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[TowerTalk] SteppIR cable capacitance (was: SteppIR problem)

To: "'W5LT'" <>, <>
Subject: [TowerTalk] SteppIR cable capacitance (was: SteppIR problem)
From: "Dick Green" <>
Date: Mon, 11 Jun 2007 12:23:32 -0400
List-post: <>
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

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 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

> -----Original Message-----
> From: W5LT []
> Sent: Monday, June 11, 2007 8:06 AM
> To:
> Subject: Re: [TowerTalk] SteppIR problem
> Dick makes some good points.
> Cable capacitance and shunt capacitance of suppressors will both
> increase the
> transient (and over-all average) current drawn from the power supply.
> A previous similar problem of motors "missing" steps (i.e. antenna
> frequently
> tuned to higher than intended frequencies), was traced to extra
> capacitance
> at the in-line suppressor.
> Bob, W5LT


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