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Charles Murray
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Blogger
Pressure in the cable
Charles Murray   3/22/2013 5:30:43 PM
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I have to admit that I was suprised by the solution in this post. I really didn't know that radio antenna cables required pressurization. Is the pressurization used just for the removal of water?

warren@fourward.com
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Platinum
Re: Pressure in the cable
warren@fourward.com   3/22/2013 7:26:59 PM
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I had the same thing happen to me several times with seagoing radars.  Sometimes you had to just drill a small hole in the bottom of the radar antenna and see if water came out.  Then I would reseal it with silicone seal.  I would go through a lot of trouble before I got to that point, however, until I had experience with water leaks in the antenna.  It never looked like there could be a leak.  But looks (leaks) are often deceiving...

notarboca
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Gold
Re: Pressure in the cable
notarboca   3/22/2013 10:58:50 PM
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Had the same thing happen with an aircraft radar waveguide.  It worked fine on the bench but when we pressurized it and leak tested we found a small pinhole caused by RF arcing.  Once airborne the leak had allowed more moisture in and pressurizing nitrogen out.

bob from maine
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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.

warren@fourward.com
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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. 

William K.
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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. 

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.

TJ McDermott
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Blogger
Re: Pressure in the cable
TJ McDermott   3/24/2013 8:55:12 PM
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Mr. Murray, I agree - it opens up all sorts of questions I didn't know needed to be asked.

How much pressure?  What source?  Plain air, or dry nitrogen?  And on and on.

jfs1970
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Gold
Re: Pressure in the cable
jfs1970   3/25/2013 10:19:10 AM
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All broadcast transmission lines that have an "air dielectric" which is virtually all broadcast transmitters, are pressurized to eliminate moisture in the lines.  It can be plain air that has been run through a dehydrator, or nitrogen from a bottle.  Dehydrators can either use a dessicant, or a condensation coil to remove moisture from the air.  Pressures are recommended by the transmission line manufacturers, and usually are in the range of 2 psi up to possibly 15 psi maximum.  High pressure is not necessary, only enough to create a positive pressure differential.  You do not want too high pressure, or you risk "bursting" the pressre window that keeps the pressure from entering either the antenna structure or the transmitter tuning cavity.  The pressure window is a thin piece of mylar type of  material (think of a 35mm photographic slide...similar material and thickness).  We generally always have the lines pressurized.  This also gives us the ability to tell when transmission line damage occurs, as the flow meter will start to increase.  Moisture in transmission lines is not good, not only because of corrosion, but when dealing with 50Kw and higher, arc overs become a real problem.

William K.
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Platinum
Re: Pressure in the cable
William K.   3/25/2013 10:22:27 AM
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The pressure in the cable does serve to keep water out, for most situations. BUT the use of dry nitrogen also has other bemefits, in that it provides a dry atmosphere, which reduces the tendancy for arcs to develop, and it avoids the effect of oxygen on both the insulation material and the conductor surface. In addition, if all of the air is displaced, the formation of ozone is inhibited, which ozone is detrimental to all of the materials used in RF circuitry. 

bdcst
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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.

William K.
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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.

bdcst
User Rank
Platinum
Re: Pressure in the cable
bdcst   3/25/2013 10:47:20 AM
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They don't necessarily need dry gas pressurization.  Many broadcast transmission systems use solid dielectric insulation and thus are not presurized.  In fact it is atypical for an AM facility to use pressurized lines.  In the past, the primary reason for using pressurized lines was slightly lower loss.  Second, FM broadcast antennas often require dry gas pressurization to insure no moisture ingress into the antenna mounted high up on the tower.

I recently had a client whose new solid state FM transmitter was unhappy with its antenna.  When the weather got cold the miss-match got worse over time.  This was supposed to be pressurized line but the client was lax about keeping the line filled with dry air.  I had them drill a small hole in the line at the lowest point outdoors on a day above freezing and lots of water came pouring out.  The temporary fix was to replace the entire air dielectic line with sold dielectric line that the tower rigger had in stock from a cellular tower job.

The client's old tube transmitter didn't have a problem with some water in the line as it could be tuned to match into almost any load.  The nature of the design of transistor transmitters, many combined smaller amplifiers, requires a precise 50 ohm match on their outputs.  While one can specify output matching networks for some brands of solid state AM transmitters, that is not the case with FM or TV.  It has to be 50 ohms or bust.

Solid line is heavier than air dielectric line.  That could raise a mechanical issue with tower loading.  Many broadcasters opt to use rigid line sections that look like copper sewer pipes.  The benefits are even lower line loss and the ability to repair a damaged section without having to replace the entire coaxial line.  But the installation labor is higher for rigid line sections.

As for troubleshooting an AM antenna system, one would normally first test the line in two conditions with a simple ohmmeter, open at the far end and then shorted or terminated in a 50 ohm load.  This will immediately detect or rule out shorts or opens in the line itself.  The old GR bridge has long been superceded by far less expensive and more informative test equipment such as vector impedance analyzers.  Many AM directional phasors come with built in operating impedance bridges similar to the old GR bridge but designed to handle full power from the actual transmitter to help maintain correct impedance matching.  This is especially important when using transmitters with tuneable output networks or matching networks as its possible to accidently adjust both the phasor input and transmitter output far from the correct 50 ohms of the interconnecting line.

I once got an urgent call from a broadcaster who had just enough knowledge to be dangerous.  He had a brand new 10 kW digitally modulated AM transmitter running at 5 kW.  He decided to swap in a different audio processor to take advantage of the superior sound capability of his new transmitter but he neglected to slowly turn it up.  The transmitter dutifully produced over 200% modulation and the excessive RF voltage was not received well by the antenna system.  The result was a bunch of alarm lights and an unhappy, but unscathed, transmitter.  Upon arrival I disconnected the transmitter from the antenna line and put an ohmmeter on it.  It might have been my old Simpson 260P VOM or my newer Simpson 488 digital multi-meter.  Dead short!  So out to the tower went I.  I disconnected the line and the short went away. Whew!  That ruled out an arc-over problem in the 500 feet of non pressurized buried line.  However, the short was evident on the output of the antenna matching network.  Indeed, after separating everything, the tower itself was still shorted to ground.  This insulated hot tower had a cell site tenant onboard with two isocouplers to permit the cellular equipment's coaxial lines to pass RF to and from their antennas without grounding out the tower.  One of those isocouplers had internally shorted.

tekochip
User Rank
Platinum
Simpson 260
tekochip   3/24/2013 1:29:36 PM
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I love any story that includes a 260.  I have a Russian friend of mine that says there was a meter in Russia that was just as fabled, but I can't remember the brand.

Tim
User Rank
Platinum
Classic equipment
Tim   3/25/2013 6:58:01 AM
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It is good to see the 1950's era General Radio bridge still in use. Sometimes older equipment is replaced just because it is old. Not so in this case. Good job finding the true cause of the issue.

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.

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.

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. 

 

 

 



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