AM – Engineering Radio

WKIP; removal of the taller tower

This is the original tower for WKIP, but not the original antenna. It was put up circa 1960 or so and like many towers from that era, has hollow legs. Thus, after 60 years or so, it is deteriorating from the inside out.

This was part of a two-tower directional array. It is odd that a class C station on 1,450 KHz would have a directional antenna at all. Even stranger still, it was directional daytime, non-directional night, both at 1,000 watts. The reason for such an odd situation; the station was co-owned with WGNY in Newburgh and the daytime coverage contours would have overlapped without a directional array. The taller tower is 215 degrees tall with top loading. During the daytime, the pattern goes to the North and it covered very well.

Vertical Bridge, the tower owner, decided it was time to replace the aging structure with a monopole. They are completing the project this summer. Our part is to move WKIP to the shorter tower and put up a temporary FM antenna for the translator. Once the project is completed, WKIP will operate from the shorter tower (which is 85 degrees) permanently, getting rid of the now unnecessary directional antenna on a class C channel. The translator antenna will move back to the monopole, once it is put up.

Problems… Yes, we have a few of those…

First, the short tower had a broken guy wire. Actually, the guy wire was fine, but the lowest grip connecting to the equalizing plate was rusted through. It is fortunate that this was discovered because the upper guy wire was getting ready to let go too. Northeast Towers was able to replace all of the grips on that set of guy wires and re-tension the tower. They did a full investigation of all of the other anchors prior to any climbing. This is in a swamp, which has flooded several times over the last few years.

Tower #2, guy wire repaired, Scala FMVMP translator antenna mounted

Next, the temporary FM translator antenna was hung on the tower. It was thought that the 3/8 sample line from the old AM sample system could be used as a temporary transmission line for this system. Unfortunately, that line turned out to be 75-ohm cable TV drop line and was not suitable for transmission of VHF. We had about 600 feet of leftover 3/8 sample line (Cablewave FCC 38-50J) from a decommissioned AM site, so we used that instead. It has quite a bit of loss on VHF, however, for temporary use, it will work.

Next, it seems this black rat snake had taken up residence in the ATU cabinet. The bottom of the ATU was full of mouse nests going back many years. One of our employees dutifully cleaned out the mouse nests unknowingly under the watchful eyes of this snake. Only after he was done, did he see the snake coiled up on the disused current meter shunt. There was a mild freakout for several minutes, but the snake left on his own and we got back to work. The black rat snakes are helpful to have around, but perhaps best if he stays outside of the ATU. We will seal up the entryway for the coax, which seems to be where all the critters are coming in.

This Kintronic Isocoil was mounted to the back of the ATU with unistrut. Even though this is a temporary installation, I have found that sometimes temporary things can last much longer than anticipated. Besides, it was easier than trying to use pressure treated 4 x 4 lumber.

Next, we measured the ATU with the fancy machine (Agilent E5061B network analyzer). In theory, the ATU input should be 50 ohms to match the incoming transmission line. No, instead it was 38 Ohms -j20.

So, a little bit of a retune was required. With the fancy machine, we were able to get it to 52 ohms -j9 or so. This is good enough for now, there will be numerous cranes in the air and the station has an STA to run at 250 watts for the project’s duration. After the new monopole is up, we will measure the base impedance of the tower and tune up the ATU for 50 ohms and then return the station to full power at 1 KW.

Smaller crane, used to assemble the larger cranes

The old tower coming down:

Top section and top loading wires separated

Two cranes were used; one to hold and lower the tower section, the other to lift two tower workers to cut away the sections. The tower was deemed unsafe to climb, therefore it had to be removed like this. It was also unsafe to drop because of the proximity to the studio building and the other tower, which is being retained.

You get the idea. These tower sections and guy wires were cut up and put in a scrap metal dumpster. They will be recycled into something else.

Now, they will work on removing the old tower base and putting up the monopole. Once that is done, we will tune up the AM on the short tower and get it back to full power.

On the subject of All Digital Medium Wave

I want to explore all digital modulation methods for Standard Broadcast (AM, Medium Wave, or Medium Frequency). The most pressing technical problem for AM reception is electrical impulse noise. Can digital modulation solve this problem? Perhaps, but I am a natural-born skeptic.

To start out; I will say up front that the hybrid HD Radio (MA1) employed on AM was (or still is) a travesty. It never worked very well and it created massive interference +/- 20 KHz of the assigned frequency, especially when employed at night. Secondly; the all-digital version of HD Radio (HDMA3) remains a proprietary system with non-standard codecs. The current owner, Experi, has a license fee structure based on station type (AM, FM, LPFM, or Non-commercial) which ranges from $5,000 to $10,000 one-time fee for a five-year period. In all fairness; DRM pays a technology license fee to Fraunhofer for MPEG codecs used by receiver manufacturers and broadcast equipment. This is estimated to be between $0.13 to $1.13 US per receiver.

Those things being said, I thought a deep dive into the technical side of HDMA3 and DRM (Digital Radio Mondial) would be interesting. I did an article comparing MA3 and DRM a while ago: All Digital Medium Wave Transmission

What challenges are there to transmitting digital radio on MW? First, there is the very limited bandwidth of the channel itself. In North and South America, AM channels are spaced every 10 KHz (9 kHz in other places). On Medium Wave, the analog channel is +/- the carrier spacing, e.g. 20 KHz (or 18 KHz) with half of that channel potentially interfering with the adjacent channels. On a 20 kHz channel, this limits data transmission rates to 72 kbps or less with DRM and 40 kbps or less with HDMA3.

Secondly, skywave propagation is a potential difficulty for all digital broadcasts. Ionospheric changes can create multipath and fading, especially as the sun rises and sets causing the D layer to form or dissipate. Changes in the E and F layers can make or completely break skywave reception. Ground wave reception is reliable out to the limits of the noise floor, and varies based on transmitter frequency, power, and ground conductivity, and electrical noise in the area.

Everything that can potentially mitigate noise and skywave reception problems is a trade-off between robustness and data throughput.

Screenshot of an HF DRM exciter from RF Mondial showing a 10 KHz wide channel on HF.

Screen Shot of an HF DRM transmission showing mask, courtesy of DRM Consortium

This is a screenshot of an SDR showing an HF DRM transmission received from a distance:

Radio Romania International, 13,650 kHz 90 KW, 7,530 KM path

The receiver is not quite on bearing for this broadcast, however, it seems to be doing well. This is Radio Romania International’s Spanish broadcast targeting South America. The Pan Adaptor shows the signal is 10.2 kHz wide, but that doesn’t mean much from a $30.00 RTL SRD. The waterfall display below shows it is spectrally dense compared to the analog signals to the left and right. Note that with DRM there is no analog carrier being sent. Instead, a series of pilot tones are attached to various OFDM subcarriers for the receiver to lock onto.

A short Primer on COFDM

The modulation method for both systems is Coded Orthogonal Frequency Division Multiplexing (COFDM), which is the same system used by mobile phones, cable systems, WiFi (802.11), ATSC 3.0 TV, etc. COFDM consists of a group of subcarriers multiplexed onto one channel. The number of subcarriers and the subcarrier spacing relates directly to the data throughput and the robustness of the signal. OFDM is a very robust method that works well in the upper VHF, UHF, and SHF bands. It can work well in lower frequencies, however, there can be issues with multipath and Doppler effect. The coded part consists of forward error correction, which may include interleaving and subtracts from the data throughput.

The ability of an OFDM signal to reject electrical impulse noise, and deal with potential fading or multipath interference is based on a few things. The cyclic prefix sets the Guard Interval for the OFDM frame. The length of the Guard Interval should be the same as the multipath delay which helps mitigate inter-symbol interference and inter-subcarrier interference. Since the Medium Wave channels are fairly narrow, the number of OFDM carriers and spacing between carriers have a great effect on robustness. The fewer carriers the more robust the signal. This comes at the expense of data throughput; the fewer carriers the less data can be sent.

Raised Cosine impulses, similar to Orthogonal frequency-division multiplexing. (2023, May 10). In *Wikipedia*. https://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing

A short Primer on QAM

Each individual OFDM subcarrier is modulated with a Quadrature amplitude modulation (QAM) signal. The advantage of this is that each individual carrier sends data at a relatively slow rate and the aggregate data rate is the sum of all the subcarriers. QAM uses two carriers 90 degrees out of phase. The amplitude of each carrier determines the resultant vector of the modulated wave to create a data bit. For example; the sum of the carriers equals +45 degrees at 25% amplitude a 1101 data bit is sent.

16-QAM Constellation diagram. (2022, December 17). In *Wikipedia*. https://en.wikipedia.org/wiki/Constellation_diagram

Both HDMA3 and DRM can use 16-QAM or 64-QAM. The larger the QAM constellation the more data can be sent. Smaller QAM constellations are more robust. HDMA3 can also transmit QPSK, which is Quadrature Phase Shift Keying. The resultant waveform from QPSK is identical to 4-QAM.

Bringing it all together

Radio Romania International, 11,975 KHz 90 KW 7,530 KM path, English service to Western Europe and North America,

A DRM-modulated HF and MF transmitter uses both sidebands to transmit unique information. There is no carrier present but rather a few pilot frequencies for the receiver to lock onto.

WFAS White Plains, NY All digital HDMA-3 signal

I like the waterfall display available with many SDR software programs. It gives a good indication of modulation density. With WFAS HDMA-3, the area +/- 5 KHz of the carrier signal has more power than the areas that are +/- 5 to 10 KHz from the carrier.

An HDMA3-modulated MW carrier sends the same data on upper and lower sidebands, effectively halving the data rate of DRM. There is a full carrier present, which represents approximately 25% of the transmitted power and does not contain any data. Currently, there are ~~four~~ three HDMA-3 stations transmitting in the US.

Both systems can make pre-corrections to the modulated signal in the exciter to compensate for amplifier non-linearities. This can greatly improve the MER and SNR.

The other perceived technical issue with AM radio is sound quality. This has to do mostly with poor-quality receivers, although there are some AM stations that are transmitting reduced-quality audio as well. There is a false notion that anything “digital” sounds better than analog. I would posit; it depends on several factors. Low-bit-rate audio codecs can sound abysmal. That being said, the newer high-efficiency audio codecs can sound quite good, but there are limits. With HD Radio, there is only one codec available; HDC+SBR. With DRM there are several; xHE-AAC, HE-AAC. xHE-AAC is designed to work with voice and can use bit rates as low as 12 kbps. It is possible for a robustly transmitted low-bit-rate codec to sound good with voice. It can sound okay with music, but not as good as analog FM.

Conclusion

Can an all-digital modulation format work well on the Standard Broadcast Band? The answer is; it’s complicated. One of the big positives of AM is that it is a very simple and well-tested system. Adding many layers of encoding and decoding is a violation of the KISS principle. That being said, using a digital modulation method that has been refined for mobile use over the years is a step in the right direction. There still is an issue with digital receivers; both HD and DRM. From what I have read, both formats are currently being included in several radio chip sets, yet I do not find those options in most car radios. There is a lack of public awareness, at least in the United States about digital radio in general. When someone says digital, most people think of streaming. When I am driving a rental car, I seldom find HD Radio, I do find Sirius/XM and all types of internet connectivity via smartphone apps.

WSM Nashville, Tennessee

We recently went on vacation in Tennessee, which is a great state. Most of the time was spent hiking around various state parks, investigating interesting places, or eating at various restaurants. I highly recommend the state parks, there are many and they are all good. All of that being said, I could not resist the temptation to swing by WSM on the way back to the airport. I am happy I did.

Historical Marker, WSM Brentwood, Tennesee

The site is right off of I-65 and easy to get to. There is a public dog park that is behind it, which is a convenient place to park.

WSM Blaw-Knox tower, transmitter building

The Blaw-Knox tower is impressive. The site is well-maintained overall.

WSM tower base, transmitter building and Aux tower

The main tower is fed with open wire transmission line. The aux tower is off the right. It was nice to stop and walk around the site taking pictures. The Brentwood Police Department even stopped by and welcomed us to the neighborhood.

WSM open wire transmission line, looks like 450 ohms

It is always interesting to stop by some of these more famous stations. It would be nice if more sites were recognized as historical places, given the role that radio played in 20th-century US society. Both of my parents grew up during the Great Depression. According to their stories; life was tough, it was a struggle to feed the family and pay rent, but they did have a radio in the living room which was switched on every night after dinner.

Cellular work on a skirted tower

Cell carriers generally do not like working on AM towers. It is out of their comfort zone and adds a layer of complexity to the project. However, sometimes they don’t have a choice, mainly when there is an existing site and they need to make changes. We have also had mixed results with tower contractors employed by various cell carriers. In one incident, a contractor showed up and re-tensioned the guy wires breaking all of the porcelain insulators. Another time, a contractor showed up to install footings 10 feet away from the tower and ripped up all the ground wires. One tower climber found one of the skirt wires was in his way, so he cut it off with a hack saw.

Those experiences demonstrate that it is far less expensive to have somebody on site while tower contractors are doing installation work on any skirted AM tower. And so it was today.

AT&T 5G sector assembled waiting for lift

This tower has two AM stations diplexed as well as two FM translators combined into one Nicomm antenna. We did before measurements on the AM stations using an OIB-3. Once the installation is done, we will do the after-measurements and then assist the licensee with any FCC 302 filings if the base impedance has changed significantly.

With each lift, I went out to the base of the tower and made sure that the skirt wires were clear of the mounting brackets and not touching anything else. The tower crew was Russian speakers. When they asked if the transmitter was still on, which was kind of comical (he motioned to the skirt wire, made like he was grabbing it, then jerked around like he was being electrocuted). I had to wrack my brain to try and remember: Передатчик выключен. The literal translation is “Not working.” The difference between “working” and “not working” is one consonant at the beginning of the second word which is pronounced soft for off and hard for on.

This site was the subject of a previous post: A tale of five signals.