tech stuff – Page 53 – Engineering Radio

WE2XRH and the NVIS antenna

WE2XRH looks like an Amateur radio call sign but it is actually the call sign of an experimental short wave station in Alaska. Transmitting DRM on 4.85 MHz, 7.505 MHz and 9.295 MHz with a Near Vertical Incident Skywave antenna system, they hope to cover all of Alaska and almost nowhere else with shortwave broadcast.

WE2XRH DART coverage with NVIS antenna system

This license was granted for two years in August of 2008 and renewed again this September until July 2012. According to the website Nextgov.com:

The company told FCC that its initial tests would be funded by and conducted for the Defense’s Joint Electromagnetic Technologies program, a classified operation whose mission is to develop technologies for use by special forces and intelligence units.

Defense also will supply surplus transmitters from the closed, Cold War-era Over the Horizon Radar, located in Delta Junction. The radar system bounced shortwave signals off the ionosphere to detect aerial targets, such as Soviet bombers, at ranges up to 1,800 miles.

The transmitters are 100 KW Continental HF units, which for this applications are running about 20 KW. According to this Yahoo Groups posting, several Japanese shortwave DXers have received the station in late 2009, but nothing recently. I shot an e-mail off to their information address, but did not receive a reply.

On High Frequency (HF) NVIS has been used for several years where line of sight VHF communications are not possible. Soldiers during the Vietnam war noticed that if a vertical whip was bent over so that it was horizontal to the ground, the signal strength was slightly less but the signals were much less prone to fading.

Near Vertical Incident Skywave antenna angle vs. distance

In this case, WE2XRH is using a crossed dipole antenna which generates a circularly polarized field. With traditional HF skywave, polarization is not a factor since the ionosphere usually causes some field rotation anyway. It is interesting that the system had this design consideration.

The NVIS is a novel approach and it may work on Medium Frequency (MF) during the night time, but daytime coverage would still have to rely on ground wave signal. The FCC has historically approached MF skywave as a secondary and unreliable transmission method. The idea being to reduce the antenna take off angle to as low as possible, hence the popularity of taller than 90 degree towers. There is good validity to that practice as mixing the ground wave and skywave components at a receive antenna will cause multipath fading.

Setting aside a new broadcasting frequency segment, say 1.6 – 1.8 Mhz, a system could be designed to transmit DRM by using groundwave during the day with a traditional 90 degree tower, and NVIS at night with a horizontal dipole antenna. Then never the two should meet. The night time NVIS system would have a small ground wave component, out to a couple of miles. In addition to that, the night time NVIS system can run on an adaptive power system, when propagation conditions are poor, more power can be applied to the antenna input and in better conditions, power reduced in accordance with a remote receive monitor that reports the Bit Error Rate (BER) back to the transmitter controller.

The best NVIS antenna is the 1/2 wave dipole positioned between 0.1 and 0.2 wave lengths above ground. In the 1.6 to 1.8 MHz band, that equates a half wave dipole antenna 260 to 292 feet long mounted between 66 to 90 feet above ground level.

This would have many advantages over the current directional antenna based MF broadcasting system currently deployed. The current system is based on pushing potential harmful signals away from a station that was licensed to the same frequency (or an adjacent frequency) earlier. This puts the onus for proper operation on the broadcast license holder. Most don’t have the know how or resources to insure that a n AM directional is operating properly. I would estimate at least half of the directional AM antennas in this country are out of tolerance. With a NVIS based night time antenna system, coverage areas would be assigned much like an FM allotment.

The BBC conducted medium wave DRM tests in 2007 with satisfactory results during the daytime, but poor reception at night time due to co channel interference. That is why DRM will not work on the current AM broadcast band and if digital radio is to be broadcast on MF, a new frequency band would be needed.

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.