tech stuff – Page 53 – Engineering Radio

Passive AM Monitor Antenna

At the place of my former employment, there is an issue with AM reception. The building is full of old, electrically noisy fluorescent light ballasts, computers, mercury vapor parking lot lights, and every other electrical noise generator under the sun. The second issue is that one of the EAS monitor assignments for two FM class B stations is WABC in NYC. Under normal conditions, WABC puts a fine signal into the area. Listening to it is not a problem at my house, in the car, and whatnot. However, at the studio, the station is audible but terribly noisy. Every time one of those FM stations ran a required monthly EAS test originating from WABC, it was full of static and just sounded bad on the air.

The state EAS folks were inflexible as to the monitoring assignment. “WABC is the PEP station for NY. You should have plenty of signal from WABC at your location,” said they.

At one time, the studio had an active loop antenna (LP-1A) from Belar, which worked but also seemed to amplify the noise. I decided that the best thing to do was go big and ditch the preamp. I made a diamond-shaped receiving loop on two pieces of two-by-four by eight-foot lumber. I wound four turns of #14 stranded wire around this frame and made a 4:1 balun to feed the unbalanced 75-ohm RG-6 coax.

That cured the noise problems and for eight years, WABC sounded pretty good on the EAS monitor.

Fast forward to about a week ago. The roof at the studio building was being redone and all the monitor antennas had to be removed from the roof. The homemade loop was not in good shape. The balun box was full of water, the lumber was cracking and falling apart, the insulation was degraded by UV exposure, etc. My boss asked, “How much to make a new one?” So I said something like forty dollars and a couple of hours. He then said, “Make it so we don’t have to ever make another one.”

Music to my ears. I started by checking my assumptions. I made a model and ran NEC to see what the electrical characteristics for that size loop were on 770 KHz. It came out better than I thought, with about 1-ohm resistance and 282 ohms inductive reactance. Fooling around a little more showed that roughly 1.3 uH inductance and 720 pF capacitance in an L network would bring this in line for a 50-ohm feed point. Since this is a receive-only antenna, that is not a prime consideration. I am more concerned with noise reduction and maintaining at least the bi-directional quality of a loop antenna.

Then, I decided to get fancy. What if the capacitance was put on the end of the loop to ground instead of the feed point? That, in effect, should make the loop directional off of the unterminated side. Driving the feed point with a 9:1 balun would also bring up the inductance on the feed point. Finally, grounding the whole thing with a separate ground lead might also get rid of some noise.

The final configuration looks something like this, which is essentially a top-loaded vertical:

Now to build it.

Once again, I felt that a non-conductive support was needed, so I used two by four by eight-foot lumber, but this time I painted them with oil-based paint. The side length worked out to be 5.7 feet per side, or 23 feet per turn for a total of 92 feet of wire.

I purchased 100 feet of PV (photovoltaic) wire (Alpha wire PV-1400), which is UV, heat, and moisture resistant and designed to last for 30 years in outdoor, exposed environments.

For the balun box, I used a metal outdoor electrical box with a metal cover. I put a ground wire jumper between the box cover and the ground common to maintain shielding. I used a water-tight bushing to feed the antenna wires and the ground wire into the box. I drilled a 3/8 hole for a type F chassis connector. Everything was given a little extra waterproofing with some silicone-based (RTV) sealant on all threaded junctions.

The spreaders for the wire windings are UV-resistant 1-inch PVC conduit. I drilled four holes, three inches apart in each spreader to run the loop wires through.

The balun is 7 trifiler turn of 24 AWG copper wire on an FT-43-102 toroid core. Trifiler means three wires twisted together before winding the toroid core.

I used all stainless steel screws and mounting hardware.

The loop is terminated with a 500 pF, 500-volt ceramic capacitor to ground. Once in place, I am going to experiment with this by jumping it out of the circuit to see what effect it has on noise and signal strength. I may also try replacing it with a 200-ohm resistor and or a 1000 pF capacitor.

The assembly was pretty easy, although time-consuming. My four-year-old son helped me paint the wood and string the wires through the spreaders.

I soldered all wire connections with 5% silver-bearing solder.

When the whole thing was assembled, I tested it out with my Drake R8 receiver. It performs much as expected, with low noise, directional away from the terminated wire loop. It does not appear to be too narrow-banded either, as the stations on the high end of the dial were also received with good signal strength.

Next was loading it on the pickup truck, driving it in, and mounting it on the studio building. I got some funny looks from my fellow travelers, then again, I usually do.

For the ground, I purchased an eight-foot copper-clad grounding rod and pounded it into the ground at the corner of the building. This area is always wet as it is the lowest area around the building and all the gutters drain there. This is not best RF ground, but for the purposes of this antenna, it should work fine. I used about 28 feet of leftover #12 stranded wire from the ground rod up to the balun box and connected it to the common ground point inside the box.

The frame itself is mounted on a standard wall-mount antenna pole. Stainless steel clamps hold the wood frame to the pole.

Once it was installed, I used my Kenwood R-2000 receiver to find the best mounting azimuth and locked everything down. I also put a toroid on the RG-6 coax coming up from the rack room to keep any shield noise from getting into the antenna.

The tuning capacitor is in there too, behind one of the loop wires.

Antenna installed. I did try substituting the 500 pF capacitor with a 220 resistor. The signal strength came up somewhat, but the noise increased more, therefore the capacitor is a good termination for this antenna.

With this antenna, the signal from WABC is nice and clean and sounds good on the FM station when a monthly EAS test is retransmitted.

What is 200 KHz divided by 400 KHz?

The standard FM channel in the United States, as defined by the FCC is 200 KHz (See CFR 47 73.201). The occupied bandwidth of an FM IBOC signal, as created by Ibiquity, Inc., is 400 KHz. See the below picture:

HD radio trace on FSH3 Spectrum Analyzer

A picture is worth a thousand words. Engineering types will understand this without explanation. For non-engineering types, here are your thousand words (or so):

On the left-hand side of the screen is the signal strength scale. Each vertical division is 10 dB. This is not absolute signal strength, it is referenced to -20 dBm. However, it gives a good relative signal strength for both the analog carrier and the IBOC carriers. The analog carrier is centered on the screen, it slopes upward like a steep mountain, peaking at -50 dBm relative. The IBOC carriers are on either side of the analog carrier, they are flat, approximately 75 KHz wide, and peak approximately 20 dBm below the analog carrier (-20 dBc). For some reason, likely the bandwidth and/or impedance match between the antenna, high-level combiner and the two transmitters, the left IBOC carrier is actually peaking around -14 dBc.

The span, as noted on the bottom right-hand side of the screen is 500 kHz. Each horizontal division is 50 KHz. The entire span of the measurable signal is eight horizontal divisions, thus 400 KHz.

As noted above, the allocated channel bandwidth is 200 KHz, thus this station is exceeding it’s allocated bandwidth by 100%. This is allowed under CFR 73.404, which is titled “Interim hybrid IBOC DAB operation.”

IBOC proponents will make the argument that FM radios work on something called “The capture effect,” which is to say that if two signals are on or close to the same frequency, only the stronger signal will be demodulated. Thus, the IBOC carriers have no effect on the adjacent channels that they are interfering with so long as the adjacent signal is stronger than the IBOC carrier. The argument is further carried forward by assuming that with a station’s protected contour (60 dBu in most cases), the IBOC carrier will never exceed that analog carrier.

That is not necessarily true, especially in areas where terrain (and buildings, underpasses, unintentional directionality in transmitting antenna, etc) can attenuate signals close in causing the IBOC signal to become equal to or stronger than the adjacent analog signal. This effect causes picket fencing. Lower powered FM stations; class A, LPFM, etc, are especially vulnerable to this effect.

Further, even in areas where the analog carrier is stronger than the IBOC carrier, the noise floor has been moved from -100 dBm or so to -70 dBm, which is a 1,000 times greater. To assume that raising the noise floor by 1,000 times will have no effect is, as they used to say in the Navy, making an ASS out of U and ME. Mostly you, in this case. This affects the receiver by making it less sensitive, it will also add noise to the demodulated signal as the elevated noise floor will show up as background hiss. Even further still, higher IBOC carrier levels, as authorized by the FCC in January of 2010 can interfere with the station’s own analog carrier.

According to both Ibiquity and the FCC, which stated in the Notice of Proposed Rulemaking, the reason for interim IBOC operations are:

iBiquity’s IBOC DAB technology provides for enhanced sound fidelity, improved reception, and new data services. IBOC is a method of transmitting near-CD quality audio signals to radio receivers along with new data services such as station, song and artist identification, stock and news information, as well as local traffic and weather bulletins. This technology allows broadcasters to use their current radio spectrum to transmit AM and FM analog signals simultaneously with new higher quality digital signals. These digital signals eliminate the static, hiss, pops, and fades associated with the current analog radio system. IBOC was designed to bring the benefits of digital audio broadcasting to analog radio while preventing interference to the host analog station and stations on the same channel and adjacent channels. IBOC technology makes use of the existing AM and FM bands (In-Band) by adding digital carriers to a radio station‘s analog signal, allowing broadcasters to transmit digitally on their existing channel assignments (On-Channel) iBiquity IBOC technology will also allow for radios to be ”backward and forward” compatible, allowing them to receive traditional analog broadcasts from stations that have yet to convert and digital broadcasts from stations that have converted. Current analog radios will continue to receive the analog portions of the broadcast.

Few if any of those goals have been met. As far as the forward/backward compatible thing, it just isn’t so unless a person actually owns an HD Radio. As noted in previous posts, few consumers have seen fit to purchase an HD Radio, nor have car manufacturers taken to installing them en mass in new cars, so there is no forward compatibility. Instead, we have FM radio stations interfere with each other and themselves in an attempt to “modernize” the audio broadcasting business. This is a bigger problem for small, community radio stations that can neither afford to install the expensive, proprietary HD Radio system nor broadcast quality receivable signals with an adjacent HD Radio signal raising the noise floor by 1,000 times or more.

I can think of no other greater threat to free over-the-air broadcasting than HD Radio and the degradation of AM and FM services that come with it. The consumer has shown that they don’t care. If given the choice between free over-the-air broadcasting that has mediocre programming and is full of interference, and some type of paid internet streaming service that sounds reasonable with good programming, they’ll go for the latter.

In short, some cobbed-together digital modulation scheme is the last thing that radio needs right now.

STL paths

I learned this one the hard way, all climates, and terrain are not equal. An important detail when planning a Studio to Transmitter Link. The RF STL is usually in the 950 MHz band, although lately, people have been using 2.4 and 5.8 GHz unlicensed systems with good results. What works well in the northeast, for example, might not work that great in Florida, where tropospheric ducting and multi-path can create reception problems.

One example of this happened in Gainesville, Florida. A station there had a 15-mile path over flat ground with tall towers on either end. It had full line of sight and Fresnel zone clearance. Ordinarily, the signal strength was -65 dB, which is about 25-30 dB of headroom for the equipment being used. However, in the mornings, most often in the late summer or early autumn, there would be brief dropouts of a few seconds. After two years of suffering through the mysterious morning dropouts, we finally rented a plane and flew the STL path, only to discover there was a swamp right in the middle that was not on the topographical map. On those mornings when dropouts occurred, it was surmised that dense fog would rise up, causing the RF path to bend and creating multipath at the receive antenna. Since it was a Moseley Starlink, the digital demodulator would unlock due to high BER. The signal strength never moved off of -65 dB.

Of course, had this been an analog STL, it would not have dropped out, although it may have gotten a little noisy for a few minutes.

I have learned to be very conservative with my STL path analysis, using software tools like RF Profiler to look at the theoretical path, but also surveying ground obstacles like trees and buildings, which are not accounted for in the USGS terrain database. There are several RF software programs out there that will do the same thing.

Last week, when a station manager insisted that an STL path was possible from a proposed new studio location, I deferred to the path study, which showed only about 50% Fresnel zone clearance. While it was true that the path is less than a mile, and it is also true that one can see the top of the transmitting tower from the roof; trees, buildings, and even an access road create problems that could potentially cause STL dropouts. We are not going down that road again. The station manager, whose background is in sales, was told to find another location or order a TELCO T-1.

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.