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Re: [RFI] Auto quieting

To: rfi@contesting.com
Subject: Re: [RFI] Auto quieting
From: CR <ka5s@earthlink.net>
Reply-to: ka5s@earthlink.net
Date: Mon, 12 Oct 2015 08:34:59 -0400
List-post: <rfi@contesting.com">mailto:rfi@contesting.com>
On 10/12/2015 2:15 AM, Roger (K8RI) wrote:
On 10/10/2015 3:15 AM, Jim Brown wrote:
On Fri,10/9/2015 10:31 PM, Roger (K8RI) wrote:
I fail to see where two runs of RG8 size cables in a car, large SUV, or long cab pickup truck would be cost prohibitive.

But it's wasted dollars. Most RF guys fail to realize the noise and RF rejection inherent in good twisted pair. For more than a century, telco went cross country on unshielded twisted pair running next to power lines. CAT5/6/7 is four high quality, 100 ohm twisted pairs, each with a different twist ratio to further minimize crosstalk between pairs.

War story.

Some years ago, I had to verify the performance of a GPS timing pulse buffer design over a range of temperatures and supply voltage. This signal was a relatively high amplitude, short, fast, unipotential pulse carried on a 50 Ohm twisted pair. The complete equipment was being automatically tested in another lab, who, not having 50 Ohm STP, used Cat 5 instead. This didn't work until I showed them they could use two Cat 5 pairs of the same length in parallel.

I would later have to deal with an angry customer engineer, who'd thought one measured the signal on 50 Ohm twisted pair by using a 50 Ohm setting or terminator at the scope, and isolated the scope from the twisted pair by using a 10 MOhm 10:1 scope probe connected "upside down" WRT to the pulse.

I talked to his tech, and didn't hear from him again.

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