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Re: [TowerTalk] Measuring Coax Loss

To: "Tower and HF antenna construction topics." <>
Subject: Re: [TowerTalk] Measuring Coax Loss
From: "Jim Brown" <>
Reply-to: "Tower and HF antenna construction topics." <>
Date: Thu, 11 Feb 2010 16:34:58 -0800
List-post: <">>
On Thu, 11 Feb 2010 19:03:51 -0500, Mike Besemer \(WM4B\) wrote:

>Still... a dB or so (or even 3) on a long run of coax isn't going to
> make a  hill of beans difference for most applications... an S-Unit 
>is ~ 6dB.

Only by someone's definition, not in reality. If you actually MEASURE the 
response of the S-meters in REAL radios, you find that an S-unit may be close 
to 6dB near S9, but is usually closer to 3dB at S5 and below. 

As to a hill of beans -- many hams have long runs of coax to their antennas, 
so loss can be a BIG deal, not a hill of beans. The loss in dB of verious RG8 
style coaxes ranges by a factor of about 2:1 from the lowest to the highest. 
Over the past several years, I've been doing a lot of little things to 
improve my station. A dB here, a dB there, they all add up. Smart operators 
know that. The difference between a 3-el yagi and a 4-el yagi of comparable 
design is only 1-2 dB, and often double the cost. That doesn't stop a lot of 
guys who have the space from putting up 4-el yagis!  

Somehow, the scientific method seems to be lost on some of those commenting 
on my post. How do you know that the MFJ (or any piece of test gear) is 
accurate if you don't compare it to another measurement or test method of 
known accuracy? I'll bet a six pack of your favorite 807s that if I had 
posted loss measuerements made ONLY with an MFJ, someone would have pooh-
poohed them because I didn't use equipment traceable to a calibration lab. 

>Forgot to mention... unless I actually SAW the 10' piece being cut off the
>longer roll I was considering buying from, I wouldn't trust the measurements

Did it ever occur to you that sometimes ham stores, even the best known, with 
the biggest ads in QST, may not have good data on what they're selling? I 
don't trust some of them any more than an anonymous vendor in a flea market. 
But there ARE some good deals out there, IF you have an open mind and know 
how to evaluate them. The point of my post was to show that you CAN get 
decent data from an MFJ259B that has been calibrated if you're measuring a 
sample that is long enough.  

How many measurements of coax loas have you actually made? How did you do it? 
How did you know that you had good data? At some point, you've got to know 
exactly how long that piece of coax is. RG8 is big and heavy, so unspooling 
enough of it to get a good measurement isn't always easy. You've got to deal 
with the length of cable sample you can get.


Jim K9YC  


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