December 2011 – Engineering Radio

Old Year SWR

This time of year is when we all sit back and assess things that we did in the past 365 or so days. It is called reflection, which is just a civilian term for SWR (Standing Wave Ratio).

Thus, I thought I would take a little time and make a few observations about the business, my part in it, and this blog.

1. The business of Radio:

Let us be honest, Radio is not what it used to be. Many times, what it used to be was somewhat of a free for all, wheeler-dealer radio station owners cutting corners and making do with less than-optimum equipment and staff. And trade, lots and lots of trade. Only in large metropolitan areas did radio stations make enough money to throw it around, but sometimes not even then. Radio was by no means a huge money-making operation and therefore, those that worked in mostly it did it as a labor of love. That may or may not have come across on the air. By far, the funniest station I ever listened to was run from a closet, with a sound reinforcement board and the program director’s CD collection. What made it so much fun was they had nothing to lose, there were no restraints placed on the staff. Once that on-air enthusiasm translated to ratings, then to revenue, the magic was gone and they were just another radio station filling a spot on the dial.

The radio business has fully transitioned from a fun, seat-of-the-pants entertainment operation to a mega money-making corporate mentality under the control of mostly non-entertainment types. Even those stations owned by smaller group owners are forced to rely on the tactics developed by the big two in order to stay in business.

Group owners will continue to extract money in whatever way they can until the money train runs off the rails. Then, radio will be replaced by something less.

2. Radio Engineering:

Engineering will continue to grow smaller, with more emphasis on computers, networking, and IT infrastructure. The future distribution of music and program material will take the form of streaming (live events), podcasts (specialty shows), and subscription services. Over-the-air free radio will become less and less relevant as younger “listeners” trend toward new media. The idea of listeners may be archaic in lieu of “subscribers” or “users.” Thus, in order to remain relevant, broadcast engineers are going to have to keep their skill sets current. I would recommend to anyone getting into the business to get current with routers, routing tables, Cisco equipment, and whatnot. The cloud is coming and will rain on all those not adjusted to the new “broadcasting” reality.

3. My part in the business:

A somewhat superannuated broadcast engineer whose skill set lies mostly within the RF and heavy-duty electrical areas, I am going back to college in January. Cisco Network Administrator is the degree I am shooting for, for that is where the local jobs, both in and out of broadcasting will be. Network Administrators are going to be the backbone of cloud computing, those that can configure routing tables will be desired.

That being said, I continue to be involved with larger RF projects and transmitter work. It is fun for me, most of the time. Having to drive two hours, one way on Christmas Eve to fix a backup transmitter, not so much, but those situations tend to be the exception, rather than the rule.

All in all, it is great fun to press the high voltage on button, not knowing if the transmitter will cycle on normally, or put on some type of display.

4. The blog:

This little thing we have here has been fun. I get a good response to most articles. I welcome all the comments and offline e-mails that come my way. My original intent, which is to provoke thought and dialog, remains unchanged. This year, I have delved into areas not covered by the trade magazines, but do have at least some bearing on radio or radio-related arts. To that end, there have been several negative responses, which is fine. I don’t pretend to know everything, if you know more, then by all means, speak up. By and large, however, the majority of responses continue to be positive.

I continue to grow the overseas audience, with roughly 36% of the page views coming from non-US IP addresses. Persons from The UK, followed by Canada, Netherlands, Australia, and Germany are the top five non-US readers of this blog.

So, I will continue to post about things in the coming year. If any of you have any suggestions or requests, shoot me an email or leave a comment.

In the meantime, have a Happy New Year!

Synchronized FM signals

How effective are they at filling in or expanding coverage for FM stations? The answer is, it depends. Most have heard of the quadcast around New York City on 107.1 MHz formed in 1996-98. It was well documented in Radio World and several other publications as a clever way to overcome the suburban rimshot problem. Four signals on 107.1 were synchronized using GPS timing data, then fed the same program material. They were WYNY, Briarcliff Manor, NY; WWXY, Hampton Bays (Long Island), NY; WWYZ, Long Branch, NJ; and WWYY Belvidere, NJ. These being four separate Class A FM stations, the 60 dBu contours did not overlap. There was some mutual interference in some areas, but there were few if any reception negative zones where the signal strength is equal between stations.

In early 2003, I was a part of the disassembly of the quadcast. In the end, it is difficult to point to any one thing that leads to the breakup. The station’s owners, Big City Radio, had filed for bankruptcy. I am not sure if the company ever had the correct formula for marketing and sales, given the strong suburban, but weak and lacking building penetration in Manhattan signal. The station initially had a country format, something that armchair quarterbacks said would not work in New York City. After a few years, Big City changed the format to Rumba, a Spanish/Caribbean music format, which did worse than Country. The fact is, that it never lived up to expectations and the station was worth more separately than together. Given the right circumstances, it could have worked.

The other synchronized FM broadcasts are those where boosters are employed. These are a good deal more difficult to configure because the booster signal is within the main station’s 60 dBu contour. Often cases, where there is severe terrain shadowing or other limitations, a well-positioned booster that is in a population center can greatly improve the signal in those areas. This was formerly the duty of an FM translator, however, those stations seem to be taking on a life of their own, without regard for the intent of the current FCC rules. Boosters can also be called a single frequency repeaters or single frequency network (SFN).

The disadvantages of an SFN are the aforementioned negative reception areas. To the receiver, this will create a multipath or picket fencing situation, which is objectionable to most listeners. The advantages are, of course, better coverage in key areas, spectrum efficiency, and the ability to create a network of common frequency systems. Think of how easy it would be if all NPR stations were all on the same frequency, for example.

The key to making a booster work is to synchronize several aspects of the RF and Audio signals:

RF carrier frequency
Stereo pilot frequency and phase
Audio amplitude and phase

The RF carrier frequency, stereo pilot frequency, and phase are locked with a GPS. Most transmitters have a 10 MHz or 1 PPS input for this.

The audio amplitude and phase synchronization are slightly more complicated. Basically, all of the audio should be coming from one audio processor and the path to the individual transmitter sites has to be very low latency. RF STLs work for this setup well, if there are suitable paths.

Once that is established, the audio timing is used to move the interference zone away from undesirable areas. There will always be an interference zone where both signals are received at the same relative strength causing dropouts.

This is the situation with WDBY in Patterson, NY. The main transmitter site is located on a hill in Patterson and has a power level of 900 Watts at 610 feet (186 meters) HAAT. The main population area is Danbury, CT, to the southeast, about 12 miles away. Between the two, there are several imposing hills, which create reception issues in Danbury. Therefore, WDBY FM1 was placed in service at the Danbury Medical Center. The booster has a power output of 1,200 Watts, at 0 feet (0 meters) HAAT (49 meters AGL).

WDBY FM-1 signal, Danbury, CT 60 dBu contour

Therefore, the southern area of the 60 dBu contour is filled in by the booster. The interference zone between the two transmitters is determined by the amount of delay in the audio between the two units. If both are time the same, the interference will occur at precisely 1/2 the distance between the transmitter sites, which in this case is 10.18 KM from the booster. Looking at the population maps, it might be better to move that more toward the north, away from Danbury.

The formula for computing audio delay time is:

A-B=C where A is the distance between the transmitters and B is the distance to the interference zone from any given transmitter. The product of that is multiplied by a constant of 3.34 to obtain the time delay in microseconds. Therefore, if the interference zone is desired to be further outside of Danbury, say 15 KM away, then the equation looks like this:

20.358 kM -15.0 kM = 5.358 KM

5.358 KM x 3.34 = 17.89 μS delay from the main transmitter site will put the interference zone out in the middle of nowhere, away from Danbury. This is the total delay between the two stations, therefore any difference in STL paths needs to be included in this figure.

Nautel has a good webinar on SFNs which can be found on their website: Single Frequency Networks Webinar

Nautel equipment has most of these features built into it, therefore, the implementation of an SFN using Nautel exciters and transmitters should be relatively straightforward.

The head smasher

I have worked in hundreds of transmitter sites over the years; AM, FM, TV, HF, Two way, Paging, Cellular, etc. So many, I have lost count. The one thing that is always annoying is equipment that is suspended from the ceiling at just the wrong height, AKA: The Head Smasher. It does not matter if warning signs are posted, I’ve seen them marked with black and yellow caution tape, and so on. If it is installed low enough for somebody to hit their head, contusions will result.

3 1/8 inch motorized coax switch mounted

Thus, when it came to installing this motorized 3 1/8-inch coax switch, there was only one way to do it. Installing it the other way would result in a head smasher behind the backup transmitter because the ceilings are low. The problem with this style of mounting is how to get to the motor and clutch assembly for servicing. There is but one inch of clearance between the top of the coax switch and the transmitter room’s ceiling. If servicing is needed, the entire switch would need to be removed, resulting in lots of extra work and off-air time.

So, an idea was formed. Why not cut the switch cover in half and put some hinges on it. The cover itself is made of aluminum. I was able to carefully mark it out and cut it with a jig saw. Then, I attached a set of hinges on the back side and a set of latches on the front. It now opens like a clam shell.

3 1/8 inch coax switch cover modification

Now, when access is needed to either the motor or clutch, the cover can be opened up and removed. Unless the actual RF contact fingers burn out, there should be no need physically remove the switch for servicing.

Cover replaced. This will not have to be removed very often, in fact, I have known some coax switches that never need service. Still, having the ability to quickly get the cover off and do some basic repairs is a good thing.

Conduit fill

It may be surprising to some, but the number of wires allowed in any given conduit is not “as many as can be jammed in there.” The National Electrical Code, AKA NEC or NFPA 70 gives specific guidance on the number of current-carrying conductors allowed in any specific size and type of conduit.

This is due to the fact that current-carrying conductors generate heat. Cables enclosed in a conduit need to dissipate that heat so that the insulation on the cable doesn’t melt, which would be a bad outcome.

Conduit fill tables are found in Chapter 9 of the NEC. There are several tables that give the number of conductors for each size and type of conduit. Then there is the general rule of thumb that for more than two cables, the maximum conduit fill is 40%. This comes in handy when several different size conductors are being run in the same conduit.

An example of this is when several circuits are going across the room to the same general location, in this case, a row of transmitters and racks. Instead of running individual conduits for all those units, one or two conduits from the electrical panel are run to a square wireway, then the individual circuits are broken out and wired from wireway to the individual loads. In this case, the following equipment is being connected:

Harris FM25K: 100 amp 3 phase high voltage power supply (#2 THHN), 30 amp 3 phase transmitter cabinet (#10 THHN)
Harris FM3.5K: 70 amp split phase (#6 THHN)
Harris MW1A: 30 amp split phase (#10 THHN)
Two equipment racks: 20 amp single phase (#12 THHN)
Coax switch: 15 amp single phase (#14 THHN)
Dummy Load: 15 amp single phase (#14 THHN)
Antenna switch/dissipation network for AM station: 15 amp split phase (#14 THHN)
Convenience outlets for the back wall: 20 amp single phase (#12 THHN)

Excluding grounding conductors, which will be addressed below, the total current carrying conductor count is thus:

#2 THHN: 3 each
#6 THHN: 3 each
#10 THHN: 7 each
#12 THHN: 6 each
#14 THHN: 6 each

Ampacities based on NEC table 310.16, THHN insulation in dry locations, maximum temperature rating is 90° C (194° F) based on the ambient temperature of 30° C (86° F)

Grounding conductors for each of those circuits, based on NEC Table 250.122 (all conductors are copper):

100 amp circuit: #8
70 amp circuit: #8
30 amp circuit: #10
20 amp circuit: #12
15 amp circuit: #14

The final conductor count is:

#2 THHN: 3 each
#6 THHN: 3 each
#8 THHN: 2 each
#10 THHN: 9 each
#12 THHN: 9 each
#14 THHN: 9 each

The plan is to use two 1 and 1/2-inch EMT conduits between the electrical service panel and the 4 x 4 square wireway. According to NEC Chapter 9, Table 4, the 40% cross-sectional size of this conduit is 526 mm2. It is easier to simply use metric measurements for this. The cross-sectional wire areas are found in Chapter 9, Table 5. Chart of various conductor sizes and areas:

Conductor	Area (mm²)	Total conductor	Total area (mm²)
#2 THHN	74.71	3	224.13
#6 THHN	32.71	3	98.13
#8 THHN	23.61	2	47.22
#10 THHN	13.61	9	122.49
#12 THHN	8.581	9	77.229
#14 THHN	6.258	9	56.322

Thus, in order to break this up into two 1 and 1/2-inch conduits, the #2, #6, and #8 (main transmitter HV power supply, backup transmitter, and grounds) are run in one conduit, the remaining circuits in the other. The idea is that the main transmitter and backup transmitter will not be running simultaneously for long periods of time. Those cable areas total 369.48 mm², well within the 40% limit of 526 mm² for 1 and 1/2 inch EMT. The rest of the circuit’s cable areas total 256.041 mm². That leaves room for additional circuits in the second conduit if future needs dictate. The extra conduit area will make pulling the wires through easy.

From the square wireway to the HV power supply, 1 and 1/4 inch conduit will carry the three #2 and one #8 ground. 1 and 1/4 inch EMT has a cross sectional area of 387 mm², the conductors contained within will be 271 mm². Less room here, but still well within the 40% limit.

Pictures will be posted when the project is done.