The AM Forum

See update at the end of this post

I have an interesting artifact on my signal which is absolutely generated by the antenna feedline.

I have actually had this problem in the past, but considered it a fluke.  Now that it has happened again, it is surely a real thing.

The situation is this:  I am using a poorly teminated (mechanically) piece of 7/8 inch, non-foam cable TV hardline for my feedline.  The feedline is does not have a helical dialectic, but is similar - with plastic insulators placed approximately every 1.5 inches within the cable.  The insulators are melted on when the cable is manufactured, and are well sealed against the shield and the center conductor.

What happens is, over time, water works its way into the cable from the antenna end, and water collects within the first one or two cavaties.

The water - apparently - boils when I transmit, modulating the signal with an odd hissing/rushing type of sound.  I have been asked if there is a jet passing overhead, or is there a fan blowing near to the microphone.  It is not a steady sound, rather, the sound varies in intensity and frequency component.

I cannot hear this effect in my local modulation monitor (which  is configured at the transmitter), but everyone listening on the air hears it.  I have received a recording of the effect, clearly audible on the signal.

It is interesting that there is absolutely no indication whatsoever at the transmitter end that anything is wrong.  No changes in SWR, power, loading, nothing in the modulation monitor - nothing - a dead carrier, but it is very apparent on the air at the far end.

Anyway, I suppose I need to take the antenna down and cut off about a foot of the feedline, and reconnect.  And, of course, try to solve the underlying problem of water penetration by somehow sealing the end (properly).

So, any ideas about the "water modulation", and/or how to properly seal the end (I cannot use a connector, but I can use sealants).

Thanks and Regards,

Steve

Update: Here is a picture of the cable showing approximately 1 inch of water still in the first segment of the cable.
(http://www.classeradio.com/water-coax.jpg)

It's probably arcing slightly, and thus amplitude modulating the signal at the antenna end, espcially on modulation peaks.  Because it's such a high frequency (and presumably low-intesnity, more like a sizzle than and arc) arc, you won't notice it on the SWR meter, etc.

That gummy black antenna sealer that Radio Shack sells is great for this.  It comes off like tape, but you can mold it to any shape and combines with itself.  You can get Andrew waterproofing kits on ebay cheaply that has this stuff in it too.

Steve

I heard this effect, along with the others, on the "carrier net" last Sunday morning.

I, too, am puzzled as to why you don't hear this on your modulation monitor... because I would have assumed that a disturbance in the coaxial cable that was large enough to produce audible (at distant receivers) modulation of your radiated signal would also produce enough variation in the impedance at the antenna end of the cable to modulate the SWR in the cable... and thus to produce an audible signal at the output of your off-air monitor's "pickup".

Here are two possible explanations:

If your off-air monitor's pickup is on the "shack side" of an antenna tuner, it is possible that variations in reflected power from the antenna/coaxial feeder would be greatly attenuated by the tuner as they travel back toward the pickup. [In effect, when they reach the tuner, they would be reflected, again, back to the antenna].

If the phenomenon in the coaxial cable is coupling/diverting rf power into harmonics of your carrier frequency... rather than just reflecting it... then not only could the above (tuner effect) be taking place... but any power that is diverted to harmonics of your carrier frequency would still be "detected" by your pickup. 

Meanwhile, as JN has suggested, these variations could be too fast to see on the particular type of SWR monitor you are using.

Stu

It's just so interesting that there is _nothing_ audible or visible at the modulation monitor.

Killing the audio, and adding another 60dB of audio gain to the mod monitor output, there is absolutely nothing (other than, at this point a very slight 120Hz power supply hum due to the very high gain).

I would expect to hear the hiss/noise, but I do not.

Oh, there is no antenna tuner.  The coax is connected directly to the transmitter output.

Regards,

Steve

Could it be that the phenomenon on the coaxial cable is modulating the radiation pattern of your antenna by coupling more or less power on to the outside of the shield of the coaxial cable? Do you have some kind of choke balun on the coaxial cable to reduce common mode current?

Yes, a whole string of 43 ferrites around through which the cable runs (I probably have 20 of them up there)...

After sealing the far end of the hard line against moisture, I would bend it so that it points straight downwards at the very end.  Water should tend to drip off the tubing, rather than migrating inside.

I had to devise a drainage system for the open wire feeders that go up through the inside of my tower.  Water was dripping off the copperweld conductors and onto the galvanised tower.  Enough copper would leach from the wire to contaminate the water, which was corroding the zinc coating on the tower, turning it green.  I ended up directing the flow away from the tower using a couple of short sections of automotive  heater hose, and the corrosion stopped.

Steve

Is the "wet" portion of the feed line "behind" (closer to the shack) the ferrites?

If so, conduction through the water can couple unbalanced current on to the outside of the shield.... and that unbalanced current would not have to pass through the ferrites

Stu

You could use some liquid electrical tape over the connections then cover with the stuff from Radio shack.  3M makes a version called joint sealer.  It is a small can with a paint brush in the lid.

I have a splice in some RG 213 using two PL 259s into a barrel connector.  I put heat shrink over the area then after shrinking the tubing used the joint sealer to cover the ends of the heat shrink.  We have quite a bit of rain in the spring and fall here and after 3 years I don't have an anomoly in my reflected power.

What happens is, over time, water works its way into the cable from the antenna end, and water collects within the first one or two cavaties.



Steve,

What you describe pretty much tells the story with air dielectric cable.  Moisture, especially in the summertime, can find its way into the cable, despite all your best efforts to seal connections.  There are only (2) real solutions, pressurize the cable or swap it out for a foam based cable.  Andrew LDF5-50A foam cable is rated for about 20KW so that should not be an issue.  If you want some cable, I can probably find you what you want from the cellphone guys.  Drop me a line via email. 

My .02 (and that is the actual value)

Pete
n1ps

You nailed it Pete!  Many years ago our company had a CATV system that used that air dielectric cable with the plastic disc spacers for trunk cabling.  It was a nightmare!  Unless you keep pressure on it, it will attact moisture, and/or the moisture already present inside the cable will condense, causing serious problems.  You can usually see big changes with changes in ambient temperature.The spacers do have tiny holes in them and it is possible to pressurize it with air, nitrogen or inert gas, but any fault in the sheath (or end connections) will lead to failure.

73,  Jack, W9GT

Just an odd thought... could the 'water' modulation be primarily phase modulation of the signal?  That would not appear on an envelope detector except where there is some degree of multipath, which would explain its invisibility to the mod monitor.

Also, what is the dynamic output impedance of your RF amp?  Not the load it sees, but its dynamic impedance, equivalent to plate resistance in a tube amp.  Maybe it is low, strongly attenuating amplitude variations at the transmitter end caused by load variations from the water further out.

A combination of the two, along with the water modulation, could be causing slight AM but lots of PM at the antenna, but essentially no water noise modulation audible in the mod monitor.  Or perhaps just the AM noise could even be doing it if the dynamic Z of the amp is really low, especially if the line length happens to be just right.

If the water noise does not vary along with the AM audio during normal signal fading, but seems to be minimum in volume when the signal fades up, and maximum while the signal is fading upward or downward, then I would suspect that the noise is mainly PM.  If instead the noise varies along with the AM audio, then I would think that it is mainly AM.  A combination of AM and PM noise would fade differently from the AM audio, but exactly how that would work would depend on how much PM and how much AM the noise produces, etc.

And maybe the feedline length makes AM noise at the antenna, look like mostly PM noise at the transmitter.

interesting problem ... commercial boys use a tank of N2 to keep feedline interior dry ...I think I'm familiar with that 7/8 hardline ... copper/copperweld inner conductor and an aluminum jacket for the outer ... as you transmit a small portion of power is converted to heat which causes the internal pressure to raise and the converse after transmititng ceases ... do this enuff and here comes the vasser ... how are you connecting to the aluminum jacket ... any copper bearing metal is out ...coupled with your very wet summer I suspect corrosion ...  hope this helps ...73 ...John

ahh you have an electrolyzer. Wait till you DO get an arc..

Yup,
Nice explosive gas mixture inside the feedline!

One little arc, and the feedline will peel like a banana!!

;D

I have heard the artifact very clearly in your signal, Steve.  Your theory of water in the coax causing the effect is just that, a theory, for now anyway.  And troubleshooting the problem is compounded by the fact that YOU cannot hear it in your station monitor.

Temporarily swapping out the cable itself will probably be the only conclusive way to determine whether or not it's the cable creating the artifact.  Since you have a uniquely designed transmitter, it is still possible that the root cause exists elsewhere.

I would be more than willing to work with you as a distant monitor while you try swapping out the coax (you know I can hear the artifact) to conclusively prove or disprove the theory of water-in-the-coax as the reason for the artifact.

You can contact me by email or phone to arrange a mutually convenient time to work on this.  I really hope I can be of assistance to you.

Eric - WB2CAU

Bacon may be on to something that the artifact might be a compound effect with selective fading.  Since selective fading is going to be different at different locations, having an off-air recording being made by 2 different stations at the same time (I can supply one of the recordings) and then comparing whether the artifact appears in the same time domain will be helpful to you.  Let me know.

Eric - WB2CAU

Hi Eric,

Thanks for the recording of the artifact !   It is definitely caused by the water.  I have had the problem in the past, and when I looked at the coax, the first couple of segments (about 3 inches) had water in them.  When I cut this part off, and reconnected, the artifact was GONE !!!

I have about a 400 foot run of this cable going from the transmitter to the antenna.  The cable has been up there for 15 years, but it may be time to replace it with something a bit better.

I just don't have any single pieces of any type of hardline which are that long (except for this one).

Regards,

Steve

Not all of them. Some use dessicated air. That works fine unless you have a compromised seal in the area of a feedline fire, whereupon the dessicator will dutifully pump dry air into the fire, ad nauseum.

Dessicators are extremely simple devices, just a bottle of silica gel beads and a little baby compressor. Don't need much more than 15-20 PSI, if that. When the beads turn pink, they're full... just microwave them until they turn blue again.

That combats condensation, and mitigates (but does not eliminate) water wicking in around the connectors. I suspect you're having the latter problem, Steve. Might be worth a shot. You're a crafty enough guy that you could probably barf one together out of odds and ends (and J.S. clip leads) you have laying about the place.

1/2 to 3/4 cup of silica beads is enough to keep the WFMX feedline dry for 2 to 3 years at a time between bake-outs.

--Thom
Keep Away One Zorched Ground Connector

Since you are certain water is the cause and you asked about sealants, in addition to some of the previous suggestions, there is a "self-fusing" silicone rubber electrical tape that works very well.  3M calls their product Scotch 70 but there are less expensive equivalents available at Wal-Mart.  The Wal-Mart version is less than $5 for a 15' roll, more than enough for your needs.  It is used not only for electrical purposes but also for plumbing.  It's got to be water tight for plumbing!

Steve,
At times, when monitoring your signal into Long Island,
I've noticed that your vfo was pulled slightly on audio
peaks and seemed to also exhibit slight fm-ing even in
the absence of audio. If the vfo has insufficient isolation
from the transmitter output, a change in SWR could cause a change in
frequency. It's possible that folks are slope-detecting the fm with
narrowband receiver filters while your direct sampled modulation monitor
would not respond to any fm.

Have you tried exciting the transmitter with other than the vfo?
Have you monitored xmtr output for any fm?

Regards,
BobbyT

Hi Bobby !

Oh, the VFO pulling was with another transmitter, and is a different issue - but it was there, too!!!  I have fixed that problem also.  I was not using that transmitter or VFO for any of these tests..

Now, for the feature presentation:  I took the antenna down and indeed the first 2 segments of the cable were filled with water.  Here is a picture of the first segment with more than 1 inch of water still remaining in the section.
(http://www.classeradio.com/water-coax.jpg)

Ain't it grand?  I haven't tried the new antenna yet - but I am 100% certain the problem is gone (just like last time) - unless there is another leak somewhere else !!!!

Regards,

Steve

Thats a great suggestion and one I use religously.

I use the original General Cable "Fused Disc" in 1" size here on 222, 432 and 1296 but with factory "pin" connectors and then screw on 75 Ohm N adaptors. The cable has been in use for 19 years with zero problems and my attenuation test every spring reads the same as when installed.

When General sold the line to Trilogy the name was changed to MC². The cable is completely bonded to the aluminum and there are no holes. The factory test is that a 1' length hold 20 psi after a 24 hour temperature cycle from -40 to +140F.

The roughly 18 feedlines here are all CATV hardline, from 1/2" to 1" and mostly foam dieletric.  Runs for 160M and up to the towers alone are 100-250' and a 750' run to the Beverages switch box and have all been trouble free.

Carl
KM1H