I have a section of air core Beldin coax, with the center feed separated by a
single wind of plastic insulator. The section is about 65 meters long. I added
enough RG58 to the feed end to get a full wave length. I then measured the
impedance using aa Autek analyzer. It read 30 Ohms.

The coaxpair program predicts for coax with a loss of about 10 db per km an
impedance of about 11.5 Ohms. The cable should be better then this.

Is this difference leaky or bad coax? Is it reasonable to assume water in the
cable? If it is water is there a practical way to get it out?

Thanks - Dan

On Mon, 26 Dec 2005 13:43:05 -0800, dansawyeror
wrote:

I have a section of air core Beldin coax, with the center feed separated by a
single wind of plastic insulator. The section is about 65 meters long. I added
enough RG58 to the feed end to get a full wave length. I then measured the
impedance using aa Autek analyzer. It read 30 Ohms.

The coaxpair program predicts for coax with a loss of about 10 db per km an
impedance of about 11.5 Ohms. The cable should be better then this.

Is this difference leaky or bad coax? Is it reasonable to assume water in the
cable? If it is water is there a practical way to get it out?

Thanks - Dan

Dan, you are a master of asking questions with partial / unreliable
information... and you have done it again!

Doesn't the Belden cable have a legible type marking?

How much RG58 did you add? (Why?)

What frequency did you make the measurement?

Was it 30+j0?

What was the objective of the measurement? If it was to determine
cable loss, does the Autek book give you a procedure for doing that?

Owen
--

Owen Duffy wrote:

Dan, you are a master of asking questions with partial / unreliable
information... and you have done it again!

Doesn't the Belden cable have a legible type marking?

How much RG58 did you add? (Why?)

What frequency did you make the measurement?

Was it 30+j0?

What was the objective of the measurement? If it was to determine
cable loss, does the Autek book give you a procedure for doing that?


Three additional questions:

What was connected to the far end of the coax when you made the measurement?

How did you determine that the combination cable was one wavelength long?

What was the angle of the impedance, or the resistive and reactive (R
and X) parts?

Roy Lewallen, W7EL

"dansawyeror" wrote in message
. ..
I have a section of air core Beldin coax, with the center feed separated
by a
single wind of plastic insulator. The section is about 65 meters long. I
added
enough RG58 to the feed end to get a full wave length. I then measured the
impedance using aa Autek analyzer. It read 30 Ohms.

I don't get it.

Roy, thanks.

1. The far end was open.

2. I used an 8405a and a directional coupler connected in reverse. I zeroed the
8405a open stub between the driven signal and the reflected signal. I then added
a coax segment and the feedline to bring the the 8405a back to near zero
deflection at a frequency near the frequency of interest. I then tweaked the
frequency to re-zero the reflected signal.

3. I believe the reading was 30R and 0j. The intention was to tune frequency and
the meter to one electrical wave length and reach zero x.

Dan

Roy Lewallen wrote:
Owen Duffy wrote:


Dan, you are a master of asking questions with partial / unreliable
information... and you have done it again!

Doesn't the Belden cable have a legible type marking?

How much RG58 did you add? (Why?)

What frequency did you make the measurement?

Was it 30+j0?

What was the objective of the measurement? If it was to determine
cable loss, does the Autek book give you a procedure for doing that?


Three additional questions:

What was connected to the far end of the coax when you made the
measurement?

How did you determine that the combination cable was one wavelength long?

What was the angle of the impedance, or the resistive and reactive (R
and X) parts?

Roy Lewallen, W7EL

On Mon, 26 Dec 2005 13:43:05 -0800, dansawyeror
wrote:

I have a section of air core Beldin coax, with the center feed separated by a
single wind of plastic insulator. The section is about 65 meters long. I added
enough RG58 to the feed end to get a full wave length. I then measured the
impedance using aa Autek analyzer. It read 30 Ohms.

The coaxpair program predicts for coax with a loss of about 10 db per km an
impedance of about 11.5 Ohms. The cable should be better then this.

Is this difference leaky or bad coax? Is it reasonable to assume water in the
cable? If it is water is there a practical way to get it out?


The hints in this guessing game a
- this might be the popular Belden 9913, lets proceed on that
assumption;
- that 65m is less than a full wave, so the frequency of the test must
less than 3.87MHz;
- the loss at the frequency of interest might be 10dB/km, implies 7MHz
for 9913 (MLL for 9913 at 3.875MHz is around 7.4dB, and less at
~3.6MHz);

Bzzzt... does not compute, inconsistent input data.

Some trivia:

The input resistance to a 65m shorted stub of 9913 at its half wave
resonance at 3.875MHz should be around 2.8+j0 ohms.

The input resistance to a 65m open stub of 9913 at its half wave
resonance at 3.875MHz should be around 900+j0 ohms.

It is anyone's guess what adding an unspecified piece of RG58 will do.

An accurate SWR meter can be used to roughly assess the loss, measure
the loss into a shorted line at the frequency of interest, and convert
the measurement(s) into return loss. The matched line loss is half the
observed return loss. The SWR looking into a 65m shorted stub of 9913
at 3.6MHz should be around 19:1, rho about 0.90, for a return loss of
~0.92dB, and a MLL of 0.46dB. (If your SWR meter does not read 100%
reflected on a s/c applied to the SWR meter terminals, don't waste
your time doing the test.)

Owen
--

I agree with Owen, something here doesn't compute. The only way I can
make sense out of it is if your line length is really an odd number of
quarter waves, and it's exceptionally lossy. As a first thing to do in
resolving the matter, I recommend measuring the velocity factor of the
main coax. For best accuracy, short circuit the far end of the line with
a low-inductance short circuit, and look for a low Z at the input. (You
can even use a scope or diode detector for this.) At low frequencies, a
single wire will do for the short; at higher frequencies, use multiple
wires radially extending from the center to the shield, or a metal
plate. At very low frequencies, the impedance will be low, increasining
with frequency. At some point it'll rise and become very high, then drop
again as frequency increases. Find the frequency where it's the lowest
-- this is the frequency where the line is exactly a half wavelength.
Physically measure the line and calculate the velocity factor. For line
constructed like you describe, the velocity factor should be around 84%.
A significantly lower factor probably means it has water in it. If it
comes out about right, let us know and we'll go from there.

Roy Lewallen, W7EL

In article , Roy Lewallen
wrote:

Physically measure the line and calculate the velocity factor. For line
constructed like you describe, the velocity factor should be around 84%.
A significantly lower factor probably means it has water in it.

Roy-

Is this correct? As described, the "dielectric" is mostly air with some
plastic used as a spacer. I would have expected something over 90%.

Back to the question of how to get the water out. If there is a large
quantity, you might be able to pour it out. Beyond that, is there a
common technique used to dry-out cable? (I suppose a shop-vac could be
connected to one end and run for a while, on a day with low humidity.)

Fred

To dry out a cable which is partially air-spaced, get a cylinder of
dry air or nitrogen and allow the gas to seep through it under a
pressure of two or three times atmospheric. It may take several days
or more for a length of 100 metres. Measure capacitance at intervals
for stability.

It speeds things up if the cable can be warmed in an oven at about 60
degrees C. You don't want to melt the polyethylene.

The foregoing is based on a vague memory of an episode about 50 years
back.
==========================================

Fred McKenzie wrote:
In article , Roy Lewallen
wrote:


Physically measure the line and calculate the velocity factor. For line
constructed like you describe, the velocity factor should be around 84%.
A significantly lower factor probably means it has water in it.


Roy-

Is this correct? As described, the "dielectric" is mostly air with some
plastic used as a spacer. I would have expected something over 90%.

I was going by the manufacturer's specifications. I've never used this
stuff myself. I think if you look at the construction you'll find more
plastic and less air than you might expect.

Back to the question of how to get the water out. If there is a large
quantity, you might be able to pour it out. Beyond that, is there a
common technique used to dry-out cable? (I suppose a shop-vac could be
connected to one end and run for a while, on a day with low humidity.)

Hopefully someone else can help you there.

Roy Lewallen, W7EL