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Emergency transmitter replacement

Bad weather or other disasters can strike any time of year.  Around these parts, the most dangerous weather events occur from early spring through late summer.  In the past twenty years or so, we have had tornadoes, hurricanes, micro bursts, flooding events and so on.  All of that got me thinking about what would happen if a tower came down, or a transmitter building was destroyed by fire, wind, water, etc.

If past events can predict future performance, there would ensue a mad scramble to replace damaged equipment and or get some type of temporary antenna into service.  That is what happened in great City of North Adams, Massachusetts when the tower that held the cell carriers, the 911 dispatch, and the local FM radio station came down in an ice storm.  Fortunately, we had a single bay Shively antenna at the shop that we trimmed up and installed on a temporary pole with 200 watts TPO.

That will cover the city of license, provided there is electricity…

What if there where an event that was so devastating that the electrical power would not be restored for months?  Think about hurricane Maria in Puerto Rico.   After that event, the infrastructure was so devastated that there was not even the possibility of getting a fuel truck to deliver diesel for the emergency generators at the hospital in San Juan.  It can happen.

With that in mind, I began poking around and thinking about how I would get something back on the air.  In the face of massive disasters, AM and FM radio is still the most effective way to communicate with the general public.  Radios are still ubiquitous in homes, cars and businesses.

Bext 30 watt exciter

Bext 30 Watt FM exciter

In a short period of time I came up with a couple of solutions.  First, the frequency agile Bext exciter uses a single solid state rectifier feeding 24 volts to the power supply board.  The audio input includes a mono balanced line level input which can be fed by a computer sound card or some other simple source.

Bext 30 Watt FM exciter power supply

Bext 30 Watt FM exciter power supply

From there +12, +15 and +20 VDC are created to run various circuits.  The heat sink cooling fan is the only thing that runs on 120 VAC, which is old and I might replace with a 24 VDC unit.

Bext 30 Watt exciter power supply voltage

Bext 30 Watt exciter power supply voltage

The power output is about 22 watts, which is not bad.  That will certainly get out well enough from a high spot and provide good coverage when the power is out because all the other in band RF noise generators will be off.

6 volt, 435 Ah batteries

6 volt, 435 Ah batteries

Then I though about the deep cycle batteries in my barn.  These 6 volt, 435 Ah units have been around for a couple of years, but last I checked, they still held a charge.  Other deep cycle batteries from things like golf carts, fork lifts, campers, boats etc could also be pressed into service.  The point is, 24 VDC should not be impossible to create.

To keep a charge on the batteries, this solar panel will work:

225 Watt, 36 volt solar panel

225 Watt, 36 volt solar panel

This setup would require some sort of 24 volt DC charge controller, which I found on Amazon for less than $15.00 US.  This charge controller has selectable 24/12 VDC output and also has two USB ports which would be handy for charging hand held devices.

I measured the power draw while the exciter was running 20 watts into a dummy load, it draws 120 Watts.

The final part would be some sort of antenna with transmission line.  For this situation, a simple wire center fed dipole hung vertically would work well.  This can be fabricated with two pieces of copper wire and a few insulators.

Simple dipole antenna

Simple dipole antenna

The lengths of each wire can be calculated as follows:

Approximate length in feet: 234/f (MHz)

Approximate length in inches: 2808/ f (MHz)

Approximate length in cm: 7132/f (MHz)

For the FM band, maximum length of wires needed will be 32 inches (81 cm).  Insulators can be made of anything that does not conduct RF; PVC, ABS, dry wood, dry poly rope, etc.

Emergency FM band dipole

Emergency FM band dipole, cut to 88 MHz, lowest FM frequency

I recommend to cut the wires slightly long, then trim little bits off of each end while watching the reflected power meter on the exciter.  To keep RF from coming back down the shield of the transmission line, make 8-10 turns, 6-8 inches in diameter of coax as close to the antenna as possible and secure with a wire tie.  This will create a balun of sorts.

My emergency FM kit consists of:

  • Bext Frequency agile exciter
  • 30 feet, RG-8 coax with N male connector on one end
  • 4 ten foot RG-58 BNC male jumpers
  • 1 four foot LMR-400 N male jumper
  • Dipole antenna, cut long
  • Solar charge controller
  • Small basic tool kit; hand tools, plus DVM and soldering iron
  • Power cords, extension cords
  • 300 watt 12VDC to 120VAC inverter (pure sine wave)
  • 20 feet audio wire
  • Various audio connectors; spade lugs, XLR male and female, RCA, 1/4 TRS, etc
  • Various RF connectors; PL-259, N, BNC, etc
  • Bag of 12 inch wire ties
  • 3 rolls of 3M Scotch 88 electrical tape
  • 100 feet of 3/8 inch poly rope

This is all kept in a sturdy plastic storage bin from the Home Depot.  If needed, the batteries and solar panel are stored in the barn along with an assortment of other goodies.

Will it ever be needed?  Well,  I hope not.  However, it is much better to be prepared to restore services than wait for somebody to show up and help.  Sitting around complaining about the government does not relieve those people in need during and after a disaster.

The Temporary AM antenna

One of those things that I have written about before, but seems to be common these days as older AM towers need to be replaced. One of our clients had just such a tower. Erected in 1960, the hollow leg stainless tower was rusting from the inside out. When the tower crew came to put up the translator antenna, they discovered that there was a hole in one of the legs and climbed back down.

The tower condition was somewhat known about, there were braces installed several years ago at certain levels to keep the tower standing. The new owner had planned to replace the tower eventually, so those plans where moved ahead.

Temporary Wire antenna, WKNY, Kingston, NY
Temporary Wire antenna, WKNY, Kingston, NY

A temporary utility pole was installed near the transmitter building and a wire was strung to another customer owned pole about 170 feet away. At 1,490 KHz, that proved to be a pretty good length. The issue with these medium wave temporary antennas is always the height above ground. In order for the radiation resistance to be somewhat reasonable, the antenna needs to be at least 1/8 to 1/4 wave length above ground. That means a minimum of 78 to 157 feet at 1,490 KHz. The utility pole installed is 35 feet AGL.

WKNY temporary ATU

Thus, the wire antenna has a fairly low resistance, with loads of inductive reactance. Something on the order of 20 ohms, +j480. Since this is temporary, we reused the existing ATU that was designed for the series excited tower. With a capacitor installed on the incoming wire to cancel out some of the inductive reactance, a simple T network was configured to match the 50 ohm transmitter output to the 20 ohm antenna.

In the end, we were able to run about 400 watts into the wire, which covered the city of license fairly well. While the new tower was being erected nearby, we had to reduce that to about 100 watts to protect the tower workers from the hazards of non-ionizing radiation.

WKNY new tower build

The new replacement tower has been constructed. It is the exact same height as the old tower, but has a twenty foot pole on top instead of a normal tower section. The pole was installed to mount the translator antenna. In addition to that, there will be other wireless services installed on this tower.

WKNY will have a six wire skirt installed in the next few days. As this tower is close to 160 degrees at 1,490 KHz, the skirt can go anywhere from 60 to 120 degrees up the tower.

Differential Audio

Most professional audio facilities use differential audio or balanced audio within their plants.  The main reason for this is noise rejection, which was discovered by the early pioneers of wired telephony back in the late 1800’s.  Balanced audio is created by generating two audio signals that are 180 degrees out of phase using either a transformer or an active device.  These are usually labelled High and Low, + and – or something similar.  Those two audio signals are then transmitted across some distance and recombined at the far end, again by a transformer or some active device.

Noise rejection, differential signaling. "DiffSignaling" by Linear77 - Own work. Licensed under CC BY 3.0 via Wikimedia

Noise rejection, differential signaling. “DiffSignaling” by Linear77 – Own work. Licensed under CC BY 3.0 via Wikimedia

When an interfering signal is picked up, it is transmitted along both sides of the balanced audio circuit until the signals are recombined.  During the re-combining process, and common mode interference is cancelled out, as it becomes 180 degrees out of phase with itself during the re-combining process.

Differential signaling is used in analog audio, digital audio (AES/EBU), HDMI, Display Port, USB, Ethernet, POTS lines, ISDN, T-1/DS-1, E-1, etc.   It is a fairly simple concept, but one of the basic building blocks in broadcast studios.

When a studio project was completed at a disused studio/transmitter site location, a certain amount of RFI was being induced on the studio microphones by the unassociated FM transmitter in the next room.  The problem with microphone level audio is the relatively low level of  microphone output, which requires a good deal of amplification.  The amplifiers in this console have active balanced inputs, which might not be exactly 180 degrees out of phase.  In this installation, microphone level audio was run about 20-25 feet on standard microphone cable then it was converted to Cat 6 cable before going into the console.  It may have been better to use the shielded Cat 6 cable for the longer runs as it likely has better common mode rejection than standard mic cable. Another option might have been Star Quad cable.  However, none of those things were done.

Western Electric was the manufacturing arm of Bell Telephone.  In their day, they made some really good equipment.  One such piece is the WE-111C repeat coil.  These can be configured for either 600/600 ohms, 600/150 ohms, 150/150 ohms,  or 300/300/300/300 ohms impedance ratios.  Since this is microphone level audio 150/150 ohms is the appropriate setting.

WE 111 repeat coil, one of the best such transformers ever made

Over the years, I have found many of these transformers discarded at various transmitter sites and studios. There are five microphones feeding this console. I mounted five of these coils in a sturdy metal enclosure and wired them with RJ-45 jacks to be compatible with the Studio Hub wiring equipment used in this studio installation.  I also grounded each unit to a piece of copper strap, which is connected to a grounding lug on the side of the unit.

Western Electric 111C repeat coils mounted in box

Western Electric 111C repeat coils mounted in box

I swept the coils from 20Hz to 20kHz:

WE 111C coils, 20Hz sweep

WE 111C coils, 20Hz sweep

WE 111C coil 20kHz sweep

WE 111C coil 20kHz sweep

This shows a 0.4 dB difference from 20 to 20,000 Hertz, thus they are all nearly flat which is a pretty cool feat of engineering.  I would estimate the age of these transformers is between 50 to 60 years old.

These coils isolate each microphone from the microphone preamp in the console.  This completely eliminated the FM RFI and solved the problem.

The Answer to Ailing Copper

I don’t know how things are in your neck of the woods, but here in the Northeastern US, our old copper TELCO networks are on their way out.  This is a problem for broadcasters who still rely on POTS lines (Plain Old Telephone Service) for transmitter remote controls, studio hot lines, etc.  The vast majority of my transmitter site access is through dial up remote controls.  There are a few locations that have web based remote controls, but to be honest; the phone part of my smart phone still gets a lot of use.  There are several locations where the old copper is just failing outright and not through a lack of effort by the repair techs.  Generally, the copper pairs get wet and develop a loud hum, which makes the remote control unit either hang up or become unresponsive to touch tone commands.

The best course of action is to get some type of VOIP line installed.  Here is the rub; many transmitter sites are nowhere near a cable system.  Several times, I have contacted the cable company to see if they will provide a VOIP phone line at a certain site.  The response is usually; sure, we can do that!  However, it will cost  you (insert some ridiculous amount of money) to extend the cable to your transmitter site.

LAN extensions to the transmitter site are a useful for a number of reasons.  More and more transmitters are equipped with web interfaces as are processors, UPSs, transmitter remote controls, security cameras,  etc.  LAN extensions can also be used for backup audio in case of STL failure.  Finally,  an inexpensive ATA (Analog Telephone Adaptor) and DID line can replace a POTS line for a lot less money.  One example; voip.ms has the following plans as of this writing:

Plan type Per month per DID number (USD) Incoming call rate (USD) per minute Outgoing call rate (USD) per minute
Per minute $0.85 $0.01 (USA) $0.009
Unlimited $4.25 $0.00 $0.009
Toll Free (800) $0.99 $0.019 $0.009

Any of those plans surely beats the standard TELCO rate of $40-50 per month per line.

Design criteria for a wireless LAN system needs to take into account bandwidth, latency and reliability.  Each VOIP phone call takes anywhere from 28-87 Kbps depending on the protocol being used.  If the wireless LAN is being used for other things such as back up STL service, access to various GUI’s, etc then the total bandwidth of all those services need to be considered as well.  Do not forget ethernet broadcast traffic such as DHCP requests, ARP, SMB, etc which can also take up a fair amount of bandwidth.

For LAN extensions, I have been using a variety of equipment.  The older Moseley 900 MHz LAN links still work, but are slow in general.  The Ubiquiti gear has proven to be both inexpensive yet reliable, a rarity to be sure.  There are several links to various transmitter sites running on various types of Ubiquiti gear, usually without problem.  One simply needs to remember to log into the web interface once in a while and make sure that both ends have all the firmware updates installed.  They are cheap enough that a couple of spares can be kept on the shelf.

The following diagram shows how I replaced all of the copper pots lines at various transmitter sites with VOIP:

Diagram of LAN extensions to various transmitter sites

Diagram of LAN extensions to various transmitter sites

List of equipment:

Nomenclature Amount Use New or used
Ubiquiti Rocket M5 3 AP and station units New
Ubiquiti AirMax 5G-2090 90 degree sector antenna 1 AP point to multi-point antenna New
Ubiquiti Rocket Dish 5G-30 2 Station antennas New
Ubiquiti ETH-SP-G2 3 Lightning protection New
Trastector ALPU PTP INJ 6 Lightning protection out door units New
Cambium PTP-250 2 Point to Point link Existing/Used
Motorola Canopy 900DA PCDD 1 AP point to multi point Existing/Used
Motorola Canopy 900DA PCDD 2 Station Existing/Used
Microwave Filter #18486 diplexer 3 Diplexer 900 MHz ISM band and 944-952 STL band Existing/Used
Cisco SPA122 ATA 9 Dial tone for remote controls New

The main studio location has the gateway to the outside world. This system is on a separate subnet from the automation and office networks. From that location a point-to-multipoint system connects to the three closest transmitter sites.  This setup uses the Ubiquiti Rocket M5’s with various antenna configurations.  Then, from one FM transmitter site, there is an existing 5.8 GHz path to another set of transmitter sites.  This uses Cambium PTP-250s.

The next hop rides on the STL system, using Motorola Canopy 900 MHz radios and Microwave Filter Company #18486 dilpexers.  These are long paths and the 900 MHz systems work well enough for this purpose.  The main cost savings comes from reusing the existing STL system antennas which negates the cost of tower crews to put up new antennas and or rent on the tower for another antenna.

There is a smaller sub system many miles away that is connected to the outside world through the cable company at the AM transmitter site.  Unfortunately, due to the distances between the main studio and those three stations, there was no line of site shots to these sites available on any frequency.

When installing the 5.8 GHz systems, I made sure to use the UV rated, shielded cable, shielded RJ-45 connectors and Lightning Protection Units (LPUs).  Short cuts taken when installing this equipment eventually come back in the form of downed links and radio heads destroyed by lightning.

Regardless, I was able to eliminate seven POTS phone lines plus extended dial tone service to two sites that previously did not have it before.  In addition to that, all of the transmitter sites now have Internet access, which can be useful for other reasons.  All in all, the cost savings is about $310.00 per month or $3,720.00 per year.

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 97.9 MHz + 96.9 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

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.

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.

Engineering Radio; the satellite reaiming tour 2017!

As previously discussed, the migration from AMC-8 to AMC-18 is in full swing. There is less than two weeks left to complete the re-aiming process.  All totalled, we have 24 of these things to re-point and all but two of them  are done.  Toward that end, I have this down to an art:

  • Go inside and make a note of the signal strength on the satellite receivers on AMC-8
  • Look up the elevation angle on dish align app for AMC-8 then compare that to what the inclinometer reads, note the difference between the calculated and actual readings
  • Look up the elevation angle on the dish align app for AMC-18, apply the difference noted above to the final value
  • Connect the XR-3 satellite aiming tool to the LNB, make sure LNB power is on and the unit is set to AMC-18, C-band
  • Elevate the dish to the AMC-18 final elevation angle calculated above
  • Note the azimuth on the dish align app, look at the satellite picture and pick out a land mark.  Swing the dish towards the land mark
  • As you start to see signal from various satellites, swing more slowly.  If the elevation angle is set correctly, when the dish passes AMC-18 at 105 degrees W, the XR-3 will lock on
  • Peak the signal (azimuth and elevation)
  • Rotate the LNB feed horn for maximum signal to noise ratio
  • Go inside, check satellite receivers, reprogram carrier frequencies as necessary

It is pretty easy. I can do the whole thing in about thirty minutes if there are no rusted bolts, etc. I wonder how many small station owners will wake up on July 1st with no satellite programming?

Comtech Satellite dishes, WABC transmitter site, Lodi, NJ

The Applied Instruments XR-3 (XR-S2ACM-01) VSAT-ACM  satellite signal meter with AMC-18 locked.  This hand held tool is great and makes aiming any dish a snap.  As the sky around AMC-18 is a wee bit crowded, it is easy to mistakenly find the wrong satellite.  With the Identify function, the satellite the dish is aimed to will be displayed, then the dish can be adjusted accordingly to the correct bird.

Applied Instruments XR-3 satellite signal meter

There are many different flavors of dishes; Comtech, Patriot, Prodelin, etc.

Prodelin 3.7 Meter satellite dish

These are all about the same to work with, the only difference is the degree of rust and deterioration on the mounting hardware, the age of the LNB and number of bees nests that need to be removed.

Axiom


A pessimist sees the glass as half empty. An optimist sees the glass as half full. The engineer sees the glass as twice the size it needs to be.

Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
~1st amendment to the United States Constitution

Any society that would give up a little liberty to gain a little security will deserve neither and lose both.
~Benjamin Franklin

The individual has always had to struggle to keep from being overwhelmed by the tribe. To be your own man is hard business. If you try it, you will be lonely often, and sometimes frightened. But no price is too high to pay for the privilege of owning yourself.
~Rudyard Kipling

Everyone has the right to freedom of opinion and expression; this right includes the freedom to hold opinions without interference and to seek, receive and impart information and ideas through any media and regardless of frontiers
~Universal Declaration Of Human Rights, Article 19

...radio was discovered, and not invented, and that these frequencies and principles were always in existence long before man was aware of them. Therefore, no one owns them. They are there as free as sunlight, which is a higher frequency form of the same energy.
~Alan Weiner

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