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.
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.
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.
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.
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.
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.
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.
I swept the coils from 20Hz to 20kHz:
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.
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:
Per month per DID number (USD)
Incoming call rate (USD) per minute
Outgoing call rate (USD) per minute
Toll Free (800)
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:
List of equipment:
New or used
Ubiquiti Rocket M5
AP and station units
Ubiquiti AirMax 5G-2090 90 degree sector antenna
AP point to multi-point antenna
Ubiquiti Rocket Dish 5G-30
Trastector ALPU PTP INJ
Lightning protection out door units
Point to Point link
Motorola Canopy 900DA PCDD
AP point to multi point
Motorola Canopy 900DA PCDD
Microwave Filter #18486 diplexer
Diplexer 900 MHz ISM band and 944-952 STL band
Cisco SPA122 ATA
Dial tone for remote controls
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.
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.
The AM stations are WMML and WENU in Glens Falls, NY. The AM stations are diplexed using a Phasetek diplexor/ATU.
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.
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.
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;
(F1 – F2) + F1 or (97.9 MHz – 96.9 MHz ) + 97.9 MHz = 98.9 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
US LTE Band 71
US LTE Band 5
US LTE Band 5
*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.