Apparently, this coaxial cable has a hot spot:
7/8 inch air dielectric coax with jacket melting off
The back story:
I received a text this morning that one of our clients station “had a lot of static on it, it might be off the air.” Upon arrival, I found the Nautel VS2.5 transmitter with 0 watts forward power and an output network fault. Reset the transmitter and the forward power and reflected power increased together, triggering another output network fault. I was able to turn the transmitter power down to 100 watts, at which point it stayed on, with 50 watts reflected power. I also noted the dehydrator running continuously and 0 PSI line pressure.
I wandered around the back of the building where the coax goes out to the tower and discovered the dripping plastic from the melted jacket. I reached up and first checked the cable to see if it was warm (it was not). Then I shook it and heard what I thought was water sloshing around inside. This is the original Andrew 7/8 inch cable from when the station signed on in 1972 or so. Very likely that further up the tower, something has chaffed through the outer jacket and shield, allowing water into the cable.
I drilled a small 5/32 inch hole at the lowest point in the cable before it enters the building. The result was a steady stream of water, which was aided by some additional pressure from a spare N2 tank. I let it drain while I ran down to town and got some lunch. I came back a half an hour later, turned the transmitter on and was satisfied to see 100 watts forward power with 1 watt reflected. I ran the transmitter up to full power for a while, then deciding discretion is the better part of valour, turned it down to half power; 820 watts which nets 8 watts reflected power.
Needless to say, the transmission line needs to be replaced as soon as possible.
It has been a year and a half since the tower collapse in North Adams, Massachusetts. Since that time, WUPE-FM (Gamma Broadcasting), WNNI and W266AW (New England Public Radio) have been operating with STAs at lower than licensed power. We have completed the installation of the combined antenna, filters and combiners and now all stations are back to full power. Here are a few pictures of the transmitter room:
WUPE-FM and WNNI transmitter racks, North Adams, MA
WUPE-FM (left hand rack) is using a Crown FM-2000 transmitter, loafing along at 1,060 watts. WNNI (right hand rack) is using a Gates Air Flexiva 2 running at 1,650 watts. Those stations are combined with a Shively Combiner:
Shively 2 way star junction combiner
We are still doing some grounding and neatening work behind the racks:
The Shively versa tune antennas that were mounted to the wooden utility pole as emergency antennas will be retained as backup antennas for both stations.
Transmitters for WUPE, WNNI and W266AW
We share the room with Access Plus, which is a wireless internet service provider in western Massachusetts. There stuff is in the open frame racks to the right of WNNI.
Transmitter racks for WUPE-FM, WNNI and W266AW
TL;DR: Tower collaspes, facility is rebuilt better than before.
Hurricane season is here. This time of year makes me fondly remember hurricanes of the past and the things we had to do to get stations back on the air; walking a mile down a sandy spit of land, wading through swamp water to get to the transmitter shack, being threatened with arrest by the Connecticut National Guard, blow drying RF modules with a hair dryer, sleeping in a camper for a week… Ahhhh, good times, great times!
The one thing that I did learn, if the disaster is big enough, expect none of the normal services to be functioning. That includes things like gas stations, fuel delivery, grocery stores, restaurants, hotels, UPS, roads, bridges, telephone service, internet service, etc.
It is not a far fetched scenario for the main FM transmitter site to be out of commission and will not be available or accessible for some prolonged period of time. There might also be mitigating circumstances such as catastrophic tower failure, destruction of transmitter building, flooding, or other major infrastructure disruptions. In those situations, calling the broadcast supply vendor of choice for a replacement might not be an option.
It has happened before…
All of these things got me to thinking about how to fabricate a reliable FM broadcast antenna from simple materials available on hand. The FCC allows for temporary operation with an emergency antenna in part 73.1680, which reads:
(a) An emergency antenna is one that is erected for temporary use after the authorized main and auxiliary antennas are damaged and cannot be used.
(b) Prior authority from the FCC is not required by licensees and permittees to erect and commence operations using an emergency antenna to restore program service to the public. However, an informal letter request to continue operation with the emergency antenna must be made within 24 hours to the FCC in Washington, DC, Attention: Audio Division (radio) or Video Division (television), Media Bureau, within 24 hours after commencement of its use. The request is to include a description of the damage to the authorized antenna, a description of the emergency antenna, and the station operating power with the emergency antenna.
(1) AM stations. AM stations may use a horizontal or vertical wire or a nondirectional vertical element of a directional antenna as an emergency antenna. AM stations using an emergency nondirectional antenna or a horizontal or vertical wire pursuant to this section, in lieu or authorized directional facilities, shall operate with power reduced to 25% or less of the nominal licensed power, or, a higher power, not exceeding licensed power, while insuring that the radiated filed strength does not exceed that authorized in any given azimuth for the corresponding hours of directional operation.
(2) FM, TV and Class A TV stations. FM, TV and Class A TV stations may erect any suitable radiator, or use operable sections of the authorized antenna(s) as an emergency antenna.
(c) The FCC may prescribe the output power, radiation limits, or other operating conditions when using an emergency antenna, and emergency antenna authorizations may be modified or terminated in the event harmful interference is caused to other stations or services by the use of an emergency antenna.
In this situation, making a circularly polarized antenna would be overly complicated, so either a horizontally or vertically polarized antenna would be the most likely scenario. There are a few antenna types that readily lend themselves to field expedient fabrication.
These are, in no particular order:
Of these, the 1/2 wave wire dipole is the easiest to construct. Cut two wires, length (in feet) determined by the formula 234/Frequency (Mhz). Attach one wire to the center conductor and one to the shield, stretch to the wires out and tune for minimum SWR by cutting or adding small lengths to the ends. The total length for such an antenna would be approximately five feet and it could be mounted horizontally or vertically. The issue with a wire dipole would be bandwidth and power handling capability.
A 1/2 wave dipole made from tubing would have better bandwidth and power handling, but tubing is a little harder to work with when it comes to tuning the antenna.
Frankly, if one is going to go through the trouble of using tubing to create an emergency antenna, the the J-Pole (end fed antenna with a 1/4 wave matching section) is probably the best. This antenna is easier to tune, does not need to work against a ground plain, has good bandwidth and a low take off angle, meaning more power is radiated out toward the horizon, giving it a good deal of gain over both a ground plane or dipole antenna. Additionally, when using standard RG-8, RG-214, LMR-400 or other similar transmission line, a well matched antenna might be able to accept about 1 KW of input power, which would net approximately 4.4 KW ERP. Not an insignificant sum, especially in an emergency situation.
Vertical radiation pattern for J-pole (1/2 wave end fed) antenna
1/4 wave ground plane vertical radiation pattern
There are many J-Pole antenna calculators available on line, but many of them include a 20 inch or so section of tubing below the tuning stub that can be electrically coupled to the supporting structure. This configuration defeats the main advantage of the antenna, creating a good deal of upward radiation. It is a better idea to use a non-conductive support piece and keep any conductive materials at least 1/2 wave length or greater from the radiating portion of the antenna.
The basic j-pole antenna looks like this:
J Pole (1/2 wave vertical antenna) diagram
The radiating part of the antenna starts above the tuning stub. Basically, the 1/4 wave stub is shorted at the bottom, the feed point is adjusted away from the shorted end until a 50 ohm impedance point is found. The center conductor of the coax is attached to the 3/4 wavelength section, while the shield is connected to the stub. The critical distances are the tuning stub length and the distance of the feed point from the shorting section. I created an excel spreadsheet (.xls) that can be used to create all the lengths required to fabricate one of these antennas. That spreadsheet can be had here: J Pole Calculator
Having a few moments of time to spare, I thought it would be fun to build one of these and put the analyzer to it. I think testing things in the real world is a good exercise and I always enjoy working with antennas anyway. Looking in the basement, I found some 3/4 inch copper tubing, a tee, an elbow and a few end caps. The complete list of parts is thus:
|¾ copper tubing
||78-96 inches (196-244 cm) (frequency dependent)
|¾ copper tubing
||26-32 inches (66-82 cm) (frequency dependent)
|¾ copper tubing
||2.5-3 inches (6.35-7.62 cm) (frequency dependent)
||Tuning stub short
|¾ copper tubing
||2 inches (5.08 cm)
||Mounting section, bottom of T to MIP threaded adaptor
|¾ copper T section
||T section for joining main section to tuning stub
|¾ copper 90 elbow
|¾ copper end cap
||End cap on tubing
|¾ to 1 inch copper MIP threaded adaptor
|1 inch PVC FPT threaded adaptor
||Insulating mounting connection
|1 inch PVC
||Approximately 20-25 inches (50-65 cm)
||Insulating mounting material
|1 inch stainless steel hose clamps
||Attaching the coax to the antenna feed point
|RG-8, RG-214, LMR-400 or other transmission line
||As needed, including 5-6 turns, six inches in diameter to form RF choke at feedpoint
||RF choke needed to keep RF off of coax shield
One important detail to remember when using the above spreadsheet, the measurements are to the closest side and not the center. Thus, if something measures 2.5 inches, it is metal to metal. Some basic soldering skills are required, but assembly is relatively straight forward. In a pinch, almost any conductive material could be used including aluminum, brass, steel, EMT, rigid conduit, or even iron pipe.
Parts cut to size for J-pole antenna on 87.9 MHz
J-pole antenna assembled
J-pole antenna on the antenna testing range
I made this particular J-pole antenna on 87.9 MHz because I didn’t feel like chopping up all my 3/4 inch tubing. Cutting and soldering the tubing took about a half an hour. Designing and fabricating the feed point system another half an hour. I’ll throw another hour in for rounding up the parts, tools, etc. Thus, entire antenna was constructed in about two hours. I used my AIM 4170D to find the proper feed point. If I were going to actually use this antenna, it would then be a matter of finding a mounting location and running the transmission line.
J-pole antenna analysis results
Actually, I was less than happy with this. While the antenna is nice and broad across several channels, there is 16 ohms inductive reactance that is impossible to get rid of. That gives an SWR of 1.4:1, which is not great. With that kind of load, I would be reluctant to run more than a couple of hundred watts into this antenna. The interesting thing is, that graph is the first one, with everything set as calculated in the spreadsheet. After that, I could make the impedance and reactance worse, but not better.
Still, in a pinch, I would use this antenna until something better could be found.
As promised, a picture of the feed point:
J-pole feed point connections
The hose clamps are not optimum, I am sure a better way to attach the feed line to the antenna can be fabricated, but again, I was thinking of an emergency situation and the parts which may be available from local sources.
After one of our clients had an FM station go off the air over the weekend, I investigated and found this:
Transformer melt down
Looks like something one might find in the reactor room at Chernobyl or Fukushima.
Transformer melted down
This is at one of those sites with three phase open delta power. Needless to say, the transformer is toast, perhaps the entire transmitter too. This will be another fun transmitter scrapping project. I was thinking about this; over the last five years, I have scrapped at least ten to fifteen old tube transmitters. The old tube types are going away fast, as are those that can still work on them.
This is a tower behind one of our FM transmitter sites. In the past, it has housed paging and two-way services. It has always been sort of a slum, in my opinion. Several times, malfunctioning or improperly installed 900 MHz paging radios from this site have caused interference with our 950 MHz STL receivers. In recent years, all those things have gone away however, to be replaced by a Wireless Internet Service Provider (WISP). Even with this change, the site is mostly overgrown and uncared for.
Yesterday, I noticed the tower was not as tall as it used to be, so I walked down the hill and saw this:
Self supporting tower after loosing top section
It appears this happened a few weeks ago. View from the other side:
Self supporting tower section resting on roof of building
Close up of tower section that failed:
Looks like the bolts that held one the flanges together failed, the tower was pushed over by a strong NE wind causing the other two legs to fail. Truth be told, the tower had been in rough shape since the mid 90’s. I am surprised that it stayed up this long.
WISP sector antennas. I don’t know if they owned the tower or were tenants. Either way, this is going to cost a few rubles to repair.
Looks like the shelter took a little bit of damage too. To be honest with you, I hope that this is it for this site. I would be nice if they take down the stump, scrap the lot of it and move somewhere else.
And some parts of Asia:
Equipment rack outlet with 220 Volts to ground.
It seems the power company has some work to do. The other leg measures 28 volts to ground, which to me means the Neutral has been lost somewhere. Fortunately, the transmitter was running on 240, which looks normal on the voltmeter. Everything in the rack; the remote control, exciter, STL, etc has been damaged or destroyed.
Then of course, there is this:
That is the power and phone line in those trees, as it leaves the road and travels approximately 1,700 feet through the woods. It is a private line and the utility will not do any work until the trees are cleared away. In all fairness to the current owners, who have owned the station for not quite a year, this situation has been like this for a long time.
Lets get started:
Results of a deer vs automobile accident
It does not look like much, however, that is about $5,500.00 worth of damage. What you don’t see is the mashed oil cooler and radiator. This happened on my way from one place to another during the early morning hours. I was traveling at about 55 MPH when a deer bolted from the woods and entered the roadway from the right. I did not have time to break.
In a ditch
A momentary lapse of attention causes loss of $80.00. I think I was adjusting the defroster as I was driving down the road when suddenly, I felt the car tilt over to an alarming degree. You can see the tow truck getting ready to pull it out. Fortunately, there was no damage to the vehicle.
Troubles with the neighbor
This is on the access road to one of our transmitter sites. The station has a legal right of way through this property, however, the neighbor seems to object. I spoke with him and showed him a copy of our deed, he has since changed plans.
One side of a balanced audio connection disconnected
This is the downside of using category cable to make audio connections. The wires are not as rugged as say Belden 8451. This was causing problems because it is at an AM studio/transmitter site.
Burned 30 amp three phase contactor
Three phase, 30 amp, 240 volt contactor installed in a 480 volt system. Lasted a few years, anyway.
White face hornets nest
New tenants on one of our towers. This is a white faced (or bald faced) hornets nest. They are really paper wasps, but that difference aside, these beasts are nasty, aggressive and have a painful sting. Normally, I am a live and let live kind of person, but in this case, they gotta go.
Dummy load attached to plywood
This is at one of our AM clients site. Somebody, quite some time ago it seems, made this test load for a 1 KW AM transmitter. It is very nice, carbon ceramic resistors, 50 ohms and surprisingly little reactance. Then they attached it to this piece of plywood. As one can surmise, the load gets quite hot under full power, full modulation conditions. We remounted this in a cage type enclosure and bolted it to the cinder block wall.
Scala PR-950U cross polarized
The client at this station is complaining of intermittent STL drop outs and low signal strength at the receive end. Found this Scala PR-950U antenna mounted for vertical polarization, but the antenna element is horizontally mounted. We’ll call it “vorizontal.”
Ribbon cable from a Cummins 135 KW generator
This was discovered during routine maintenance and thankfully not during a power outage. Mice got into the control box of a newish Cummins 135 KW generator and chewed through what looks like a data buss cable. The generator would not run and the cable and control board needed to be replaced.
There is more bulging capacitors removed from flat panels monitors.
And so on…
Sometimes it is obvious and relatively easy, other times not so much. This summer we have had wave after wave of afternoon thunderstorms. It is almost like living in Florida; almost, but not quite. Anyway, with the storms occasionally comes some lightning damage. At most of the transmitter sites we service, every step has been taken to ensure good grounding and adequate surge suppression. This is especially true of sites that have been under our care for a few years. Even so, occasionally, something gets through. After all, those five hundred foot steel towers do attract lightning.
Broadcast Electronics AM5E output tuning section
This is the output section of the BE AM5E transmitter at WROW. The transmitter got pretty trashed; a bad PA module and power supply and this capacitor in the output section. This particular transmitter is 14 years old and this is the first major repair work we’ve had to do it.
Broadcast Electronics AM5E output tuning capacitor
The capacitor was fairly easy to change out. As a general precaution, both capacitors were changed. There was a spare PA module and power supply on the shelf, thus the transmitter was returned to full power relatively quickly.
Broadcast Electronics AM5E output forward and reflected power meters
The rest of the antenna system and phasor were inspected for damage, a set of common point impedance measurements taken, which showed that no other damage was sustained.
Next, the 30 year old Harris SX2.5 A transmitter at WSBS. This failure was slightly more exotic; the transmitter started randomly turning itself off. The culprit in that case was this:
Harris SX2.5 remote control interface bypass capacitor
Literally, a two cent part. The transmitter remote control uses opto-isolators. The inputs to these opto-isolators are RF bypassed to ground on the back of the “customer interface board.” After determining that the remote control was not malfunctioning, it was down to either a bad opto-isolator or something really silly like a bypass capacitor. This capacitor was on the ground side of the remote off terminal. It shows short on the capacitance meter and 4.1 K on the ohm meter, just enough to randomly turn the opto-isolator on and shut down the transmitter. Being a Harris transmitter, removing and replacing the “customer interface board” was no easy matter. Overall, it took about three hours to find and repair this problem.
As promised in an earlier post, here is an update on the progress at the North Adams tower site for the restoration work on WUPE-FM and WNNI. For those unfamiliar, refer to this post: North Adams Tower Collapse.
A contractor installed a 70 foot wooden utility pole last week. We ordered new Shively Versa2une FM antennas as replacements for the antennas destroyed when the tower fell last March. These new antennas are field tunable, which is a nice feature. The idea is that this pole will be used until the replacement tower is constructed, which is many months away. After the new tower is up, I would like to keep the pole in place as a backup facility for both stations.
North Adams restoration work
The bucket truck arrived but the driver had a bit of bad news; there is room for only one person in the bucket. The boss pipes up and says “Oh, that’s okay, Paul can go up and run the bucket”
Are you sure this is a good idea?
So anyway, it turns out running a bucket truck is not a huge deal; there is a joy stick of sorts that moves the booms around, up down, sideways, etc. Once you get the feel for it, it is pretty easy and three dimensional movement becomes second nature. That being said, at 70 feet in the air, everything gets a little wobbly, so it is best not to jerk the controls around.
The antennas were mounted on a 2 inch pipe which was attached to the pole with 1/2 inch threaded rod. We left a little bit of pipe sticking up above the top of the pole to get the FM antennas as high a possible.
Mounting pole to tower
Mounting pole to tower
Some dude in a hang glider checking out the work
Getting photobombed by some guy in a hang glider is a new experience. No day is exactly like another in this line of work.
WUPE-FM and WNNI temporary antennas
The antennas were tuned up once they were up on the pole. We did this with the network analyzer, which made the job very easy. WUPE-FM (top antenna) started using this antenna on Wednesday afternoon (5/7) with greatly increased power output. This gets the station almost the same coverage area as they had before the tower collapse. We tested WNNI (bottom antenna) and it all looked good. WNNI is still waiting for a temporary wireless internet feed for program delivery. Once that is established, we will have to do the intermod measurements one more time before they can go on the air.
Here are some pictures of the cleaned up site:
North Adams, fallen tower removed
North Adams, fallen tower removed
The temporary monopole being used by the cell providers:
North Adams temporary cell tower
Basically the pole is ballasted in place by those huge concrete blocks.
High winds seem to be the culprit in the collapse of two towers in North Adams. According to the Motorola system technicians, it happened at about 12:30 am Sunday morning, which is when all their link loss alarms started going off. The larger, self supporting tower broke from it’s mounting plate and tipped over into the smaller guyed tower next to it. Effected are WUPE-FM and W226AW (WFCR New England Public Radio) as well as NEPR new station WNNI which has not officially signed on.
Cellular service for ATT, Verizon and Sprint/NEXTEL were all knocked off line as well internet services and E911 dispatch. Those services are coming back on line, with temporary modular cell units en route. News report from WWLP channel 22, Springfield, MA:
Here are some pictures:
North Adams Cell Tower
WUPE-FM antenna on the ground
WUPE-FM STL dish
Base of WUPE-FM (formerly WMNB) tower
WUPE-FM, WNNI, and W266AW transmitter building
North Adams Cell Tower
North Adams Cell Tower
North Adams Cell Tower
North Adams Cell Tower
Tower base mounting plate, apparent failure point
Tower base mounting plate
Tower Base Mounting Plate
For pictures of the towers during happier times, refer to this post: Filtering for co-located FM transmitters.
Restoration work is underway with WUPE-FM expected to return to air at low power by Monday afternoon.
WUPE-FM was returned to air at low power by about 1pm on Monday 3/31. We took an unused Shively 6812 antenna that was tuned to 94.1 MHz and retuned it to 100.1 by cutting 1/4 inch pieces from the end of the elements until it was on frequency. It took a bit of doing, but with a network analyzer, we were able to get it to 1.2:1 SWR with symmetrical sidebands. Running 600 watts, it covers the city of license and then some.
WUPE-FM temporary antenna, Shively 6812
The STL antenna is a survey antenna mounted on the side of the building. In this configuration, with the leaves off of the trees, we are getting about 250 uV signal, which is pretty good.
WUPE-FM temporary STL antenna
The site is now crawling with insurance investigators, cell site technicians, North Adams fire department, Berkshire County Sheriff’s officers, tower workers, etc. After we finished this work, we cleared out to make more room for everybody else. Estimated restore time for W266AW is Wednesday 4/2.
Planning for the replacement tower is already in progress, I’d expect it to happen fairly quickly. The next step for the broadcasters is to put up a 70 foot utility pole and get a full powered antenna for WUPE. This should happen in the next two weeks or so. That will serve as the temporary facility until the new tower is constructed.