tech stuff – Page 55 – Engineering Radio

The folded Unipole antenna

In the 1990s, the folded unipole antenna was touted by many to be the savior of AM radio. There were many claims that a folded unipole antenna did not need a complicated ground system, a simple ground rod at the base of the tower would work fine. That turned out to be not exactly the case. Kintronic did a study (.pdf) that basically dispelled that notion, along with several others. The folded unipole antenna performed within a few percentage points of a series-fed tower under the same testing conditions.

three wire folded unipole on a guyed tower

Folded unipoles do have the advantage of a grounded tower. Grounded towers have a distinct advantage in lightning-prone areas, such as central Florida. I can attest through my own experience, a series-fed tower is much more likely to induce lightning damage to a transmitter or ATU. Folded unipole tower systems can also be used to co-locate other antennas, such as STL, cellular, PCS, etc. Making some extra rental money on an AM tower is not a bad way to go.

I began fooling around with MANNA-GAL, which is a NEC-2-based program. It is a free ham radio program, so it is a little clunky to use and it took a while to figure out, but once I did, it is fun. I modeled a unipole antenna for medium wave use and the results are pretty interesting. First of all, I drew out X-Y part of the system on graph paper because the program requires all wires (elements) to be entered in a coordinate-based format. The Z axis is the tower, since there is only one of those, that was easy. I played around with series vs. unipole systems and the results were fairly close to what they are supposed to be. One of the nice things about MANNA-GAL is it allows the user to change the ground conditions. To add a unipole to the tower, I put 3 wires spaced one to two meters away from the primary Z-axis wire, connected them to the top of the tower, and changed the drive point to the skirt wires.

The interesting part is when I added an above-ground counterpoise instead of a buried radial ground system. I think Ron Nott, of Nott, ltd. did much of this work too. What I found was that with between 5 – 10 above-ground radials of 90 degrees or greater, the efficiencies are within about 10 percent of theoretical for a 120-buried radial system. Again, the ground conductivity plays a big role in this, poor ground conductivity will reduce efficiencies equally for both systems.

As the tower height approaches 110 degrees or so, depending on the spacing from the tower of the skirt wires, the bandwidth really starts to open up. At 110 degrees the base impedance is about 120 ohms with about 80 ohms inductive reactance. Both the impedance and reactance slope slightly upward with frequency but are linear +/- 50 KHz of the carrier. This slight asymmetrical sideband distribution can be easily canceled out in the ATU with a few degrees of negative phase shift through the T network.

Again, all of this is theoretical, but I have found that NEC is usually within +/- 10% of real-world values. It is difficult to get a handle on ground conductivity unless measurements are taken. Even from season to season, that can change.

The above-ground counterpoise requires a partial proof, according to FCC 73.186. If this were a directional station, this would be required anyway. For a non-directional station, it is pretty easy, for six radials, it would probably take about one to two days of driving around with a FIM 41. The other consideration is public exposure to RFR from the radials. This can easily be measured with a NARDA meter. More radials will spread the induced currents out more, for higher-powered stations, 10 above-ground radials might be required.

There are several radio stations in the country that are successfully using above-ground counterpoises. It seems to be a good system and requires much less material and labor to install than the traditional ground system.

Therefore, if I were designing a new AM station, I’d use a grounded tower between 105 and 110 degrees with a unipole and 6 above-ground radials 90 degrees or greater.

The surreal trip to the WICC transmitter site

What could be so bad about going to an AM transmitter site on a peninsula off of the Long Island Sound. Sounds pretty nice, right? It began just so, driving through the town of Stratford Beach parking lot to the construction gate, the towers were visible off in the distance. A nice crushed gravel road across the barrier island, I have certainly been to worse places.

WICC towers pleasure beach island — WICC towers Pleasure Beach Island, CT

And then, things begin to look a little bit different. It is really hard to put into words, seems like some other country.

Pleasure Beach Bungalows — The beginning of the Pleasure Beach Bungalow Colony

It turns out this is not quite a nice trip after all.

Pleasure beach lawless zone — Pleasure Beach Lawless Zone

I’ve been to several so-called “developing areas” like Port Au Prince, Hatti for example. Nothing ever looked this bad.

Pleasure Beach ocean side bungalow — Pleasure Beach Ocean Side Bungalow

I can imagine some family coming here every summer to spend time at the beach.

What anarchy looks like.

Pleasure beach burned out cottage — Pleasure Beach burned out cottage

The back story is this: From the 1920s until 1996, Pleasure Beach was a nice seasonal oceanside bungalow colony, complete with an amusement park. These cottages (but not the land they were on) were owned by people from the surrounding cities and towns and the entire area appeared to be quite nice in its day. Then, in 1996, the wooden bridge that connected Pleasure Beach to Bridgeport burned. There are several theories; crackheads, radical environmentalists, etc. The city of Bridgeport did not rebuild the bridge, which meant the only access was by walking from the Town of Stratford beach parking lot, a trek of at least a mile or longer. In 2007, the town of Stratford decided not to renew these land leases, and the building owners were forced to remove any remaining items they wanted by barge. Soon thereafter vandals began walking down the peninsula from Stratford. Slowly, most of the bungalows were broken into and several were burned. This is mostly the work of “kids,” who, because they are under the age of 18, get a slap on the wrist and returned to their parents. Oh, those wacky kids, what will they do next?

Truth be told, they should be the ones out here cleaning this up, for free.

Finally, this year, the city began tearing down and cleaning up the remaining buildings, trying to put the former bungalow colony “back to nature.”

The transmitter site for WICC moved here in 1932. This building contained a nighttime operating studio, kitchen, bathroom, and bedroom. I can imagine hanging out here some summer night, spinning tunes, and having a good time. The former amusement part is just out of the picture to the left. At the amusement park, there was a carousel, a big snack bar, a dance hall, and an area for portable rides like Ferris Wheels and such.

Now the building is full of disused gear, old carts, transmitter, and tower parts, the water has been shut off and I’d not want to be out here at night under any circumstances.

The antenna array consists of two 300-foot Milliken towers, originally installed at WNAC (Now WRKO) Boston, MA. They were moved to Pleasure Beach Island in 1932. Many people mistakenly think these are Blaw-Knox towers. Milliken preceded Blaw-Knox by several years. They built and designed towers around the world for radio and electric transmission. In the late 1930’s they were bought out by Blaw-Knox, which kept the design. I love these tapered self-supporters, they have survived several major Hurricanes since 1932. The south tower is about 150 yards from the Long Island Sound. Salt air seems to do them no harm, either.

WICC Milliken tower, south looking up — WICC Milliken south Tower, looking up

The station operates at 1 KW day, 500 watts night, DA2. The towers are 60 degrees tall, spaced 149 degrees. That is a little short, however, they are surrounded by salt water, so the signal goes like gangbusters. Because they are short, the impedances are low, about 10 ohms for nighttime and 30 ohms for daytime. Since the towers are so wide, the impedances are flat far beyond 50 kHz on either side of the carrier, which makes it a nice broad-banded antenna system. The 1932 phasors and ATUs were redone in 1972. All of the common point impedance measurements are still posted on the wall.

WICC Harris SX-1A, Phasor and Harris BC1H — WICC Harris SX-1A, Phasor and Harris HC1H

The main transmitter is a 1990 Harris model SX-1A. It seems to be reliable enough, my experience with the SX-1 is it has an overly complicated control system. The backup is a Harris BC1H, a sort of hybrid solid-state tube unit, which is also reliable.

Frequency voltage meter — WICC frequency and voltage meter

This high-tech test and measurement center is attached to the incoming electrical service. Over the years, there have been some quality control issues with the incoming electrical service, mostly due to Osprey building nests on the crossarms. During rain storms, these nests catch on fire and kill the power to the site. The power company is in the process of redoing the electrical service to the building.

This is a video of the former amusement park and cottages shot two years ago when the cottages were more or less intact. It is a bunch of stills set to Pink Floyd music:

Looks like they all just got up and left.

Vintage Texar Audio Prisms

I give to you, the original Texar Audio Prism:

I love the sound of these units when coupled with an Optimod 8100A. Many people have (or rather, had) difficulty setting these things up. I found them to be very easy to deal with, just follow the instruction manual. If that doesn’t sound good, then there is something wrong with the unit. Over the years, there are only a few consistent problems. The first thing is with the voltage regulators. They have heat sinks attached with nylon screws. The screws get brittle and fall apart, making the regulator overheat and go bad. I have taken to replacing the nylon screws, and if the heat sink has fallen off, the entire regulator. There are also a few electrolytic capacitors in the power supply and on the audio board, it is always a good practice to replace those. Otherwise, unless the unit has been blown up by lightning, it should work.

As for set up, follow the directions in the manual:

Bypass the units using the bypass switch
Turn on the onboard pink noise generator
Using the test ports on the front of the unit, plug a Simpson 260 VOM set on 2.5 VAC important: use the ground port on the front of the unit, not the case
For use with an Optimod 8100A, using the dB scale on the Simpson 260, set all the bands for a 4.0 reading. Set the density to 3/4.
Turn off the pink noise generator and switch out of bypass mode.
Make sure the levels in the studio are where they should be.
Adjust the input gain so the “Buffer Active” light does not come on during normal-level programming.
Adjust the output levels so that the input buffer on the Optimod reads between -7 and -3 vu.

The rest of the settings are on the Optimod:

Clipping = 0
HF limiting = 5
Release time = 2
Bass coupling = 2
Gate = 0
Set the input attenuators for about 10 dB total gain reduction, with peaks around 15 dB or so.

Then set the L-R null. To do this, make sure the program material is in mono, then adjust the L or R input attenuator for minimum reading. Also, if the Audio Prism has PR-1 (phase rotators) installed, bypass the phase rotator in the Optimod. There is also a replacement card 5 made by Gentner called the RFC-1 for the Optimod 8100A. I notice little difference between a stock Optimod and an RFC-1 Optimod.

That is a good starting point. Most people are quite happy with this, but if needed, the high and low settings on the Prism can be adjusted slightly to suit the station equipment. When properly adjusted, this equipment rides gain, and adds a certain amount of loudness, while keeping the programming material natural sounding. Further, unlike some “modern” air chain processors, it does not boot up and it does not occasionally lose its mind, requiring a reboot.

The best paragraph in the manual, or any broadcast equipment manual is this:

There is a wealth of information available in the LED display. A few minutes of watching them in reduced light (emphasis added) while listening to a familiar program input will greatly help in understanding their action.

It will also greatly enhance your buzz, dude. It was the 70’s.

Digital Radio Mondiale, an alternative to HD radio?

Could be. Digital Radio Mondiale, or DRM, is a modulation scheme that a group of broadcasters and transmitter manufacturers have been working on since about 1997 or so. There are numerous shortwave broadcasters; the BBC, the CBC, Deutsche Welle, Radio France Internationale, the VOA, and others have been using DRM on shortwave for several years now. I can state that the shortwave DRM system works well, I have a software decoder and use the sound card input on my computer to decode and listen to DRM shortwave broadcasts.

The goal of DRM is to establish a worldwide open standard for digital broadcasting in the LF, MF, HF, and VHF bands. In the early years of development, DRM was designed for digital broadcasting on bands below 30 MHz. This system is now known as DRM30. Since then, the DRM consortium has expanded that to the VHF band (up to 174 MHz) as well (meaning where the current FM band is located) and has called that system DRM+.

DRM uses COFDM (Coded Orthogonal Frequency Division Multiplex) modulation, which is the same as IBOC HD radio^TM. This is a robust modulation system that employs multiple carriers at lower power (than an analog carrier) spread across the entire allotted bandwidth.

One of the claims is DRM transmits less power and is more energy efficient. In general, digital radio modulation does transmit less power, that is true. However, transmitters have to be run more linearly for digital due to the increased bandwidth. This may not translate to greatly increased efficiency from the AC mains to RF standpoint. Because of that, there is more waste heat, and thus more air conditioning is needed to cool the transmitter room.

Some of the advantages of DRM over Ibiquity’s HD radio^TM are:

Open source system. Royalties are paid by the transmitter manufacturers only (and to date, most major US transmitter manufacturers have already paid these). There is no royalties paid by the broadcaster to install DRM or by the consumer when purchasing a DRM-capable receiver. One company does not own the rights to the modulation system for all the broadcasters in the country.
Universally standard; accepted by the European Telecommunications Standards Institute (ETSI), International Electrotechnical Committee (IEC), and the International Telecommunications Union (ITU).
The CODEC is HE-AAC 4, which is widely used worldwide.
DRM30 and DRM+ fit into existing band plans and will not interfere with other users on adjacent channels. DRM30 is designed for 9 KHz channel spacing and DRM+ is designed for 100 kHz channel spacing, all of which comply with existing FCC regulations.
Standardized receiver profiles are things that must be included in all DRM receivers. There are several advanced options as well, such as a media-rich system that includes video.
DRM+ has several added features: DRM text, which is similar to RBDS. EPG or electronic programming guide, which shows what is coming up next and a searchable schedule of when programs may be heard out to seven days. Some DRM+ receivers will have a TIVO-like recording device that allows the user to record programs and playback later.
Traffic reporting and routing

In addition to that, DRM30 stations have the ability to transmit low frame rate H. 264 video. This is a distinct advantage for short-wave stations that are seeking a way around firewall blocking. The video image is small, 176 x 144 pixels and it is 8 frames per second, which is about as good as can be expected using a 9 KHz channel.

In some cases, DRM is capable of a hybrid mode (ed note: DRM calls this “Simulcast mode”), but what have we learned about hybrid mode digital radio: It doesn’t work very well. In short, it would be better if DRM were employed in the digital-only mode. To many, this is a distinct disadvantage, but I don’t see it that way. There have been many that have made the IBOC rollout/FM broadcasting rollout analogy. Frankly, those arguments don’t hold water. When FM was introduced, no attempt was made to shoehorn it into the existing AM (Standard Broadcast) band, it was not designed to interfere with other stations or itself, power levels were sufficient for good reception using existing technology, quality over AM was markedly improved and programming was often separate (simulcasting with existing AMs did not start until later). My point here is that any digital broadcasting should be introduced on a separate set of frequencies. Some have proposed using TV channels 5 and 6, which makes some good sense. Whatever the outcome is, we have learned, the hard and expensive way, that hybrid digital broadcasting does not work well.

A brief video about DRM30.

Currently, DRM30 is only allowed on shortwave broadcast frequencies in the US. I asked a product development engineer from a major reputable broadcast transmitter manufacturer about this, and his response was:

Medium-wave broadcasting in the US already has HD radio, so the FCC would be disinclined to allow a new standard
One might be able to apply for an experimental license to broadcast DRM, but it would likely have an expiration date
It is possible to operate DRM in a hybrid mode on the AM band and occupy the same bandwidth as HD radio^TM (30 kHz), it might also be possible to squeeze that down to 20 KHz.
Most modern (read: solid-state) AM broadcast transmitters should be able to transmit DRM without modification (antenna systems may be a different matter).

It might be fun to apply for an experimental license to broadcast somewhere in the 1600-1700 KHz range with DRM30 only and no analog modulation, except for an hourly station ID in Morse. A 1/4-wave tower in the middle of that band would be 141 feet tall. With the use of a skirt, a grounded tower can be employed. That and a few above-ground radials and the system would likely be pretty efficient. Part of the experiment would include driving around and taking signal strength readings while recording the programming material. This would give some real-world testing on how the system would perform in widespread use.

Of course, this would require a major about-face by the FCC, which is not likely unless someone there grows or somehow acquires a backbone.

Some people question the need to do any type of digital broadcasting. I am a realist, in one way, shape, or form, digital radio broadcasting will (or already is) take(ing) place. It would make the most sense if the best system were used, which is not necessarily the first system proposed. The big question is, will today’s terrestrial broadcasters be involved, or out of business?