Engineering Radio – Page 59 – One Hundred percent Human Generated Content for your Reading Enjoyment

Those Shortwave Sites

It is a joke circulating in Russia at the moment. Kind of funny when you think about it.

In light of the developing situation in Eastern Europe, it may be wise to retain some of those HF broadcasting (AKA Shortwave) sites. It may be too late for Canada, however, the US government still has a few high-powered HF sites that they may want to hold onto for a while. There are several ways that shortwave broadcasting can be beneficial.

Like all radio broadcasting, quality content is needed to attract listeners. Most of what is available on shortwave are religious or transparent government propaganda. There are exceptions to this, but they are rare. Introduce quality programming, and shortwave listenership will increase.
DRM 30 (Digital Radio Mondial) is still in its experimental phase. It has been demonstrated to work reasonably well on HF. Several digital data formats are successfully being used on HF; HFDL, ALE, STANAG 5066, PACTOR and others. DRM 30 has the advantage that H.264 video can also be transmitted.
The VOA has been experimenting with images transmitted via MFSK, AKA the “VOA Radiogram.”
HF is always susceptible to changing propagation. However, it can be reliable enough, especially when frequency diversity is employed, to overcome these issues when no other method of communication is available.
DRM and MFSK can be decoded using a simple shortwave radio and a computer sound card. A DRM CODEC is required, but those are readily available for download.
Analog shortwave broadcasting using AM is still viable. AM has the advantage of being extremely simple to receive and demodulate. Simple receiver kits can be built and run on 9 volt a battery.
While the Soviet Union had an extensive jamming network, those sites have long since been non-functional. Most countries have discontinued the practice of jamming with the exception of China, North Korea, Cuba, and perhaps some countries in the middle east (the usual suspects).

Sample of DRM reception via shortwave:

If the internet is censored or somehow becomes unavailable in that part of the world, shortwave may be the only method to convey an alternate point of view.

Hopefully, things will settle down and return to at least a civil discourse. However, it never hurts to have a plan.

Decomissioning a NEXTEL site

Remember when “NEXTEL (b-b-b-beep), how business gets done…” Well, not anymore. NEXTEL was purchased by Sprint in 2005 and their product lines were combined. Thus, all of these old NEXTEL sites have become redundant and switched off. This particular site was co-located with one of our FM radio clients, which required a power reduction while the old equipment was removed from the tower. I took the time to grab a few pictures of the process:

Former NEXTEL communications equipment room

All of the equipment was removed from the equipment shelter. This site has been switched off since June 2013 and everything in it is destined for the scrap yard. This equipment worked on the 800 MHz bands, which have been re-purposed for Public Safety and Critical Infrastructure, e.g. government users. These racks and radios look like they were expensive:

Speaking of expensive, this site had over 4,500 feet (1,370 meters) of 1 5/8 inch foam coax, which was cut up and scrapped. At today’s prices, that cost $13.25 per foot.

The tower was rigged:

Each of the three panel sector mounts were removed and lowered to the ground.

The NEXTEL antennas were mounted at the 260-foot (80 meter) level of a 395-foot (120 meter) tower. It took some time to remove all of the antennas and equipment from the tower.

Cellular panel antenna array being removed from a tower

I looked on the Sprint website and could not determine if they still offer a push to talk service option (direct talk). With all of the communications options available today, I do not expect there would be much call for it.

For old times sake, here is an old NEXTEL commercial from many years ago:

They did have a good marketing department…

Engineer Killer

That was the title of the email with this photo attached:

Disabled high voltage shorting bar, Collins 820D-2 AM transmitter. Courtesy Pete Partinio

That seems about right.

For many, many reasons, this is a bad thing to do. First of all, the shorting bar is the last point of discharge for the high-voltage power supply. When all else fails, this is designed to route the 3,500-volt plate supply safely to ground. Having a stray 3,500 volts floating around inside a transmitter is never a good idea. Fortunately, it was spotted and removed before anything bad happened.

Secondly, it looks like somebody used a 12 VDC cigarette lighter plug as an insulating device. Wow, did they get lucky. This could have started a fire.

As to exactly why it was there in the first place, I cannot rightly say.

And this is why only properly trained people should be working on transmitters, especially tube-type ones.

The AM Receiver problem

The technical problems with AM broadcasting can be broken down into three broad categories:

Interference from other AM stations
Interference from unintentional radiators (AKA electrical noise)
Poor receivers

Much of the poor fidelity issues with AM broadcast audio come from the narrow IF bandwidth of the typical AM receiver. Older AM receivers had much wider IF bandwidths, sometimes as much as 15 KHz +/- carrier. As the AM band was overfilled with stations starting in the late 1940s, this became a big problem. The tube-type front ends with great sensitivity but not very much selectivity was unable to cope with adjacent channel interference, leading to what was known as “monkey chatter.” This type of interference can be technically described as the higher audio frequency peaks from adjacent channel stations being demodulated. Those hearing this type of interference found it very annoying and rightly so. Thus, receiver manufacturers were deluged with complaints about the poor quality of their units. The solution was simple; narrow the bandwidth until the “monkey chatter” disappeared. This new de facto standard IF bandwidth turned out to be +/- 3 KHz carrier.

It does not take a rocket scientist to see that 3 KHz audio is slightly better than telephone quality. This was the beginning of the perceived AM low fidelity problem. In the meantime, FM broadcasting, after years of lagging behind in spite of its superior audio, made great strides into mainstream acceptance.

NRSC-1 was supposed to reduce this type of interference by limiting AM broadcasting stations’ audio bandwidth to 10 KHz. The idea was to attempt to keep the modulation index somewhat within the allotted channel. This standard was mandated by the FCC in 1989, after which receiver manufacturers were to change their design to allow for broader IF bandwidths, thus improving AM fidelity. There was even an AMAX standard adopted by some receiver manufacturers. Unfortunately, by this time, the majority of AM stations were transitioning from music to talk radio. The new standards were too little much too late.

A quick scan with a quality AM receiver shows that many stations are transmitting high-quality audio, which, with a properly adjusted IF bandwidth can sound remarkably good:

Screen shot - WEOK true oldies channel — Screen shot – WEOK True Oldies Channel

This is a screenshot from an SDR (Software Defined Radio) showing WEOK, Poughkeepsie, NY broadcasting the True Oldies Channel. The signal strength is slightly low, but this is a rural area and the noise floor is also low. I limited the bandwidth to +/- 7.5 KHz carrier because of the pre-emphasis used on most AM stations makes the high-end sound strident. Looking at the spectral display, there is more audio available beyond what I am listening to. This brings me to this; AM fidelity is not inherently inferior, it can sound quite good. There is no reason why AM receiver manufacturers cannot improve their products to include some advanced features;

Variable IF bandwidth based on signal strength
Variable user selected IF bandwidth
Sharp selectivity – adjacent channel rejection
Selectable sideband demodulation (carrier plus upper or carrier plus lower sideband)

While this will never sound as good as FM stereo, it still can sound pretty good, especially with older music recorded before say 1975 or so.

Manufacturers would have to have some impetus to include these features in their chipsets, such as multiple requests by listeners who are looking for better AM quality, which leads us back to programming…

The other issues with AM electrical noise reception and interference from other radio stations are surmountable, so long as there is a reason to. This, leads us back to… programming.