Message-ID: <001301c47494$f085d120$6507a8c0@Main>
From: "Alan Melia" <Alan.Melia@btinternet.com>
To: rsgb_lf_group@blacksheep.org
References: <41067F3D.3229.595E2B@localhost> <000601c473f2$59f7abe0$6507a8c0@Main> <040b01c47403$f00ca160$4d540150@captbrian> <000901c47426$2b878ee0$6507a8c0@Main> <002101c47469$de0564a0$13bc0150@captbrian> <000401c47490$d4a65640$6507a8c0@Main>
Date: Wed, 28 Jul 2004 12:20:47 +0100
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Subject: LF: Re: DC ground resistance & bridges correction
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a little finger trouble changed the sense !! see below
----- Original Message -----
From: "Alan Melia" <Alan.Melia@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: 28 July 2004 11:51
Subject: LF: DC ground resistance & bridges


> Hi again Bryan, I am not really sure of that one as I have not tried to
> correlate the two measurements yet. Genuine DC measurements are fairly
> meaningless due to polarisation effects, but some sensible values can be
> achieved using low (audio) frequencies, even 50Hz. I suspect that one is
> mainly measuring the contact resistance of the system to ground whereas
what
> is important at LF is the lossy capacitor. There will be some similarity
but
> I would suggest that reductions seen in the 50Hz value would NOT
necessarily be
> seen at 136kHz. It all depends on how you measure it. For instance a wire,
> insulated from ground, under the top-wire may well reduce the 136kHz loss
> without affecting the "DC" value. Connecting to a steel water pipe running
> in the opposite direction to the aerial may well reduce the "DC"
resistance
> without affecting the loss at 136kHz much.
>
> The RF bridge is a useful gadget as it can help you tune and match the
> aerial later. A 555 can be used as a drive source, RX as detector. Lloyd
> Butler has a full description of a suitable unit at
> http://www4.tpgi.com.au/users/ldbutler/LFBridge.htm  His bridge is
designed
> for the Oz segment at 160 to 180kHz but a slight change in the 555
> components will allow to to tune 136kHz. Calibration, always a problem, is
> simple with this design, just measure the reference components with the C
or
> R range on a digital multimeter.
> He uses an op-amp to drive the bridge with what is effectivly a floating
or
> balanced drive. This avoids the effects of stray reactance between the
> primary and secondary windings, which affect my simple design if driven
> single-ended. You could try using an EMC common-mode choke as a balun.
Many
> of the EMC ferrite components which are very cheap are usable at LF as
> chokes and tranformers, and can often be a cheap way of buying toroids
(CPC
> or Rapid). One low current common-mode choke I found has a winding
> inductance of 50mH and enough window space to wind on a "secondary" which
> makes a cheap and adequate (not optimum, as it is beginning to role off at
a
> few kHz) audio isolation transformer between PC and radio, for less than
Ł1.
> It also works well as a low power 600 to 50 ohm transformer at 136khz.
>
> Now's the time to get the aerials sorted for lots of activity in the
autumn
> (I hope !!)
>
> Cheers de Alan G3NYK
>
> ----- Original Message -----
> From: "captbrian" <captbrian@ukonline.co.uk>
> To: <rsgb_lf_group@blacksheep.org>
> Sent: 28 July 2004 07:12
> Subject: LF: Re: Re: Re: Re: Found earth rod source
>
>
> > Thanks for that.
> > I am well aware that resistance at RF is different from that at DC BUT
can
> > we assume (oops.... justifiably guess) that changes which reduce DC
> > resistance  to ground also reduce RF resistance to ground, ? I exclude
> > inductances obviously.
> >
> > Bryan G3GVB
>
>
>