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SherpaDoug
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Gold
Re: Pressure in the cable
SherpaDoug   4/15/2013 10:44:20 AM
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In the 1980's when I learned about installing RADAR on fishing vessels I was taught to drill a drain hole in the lowest part of the waveguide to let water out.  I was told not to worry about how water would get in, that was inevitable.  Either let the water out or it will corrode its way out.

jljarvis
User Rank
Gold
Water in the coax
jljarvis   4/2/2013 4:43:03 PM
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Paid my way through college doing Bcdst Engineering. 

Two similar stories.  In one case, we had a 1kw FM on a single AM tower.

The 2" FM heliax had a pressure leak near the antenna, so they ran it unpressurized.  Water leaked in at the antenna, and collected in the horizontal feedline at the base of the tower, tripping off the transmitter.


The Chief Engineer's solutioin?  Drill a 1/4" hole at the base of the vertical run, to let the water drain out.   Broadcast stns are notably cheap.

In another case, we were installing a 4 tower AM array, requiring work during the experimental period, after midnight.  This array was in a river bottom pasture, and the consulting engineer and I were tuning up one of the towers, up in the doghouse, when one of the farmer's bulls moseyed up and let out a bellow.

You can picture the scenario from there.  Suffice it to say it required a lot of waiting, followed by running like hell when the bull got far enough away. 

 

 

 

bdcst
User Rank
Platinum
Re: Air dielectric coax
bdcst   3/26/2013 9:49:11 AM
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Ironically, even AM directionals utilizing air dielectric line for greater phase stability, will be monitored with solid dielectric small diameter sample lines.  Those sample lines are often specified as phase stable.  Sample lines are cut to the same length with the excess from the shortest tower run coiled up.  Thus aging and temperature won't affect the accuracy of the relative phase measurement.

One could also make the main power carrying coaxial lines identical in length if one can tolerate additional power loss and extra material cost.  A rare AM directional might have the phasor situated in the middle of the array, especially if the transmitter building were so located, thus eliminating uneven phase shift over time and temperature due to differing line lengths.

Phasors are adjustable to compensate for seasonal variations of the array.  So, having reliable and stable RF samples from each tower fed back to an antenna monitor is crucial.  Those monitors compare both RF voltage or current depending upon the pickup and phase at each tower.  It is the relative measurement between the various towers and one reference tower that enables the pattern to be kept dialed in.  Initial array monitoring measurements become the daily measurement references only after real world field strength measurements are made at hundreds of locations down radial lines from the array.  This is called a full antenna proof and is very time and labor intensive.  Recently the FCC has started to permit theoretical mathematical proofs in place of the full antenna proof as computer modeling has proven to be sufficiently accurate so long as the array was properly built and monitored.

With the shorter wavelengths associated with FM & TV broadcasting, monitoring the temperature of coaxial line that is handling high power can avert disaster.  Depending upon where a directional coupler is placed on the line you might not detect a standing wave peak, i.e. reflected power from a missmatch.  But the current peaks in the line will disspate more energy as resistive heat while voltage peaks will promote arc-overs.   I have seen my share of line and antenna burnouts due to damaged line section couplings, bullets and cups, mostly due to moisture ingress.  Getting a mid-evening phone call from the local constabulary that a tower was lit up with flaming melted plastic jacketing dripping down to the grassy field below, is not relaxing.  Fortunately, today's high powered transmitters do respond very quickly to load faults and will not permit poorly trained personnel to keep them on the air.

Frank Karkota
User Rank
Iron
Air dielectric coax
Frank Karkota   3/25/2013 9:19:35 PM
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Many years ago, most stations used solid dielectric coax.  The problem was that as the coax aged, the velocity factor changed.  With over a quarter mile of coax to the antenna, this resulted in a slight phase change which affected the pattern.  To make matters worse, the sampling loop was also affected by the change in the velocity factor and it was impossible to keep the station within FCC regulations.  The ratios, as measured, were part of the license and readjusting to bring the null into tolerance would have caused incorrect readings on the antenna monitor meter.  Using air dielectric provides better phase stability.

One point on corrosion is that the inner conductor and probably the shield had a thin layer of plastic to prevent corrosion.  This is why my Simpson meter did not measure the resistance of the water and only showed the resistor at the distant end.  The pressure in the coax was very small, just enough to stop leaking.  The leak may have been caused by sloppy installation, shifting during the winter frost, or a subterranean rodent.

This station had a sister FM with a three inch (as I recall) hard line to the antenna.  At every place where there was a dip, the coax was warm.  Since I was hired to solve problems, I pointed this out to the chief and considered my responsibility complete.

William K.
User Rank
Platinum
Re: Maine-Bob
William K.   3/25/2013 9:16:27 PM
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Bob, I agree that it is usually the engineers who tend to consider the downstream results of those changes that others would make without ever thinking. RoHS is another example of changes made without considering the secondary and tertiary results. 

William K.
User Rank
Platinum
Re: Pressure in the cable
William K.   3/25/2013 9:13:39 PM
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@bdcst, it is certainly true that what works in systems intended for pressurizing would not nessesarily be suitable for systems that were not designed for it. One size seldom fits all equally well, and in cables that is certainly true. It works very well in those cables with the ceramic washers though.

Charles Murray
User Rank
Blogger
Re: Pressure in the cable
Charles Murray   3/25/2013 8:24:56 PM
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Thanks for the info, William K. It all makes sense, but I have to admit I never thought about it before. Judging by some of the other posts here from our very smart commenters, I'm thankfully not alone.

warren@fourward.com
User Rank
Platinum
Maine-Bob
warren@fourward.com   3/25/2013 4:39:05 PM
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I would always worry about people adding things to my worksite.  It often causes a problem when someone doesn't realize how their efforts affect the overall site.  The line or pole that prevents a radar from turning, the stucture that destroys the radiation pattern of an antenna, the moving of a wire that removes the ground connection.

In these troubled times, when management is looking for every way to squeeze another penny for upper management perks, they perhaps not realize the effect their decisions would have on the overall business.  I firmly believe engineers need to be involved in all aspect of the company.  We see things few others see and in a way that often isn't obvious to non-technical types.  Plus, it gets us noticed in a non-Dilbert way. 

bob from maine
User Rank
Platinum
Re: Pressure in the cable
bob from maine   3/25/2013 2:49:48 PM
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Oh the joys of megawatt radars trying to transmit through water-filled waveguide. Eventually the water will boil and turn to steam which occasionally finds its way into the radar transmitter itself. High power RF and water do not co-exist peacefully. Ground based HF antenna tuners also had a tendancy to fill with water and the resultant explosion would launch the antenna like a small missile. The only AM transmitter I ever worked on had an alarm when the pressurizaton failed. Good troubleshooting article.

bdcst
User Rank
Platinum
Re: Pressure in the cable
bdcst   3/25/2013 10:51:22 AM
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Actually, some coaxial line manufacturers caution against using dry nitrogen in their air dielectric lines.  Apparently under certain conditions the nitrogen will react with the Teflon spacers/insulators while a normal air mixture will not.

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