[TowerTalk] Measuring Coax Loss

[Mike Besemer (WM4B)]

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.  

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?  

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.    

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.

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. 

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

WM4B

  
-----Original Message-----
From: towertalk-bounces at contesting.com
[mailto:towertalk-bounces at contesting.com] On Behalf Of Jim Brown
Sent: Thursday, February 11, 2010 7:35 PM
To: Tower and HF antenna construction topics.
Subject: Re: [TowerTalk] Measuring Coax Loss

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
>anyway.

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.

73,

Jim K9YC  


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