a little finger trouble changed the sense !! see below
----- Original Message -----
From: "Alan Melia" <[email protected]>
To: <[email protected]>
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
£1.
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" <[email protected]>
To: <[email protected]>
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
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