More AM work, Part V

I don’t know how many parts there should be in this series, five is a guess.

While some AM stations surrender their licenses to the FCC, others are undergoing needed repair work to stay on the air for a while longer.

WBEC-AM in Pittsfield Mass is the topic of today’s post. This station is Non-Directional day, Directional Night with a two-tower array. This site was built around 1956 when the station moved from Eagle Street, near downtown Pittsfield.

This Shively 4-bay antenna is the backup antenna for WBEC-FM. It is being replaced with a 3-bay antenna. The new antenna will serve W277CJ which is relocating from downtown and as a backup for WBEC-FM. A set of signal strength measurements for the nighttime directional array is required along each of the three monitor point radials before and after that work is completed.

The issue these days is the nighttime directional system, which is somewhat erratic when in use. The towers are 180 degrees tall with 35 degrees of top loading making the towers 215 degrees tall or just under 5/8 wavelength. As such both current and voltage are near maximum at the tower base, which makes them very sensitive to any changes at the base.

First, there was an issue with the tower lighting system. It seems that somewhere on the non-reference tower, the beacon conductor is shorted to the tower. When the tower lights are on, the loop current rises and falls in time with the beacon light. Because these are series excited towers, the fuse for the beacon does not blow, but the 60-cycle AC current does show up on the tower loop current reading. Rather than try to repair things on an almost 70-year-old tower light system, it was decided that both towers should get new LED lighting systems.

Next, mice chewed through several AC supply cables for the phasor at the base of tower #1. When switching from day to night patterns, some or all of the contactors would not move or get hung up between states taking the station off the air.

It is unfortunate that the phasor is at the base of one of the towers and the antenna monitor is back inside the building with the transmitter. It takes two people to make adjustments to the nighttime array.

The tower #1 ATU/phasor building is full of old unused equipment, a mouse haven. It also blocks access to several points that need to be sealed up to keep mice from getting in in the first place.

What is interesting is, that the original wiring and photocell failed at some point, and someone simply wired up a new photocell, leaving all of the old equipment in place. Thus, it became difficult to troubleshoot and ID the conductors in use vs the ones abandoned. With the replacement of the tower light systems, I decided that everything must go. The new Slatercom A-1 replacement lighting systems have individual controllers each with its own photocell. In addition, they have wireless links for tower light monitoring. It is great to get rid of the dry pairs going back to the transmitter room, which always creates RF and lightning headaches.

The station will also be saving some money on electricity. The new lighting system draws 88 watts total vs the old incandescent system which drew 1,000 watts with a 50% duty cycle on the beacons. The old system was on all the time due to photocell failure. I estimate they used 17,520 KWh per year on tower lights, at $0.20 per KWh which is $3,504.00 per year vs $308.00 per year for the LED systems. The added benefit of LED fixtures is that they should last much, much longer than incandescent fixtures.

This lovely Harris FM1H3 was donated to the scrap yard. I believe that this is the second transmitter (1974) for the original 105.3 WBEC-FM which signed on in 1967.

The building clean-out, unwiring, and rewiring process took about a day and was well spent, in my opinion. Working in a building that is not full of mouse nests, droppings, and stinks like mouse urine is nice. I plugged several holes in the building with stainless steel pot scrubbers and spray foam.

After removing and replacing the old tower lighting system, the Nautel Amphet 1 transmitter would not run into the nighttime array. This was likely due to the changes at the tower base. I used a VNA to measure each tower’s base impedance which is 42.5 ohms and -j139, 44.5 ohms -j155 respectively.

Then the daytime and nighttime common point impedance was measured. Both were off, but the nighttime was more so than the daytime. I adjusted the R and X until both were reasonable and the transmitter would run into both patterns. According to Nautel, the Amphet 1 transmitter runs best into a load of 50 ohms j+5 measured at the transmitter output.

The daytime antenna is non-directional and there is no “common point,” however, there is an R and X adjustment in the phasor for the daytime tower. According to the file I found, it used to be a directional daytime until 1967, when they could make the daytime antenna non-directional. It is an interesting setup.

With the array properly adjusted, readings could be made along the monitor point radials. This station has three monitor points, two are in the nulls and one is in the main lobe. It is the nulls that are the greatest concern. Fortunately, much of the documentation from the original proofs was found in a filing cabinet. While the maps are nice, they date from 1950 and are woefully out of date. However, I was able to find good reference points on the 1950 maps and redraw radials on a Google map.

I found these photos of the monitor points as they were in 1956:

Those were great because I was able to verify the locations of the monitor points today:

Based on that, I made three maps with radials on the monitor point azimuths:

The consulting engineer wanted about 10 readings on each radial. I created an individual map for each radial, marking points where the radial crosses a public road. When it came time to do the monitor points, I loaded the map into my smartphone and followed the directions to each point. It worked very well.

We will return and make post-installation measurements once the new FM antenna is installed.