Re: [QRP-L] Elmer Question: Antenna feedline (long)

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From: George, W5YR (w5yr@att.net)
Date: Sat Feb 28 2004 - 12:34:49 CST


Tom, coax problems usually involve (a) connectors and their attachment to
the line and/or (b) increased line loss usually due to moisture getting into
the line and corroding the shield. Very old coax might also suffer from UV
effects such that the polyethylene dielectric has changed properties and
increased its contribution to line loss.

Checks of the line should include a close visual inspection of the
connectors and the solder joints to the coax at each end. The effects of
moisture and/or UV exposure can be checked by measuring the loss of the line
and comparing it to the expected loss based on published loss in dB/100-ft.

Of course, ohmeter checks for shorts and opens should be made if feasible,
although it is unlikely that a line that has worked for several years has
suddenly developed a short or open, unless a poor connector joint is
involved.

A more direct though invasive check for corrosion is simply to remove some
of the outer covering and examine the braid directly in the vicinity of the
connector at the antenna end. Any signs of corrosion there usually indicate
that more is present along the line and it should be replaced.

Line loss is easily measured with a wattmeter if you have or can borrow one.
The procedure is to first measure the power delivered to a dummy load from
the transmitter (any convenient power level will work but let's use 100
watts as an example) connected by a short length of known good coax. Then
connect the coax to be tested to the transmitter and move the wattmeter and
dummy load to the antenna end of the coax. Repeat the test measuring the
power delivered through the coax to the dummy load. It is important that
the transmitter power output not change while this test is being done.

Armed with the two power values (one at the line input and the other at the
line output) a subtraction of the smaller from the larger gives the measured
line loss in watts.

Now, we get a little technical. The specs for your RG-8 give matched line
loss in dB per hundred feet. You need to know the approximate length of your
line which you divide by 100 to find the loss in dB for that actual line
length. That is what presumably good new coax would do.

Now to find your measured line loss in dB, use the formula

                           dB loss = 10 x log(Pi/Po)

where Pi is the input power - say 100 watts as an example

          Po is the measured output power - say 90 watts

The power ratio Pi/Po is equal to 100/90 in our example which is 1.111.
Using a calculator, find the log of 1.111 which is.0.0457. Multiply by 10 to
get the line loss as 0.457 dB.

The matched loss for RG-8 (Belden 8237) is 0.662 dB/100 ft at 14 MHz . Now,
let's assume that your line is 50 ft long. You can measure the line length
with your antenna analyzer if you don't know it.

So you would expect the loss for new, good RG-8 to be 0.331 dB. Your
measured loss is 0.457 dB which is 0.127 dB larger.

So, now comes the judgment part. How important is an added matched line loss
of 0.127 dB? We would expect that such a small fraction of a dB is likely to
be relatively unimportant. We know immediately that since it is a small
number that there is nothing seriously wrong with the cable and connectors.
Corroded coax or poor connector joints usually show larger losses.

To put it in perspective, if you input 100 watts to the line then the
expected loss of 0.331 dB represents a power ratio of 100/Po, so we need to
solve for this value of Po. With our calculator we solve the equation

                    10^(0.331/10) = 100/Po

or

                    Po = 100/(10^0.0331) = 92.66 watts

But we measured 90 watts of actual Po, so we know that the coax has an
additional power loss of 92.66 - 90 = 2.66 watts. This is 2.66% of the input
power of 100 watts.

Most folks would agree that an added loss of less than 3% is hardly cause
for concern, especially since the wattmeter can be in error by that much or
more.. Thus we can conclude that the coax is probably still in good shape,
although showing possible small signs of deterioration.

On the other hand suppose that we had measured only 40 watts output.
Immediately we know that we have a loser coax since 60 watts is being
dissipated in the line. This equates to a loss of 3.98 dB or over ten times
the expected loss for good RG-8. If the connectors check out, then something
is wrong with the line itself and replacement is indicated.

I have known coax to be good after 30 years in the sun and weather. So age
is not always the criterion. On the other hand, coax is relatively cheap, so
if you have doubts and lack the equipment to make these tests, the easy way
is just to run new coax. I suggest RG-8X for100 watts or less and RG-213 for
QRO.

I use RG-8X for all coax applications and find it to be very convenient with
its smaller size and weight. I would advise against RG-174 unless the line
is short and you need its small size and weight for some reason.

All this has told you how to build a clock because you asked for the time.
[g]

But. I wanted to overview the steps involved in making this test since you
posed it as an Elmer Question. Those get my attention!

73, George W5YR
w5yr@att.net
http://www.w5yr.com

 ----- Original Message -----
From: "Tom Mc" <redmen1969@optonline.net>
To: "Low Power Amateur Radio Discussion" <qrp-l@Lehigh.EDU>;
<QRP-L@mailman.qth.net>
Sent: Saturday, February 28, 2004 10:06 AM
Subject: [QRP-L] Elmer Qustion: Antenna feedline

> Hi gang,
> I have an antenna question which is of an Elmer nature as I'm sure its
> pretty basic. I appreciate any help you may be able to provide.
>
> It's almost March so I'm thinking that a spring project - once the weather
> warms up - for me will be to replace my antenna feedline. I am using a 40
> meter Windom antenna and am feeding it with RG-8. This feedline is
several
> years old and my question is how can I tell if it needs replacement? Here
> are some facts, to help you understand my specific situation:
>
> 1) I can easily access the antenna end of the feedline, it is connected to
> the Windom by a UHF connector which I can unscrew.
>
> 2) I have some test equipment including a DMM and a generic version (I
> think "Vectronics") of the MFJ antenna analyzer.
>
> 3) I'm not sure what I am looking for, other than a direct short or an
open
> line.
>
> 4) I really have no reason to think that the RG-8 is bad, but every time
> the bands are dead I scratch my head and look at the antenna to see if its
> still there.
>
> 5) I realize that RG-8 probably wasn't the best choice, but it is what I
> have, so if I replace it, I'll get something better, but if it doesn't
need
> replacement I don't want to, naturally.
>
> If you'll need any more info, please let me know. Thanks in advance for
any
> help you might provide.
>
> Tom McCulloch
> WB2QDG
>
>
>
>
> If only God would give me some clear sign! Like making a large deposit in
my
> name in a Swiss bank.
> -Woody Allen
>
>
>
> ______________________________________________________________
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