It is a little bit blurry, but the real problem is that none of the indicator lamps on the phasor or antenna monitor are working. Those little incandescent 387 bulbs burn out frequently. It is difficult to tell, at a glance, whether the phasor is in daytime or night time mode. One also cannot tell which tower or mode is selected on the antenna monitor.
It is a small job to replace them, but it does take some time. They currently exist in older transmitters, studio consoles, meter back lighting, and other control indicators. Since I began working in radio, I have replaced hundreds of these little lamps. I would rather spend my time on more interesting projects.
The 387 bulbs cost about a dollar each and last less than a year, in most cases. Fortunately, there is a solution to all this. Enter the based LED replacement lamp. These little guys have the long life of an LED (100,000+ hours) in a package that is a direct replacement for the Incandescent lamp. They run about $5.31 each.
Dialight makes a very handy cross reference:
The entire cross reference section is three pages long and is a part of their PMI catalog. The full cross reference .pdf can be found here.
Those Dialight LED lamps are available from Mouser, Allied and Newark Electronics.
Time is money. There are much better things to be doing than going around replacing incandescent indicator bulbs in various pieces of equipment. At the same time, it is important to know what the status of that equipment is at a glance, which is the reason for using any type of indicator in the first place. Using drop in replacement LED indicating lamps with certainly save time and money in the long run.
I have been working on an AM station lately. WBNR signed on in 1959 and follows the now familiar AM trajectory; after making bank in the 60’s, 70’s and 80’s, revenue declined, maintenance deferred, yada, yada, yada…
After a stint with a news talk format, the station changed to “Real Country,” a few years ago. WAT! Music on the AM? Actually, it is doing quite well. The perception is that AM sounds terrible and nobody listens to it. The stock AM radio in my Subaru (made by Pioneer) sounds pretty good on AM. I have noticed that when I first tune a station in, it sounds narrow banded, slightly better than a telephone. However after a second or two, the bandwidth opens up and it can sound quite good. I have also heard this station playing at several local businesses. When we turn it off to do maintenance, the phone starts ringing. Clearly, somebody is listening…
This station is part of a three station simulcast. The AM station to the north got rid of its directional antenna and added an FM translator a few years ago. That has made a big difference. Thus a translator was acquired for this station as well.
The translator was held up by an informal objection filed by Prometheus, Et. Al. as part of a blanket filing against all new translator licenses by the LPFM advocate. In any case, the Construction Permit has been on hand for a while, so the owner felt it was time to move forward with building out the new FM signal.
Installing the single bay Shively 6812 antenna on the side of one of the night time towers triggered some other things. A bit of the deferred maintenance was addressed; new stockade fences around all the towers replaced the original fences put up in 1988. Those original fences were falling down.
The antenna system for WBNR is actually quite elegant, perhaps even beautiful. A simple two tower system for the daytime array and a separate two tower system for the night time array. The night time towers are top loaded, adding about 30.7 degrees in electrical height.
The CP for the translator required some extra steps because of the mounting on the night tower of the AM array. Before and after impedance measurements need to be taken on the tower in question. Another requirement of the CP, a set of before and after monitor points need to be taken.
While I was measuring the base impedance, I decided measure all the towers instead of just the night time tower that has the translator antenna mounted on it. This is a good point of reference if any problems arise in the future. Often, this information can be found in the technical paperwork from the original license application. Those files can be a treasure trove of information. Unfortunately, it appears that a good portion of the original paper work is missing.
The Phasor and ATU’s are a late 80’s Harris product. They are actually in remarkable shape, all things considered. All of the RF contactors are Harris HS-4P motor driven units. They are rated at 30 Amps, RF-RMS. I don’t think that they are supported by GatesAir. I have a small stock of spare finger stock and contact bars. I suppose, if I had to, I could make or adapt parts to repair.
Looking at the base currents and the base current ratios for both the day and night patterns (base current ratios are on the station license), the tower impedance has changed very little over thirty years. That is good news, especially with those 215 degree tall night time towers.
The WBNR license application did contain an overall system diagram showing the Phasor and all the ATU’s. It did not contain any component ID’s or other information. I scanned that in, created a vector graphics file and expanded it to a 24 x 36 inch size. I was able to fit all the component values and other information on the diagram.
The other issue is the monitor point descriptions. They include statements such as “Point is marked with yellow and white paint on a tree,” or “In the northeast corner of the Texaco research facility parking lot.” Those references are long gone and I would prefer to use a set of GPS coordinates. Using the topographical maps from the proofs, I found each monitor point and then recorded a set of GPS coordinates for each. In the future, they will be much easier to find. If anyone is still doing monitor points, I would recommend this method.
Yet another problem; the phasor control system was damaged by lightning. The overly complicated Harris Phasor control card was replaced with something a little more straight forward and reliable. I designed a simple set of relays, one for daytime and one for nighttime, to change the antenna system over. The transmitter interlock goes through the relay contacts, so the transmitter PDM is killed while the power changes. Tally back from each of the towers is handled by a set of relays for each pattern, which is also interlocked with the transmitter. All of this prevents the RF contactors from switching hot, something that has caused some damage in the past.
W243EM is 100 ERP watts, non-directional with a 1 bay Shively 6812-1R antenna installed at 381 feet (116 Meters) AGL on one of the night time towers.
Transmitter is a BW Broadcast TXT-600. The power calculation is as follows:
ERP 100 Watts = 50 dBm
System gains and losses:
Transmission Line loss, 500 feet (152.4 Meters), RFS LCF78-50JA = -1.75 dB
Isocoupler loss, Kintronic ISO-170-FM = -0.8 dB
Antenna gain, Shively 6812-1R = -3.39dB
Total system losses and gains: -5.94 dB
TPO: 55.94 dB or 393 Watts
With all that work completed, the license application was filed to cover the construction permit. Once that was accepted by the FCC, program test authority was granted and the transmitter was turned on. Hopefully, with the translator on the air, the perceptions regarding listeners will change and the station can bill more.
I really enjoy working on Medium Frequency antenna systems. I don’t know why, but antenna systems in general are always fascinating to me.
This is an interesting project currently underway at one of our client’s AM sites. They have decided to go all in and create a WISP (Wireless Internet Service Provider) for the community around the AM tower. I thought it would be interesting to explore this topic, as there are not many opportunities for AM towers to lease vertical real estate.
First a few basic ideas. For an AM broadcaster, (aka medium wave or standard broadcast band) the entire tower is part of the transmitting antenna. There are two types of towers; series excited and shunt excited. A series excited tower has a base insulator, like this:
A shunt tower usually has a series of wires called a skirt, separated from the tower by standoffs, which go to the top of the tower or nearly to the top of the tower. The base of the tower is grounded, like this:
A shunt excited tower has distinctive advantages for co-location opportunities in that the tower itself is grounded, greatly simplifying placing additional antennas on the towers. That is not to say that antennas can not be installed on series excited (insulated) towers, it just requires an extra step of using isolation coils.
In all cases, the tower should have a structural study done to insure that the additional antennas do not overload the tower and cause structural damage or collapse.
In this case, the tower is new and was designed for the extra load.
The plan is to create a sectorized wireless internet system using four 90 degree panels, each with three access points. A tower mounted sixteen port switch is mounted behind the panel antennas and the switch communicates with the ground mounted router through two fiber optic cables. A 54 volt DC supply powers the switch, access points and point to point radios mounted on the tower. There are two fiber runs, one is for subscriber traffic and the other is for radio management. This system is using Ubiquiti gear.
A word or two about Ubiquiti gear. Ubiquiti specializes in cheap equipment manufactured in China. That is a double edged sword. On the plus side, if anything breaks or gets damaged by lightning or whatever; throw it out and install a new one. On the negative side, I have seen Ubiquiti gear do some strange things, particularly after a firmware upgrade. The newer stuff seems to be better than the older stuff. All that being said, as this is a brand new operation and seems to be a proof of concept, then the Ubiquiti gear will be fine to start with.
The tower crew made quick work of installing the sectorized access points.
Going up the face of the tower, there are the aforementioned fiber cables, the 54 VDC power cable and one backup Ethernet cable. All of the Ethernet jumper cables used to connect the access points to the switch are UV rated, shielded Cat 5e and use shielded connectors. This is very important on a hot AM tower. Due to the skin effect, the shield on the shielded cable protects the interior twisted pair conductors from the high AM RF fields present on the tower.
At the base of the tower, the DC power cable and the Ethernet cable go though high quality lightning protection units. These are Transtector 1101-1158 Ethernet and 1101-1025 48 volt outdoor DC power units. Even though the DC power supply is 54 volts, the 48 volt LPU’s will function adequately. The TVSS devices used in the LPU circuit are rated for 88 volts maximum continuous voltage.
In addition, I made a service loop on the DC cable with also creates an RF choke. Several (12-14) turns of cable 18-20 inches (45 to 50 cm) in diameter act to keep the induced RF at the input terminals of the LPU low so the protection devices do not fire on high modulation peaks. This also helps to keep the AM RF out of the 54 VDC power supply in the rack.
The backup Ethernet cable has a similar setup. Regarding the Ethernet cable and induced RF, this station runs 1 KW. As long as the shielded RJ-45 connectors are applied properly and the tower mounted switch is grounded along with the LPU, then all of the RF should be on the very outside of the cable shield (due to the skin effect).
This principal also applies to lightning strikes. Although lightning is DC voltage, it has a very fast rise time, which makes it behave like AC on the initial impulse of the strike. The voltage induced on the shield of the cable will not effect the twisted pairs found deeper within the Ethernet cable. Of course, all bets are off if there is a direct strike on a piece of equipment.
AM stations running powers more than 1 KW, Superior Essex makes armored shielded cable called BBDG (the new trade name is EnduraGain OSP). This cable comes with a heliax like copper shield with an optional aluminum spiral armor. This cable looks very robust.
On series excited towers (those with an insulated base) fiber optic cable can be used to cross the base insulator without any problems, as long as there is not any metal in the cable (armor or aerial messenger).
DC power can cross the base insulator using something called a “Tower Lighting Choke.” This device is a set of coils wound around a form which passes the DC power but keeps the AM RF from following the DC power cable to ground. These work relatively well, however, lightning protection units still need to be installed before the DC power supply.
I went to do maintenance at one of our sites and noticed that a certain transmitter was running at half power. Followed the path of the fault log and found this. When I mentioned it to the station staff, they said, “Yeah, we noticed it sounded a little funny…”
This is the second time this has happened with this particular transmitter. In any case, this is what I get paid for, so I am certainly not complaining. If only every problem where this easy to find.
When I get back out there to replace this, I will bring out my network analyzer and sweep the antenna and transmission line to make sure there are not issues with that. In addition, I will double check all the grounding to make sure the copper thieves have not made off with any critical components like the ground buss bar or #2 solid down lead wires.