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A tale of five signals

I am currently finishing an interesting project involving putting up two translators on a diplexed AM tower which also holds a mobile phone/data tenant as well.  All-in-all, this seems to be a very efficient use of vertical real estate.

WMML WENU tower, Glens Falls, NY

WMML WENU tower, Glens Falls, NY

The AM stations are WMML and WENU in Glens Falls, NY.  The AM stations are diplexed using a Phasetek diplexor/ATU.

Diagram showing WENU/WMML tower with W250CC/W245DA antenna installed

Diagram showing WENU/WMML tower with W250CC/W245DA antenna installed

Diplexor diagram, WENU/WMML Glens Falls, NY

Diplexor diagram, WENU/WMML Glens Falls, NY

The translators are W250CC and W245DA which are using a NICOM BKG-77/2 two bay 3/4 wave spaced antenna mounted at 53 meters AGL.  The translators use a Shively 2640-04/2 filter/diplexor which as a broad band input port in addition to the translator input ports.  Since these translator signals are only 1 MHz apart, the higher power Shively filter was installed because it has better rejection characteristics.  The broadband input port allows the NICOM antenna to be used as a back up for any of the three FM stations; WKBE 107.1, WNYQ 101.7, or WFFG 100.3.  Two transmitter sites for those stations are mountain top locations which are very difficult to get to in the winter time.  Having a backup site available takes some of the pressure off during storms or other emergencies.

Shively 2640 -04/2 filter for W250CC and W245DA

The NICOM FM antenna was mounted on the tower when W250CC went on the air in October of 2016.  When it was installed, the base impedances for both AM stations were measured.  For some reason, WENU 1410 KHz seems to be more sensitive to any changes on the tower, thus the WENU ATU needed a slight touch up.  When working on diplexed AM systems, it is also important to make sure that both trap filters, which are parallel resonant LC circuits, are tuned for maximum rejection of the other signal.  During this particular installation, nothing was added to the tower and no change in the base impedance for either station was noted.

Shively Filter, connected to transmitters and antenna

Shively Filter, connected to transmitters and antenna

As a condition of the construction permit, measurement of spurious emissions of all stations sharing the common antenna needed to be completed to ensure compliance with FCC 73.317(b) and 73.317(d).  I made careful measurements of the potential intermod products between the two translator frequencies.  This measurement was completed with my TTI PSA6005 spectrum analyzer.

The primary concern here is mixing products between the two transmitters. Both transmitter are BW TXT-600 with low pass filters before the output connector. There are three frequencies of interest;

  1. (F1 – F2) + F1 or (97.9 MHz – 96.9 MHz ) + 97.9 MHz = 98.9 MHz
  2. F2 – (F1 – F2) or 96.9 MHz – (97.9 MHz – 96.9 MHz) = 95.9 MHz
  3. F2 + F1 or 100.1 MHz + 98.9 MHz = 194.8 MHz

That, plus harmonic measurements out to seven or eight harmonics of the fundamental frequency should be enough to demonstrate compliance with FCC out of band emissions standards. Being that this site has LTE carriers, it is very important to measure the harmonics in those bands. Mobil data systems often use receiver pre-amps, which can amplify harmonics from the FM band and make them look out of compliance. Having a base set of reading to fall back on is always the best course in case the “out of tolerance” condition gets report to the FCC.

Measurements on these frequencies must meet the emissions standards outlined in FCC 73.317 (d), which states:

Any emission appearing on a frequency removed from the carrier by more than 600 kHz must be attenuated at least 43 + 10 Log10 (Power, in watts) dB below the level of the unmodulated carrier, or 80 dB, whichever is the lesser attenuation.

Harmonic frequencies to be measured:

Harmonics for 96.9 MHz fundamental Harmonics for 97.9 MHz fundamental Comments
193.8 195.8
290.7 293.7
387.6 391.6
484.5 489.5
581.4 587.4
678.3* 685.3* US LTE Band 71
775.2* 783.2* US LTE Band 5
872.1* 881.1* US LTE Band 5
969.0 979.0

*Frequencies that fall within the mobile data LTE bands. Traces where recorded and saved for these frequencies.

All of that information, once compiled is attached to the FCC form 350-FM, which, once filed grants Program Test Authority.

BW TXT-600 V2 translator transmitters

BW TXT-600 V2 translator transmitters under test and measurement

GatesAir FLX-40 one year in

I was at the WEBE transmitter site recently and took the time to look over transmitter we installed last year:

GatesAir FLX-40 transmitter, WEBE Bridgeport, CT

GatesAir FLX-40 transmitter, WEBE Bridgeport, CT

Overall, I would say that this transmitter has been very reliable.  We had to install a UPS for the exciter and HD Radio exporter, but that is not a big deal. During the first power outage, the exciter went dark first. It took longer for the transmitter controller board to lose power, in the interim the controller turned the transmitter power all the way up. When the generator came on line 10 seconds later, the transmitter returned to operation at 41.5 KW. This, in turn, caused one of the other field engineers to freak out and nearly lose his mind (stay away from the brown acid, FYI).

I installed the UPS a few days later.

WEBE TPO 35.3 KW with HD Radio carriers on

WEBE TPO 35.3 KW with HD Radio carriers on

Transmitter power output is 35.3 KW, which is getting into the semi-serious range. The reflected power goes up when it gets warm out and goes down in colder weather.  Over the winter, it was running about 50 watts.  Even at 138 watts, that represents 0.004% reflected power. The TPO forward goes to the 6 bay, 1/2 wave spaced antenna side mounted, 470 feet (143 meters) AGL. The station covers pretty well.

WEBE Pump station

WEBE Pump station, pump is running 2/3 speed and fans are running at about 1/2 speed

Overall, I would give the liquid cooling system an A grade. The transmitter still dumps a fair amount of heat into the room from the RF combiners and PA power supplies. Most of the heat, however, ends up outdoors. Previously, we had two Bard 5 ton AC units running almost full time. Now, only one AC unit cycles on and off except for the hottest days of the year. Outside temperature when this picture was taken was 81 degrees F (27.2 C).

Next year, we will have to send a sample of the coolant off to be analyzed.

Gates FLX-40, WEBE Bridgeport, CT

Gates FLX-40, WEBE Bridgeport, CT

I have had good experiences with the GatesAir FLX/FAX series transmitters. I would recommend this to a friend.

Status of AM revitalization

It has been about five years since the AM revitalization initiative was first proposed by the FCC and about five years since the first rules changes took place.  Those rules changes included:

  1.  FM translators for AM stations
  2. Allowing stations to use MDCL (Modulation Dependent Carrier Level)
  3. Changing some of the antenna radiation efficiencies requirements
  4. Changing some of the allowable interference towards other stations requirements
  5. Loosening some rules regarding proofs, MOM, night time coverage over city of license, etc

Things that were not addressed:

  1. Receiver quality and technical advances
  2. Ambient noise levels on Medium Frequency (among other) bands
  3. HD Radio or any other digital modulation scheme

Things that were discussed then changed subsequently as a separate initiative:

  1. The main studio rule, which was eliminated for all broadcasting stations

What has been the net effect of these changes?  Has any of this revitalized AM radio?  The net effect has been approximately more of the same.  There have been many stations that have applied for and received licenses for FM translators.  Those stations, in most cases that I am aware of, receive some benefit of extra revenue because of this.  Stations with carrier power levels of 10-50 KW have taken advantage of MDCL technology to save some money on their electric bill.  Nothing wrong with that.

For stations that use a directional antenna, proofs of performance and other DA matters with the FCC have become slightly easier.  Medium Frequency (MF) directional antennas are very large, require a lot of land, are expensive to build, license and maintain.  I know of several stations which have downgraded from a class B station with a directional antenna to a class D station with a single tower and greatly reduced night time power.   Those downgraded stations certainly benefit from an FM translator.

I have heard from more than one AM station owner who says after four years, they are going to “turn in their AM license and just keep the FM.”  I am sure that they are not informed regarding translator rules.  Perhaps, however, the FCC will allow this in the future; a sort of back door commercial low power FM station classification.

The AM band zenith occurred in November of 1991, when there where 4990 licensed AM stations in the United States.  As of June 30, 2018, the total stands at 4633.  That is a decline of 357 stations.  There are currently 90 AM stations listed as silent.  That represents a decline of approximately 9 percent or less than 1/2 of one percent per year.

The last number of AM stations actually transmitting HD Radio that I found was approximately 110, which differs from the iBiquity (and FCC) number of 240.  The FCC data base includes stations which are currently dark, or stations which where transmitting HD Radio at one time but have since turned it off.  Either way, it is a small percentage of licensed stations.  As of this time, AM HD Radio appears to be a non-starter.  In other parts of the world, Medium Frequency DRM seems to be doing well.  The difference seems to be that the DRM operation is all digital and the digital carriers have a much higher power level than that of the hybrid AM HD Radio being used here.

Of those 4633 standard broadcast stations, approximately 260 belong to iHeart radio, Cumulus owns approximately 120 and Townsquare owns approximately 80.   That accounts for 460 stations.  The remaining 4000 or so stations currently on the air are owned by medium sized corporations or individual owners.  The reason for the distinction; I have noticed that the large corporate owners tend to concentrate resources and effort on those licenses that will make the best return, e.g. FM stations.  Of course, there are a few exceptions to that trend, often in major markets.

Of those 4000 or so remaining AM stations, most seem to be treading water.  They are making enough money to stay on the air.  There are a few AM stations that are doing remarkably well.  Those are the ones with primarily  local content.  The vast majority of AM stations are running some type of syndicated talk.  News/talk and sports radio are the two most common formats.  Conservative news/talk seems to be the bread and butter.  Liberal news talk has been tried, but none have succeeded.

Last May, the Supreme Court overturned the Professional and Amateur Sports Protection Act of 1992.  That federal law prevented gambling on outcomes of professional and college sports games.  With the overturn of that rule, individual states can now legalize sports betting.  It will be interesting to see what states allow legalized sports gambling and whether that has any effect on the various sports radio formats.  I can see where individuals and odds makers may want to get good inside information regarding team dynamics and so on.  The sports network that can furnish such information may be in a good position to carve out a niche.

Music can and does sound good on AM when it is done correctly.  There is a great misconception that AM fidelity is poor.  That is not necessarily so.  There are a good many AM receivers these days which have much better bandwidth than the previous generation receivers.  I am noticing that car radios in particular sound much better.  Yes, there are still problems with electrical noise and night time interference.  There are still technological improvements that can be made for analog AM on the receiver side.

In summary; the revitalization efforts have benefited some AM stations in some areas.  The truth is, that many AM stations have been let go for so long that there is no saving them.  Other AM stations that are still viable are making a go of it.  In nautical terms; there is six feet of water in the hold, the pumps are working and the ship is not sinking… for now.

WKIP

This was the radio station that I listened to (or rather, my parents listened to) when I was a very young kid.  From this source, things like school closings, weather, lunar landings, news, sports and traffic could be heard.  At one point, there was a guy called the “Traffic Hawk,” (real name Don Foster) who flew in a Cessna 172 east and west over main street in Poughkeepsie advising drivers of any slow downs in the area.  That’s right, Poughkeepsie, New York, population 30,000, had it’s own eye in the sky, broadcasting live from the aircraft overhead.  Actually, I think he also flew up and down South Road (US Route 9) in the vicinity of the IBM plant, which employed quite a few people in those days.

There was also a guy who tried to break the Guinness Book of World Records by staying awake the longest, this happened several times.

For me, it was the school closings.  I hated school with an absolute passion.  Everyday, I would ride the school bus and say a little prayer; “…please God, make it today.  Make the boiler stop working, or the electricity to go out.  Make the kitchen catch on fire or the roof to cave in.  You are a great and mighty God and I don’t ask for much.  Please destroy my school today.”  Alas, God did not seem interested in this.

Anyway, back to the topic at hand.

WKIP first signed on in 1940 with the studios and transmitter located at The Nelson House, 42 Market Street, Poughkeepsie.  That building is long gone and  the location appears to be the parking lot for the Dutchess County Office building.  Being neighbors with some influential guy from Hyde Park made for a nice dedication speech:

It signed on with a power of 250 watts on 1,420 KC on June 6th, 1940. Soon thereafter, it changed frequency to 1,450 KC as a part of the AM band shift brought about by NARBA.

Over the years, the station went through several ownership changes. The first major technical change came in 1961, when the station transmitter site moved to it’s current location, then called Van Wagoner Road, now Tucker Drive. The station increased power to 1,000 Watts and installed a direction antenna for daytime use.  It is one of those rare night time non-directional, day time directional stations.

The directional antenna consists of two towers; tower one is 180 degrees tall (103.4 Meters or 340 feet) with 35 degrees of top loading.  That is used for both the day and night time array.  Tower two is 85 degrees tall (48.8 Meters or 160 feet) and is used only for the daytime array.  This pushes the major lobe of radiation towards the north.  I don’t know the reasoning behind that, but somebody spend a good amount of money to make it so.

Here is a air check from the early 1980’s.  Weather on that day was “Sunny, cloudy, whatever… take your pick.”

Good old Steve Diner.

Today, the station looks like this:

The 1961 WKIP transmitter building with tower

The 1961 WKIP transmitter building with tower

When I was growing up, my cousins lived within walking distance of this. We used to come over than throw rocks at the tower when the station was unmanned on Saturdays and Sundays. At least, I think it was unmanned because no one ever came out and yelled at us.

WKIP backup transmitter, phasor and main transmitter

WKIP backup transmitter, phasor and main transmitter

Mid 1980’s MW-1A still runs. The BE AM1A is the main transmitter. The phasor is the Original 1960’s Gates Phasor.

This video shows how the studios used to look, before they were rebuilt by Clear Channel Circa 2002 or so. At about the 2:02 mark, you will see the room pictured above as it looked in 1990.

The space between the video above and the picture below looked bad with nothing in it. It looks better now.

WKIP clock

WKIP clock

That clock is a collectors items and belongs in a museum.

Installing a satellite dish

This is a replacement dish for the Comtech dish destroyed in a downburst event a few weeks ago.  The first part of the job entailed placement of the new dish down on the ground.  The town code enforcement officer was much happier with this idea than mounting it up above roof level along back the building as the old one was.  Of course, this is possible due to the shift in satellites last year to AMC-18.

Finding a good spot on the radio station property was fairly easy.  The studio is located in a business district, thus the side yard requirements where zero feet, which is great.  The building inspector required that we dig a test hole to see what type of soil was there.  It turned out to be fill.  That required the footing design be changed somewhat and stamped by a licensed engineer.  Not a major problem.

Satellite mount pole, waiting pre-pour inspection

Satellite mount pole, waiting pre-pour inspection

The footing is 36 inches wide by 7 feet deep.

A little bit of water in the bottom of the hole

A little bit of water in the bottom of the hole

The mounting pipe has flanges welded to the side of it to prevent it from spinning in the concrete.

Footing poured and cured

Footing poured and cured

After the pour, we let the concrete set up over the weekend.

New dish bolted together

New dish bolted together

The dish is assembled and waiting for lift.  We used a back hoe to lift the dish onto the mounting pole, unfortunately, I was not able to take a picture as I was on a ladder attaching the dish to the pedestal with U-bolts.

Viking 1374-990 3.7 Meter R/O dish installed

Viking 1374-990 3.7 Meter R/O dish installed

Here it is installed and aimed at AMC-18. I used the Satellite Buddy, which makes the aiming job much easier. Once the signal is acquired, I like to peak the Eb/No on the West Wood One carrier, which seems to be the most sensitive to any type of change.

Viking 1374-990 3.7 Meter satellite dish, back view

Viking 1374-990 3.7 Meter satellite dish, back view

Register those C band satellite dishes!

UPDATE:The registration deadline has been extended to October 17th, 2018. Switch back to procrastination mode…

Satellite dishs, WABC transmitter site, Lodi, NJ

Unless you have been sleeping under a rock, you should already be aware of the FCC request to register the C band Receive Only (RO) satellite dishes. This development comes from the never ending drive for more bandwidth from the mobile phone/data networks (remember the desire to use GPS frequencies for mobile data a few years ago).  Normally, this type of registration would require a full frequency coordination study, however until July 18th, this requirement has been waived.  The registration is completed online with the filing of FCC form 312 and a $435.00 filing fee.  West Wood One has supplied and example form (.pdf) which shows the required information for each dish.  Schedule B of FCC form 312 requires quite a bit of technical information required for each dish:

  • Site Coordinates (must be NAD27 according to the instructions on the form)
  • Site elevation AMSL in meters
  • Dish height to top of dish in meters
  • Dish make and model number
  • Dish size
  • Dish mid band gain
  • Emission designator (WWO uses 36M0G7W other providers may be different)
  • Eastern and Western arc limits
  • Eastern and Western arc limit elevation angles
  • Eastern and Western arc limit azimuth angles

Most of this is intuitive.  There are several steps to getting the information in the correct format.  Google maps (or other mapping programs) will give coordinates in decimal format.  To convert to Degrees Minutes Seconds in NAD27 use NADCON.  Site elevation can be found using free map tools elevation finder.  To determine the arc, a smart phone app such as Satellite Finder or Dish Pointer can be used.  If not actually on site, then Dishpointer.com can be used to determine the arc.

My best suggestion is to include as much of the arc as possible for each location.  The future cannot be predicted with any degree of accuracy and it is entirely possible that the current satellite position may not be used forever.

Wind damage

This satellite dish nearly broke off of its mount during a “macroburst” event.  According to the National Weather Service:

A macroburst is a thunderstorm downdraft affecting an area at least 2.5 miles wide with peak winds lasting 5 to 20 minutes. The macroburst is a straight-line wind phenomena not associated with rotation…used to differentiate from tornadic
winds. Macrobursts can produce as much if not more damage as tornadoes due to the size and scope of a macroburst.

On May 15th a large group of severe thunderstorms triggered at least three tornadoes and one macroburst event in eastern New York and Western Connecticut.  Winds in the macroburst area were estimated to be in the 85 to 105 MPH range.

The next morning, it took a long time to get the the clients studio.  Trees where down everywhere, roads were closed, traffic lights not working, etc.  This created numerous detours and traffic jams.  When I finally arrived at a clients studio facility, this was the first thing I noticed:

Comtech 3.8 meter dish with broken mount

Comtech 3.8 meter dish with broken mount

That is an older 3.8 meter comtech dish hanging on by one 3/8 inch stainless steel U bolt.  The funny thing is, they did not complain about this or the lack of satellite service.  The main complaint was that the studios were on generator and some of the lights and air conditioners were not working.

Comtech 3/8 inch stainless U bolt holding up 650 pound dish

Comtech 3/8 inch stainless U bolt sheared off

Comtech 3.8 meter dish support bracket twisted

Comtech 3.8 meter dish support bracket twisted

This dish had originally been put up when AMC-8 was the main commercial radio network bird in the US.  The dish elevation was only 9 degrees above the horizon, so this had to be put up next to the building at roof top level to clear the trees and see 139W.

I was attempting to secure the dish but in the end, the 650 pound dish was too tenuous and the weather was still unstable.  There was other damage to the dish thus we decided to take it down instead.  Even that took a bit of doing.  We were trying find a crane or bucket truck, but all that type of equipment had been pressed into service with recovery efforts.  We finally undid all the bolts and bracing and fell it like a tree.

Comtech dish on the ground

Comtech dish on the ground

The dish was then cut up and put in the dumpster.

The new satellite dish will be installed next to building in a lower position.

AM station downgrade

I have been working on another formerly direction class B AM station, this one is in Rutland, VT.  WSYB has been on the air since 1931 with the same call letters serving the east central part of Vermont.  In 1931, it was operating on 1500 kc with 100 watts of power.  In March 1941 it moved to 1490 kc with 250 watts before settling, a few months later, on 1380 with 1,000 watts, directional night time protecting CKPC in Brantford, Ontario, Canada.

The transmitter site was first located at 80 West Street (now known as BUS US 4), in Rutland.  It was moved to its current Dorr Drive (Formerly Creek Road) location in 1938, when the station was requesting a power upgrade to 250 watts.  Whilst cleaning out the old transmitter building, a copy of an operating log, dated December 7, 1945 was discovered in the attic above the transmitter room:

WSYB transmitter log, 1945

Back from the time when readings were required every 30 minutes.

In 1956, WSYB was allowed 5,000 watts daytime non-directional with 1,000 watts night time directional.

At some point in the early 1990’s, the original towers were replaced with solid leg Pirod towers, each 195 feet tall.

After that, things went the way things do; AM steadily declined in favor of FM, local programming was mostly replaced by syndicated satellite stuff, there were several transfers of ownership, etc.

A translator on 100.1 MHz was added in 2016; the two bay Shively antenna was installed at the top of the South West tower.   There is local programming on the station from 6am to noon on weekdays.  There may also be some gardening shows and other such programming on weekends.

The current owner has decided, like they have done in other markets, that AM directional antenna systems are a maintenance nightmare, the risk of FCC sanctions are high for an out of tolerance antenna array, the ratings and income from the station do not justify the risk/cost.  Thus, non-directional night time operation was applied for and granted.  The station is now a Class D with 25 ass kickin’ night time watts.

WSYB had a two tower night time antenna system.  The tower closest to the building (SW) was also the daytime, non-directional tower and it now holds the FM translator antenna and STL antenna.  Thus, it was decided to ground that tower and keep those antennas in service.  The far tower (NE), which was the second tower of the night time array would become the AM antenna.  The night time ATU was built for less than 1,000 watts input power, so several components needed to be upgraded for 5,000 watt operation.

WSYB rebuilt ATU

WSYB rebuilt ATU

I had available these nice vacuum capacitors that came out of another decommissioned antenna system.  The vacuum capacitors are great because the voltage/current ratings are much higher than the mica capacitors that were in the circuit before.  You can see black goop where one of the Sangamo mica capacitors on the input leg failed several years ago.  These vacuum capacitors are rated at 15 KV and the current rating at 1.38 MHz is probably in the 70-80 amp range.  I had to move the base current meter from the former daytime (SW) tower out to the NE tower.  The day night switch was taken out of the circuit.  The transmission line to the far tower was replace with 7/8 inch foam dielectric cable.  A slight touch up of the coil on the input leg of the T network was all that was required to bring it into tune.

The electric lines to the tower have been temporarily disconnected.  As soon as they are reconnected, I will vacuum out all the mouse crap and other debris.  The ATU building also needs some work sealing in up against the elements.

The tower base impedance is 75 ohms, +j95 making the base current 8.6 amps daytime and 0.58 amps night time.

WSYB radiating element

WSYB radiating element

For me, the magic of radio exists at that boundary between the real objects (towers and antennas) and the ether.  The transference of electrical voltages and currents into the magnetosphere is something that still fascinates me to this day.  Coupling a 5,000 watt medium wave transmitter to a tower and watching it work is something that I will never grow tired of.

Fixing another AM station’s antenna system

I have done several of these posts in the past, but it always seems to be of some interest, so it bears repeating.  AM antenna systems are not black magic.  They are actually pretty easy to understand if the fundamental knowledge is in place.  Medium Wave frequency wavelengths are fairly large compared to other broadcast frequencies.  Thus, the components are larger.

The three basic components of an AM antenna system are the tower, the ATU (antenna tuning unit) and the transmission line (AKA Coax).  The tower is the radiating element and they come in a variety of flavors; uniform cross section guyed, self supporting, series excited, shunt excited, etc.   A series excited tower has a base insulator and is fed directly from the ATU.  A shunt excited tower has a grounded base and uses a skirt or folded monopole design to transfer the RF to the main radiating element.  This design has an advantage as the tower can be used for other wireless and broadcast services.

The antenna work in question for this project is WINE, 940 KHz, Brookfield, CT.  The skirted tower is used for WRKI.  It also has two way and cellular clients.  The issue is instability of the WINE antenna system, which is likely due to improperly attached shorting wires between the skirt at the tower.  Over the years, the impedance of the skirt has gone way up.  The tower itself is 152.1 meters (499 feet) tall, or 170.3 electrical degrees.  The skirt length is about 82 electrical degrees and it is shorted at about 72 degrees.  There have been several papers written about folded monopoles for Medium Frequency (AKA AM or Standard) broadcast service.  The recommendations state that for best performance, the short to the tower should be between 62 and 90 electrical degrees.  Since the existing system falls in that range, there must be other problems with the antenna skirt and or shorting wire to the tower.

WINE skirted tower diagram

WINE skirted tower diagram

If one looks at this diagram, that configuration should look something like a gamma match, often used on dipole and yagi type antennas.  A gamma match can be thought of as a stub of transmission line which is bonded to the radiating element at some favorable wave length corresponding to the desired radiation resistance.  This is one of several configurations for folded monopole antennas and this type is most often seen on towers that support other wireless service antennas such as cellular and two way systems which are installed above the skirt.

There are a few interesting data points when looking at these type of antennas.  First is the ratio of the diameter of the skirt over the height of the tower, or D/H.  The larger this ratio is, the better the bandwidth characteristics of the antenna system are.  This makes sense, when you think about it. In this instance, the tower is 151 meters (495.4 feet) tall and the skirt is 3.3 meters (10.83 feet) wide, thus the ratio is 0.0218.

The licensed base impedance if 234 ohms with a good amount of inductive reactance. When Sprint and T-mobile changed their configuration on the tower, that impedance shifted dramatically.  The existing skirt is in fairly rough condition.  The bottom ring that connects to the ATU is made out of copper tubing.  It is attached to the skirt wires with steel saddle clamps, all are rusted and all of which are lose and can slide around.  At some point, the tubing filled up with water, then froze causing the tubing to split open.  At the top of the skirt, the jumper wire looks suspicious and the top ring does not go all the way around. The shorting stub to the tower looks like it is made out of battery jumper cable.  I purchased new cross wire clamps and found some spare copper weld skirt wire at another site.  Both the bottom ring and top ring were replaced as well as the shorting stub to the tower.

After the repair work was done, I had the tower crew reattach the short slightly below the last skirt to tower bonding point.  In that position, I found the impedance went way up.  Thus, going lower was going towards a resonance point.  I had them move the short up to the former shorting point and remeasured and found the impedance was 235 ohms, only 1 ohm off from the previously licensed values.

Initially, I thought it would be nice to find a better position for the shorting stub and get a lower base impedance.  This would make the whole antenna system work better (improve bandwidth, stability, etc).  However, there was a set of guy wires above the bonding point.  The tower crew would have had to disassemble the top ring to move above the guy wires.  We were running out of daylight and weather so I had them lock everything down where it was.  On a station running an all sports format that has no listeners and does not make any money, it does not make a lot of sense to spend gobs of money and time to rebuild the ATU for a new base impedance.  When I got the impedance back to within 0.11% of the licensed values, it was time to declare victory and go home.

Working with rigid transmission line

Installing transmitters requires a multitude of skills; understanding the electrical code, basic wiring, RF theory and even aesthetics play some part in a good installation.  Working with rigid transmission line is a bit like working with plumbing (and is often called that). Rigid transmission line is often used within the transmitter plant to connect to a four port coax switch, test load, backup transmitter and so on.  Sometimes it is used outside to go up the tower to the antenna, however, such use has been mostly supplanted by Heliax type flexible coax.

We completed  a moderate upgrade to a station in Albany; installing a coax switch, test load and backup transmitter.  I thought it would be interesting to document the rigid line work required to complete this installation.  The TPO at this installation is about 5.5 KW including the HD carriers.  The backup transmitter is a Nautel VS-1, analog only.

This site uses 1 5/8 inch transmission line.  That line is good for most installation up to about 10-15 Kilowatts TPO.  Beyond that, 3 inch line should be used for TPO’s up to about 30 Kilowatts or so.  Even though the transmission lines themselves are rated to handle much more power, often times reflected power will create nodes along the line where the forward power and reflected power are in phase.  This can create hot spots and if the reflected power gets high enough, flash overs.

Milwaukee portable band saw

Milwaukee portable band saw

Working with rigid line requires a little bit of patience, careful measurements and some special tools.  Since the line itself is expensive and the transmission line lengthener has yet to be invented, I tend to use the “measure twice and cut once” methodology.   For cutting, I have this nice portable band saw and table.  This particular tool has saved me hours if not days of work at various sites.  I have used it to cut not just coaxial line and cables, but unistrut, threaded rod, copper pipe, coolant line, conduit, wire trays, etc.  If you are doing any type of metal work that involves cutting, this tool is highly recommended.

Milwaukee 6230N Band Saw with cutting table

Milwaukee 6230N Band Saw with cutting table

Next point is how long to cut the line pieces and still accommodate field flanges and inter-bay line anchors (AKA bullets)?  The inner conductor is always going to be sorter than the outer conductor by some amount.   Below is a chart with the dimensions of various types of rigid coaxial cables.

Length cut chart for various sizes of rigid coaxial cables

When working with 1 5/8 inch rigid coax, for example, the outer conductor is cut 0.187 inches (0.47 cm) shorter than measured distance to accommodate the field flange. The inner conductor is cut 0.438 inches (1.11 cm) shorter (dimension “D” in the above diagram) than the outer conductor to accommodate the inter bay anchors. These are per side, so the inner conductor will actually be 0.876 inches (2.22 cm) shorter than the outer conductor.  Incidentally, I find it is easier to work in metric as it is much easier to measure out 2.22 CM than to try and convert 0.876 inches to some fraction commonly found on a tape measure.  For this reason, I always have a metric ruler in my tool kit.

Altronic air cooled 20 KW test load

1 5/8 inch rigid coax run to Altronic air cooled 20 KW test load

1 5/8 inch rigid coax and 4 port coax switch mounted in top of Middle Atlantic Rack

1 5/8 inch rigid coax and 4 port coax switch mounted in top of Middle Atlantic Rack

The next step is de-burring.  This is really critical at high power levels.  I use a copper de-burring tool commonly used by plumbers and electricians.  One could also use a round or rat tail file to de-bur.  The grace of clamp on field flanges is they have some small amount of play in how far onto the rigid line they are clamped.  This can be used to offset any small measurement errors and make the installation look good.