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VK3ABK > TECH 02.06.06 11:41l 77 Lines 3430 Bytes #999 (0) @ WW
BID : 31112_VK3HEG
Read: DL1LCA GUEST
Subj: Capacitors and Resistivity.
Path: DB0FHN<DB0MRW<DB0RGB<OK0PPL<DB0RES<DK0WUE<7M3TJZ<SP7MGD<VK7AX<VK3HEG
Sent: 060602/0403Z @:VK3HEG.#WEV.VIC.AUS.OC #:31112 [Ballarat] $:31112_VK3HEG
From: VK3ABK@VK3HEG.#WEV.VIC.AUS.OC
To : TECH@WW
Hello all Pendants :-)
The topic of capacitive dielectric (Ralph v ARRL) may not (yet) exceed that
of resistivity in endless quoting, but I think it is more intriguing.
I sent a tentative bulletin about the use of 'dielectric constant', both
'directly proportional' in the case of a parallel plate capacitor, and the
'square root of' in a coaxial cable.
I must thank Maurizio, IW6DFW, for his interesting comments....
He wrote....
Reading my physics books about e.m.f. I'm sure that a direct correspondence
between dielectric constant and capacitance there is with certainty only when
the place where is the field (dielectric medium) is homogeneous, so in
mathematical words, permittivity can be treated as a constant then brings out
from the integral operator.
If these conditions doesn't occurs, it's necessary to use the linearity
property of the integral operator, so split the formula into a run of sums of
a number of addendums equal to the number of interphaces between inhomogeneous
materials. In this way no direct proportionality is insured.
I hope I've been clear.
Yes, Maurizio, that's perfectly clear, and would be important with many
dielectrics. Years ago, I dealt with some most inhomogenious dielectrics!
In my comments, I tried to understand the reason for using the square root
of the dielectric constant (or permittivity if quoted) in (circular) coax
cable. We don't always get an explanation for equations and constants used
in text books. I proposed my own thoughts and reasoning.
Now, to comment on the latest bulletin from Ralph, VK2ZRG, which is relevant
in context.
Ralph wrote, in part....
In your bulletin of 24th May titled "Resistivity again (VK3ZRG)" , there
were resistivity figures of 2.6 Ohm cm x 10^-6 for chromium. Maybe the number
is missing a 1 and should be 12.6 instead. 12.6 is close to the numbers from
the CRC handbook that Pete, G6KUI posted on packet, and close to other numbers
I've found since my original bulletin. If chromium really did have a low
resistivity, could you imagine some advertising agency passing up the
opportunity of saying "Our xyz RF product is chromium plated for good looks
and low surface resistivity." I've never heard such a claim, have you?
My comment....
The references I quoted as "2.6 Ohm cm x 10^-6 for chromium" were text books
by reputable authors and publishers. The fact that 'two' such sources gave
the same figures for specific resistivity, seems to have merit. But, as we
have seen from several other sources, there is some doubt about the 'true'
resistivity. If you normalize your ARRL 'relative to copper' figure, the plot
thickens, as they say in any (other) good mystery.
One thing that you should be careful with is the 'information' that is
printed in the 'popular' magazines, often by amateur authors who are not
qualified, or are quoting some other unqualified source. This is how many of
our myths ('memory' in Nicads, SWR and power 'loss', etc.) are promulgated.
I mentioned,in a previous bulletin, the KuleRod (spelt with a - over the u)
antenna advertised many years ago, and the claim for "low resistance loss"
(therefore 'cool') and surmised that it have been might be chromium plated
on account of the low figures given for it's resistivity.
So, we have good reason to talk about some things of real interest on packet.
Long may it reign!
73. Dick. VK3ABK.
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