Two things Dick Green WC1M wrote about:
"I have a 4-el SteppIR at 96' at the end of a 350' cable (plus or minus 10
feet or so.) It's the stock 16-conductor cable from SteppIR. By the way,
it's not been mentioned yet, but the cable consists of eight twisted pairs
with a heavy braided shield around the entire bundle. If I'm not mistaken, a
twisted pair reduces the amount of capacitance between the wires in the
pair."
And...
"I mention this because I'm wondering just how much capacitance the extra 150
feet of twisted pair would add in the 500' system, compared with the
parallel capacitance in my 350' system."
The answer to your second query is "it depends"... Are these identical cables
from the same manufacturer and production run? Using a single 1,000 foot spool
and chopping it into several different lengths would be a reasonable test. But
if this were a scientific test, you would want to sweep that spool of cable
before you chopped it up and then make sure you were running the tests at the
same temperature, humidity, atmosphere, etc. But that's way over the top for
this situation.
Regarding the capacitance per unit length, yes this will change depending upon
the turns-per-inch. In the early computer days (Cray1), the clocks and critical
circuit timing was "tuned" by varying the length and TPI of the interconnecting
wires on the backplane.
But you triggered another thought when you stated the cable is a twisted pair,
shielded bundle.
Without a schematic, I will speculate the twisted pairs are being driven as a
true differential circuit. If this is the case, it is possible to have proper
ELECTRICAL continuity but have mis-matched pairs.
For example, wires 1&2 should be a pair and 3&4 should be the second pair. But
if you wire 1&3 as a pair and 2&4 as a pair, the differential circuit will
"fall apart" because you have lost the benefit of the twisted pairs. Sometimes
you can get away with miswired twisted pairs (short runs), but many times the
system will be "funky" at best.
Here's an article on twisted pair with an illustration of the miswiring I was
talking about. http://www.cirris.com/testing/twisted_pair/twist.html Scroll
down about half way to see the diagram.
I would triple check the color coding on both ends of that cable. Many men have
partial color blindness (I can't distinguish light green and light brown). Even
though there is electrical continuity on all wires from one end to another,
that doesn't necessarily rule out a single wire swap between pairs.
73 de Bob - KØRC in MN
------------------------------
ORIGINAL MESSAGE:
Message: 3
Date: Mon, 11 Jun 2007 12:23:32 -0400
From: "Dick Green" <wc1m@msn.com>
Subject: [TowerTalk] SteppIR cable capacitance (was: SteppIR problem)
To: "'W5LT'" <W5LT@tx.rr.com>, <towertalk@contesting.com>
Message-ID: <BAY133-DAV17B6A28497E3161AFB40D18D1A0@phx.gbl>
Content-Type: text/plain; charset="us-ascii"
I don't want to shift the discussion too far from the original problem, so I
started a related thread for everyone to chew on.
I have a 4-el SteppIR at 96' at the end of a 350' cable (plus or minus 10
feet or so.) It's the stock 16-conductor cable from SteppIR. By the way,
it's not been mentioned yet, but the cable consists of eight twisted pairs
with a heavy braided shield around the entire bundle. If I'm not mistaken, a
twisted pair reduces the amount of capacitance between the wires in the
pair.
There are lighting suppressors at the base of my tower and the side of the
house, which are separated by about 225' of cable. The suppressors are K5FD
SP12 Surge Protectors (see http://k5fd.com/surge.html.) As you can see from
the picture, for each wire in the cable, the two suppressors add four MOVs
and two .1uf capacitors in parallel (total of .4uf for the caps), plus two
ferrite filter chokes and two 5A fuses in series.
I'm aware of a situation like the one Bob mentioned (it might be the same
one!) I believe in that case the suppressors in question were made by ICE.
They added too much parallel capacitance and were replaced with K5FD SP12
suppressors. I was concerned about the filter caps when I selected the K5FD
suppressors, and planned to snip them off the boards if I ran into problems.
So far, I've not had to do that.
The SteppIR consistently positions the elements in such a way that I'm
confident they're at the specified lengths. How do I know? Well, the SWR on
the driven element is always the same for a given frequency, and is within
nominal parameters. It's also about the same as what I saw when I had the a
4-el SteppIR at the end of a somewhat shorter cable using Polyphaser
suppressors at only the shack end. The Polyphaser suppressors contain only
MOVs. Also, the 4-el behaves like it should from a gain, F/B and F/S
perspective, even when I compare it with a Force-12 4-el monobander mounted
at 72 feet on another tower (the SteppIR is never less loud than the
monobander, and is often up to about 1/2 S-unit louder -- probably due to
the additional height and significant terrain differences.)
I mention this because I'm wondering just how much capacitance the extra 150
feet of twisted pair would add in the 500' system, compared with the
parallel capacitance in my 350' system.
73, Dick WC1M
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