Return-Path: Received: (qmail 18682 invoked from network); 16 Dec 2002 16:23:36 -0000 Received: from netmail02.services.quay.plus.net (212.159.14.221) by mailstore with SMTP; 16 Dec 2002 16:23:36 -0000 Received: (qmail 16236 invoked by uid 10001); 16 Dec 2002 15:31:48 -0000 Received: from post.thorcom.com (193.82.116.70) by netmail02.services.quay.plus.net with SMTP; 16 Dec 2002 15:31:47 -0000 X-SQ: A Received: from majordom by post.thorcom.com with local (Exim 4.10) id 18NxD2-0005l4-00 for rsgb_lf_group-outgoing@blacksheep.org; Mon, 16 Dec 2002 15:31:08 +0000 Received: from [24.153.64.2] (helo=smtp.comcast.net) by post.thorcom.com with esmtp (Exim 4.10) id 18NxD1-0005il-00 for rsgb_lf_group@blacksheep.org; Mon, 16 Dec 2002 15:31:07 +0000 Received: from yourw92p4bhlzg (pcp02064100pcs.nrockv01.md.comcast.net [68.83.215.165]) by mtaout06.icomcast.net (iPlanet Messaging Server 5.2 HotFix 1.07 (built Nov 25 2002)) with SMTP id <0H7700DL0XOKN0@mtaout06.icomcast.net> for rsgb_lf_group@blacksheep.org; Mon, 16 Dec 2002 10:29:10 -0500 (EST) Date: Mon, 16 Dec 2002 10:29:08 -0500 From: "Ralph Lane" To: rsgb_lf_group@blacksheep.org Message-ID: <002301c2a517$e080e650$0000a398@yourw92p4bhlzg> MIME-Version: 1.0 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 X-Mailer: Microsoft Outlook Express 6.00.2600.0000 X-Priority: 3 X-MSMail-Priority: Normal References: <5.1.0.14.0.20021213133819.00ac0c30@POP3.freeler.nl> Subject: LF: Re: Measuring earth resistance Content-Type: text/html; charset=windows-1252 X-Spam-Status: No, hits=1.9 required=5.0tests=BIG_FONT,FROM_ENDS_IN_NUMS,MAILTO_LINK,REFERENCES, SPAM_PHRASE_00_01,SUPERLONG_LINE,USER_AGENT_OEversion=2.42 X-Spam-Level: * Sender: Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group Content-transfer-encoding: 8bit

Dick, conventional wisdom says that the soil resistance becomes lower as the soil becomes wetter. However your measurements indicate the antenna ground loss becomes greater with increased wetness. Perhaps there is another coupling mechanism here. For example, if one had a loop antenna suspended in free space, would the ground loss resistance become zero?

Ralph Lane

----- Original Message -----

From: Dick Rollema

To: LF-Group

Sent: Friday, December 13, 2002 7:39 AM

Subject: LF: Measuring earth resistance

To All from PA0SE

There were some requests for info on the way I measured the earth resistance of my aerial system.

In 1988 I made an impedance bridge with a noise source. In the first version of this e-mail the circuit diagram was included but it did not pass the reflector.
Most bridges of this type have a transformer between the noise amplifier and the bridge circuit. But I found it impossible to make the bridge frequency independent up to 30MHz. By putting the transformer between the bridge circuit and the detector I managed to make the readings reliable up to 30MHz.

For measuring earth resistance I connect the bridge between the bottom end of the loading coil and the earth connection, after disconnecting the transmitter from the coil. The X-control is left at the zero position
The detector (receiver = "ontvanger " in Dutch) is tuned to 136.5kHz and the vacuum capacitor in parallel with the tuning coil and the R-control of the bridge tuned for minimum noise. The minimum is so frequency-dependent that it becomes deeper as the bandwidth of the receiver is reduced!
The total resistance of the aerial system can then be read from the R-control. But for better accuracy I measure its value with a digital multimeter.
From the value so found I subtract the loss resistance of the loading coil. What remains I call the earth resistance. Of course in that value are also included the radiation resistance and the resistance of the aerial and earth wire, But these quantities are negligible as compared to the earth resistance.

In the earth resistance will also be included losses in the aerial insulation and in the capacitance of the aerial to surrounding objects like trees (no trees in my case). But these losses are impossible to separate from the real earth resistance.

Measuring earth resistance at 50Hz has no meaning for the loss at 136kHz because the earth resistance (impedance is more correct) is frequency dependent. At low frequencies earth acts like a resistor, at high frequencies like a capacitor in parallel with a resistor.

I find the loss resistance of the loading coil with its tuning capacitor in parallel as follows.

I use my W & G signal generator and tracking selective voltmeter type PSM-5. The generator is connected to a one-turn loop that is positioned at some 35cm from the bottom end of the coil. An oscilloscope probe is connected to the level meter and hung near the top end of the coil. The aerial is replaced by a variable capacitor (broadcast receiver type). The PSM-5 is adjusted to 136500Hz and the variable capacitor tuned for maximum voltage over the coil and its value noted. The PSM-5 is then tuned higher and lower to the frequencies where the voltage over the coil has dropped to 0.707 (-3dB) of the maximum. The frequency difference between these two frequencies is B (I use a frequency counter to improve accuracy; B is rather small). This yields Q = 136.5/B (Q = 350 in my case).
The value C of the variable capacitor is then measured. This yields the reactance of the capacitor
X = 6.28 * f * C ohms. Because there was resonance this is also the reactance of the loading coil in parallel with its tuning capacitor.
We now find for the loss resistance of the loading coil: R = X/Q.

My measurements support the findings by Alan Melia. The earth resistance is higher in winter than in summer and is maximum when the earth is soaked with water. When the water freezes the resistance goes down.

I hope this info is of some use.

73, Dick, PA0SE