FM transmitter site – Page 8 – Engineering Radio

North Adams tower update

As promised in an earlier post, here is an update on the progress at the North Adams tower site for the restoration work on WUPE-FM and WNNI. For those unfamiliar, refer to this post: North Adams Tower Collapse.

A contractor installed a 70-foot wooden utility pole last week. We ordered new Shively Versa2une FM antennas as replacements for the antennas destroyed when the tower fell last March. These new antennas are field tunable, which is a nice feature. The idea is that this pole will be used until the replacement tower is constructed, which is many months away. After the new tower is up, I would like to keep the pole in place as a backup facility for both stations.

The bucket truck arrived but the driver had a bit of bad news; there is room for only one person in the bucket. The boss pipes up and says “Oh, that’s okay, Paul can go up and run the bucket”

WAT!

So anyway, it turns out running a bucket truck is not a huge deal; there is a joy stick of sorts that moves the booms around, up down, sideways, etc. Once you get the feel for it, it is pretty easy and three dimensional movement becomes second nature. That being said, at 70 feet in the air, everything gets a little wobbly, so it is best not to jerk the controls around.

The antennas were mounted on a 2 inch pipe which was attached to the pole with 1/2 inch threaded rod. We left a little bit of pipe sticking up above the top of the pole to get the FM antennas as high as possible.

Some dude in a hang glider checking out the work

Getting photobombed by some guy in a hang glider is a new experience. No day is exactly like another in this line of work.

WUPE and WNNI temporary antennas — WUPE-FM and WNNI temporary antennas

The antennas were tuned up once they were up on the pole. We did this with the network analyzer, which made the job very easy. WUPE-FM (top antenna) started using this antenna on Wednesday afternoon (5/7) with greatly increased power output. This gets the station almost the same coverage area as they had before the tower collapse. We tested WNNI (bottom antenna) and it all looked good. WNNI is still waiting for a temporary wireless internet feed for program delivery. Once that is established, we will have to do the intermod measurements one more time before they can go on the air.

Here are some pictures of the cleaned-up site:

The temporary monopole being used by the cell providers:

Basically the pole is ballasted in place by those huge concrete blocks.

WEBE pictures

WEBE is fairly unique in that its antenna is mounted on the side of a 500-foot smoke stack. I took a few pictures last winter:

This is a close-up of the Antenna:

WEBE main antenna, Shively 6 bay half wave spaced, ERP 50 KW

Here is an even closer view from a different angle:

From this angle, one can see the mounting brackets and the wire mesh reflector installed on the smoke stack. From the first picture, one can see that the 400 MW PSEG coal fired power plant puts out a lot of combustion products when on line. Combustion is an exothermic chemical reaction that looks like this:

Hydrocarbon Fuel + Oxidizer + Nitrogen → Heat + CO₂ + H₂O + NO_x

Included in this are any trace elements that are found naturally in the coal that is being burned. These include things like Mercury, Nickel, Uranium, et cetera. These trace elements can concentrate around the smoke stack because they fall out of the particulate quickly and these plants burn a lot of coal. The above picture was taken on a very cold day, most of what is coming out of the smoke stack is steam.

The issue for the radio station is when the particulate matter accumulates on the antenna, effectively shorting it out. The solution was to place the RADOMES around the elements and then constantly purge the RADOMES with nitrogen. Thus, this liquid N2 tank is vital for the operation of the radio station:

Each element of the antenna has a small hole in the feed line. N2 is fed continuously into the transmission line at a pressure of about 1.5 inches water column which then purges the RADOMES keeping any combustion products out of the RADOMES. The N2 tank needs to be changed out every 18-21 days and weights over 650 pounds when full.

North Adams Tower Collapse

High winds seem to be the culprit in the collapse of two towers in North Adams. According to the Motorola system technicians, it happened at about 12:30 am Sunday morning, which is when all their link loss alarms started going off. The larger, self-supporting tower broke from its mounting plate and tipped over into the smaller guyed tower next to it. Effected are WUPE-FM and W226AW (WFCR New England Public Radio) as well as NEPR’s new station WNNI which has not officially signed on.

Cellular service for ATT, Verizon, and Sprint/NEXTEL were all knocked offline as well as internet services and E911 dispatch. Those services are coming back online, with temporary modular cell units en route. N

Here are some pictures:

WUPE-FM WNNI and W266AW transmitter building — WUPE-FM, WNNI, and W266AW transmitter building

Tower base mounting plate, apparent failure point

For pictures of the towers during happier times, refer to this post: Filtering for co-located FM transmitters.

Restoration work is underway with WUPE-FM expected to return to the air at low power by Monday afternoon.

Update:

WUPE-FM was returned to air at low power by about 1pm on Monday 3/31. We took an unused Shively 6812 antenna that was tuned to 94.1 MHz and retuned it to 100.1 by cutting 1/4 inch pieces from the end of the elements until it was on frequency. It took a bit of doing, but with a network analyzer, we were able to get it to 1.2:1 SWR with symmetrical sidebands. Running 600 watts, it covers the city of license and then some.

The STL antenna is a survey antenna mounted on the side of the building. In this configuration, with the leaves off of the trees, we are getting about 250 uV signal, which is pretty good.

The site is now crawling with insurance investigators, cell site technicians, North Adams fire department, Berkshire County Sheriff’s officers, tower workers, etc. After we finished this work, we cleared out to make more room for everybody else. Estimated restore time for W266AW is Wednesday 4/2.

Planning for the replacement tower is already in progress, I’d expect it to happen fairly quickly. The next step for the broadcasters is to put up a 70-foot utility pole and get a full-powered antenna for WUPE. This should happen in the next two weeks or so. That will serve as the temporary facility until the new tower is constructed.

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.