Engineering Radio – Page 48 – One Hundred percent Human Generated Content for your Reading Enjoyment

Repairing the Nautel VS2.5 transmitter

The newish Nautel VS2.5 transmitter installed at WJJR had an RF module failure. This particular model transmitter does not have slide-in RF modules as other Nautel transmitters do. To fix this transmitter, it has to be pulled out of the rack, flipped over, and opened from the bottom. The module replacement is very straightforward, there are five solder pads that connect to wires carrying the input, output, power supply, and bias voltages.

Nautel VS2.5 transmitter RF modules and combiner

The troubleshooting guide gives good instructions on how to check the PA MOSFETS with a DVM. I found that 1/2 of the device in PA1 was bad:

All in all, not a very hard repair. This was under warranty, so a replacement RF pallet was sent to the station without charge. The problem is more about where the transmitter is located:

Killington Peak is the second tallest mountain in Vermont, topping out at 4,235 feet (1,291 meters). In the winter, one can take the chair lift to the top. In the summer, the road is drivable with a four-wheel drive. In those in between months, access to the top can be very tricky at best. We had a pretty wet spring this year, so the roads up the mountain are just now becoming passable for vehicles.

Even after reaching the parking lot, there is still a 10 minute walk to the peak, another 200 or so feet up a steep, rocky trail.

Further complicating things, this transmitter is wedged into this little shack, which holds; a BE FM3.5A transmitter (defunct WJJR), a Harris HT3 transmitter (WZRT), an ERI combiner, two racks of equipment (STLs, Exciters, remote controls, etc) a backup QEI transmitter, an Onan generator transfer switch:

Killington Peak fire tower, WJJR WZRT transmitter building

Both stations run into this ERI half-wave spaced antenna:

It is very tight in this transmitter room. There is a new tower on Killington Peak, which is still under construction. At some point, the plan is to move into the larger building next to the new tower.

On a clear day, the view from the top is spectacular. On this day, the peak was in the clouds, so not so much:

It is a great site, the HAAT is 2590 feet (790 meters) and the stations carry forever on relatively low power outputs.

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 Shively 6710 Antenna

Perhaps that is one Shively Antenna that you haven’t heard of. They were an oddball combination of a horizontally polarized antenna with an adjustable vertical element. This design allowed the station to adjust the ratio of horizontal to vertical power from a range of 1:1 to about 4:1 (H:V). Why would this be a desirable feature?

Back in the early days of FM broadcasting, almost all stations had horizontally polarized antennas. This system worked remarkably well, stations could broadcast at moderate power levels over fairly long, line-of-sight (or mostly line-of-sight) paths. Most FM receivers were stationary units installed in people’s homes often with outdoor antennas.

It was not until the late 1960s and early 1970s that FM radio receivers became a stock option in most low and mid-cost automobiles. It was then that a slight problem with FM broadcasting was discovered; car antennas are vertically polarized. People driving around in their new machines found that the FM reception was not all that great. Stations began adding a vertical component to their signal to help improve the mobile reception situation.

I found this Shively Brochure in a file cabinet drawer at the WFLY transmitter site. This model antenna was ordered and installed by that station in 1970. It had a 3:1 horizontal-to-vertical ratio. Why not install a fully circularly polarized antenna? Because often that necessitated installing a new, more powerful transmitter. Every watt of power taken from the horizontal plane and added to the vertical plane reduced the ERP by that much and had to be made up with more transmitter power output. Oftentimes, the ratio of H:V power would be adjusted to take up whatever headroom there was in the transmitter and the station would run that way until the next transmitter replacement cycle.

I found the remains of this antenna in the woods, northeast of the tower.

This section looks pretty well destroyed. It is probably better to dispose of these types of things by scraping, them rather than dumping them in the woods. While there is not a lot of scrap value to this unit, it can become attractive nuisance to copper thieves and other vandals if it is left laying about.

It is a strange-looking piece of kit, a sort of make-do until the situation could be fully rectified. I think this antenna was in service until 1986 or 87 when it was replaced with a circularly polarized ERI.

Filing an STA

FCC rules stipulate that when a station is operating at variance from its licensed parameters for more than 10 days, Special Temporary Authority (STA) is required. The reasons for requesting an STA are varied but could include things like:

Damaged transmission equipment
Loss of transmitter site or building use
Loss of tower
Eviction
Facilities upgrade or renovation
Natural disaster

The loss of the transmission tower at WUPE-FM falls into one of those broad categories. Thus, we have filed an STA with the FCC for temporary transmission facilities while a new tower is being constructed. Since the old tower is completely lost, we specified a new tower location, new height above average terrain (HAAT), new ERP, and environmental certification. To gather that information, several steps were needed:

Obtain a new tower location. This was done with a GPS receiver and verified on itouchmap.com. Once the NAD83 position was obtained, it needed to be converted to NAD27 for the FCC filing. The FCC has a conversion tool on its website.
HAAT calculation is fairly simple, use the HAAT calculator tool on the FCC website. For this, the antenna radiation center height Above Mean Sea Level (AMSL) is needed. Using a topographical map, find the ground level AMSL, convert it to meters, then add the radiation center height above ground level (AGL).
The Effective Radiated Power (ERP) calculation is also simple; Transmitter Power Output (TPO) minus system losses (transmission line and antenna gain). It is easiest to do this in dBm, e.g. convert the TPO from Watts to dBm, then add or subtract the gain or losses in dB, and convert the final product back to Watts.
The environmental statement is slightly more tricky. Basically, the filer is certifying that the STA complies with all environmental regulations including OET-65 (RF exposure limits). Since the temporary antenna is significantly lower than the original, some investigation is required. For this, there are two methods to demonstrate compliance; ground measurements with a NARDA meter, or RFR worksheets which are a part of the broadcast station renewal form, FCC-303s.

I have taken the RF worksheet sections out of the 303s and separated them into the FM RF Worksheet and the AM RF Worksheet. These worksheets are not effective for large tower farm-type sites where there are too many variables and RF contributors to be accounted for. The calculations on the worksheets are not conclusive, however, if the facility in question falls under the limits, it is generally accepted as being in compliance. Taking ground measurements with a NARDA meter is the definitive method for determining RFR compliance. Since this is a relatively simple site, the worksheet calculations should be sufficient.

The worksheet calculations show that the RFR is within both the controlled occupation limits and the uncontrolled general population limits.

The position of the new temporary pole was verified on itouchmap.com:

It is never good to be operating at a varience from licensed parameters without notification of the FCC. Such things could lead to fine or other problems for the broadcaster.