40000Vpp

using the twentieth of max TX power. 40kV? Is that possible?

the current from the coile foot into the radials was 0.145A (3 A with max power).

I cant measure the Tesla voltage when the  power is raised because the oscilloscope or other

instruments indicate results without connected to the object under test.

 

the station is on the air in mode QRSS3600 on 8970.022Hz since today 1640UTC.

 

current into the radials: one Ampere.

 

who will try to calculate the ERP?

the aerial in use can be seen at

 

http://dj8wx-dl.de/hompage500kHz/TX%20equipment.htm 

fourth pic on the site. The shown 23m long wire has been lengthened to 43m.

 

best regards

Uwe/dj8wx