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Re: [TowerTalk] Measuring Coax Loss

To: "Tower and HF antenna construction topics." <>
Subject: Re: [TowerTalk] Measuring Coax Loss
From: "Roger (K8RI)" <>
Reply-to: "Tower and HF antenna construction topics." <>
Date: Thu, 11 Feb 2010 21:44:47 -0500
List-post: <">>

Dan Schaaf wrote:
> Having been a calibration Engineer in my early days,
As was I
>  I concur with Jim.
> In addition, each coax connector inline introduces losses.
True, but the loss between a couple connectors and the 10 I have between 
the rig and top of the tower is only a couple tenths of a db, far less 
than they tell me it's supposed to be.
>  They all add up.
> Definitely worse in VHF/UHF.
I'm talking about six and two meters.  440's a different animal.
The number of connectors adds up in a hurry and can get expensive.
Starting from the rig I have a patch panel. That's two male connectors 
and a bulkhead connector.
At the tower the cable exits the equipment box which is two more 
connectors and another bulkhead connector for grounding near the base of 
the tower.  Then at the top of the tower the coax is grounded again for 
two more male connectors and a bulkhead connector. Then, one of those 
connectors was on a 28' pigtail which goes to a power divider which 
feeds two antennas with two coax connectors  but to an individual 
antenna there is the coax between the divider and the antenna with two 
chassis connectors (one at the divider and one at the antenna), plus the 
coax between them for two more connectors.  Overall length from rig in 
ham shack to antennas is 228' with a total of 14 connectors

160 through six has a six pack added for SO-2R for 12 including the 
antenna...except 75 and 40 are fed through remote antenna switches for 
an additional 4 connectors or a total of 16, but at those frequencies 
it's still only a couple of tenths. However if you are only talking 
about connection points the number drops from 14 to 9.
>  Some folks say you can't hear the difference, 
> but when you are trying to hear and work a weak signal in/above the noise, 
> every db counts.
Working aurora where the signal can't be seen on a scope and only shows 
up as a change in the background noise is where every db really counts.<:-))


Roger (K8RI)
> Dan Schaaf
> "In the Beginning there was Spark Gap"
> ----- Original Message ----- 
> From: "Jim Brown" <>
> To: "Tower and HF antenna construction topics." <>
> Sent: Thursday, February 11, 2010 7:34 PM
> 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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