STL – Page 2 – Engineering Radio

The 16 channel bi-directional STL

As a part of our studio build-out in Walton, we had to install a high-capacity STL system between the studio and transmitter site. Basically, there are five radio stations associated with this studio and the satellite dish and receivers are going to be located at the transmitter site.

The audio over IP gear is getting really sophisticated and better yet, more reliable. For this application, we are using a Cambium networks (Motorola Canopy) PTP-250 radio set and a pair of Wheatstone IP88 blades on either site. Since there is quite a bit of networked gear at the transmitter site, the IP88s will live on their own VLAN. The PTP-250s will pass spanning tree protocol, rapid spanning tree protocol, 802.1Q, and other layer two traffic.

The Wheatsone IP88A blades are the heart of the system. Not only do they pass 16 channels of audio, we can also pass 8 logic closures bi-directionally. This is key because we are shipping satellite audio and contact closures back from the transmitter site. The IP88A setup is fairly easy, once the IP address is entered. The web GUI is used for the rest of the configurations including making the connections between units.

Pair of Wheatstone IP88A AoIP interfaces

The switches are managed units. The switchports need to be set up via command line to pass VLAN traffic. There is an appendix in the IP88 manual that outlines how to do this with various managed switches. This is the most important step for drop-out free audio. The switchports that connect to the two radios are set up as trunk ports using either VTP or 802.1Q.

The PTP-250 radios were already on hand, new in the box. They are built really well and look like they should not break in a year or so. These particular units are connectorized, therefore an external antenna was needed. There are many such antennas, this system ended up with an RF Engineering & Energy 5150-5850 MHz dual-polarized parabolic dish with RADOMES. RADOMES are necessary to prevent ice or snow build up in the winter.

RF Engineering & Energy 5150-5850 MHz dual polarized parabolic dish with LMR400 jumpers

Since the path is only 3.37 miles (5.43 kilometers), I set them up with a 40 MHz wide channel. This is a rural, small-town setting. When I looked at the 5.8 GHz band on a spectrum analyzer, it looks fairly uncongested. These are MIMO single or dual payload selectable. I will try them as single payload units since the path is short and the band is uncongested. This should keep the throughput high.

Studio to transmitter site LAN extension

The PTP-250s use POE injectors in mounted in the rack rooms. CAT5e shielded cable with the proper connectors properly applied is a must for lighting protection. The PTP-250 units came with Cambium PTP-LPU lightning protectors. I also installed Polyphaser AL-L8XM-MA type N surge suppressors on each RF port of each PTP-250.

MPX over IP

In the progression from Circuit Switched Data to Packet Switched Data, I can think of many different applications for something like this:

The FMC01 MPX to IP encoder can be used for multi-point distribution (multi-frequency or same-frequency network) of FM Composite audio, or as a backup solution over a LAN bridge, LAN extension, or public network. I can think of several advantages of using this for a backup when composite analog STLs are in use. There are many compelling reasons to extend the LAN to the transmitter site these days; Transmitter control and monitoring, security cameras, office phone system extensions, internet access, backup audio, etc. I would think, any type of critical infrastructure (e.g. STL) over a wireless IP LAN extension should be over a licensed system. In the United States, the 3.6 GHz WLAN (802.11y) requires coordination and licensing, however, the way the rules are set up, the licensing process is greatly simplified over FCC Part 74 or 101 applications.

Another similar CODEC is the Sigmacom Broadcast EtherMPX.

Features include:
• Transparent Analog or Digital MPX (MPX over AES), or two discrete L/R channels (analog or AES).
• Built-in MPX SFN support with PTP sync (up to 6.000km in the basic version). No GPS receivers!
• Unicast or Multicast operation to feed an unlimited number of FM transmitters with MPX from one encoder.
• Linear uncompressed PCM 24-bit audio.
• Very low audio latency: 2,5mS in MPX mode.
• Perfect match with Sigmacom DDS-30 Exciter with Digital MPX input.
• Can be used with high-quality 802.11a/n Ethernet links.
• DC coupled, balanced Analog inputs & outputs with -130dBc noise floor.
• No modulation overshoots due to compression or AC capacitor coupling.
• Decoder provides simultaneous Analog & Digital output for transmitter redundancy.
• Aux RS232 serial transparent link, Studio to Transmitter.
• Auto switchover to Analog input when Digital signal is lost.
• Centralized remote control & management software

One last thought; separating the CODEC from the radio seems to be a good idea. It allows for greater flexibility and redundancy. Using an MPX-type STL allows sensitive air chain processing equipment to be installed at the studio instead of the transmitter site.

Fifth Generation WLAN

Like all data-carrying technology, WLAN, or WiFi, continues to evolve into a better, faster, and more robust platform. The IEEE wireless ethernet specification 802.11ac combines all of the past developments, plus some added features, into one specification. Here are some of the highlights:

Operation on 5 GHz only. Many more available channels in this spectrum than in 2.4 GHz
Increased channel bonding makes wider channels carry more data. In the 5 GHz spectrum channels are 20 MHz wide and do not overlap. 802.11ac allows for 40, 60, 80 or even 160 MHz channels. This is great for short distances, longer distances will be prone to greater interference over wider channels
Modulation schemes that allow up to 256 QAM. A 256 QAM constellation is going to look pretty crowded unless it is on a wide channel. Again, this would be good for short distances.
Increased MIMO. Up to 8×8 MIMO (Multi In Multi Out) which can greatly improve throughput. MIMO means multiple transmitters and antennas in the same unit. The first number is the transmitter count the second number is the antenna count. Thus an 8X8 system will have eight transmitters and eight antennas. This allowed beam forming by use of phased antenna arrays, which can greatly reduce multi-path
MU-MIMO (Multi-User MIMO). Basically, the access point sends the data frame only to the desired host, thus instead of acting like an ethernet hub sending the frame to every connected host, the AP is acting more like an ethernet switch.

The goal of all of these modifications is to get gigabit transfer rates over WLAN.

What does all of this have to do with radio broadcast, one might ask? That is a good question.

There are several applications that have to do with remote broadcasting. Many sports areas, nightclubs, or other likely places to be broadcasting from have WIFI installed. Using a laptop with an AoIP client installed not only can connect to the studio for audio delivery, but the same laptop can also use RDP or VNC to control the station’s automation computer as well. This means easier integration of the remote into voice-tracked or syndicated programming.

Secondly, wireless LAN bridges between the studio and the transmitter site can act as an STL, a backup STL, a remote control return link, a bridge for a network-connected transmitter, a VoIP phone link, IP security camera backhaul, or almost anything else that can send ethernet data. I have found it useful to simply have a computer available at the transmitter site, even if it is only to download manuals and whatnot. We have taken several old Windows XP machines and reloaded them with a Linux variant and installed them at various transmitter sites. It saves the trouble of having to download a manual on the smartphone and then page back and forth across a really small screen to read it. As for using unlicensed WiFi to link to a transmitter site; the link between the WICC studio and transmitter site runs 78 Mbps most days. This is a two-mile link over mostly water. I will say, when there is fog, the link rate drops to 32 Mbps, which is still pretty good, all things considered.

Of course, office network applications; laptops, tablets, smartphones and other personal devices.

Finally, Broadcast Engineers really need to keep abreast of networking technology. There are many, many applications for WiFi units in the broadcast industry.

The Kintronic Isocoupler

Had a problem with this Kintronic FMC-0.1 isocoupler the other morning.

Kintronic FMC-1.0 STL ioscoupler — Kintronic FMC-1.0 STL isocoupler

After an overnight drenching heavy rain and very high wind, the STL transmitter associated with this unit was having high VSWR faults. This isocoupler crosses a base insulator of an AM 50 KW directional antenna. This particular tower has negative impedance, which is to say, it sucks power out of the pattern and feeds it back to the phasor. An interesting discussion for another time, perhaps.

Using a dummy load, we isolated the problem to the isocoupler by first connecting the load to the output on top of the unit (the problem still exists) and then to the transmission line prior to the unit (the problem went away). Of course, the AM station had to be taken off the air to do this work.

Once the issue was confirmed as the isocoupler, I opened the unit up and found that water had entered and pooled in the top of the bottom half of the isolation transformer.

The isolation transformer consists of two loops to ground capacitively coupled through air dielectric. The issue is with the opening around the top of the unit, under the lip of metal lid. Apparently, this allowed water in.

Kintronic isocoupler isolation transformer

It is difficult to tell with the lighting in this photograph, however, the bottom part of this isolation transformer has water pooled around the center insulator. Using a rag, I cleaned out the water and dirt from the center insulator. After reconnecting the antenna and transmitter transmission line, a quick check revealed the problem was much better, but still not completely gone. I suspect water seeped further down into the bottom half of this unit. The repair work was good enough, however, to return both stations to the air.

Glad to get that bit of work done while it was still relatively warm out.