Paul Thurst – Page 58 – Engineering Radio

Filtering for co-located FM transmitters

Well-sited FM transmitter locations usually want some height above average terrain. This means either a tall tower or a high hill or mountain. Once a site is developed, co-location of other FM transmitters often happens because sites are expensive to develop. A second station can save money by using existing facilities.

For all those newly permitted LPFM stations; pay attention. If you are going to be co-located at an existing FM broadcast site, you may need to do this too.

Interference from intermodulation mixing products can develop when FM transmitting antennas are in close proximity. This is especially true with solid state, broadband PA commonly used in today’s VHF FM transmitters. Thus, when antennas are closely placed, external filtering is required.

WUPE FM transmitter site, North Adams, MA

This is the case with a current project in North Adams, Massachusetts. New England Public Radio is placing WNNI on the air from the WUPE-FM site. WNNI is using one of those new Harris (now GatesAir?) Flexiva transmitters and WUPE-FM uses a Crown FM-2000A. The antennas are on separate towers, but the towers are in very close proximity, about 30 feet apart. In order to avoid any possible problems, a Shively 2602-3A-FB 3 pole filter was installed on each station. The filter is a bandpass for the station installed and a notch for the other station.

The primary concern here is mixing products between the two transmitters. Both have broadband solid-state amplifiers with low-pass filters before the output connector. There are three frequencies of interest;

(F1 – F2) + F1 or (100.1 MHz – 98.9 MHz ) + 100.1 MHz = 101.3 MHz
F2 – (F1 – F2) or 98.9 MHz – (100.1MHz – 98.9MHz) = 97.3 MHz
F2 + F1 or 100.1 MHz + 98.9 MHz = 199 MHz

That, plus harmonic measurements out to three or four harmonics of the fundamental frequency should be enough to demonstrate compliance with FCC out-of-band emissions standards.

Measurements on these frequencies must meet the emissions standards outlined in FCC 73.317 (d), which states:

Any emission appearing on a frequency removed from the carrier by more than 600 kHz must be attenuated at least 43 + 10 Log10 (Power, in watts) dB below the level of the unmodulated carrier, or 80 dB, whichever is the lesser attenuation.

It is also noted that this site has several cellular carriers and no doubt has or will have LTE at some point. We all know that rural LTE installations can create self-induced problems, which are then conveniently blamed on the nearest broadcast station because, hey, why not?

To further complicate matters, New England Public Radio also has a translator, W266AW (101.1 MHz) on the same tower as WNNI. The same measurements noted above must be repeated for the translator.

WNNI equipment rack. This is one of those new Harris (GatesAir?) Flexiva FM transmitters.

WUPE FM filter installation

WNNI 4 bay half wave spaced Shively antenna. Antenna for W266AW below

New WNNI antenna mounted on cell tower next to WUPE-FM tower. The W266AW translator antenna is directly below WNNI’s main antenna.

WUPE-FM 3 bay half wave spaced Shively antenna

WUPE-FM antenna installed on the original broadcast tower. I believe the tower dates from 1959 or so.

It is important to get this type of installation right the first time. Creating interference all around or above the FM band is never a good strategy. Going back to ask for more funds to make something right is also highly frowned upon.

A brief Pictorial History of early micro-computers

I enjoy history, perhaps more so than others. My first computer was an Apple IIC, purchased in 1985. It had a single 5 1/2 inch floppy drive, no hard drive, and a wee little monochrome monitor. Basically, it was a glorified word processor until I figured out how to connect it to my Amateur Radio transceiver. Here is a picture of my radio room, circa 1988 on Guam:

Finding more things to do with it became a hobby of sorts. After a while, I realized that what I wanted was what we termed an “IBM clone,” what is known as a PC today.

When I came across this blog post; History of Computers, I found it interesting and thought I’d share.

Enjoy!

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…