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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 17:35:14 -0800
List-post: <">>
On Thu, 11 Feb 2010 19:59:11 -0500, Mike Besemer \(WM4B\) wrote:

>Okay... I'll accept your argument about S-meters because frankly it's not
>worth arguing about.  3 dB or 6 dB is still not a great deal of loss at HF.
>I doubt that most of us can hear 3 to 6 dB 99% of the time.  

A dB or two can make the difference in a marginal circuit, whether it's 
or 160M. More than once, while in search and pounce mode during a contest, I've 
failed to make a contact with a weak station after several calls at 900 watts, 
then made the Q by retuning the amp for full power. That's only 2 dB. 

>I'm not sure what you call a 'long run' of coax', but I have a 125' run to
>my relay point plus the additional length to the individual feedpoints.  Is
>that what you consider long?  

That's a short run by some standards. There are guys with 500 ft runs! My 
tribander is a 380 ft run to the top of the tower, and its 1/2-inch heliax. 

>I agree that it's not good to WASTE dB's but I also would not spend a bunch
>of money for a couple dB... be it on an antenna or other components to
>reduce loss... not for a standard HF station anyway.  I already qualified my
>previous statement saying that it did not apply to microwave/weak signal
>work.  The fact that you implied that some of us are not 'smart operators'
>is rather assumptive and could be considered by some as offensive.  As for
>me, I just don't find it worth while to worry about what I consider
>negligible loss.  

Offensive? I don't know you at all, but some of the smart operators I'm talking 
about include some very competitive contesters and DXers -- K6XX, N6RO, N6BV, 
K3LR, W8JI, W4ZV, K9DX. These guys do not leave tenths of a dB laying around! 
That's a part of what it takes to win contests (and working the weak ones, or 
breaking pileups).    

>Comparing the MFJ to another measurement is useless unless the other
>instrument is traceable to a NIST standard, so your comment about comparing
>it to another measurement is invalid... that comparison is ONLY valid if one
>of the instruments is of known accuracy.  Comparing to another instrument is
>like having two watches... you never know what time it is.

You owe me a six-pack! Make it Fat Tire. :)

>As for my qualifications to measure coax loss, I spent 24 years in the
>military working on avionics systems.  I made literally hundreds (probably
>thousands) of loss measurements on systems that required precise (and I mean
>precise down to a gnats-a$$) loss measurements.  Depending on the
>application, we used HP power meters, spectrum analyzers, VNA's, and several
>other calibrated methods.  These were applications that could result in
>loss-of-life if done improperly.  By that standard, a couple of dB on an HF
>station does NOT mean a hill of beans. 

That's great if you HAVE all that test equipment, and if you have the budget to 
keep it calibrated. 

>So, it's all a question of perspective.  Before you impeach anybodies
>thoughts or qualifications again, you might want to think about that.

I think about it all the time, which is why I made the statements I did. I've 
worked for many years in pro audio, and am a member of the Acoustical Society. 
Some of us know a LOT about the value meaning of a dB in terms of human 
perception, and as K3ZXL noted, 1-2 dB can be quite obvious under certain 
conditions. Like the conditions I cited in my first paragraph above. 


Jim K9YC


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