tech stuff – Page 54 – Engineering Radio

WSPK antenna replacement, part I

WSPK is located on North Mt. Beacon, which is the highest point for miles around. It has a fantastic signal. The site is a little difficult to get to, however, especially in the winter. In previous years, the road has been impassable four months out of the year. Some engineers have hired a helicopter to get up there when the snow is deep. For that reason, it is important to keep the equipment in good shape.

WSPK Shively 6810 antenna with damaged top radome

After last February’s snow/rain/ice storm, it was noted that the top antenna radome was missing its top. A tower climber was sent up to look at it and it was also discovered that the top bay was bent down and the element was almost cracked in half. A result of falling ice, likely from the big periscope microwave reflector (passive reflector) mounted above it.

The periscope reflectors went out of service in 2007, but the tower owner did not want to pay to take them down, thus a problem was not being solved. It was decided to replace the 25-year-old Shively 6810 antenna with a new one, during which work, the radio station would pay to remove the reflectors from the tower. In exchange for that work, the radio station would then be able to repair and remount the old Shively antenna below the new one, thus having a backup antenna. Problem solved, except for, you know: The actual work.

The tower and the periscope microwave system were installed in 1966, operated on 12 GHz, and were used by the Archdiocese of New York to relay their educational television programming from their Yonkers headquarters to the various schools in the Hudson Valley. Sometime around 1975 or so, the FCC mandated that periscope microwave systems could no longer be used due to all the side lobes and interference issues they caused. They were to be taken out of service as soon as possible. The Catholic Church, being a multi millennial organization figured “as soon as possible” meant within the next fifty years or so. Anyway, somebody else needed that frequency, therefore in 2007, they bought the Archdiocese a new digital microwave system.

The problem with the reflectors; they are big. They are also heavy, and present a huge wind area. They are also 300 feet up in the air.

WSPK tower periscope reflectors seen from ground level

Finding a day with lite winds on top of Mount Beacon can be a problem. Luckily, the weather was with us. Still, it took a while to get this work moving along. The other consideration is RFR and tower climber’s safety. There are two digital TV stations, WSPK, several cell carriers, something called “Media Flow,” and a bunch of two-way radio repeaters. The main concern was WSPK, the DTV, and Media Flow since the top of this tower is right in the aperture of those antennas. All either went way down in power or off the air while this work was ongoing.

Rigging a gin pole and getting it to the top of the tower was a chore. The gin pole needed to be threaded through those torque arms like a needle.

The tower riggers truck had two winches, one a basic 120-volt capstan, the other a hydraulic winch in the bed of the truck with 1/2 inch steel cable.

The bolts holding the reflectors in place had to be cut with a saw, you can see the tower climber working on the left-hand reflector, which gives you an idea of size. If this reflector were to fall off the tower, chances are good that major damage and or injuries would result on the ground. Proceed with extreme caution.

Cutting bracket mounting bolt on periscope reflector

Carefully lowering reflector past Shively 6810 FM antenna and Scala PR-950U microwave antenna. During this phase, the tower climbers had to push the reflector out away from those obstacles with their legs. You can see the gin pole at the top of the tower.

lowering periscope reflector — Lowering Periscope reflector

Another view:

Another view:

Almost down to the ground. This measured 15 by 10 feet and ended up weighing 830 pounds.

One down, one to go. I can’t believe those gigantic things were at the top of this tower, on the top of this mountain for 43 years and the tower is still standing. This is going to change the appearance of the mountain top from down below. For years, it looked like a pair of mickey mouse ears, now it will only look like a tower. I wonder what the environmentalists will think.

I will make a second post with the antenna pictures as this one is getting a little long.

Care and feeding of Propane Fueled Generators

Broadcasters historically have tried to remain on the air during emergency events like major storms, earthquakes, and other forces of nature. Oftentimes, commercial power is interrupted, and thus, the backup power generator is installed. Propane-powered generators for medium duty (power up to 45 KW) are popular because of the decreased environmental hazards, availability and expense of fuel, and ease of maintenance and repair. This sized generator can run the critical loads of a studio facility or a transmitter site with TPOs between 5 and 10 KW.

Katolight 45 KW generator w/outside housing

Most propane generators use a gasoline engine modified to use propane. These generators can also use natural gas, however, because natural gas has slightly less energy, the generator’s service rating is reduced by about 10 percent.

The biggest error I consistently see with propane generators is improper fuel tank sizing. It might seem just fine to plop a 500-gallon tank down next to a 45 KW generator and expect everything to be just fine. 500 Gallons may sound like a lot of fuel, but the more important consideration is tank vaporization, that is to say, how fast the liquid propane can be removed from the tank for use. Propane fuel companies should be able to size these things correctly, most of them have books and charts that tell what capacities and sizes are needed. However, as a general troubleshoot guide, the following information is provided:

Generator manufacturers will specify how many BTU per hour a generator will require under full load. If not, these are some conservative rules of thumb:

For every 1 KW of electrical generation, 2 horsepower of the engine is needed*
Under full load, each horsepower will consume 10,000 BTU per hour*
Propane has 92,000 BTU per gallon
Propane weighs 4.2 pounds per gallon

*Note: These are not the figures you will find in your engineering handbooks, they are adjusted for generator winding and engine efficiency.

Propane Tank Vaporization Rates (Continuous BTU/hr vs volume at tank temperature):

Size propane in a tank (assumes 1/3 full)	Maximum continuous BTU/hr at degrees F
Size propane in a tank (assumes 1/3 full)	0°	20°	40°	60°	70°
120	129,600	188,640	247,680	308,160	338,400
150	146,880	213,790	280,700	349,200	383,520
250	253,800	369,400	485,000	603,480	662,700
325	321,300	467,670	614,000	763,900	838,900
500	396,270	567,700	757,300	942,240	1,034,700
1000	708,480	1,031,230	1,353,980	1,684,600	1,849,900
1450	816,120	1,253,400	1,645,690	2,047,550	2,248,480

Note: Tank vaporization depends on fuel level, tank temperature, and withdrawal rate. The above chart is a conservative generalization and represents a safe median value.

If a propane tank cannot vaporize fuel fast enough, the generator will begin to run lean, eventually overheat, and shut down. The vaporization rate depends on the tank temperature, which drops as fuel is withdrawn. For the above-cited 45 KW generator called to duty after a severe winter storm, the tank would need to vaporize: 45KW x 2 HP = 90 HP. 90 HP x 10,000 BTU/hr = 900,000 btu/hr. A 500-gallon tank is too small for that size generator.

As the tank temperature drops a propane tank can develop frost on the outside of the tank, even on a hot summer day, which compounds the problem.

The correct size tank for a 45 KW generator is 1000 gallons. This can also be two five-hundred-gallon tanks connected in parallel via a high-pressure line.

45 KW propane generator with two 500 gallon tanks — 45 KW propane generator with two 500-gallon tanks

Also note, the generator’s radiator is facing the tanks so that when the unit is running, hot air is blowing on the tanks, warming them up. This particular generator is about 25 years old, which is why it looks a little worn. It still carries the load and mechanically is in sound condition.

Most propane fuel systems have two regulators; one high-pressure regulator on the tank, which takes the variable tank pressure and steps it down to about 10 PSI, and the vaporizer which steps the pressure down to a few ounces per square inch (or inches of water column) and adds air creating propane gas for the generator to burn.

High pressure propane tank regulator — High-pressure propane tank regulator

It is important that the vaporizer be mounted above the snow line and that there is a little screen on the air intake, otherwise, mud wasps will build a nest in the air intake and the next time the generator is required to run, it won’t start.

Low pressure propane regulator/vaporizer — Low-pressure propane regulator/vaporizer

Fuel piping is also a concern, if the low-pressure lines are not large enough to handle the required BTU, the generator will run lean, creating the same problems as an improperly sized tank. Different piping has different capacities, see the following charts:

Propane steel pipe sizing diagram — Propane Schedule 40 steel pipe sizing diagram

Propane copper pipe sizing diagram — Propane copper-K pipe sizing diagram

Assumes pressure less than 1.5 PSI, one MBTU is equal to 1,000 BTU per hour.

Once the generator is installed, maintenance is required. As a minimum:

Exercise the engine bi-weekly for 15 minutes. Propane generators do not need to run under load.
Check fuel, oil, and antifreeze levels monthly, more often if heavy use.
Change the oil, oil filter, and air filter, and check the antifreeze freeze point, and battery electrolyte specific gravity yearly
Change out belts and hoses as needed, and pay close attention to the block heater hose, this is where leaks often develop
Clean out mice nests and droppings as needed

Mice love generators.

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.