tech stuff – Page 35 – Engineering Radio

Whatever can happen, will happen

This is a universal truism that can also be expressed as “Murphy’s Law.” I don’t rightly know how Murphy received credit for this, however, I chalk it up to either the luck of the Irish or the gift of self-promotion. Either way, that principle was demonstrated again with a 950 MHz STL link between Mt. Beacon and Peekskill, NY for WHUD.

I noticed, while doing some transmitter maintenance, the receive signal strength of the STL had dropped from 300 µV to 30 µV. That is an alarming development. Therefore, we scheduled a tower crew for the next day, not wanting to go off the air over the coming holiday, which would be a sure bet otherwise. Upon arrival, the tower crew noticed a strange thing in the STL transmission line at the base of the tower, which looked like some type of a splice. Truth be told, I have been associated with this station since 1999 and had never noticed the splice before. This STL system was installed in 1998 when the station’s studio moved from Peekskill to Beacon. I can say, of all the things that have gone wrong over the years, this STL system was always very reliable. Regardless of that, I quick check with a spectrum analyzer showed a 3 dB return loss at 137 feet (41.75 m), exactly the distance from the transmitter room to the base of the tower.

3 dB return loss, distance to fault 137 feet

A 3 dB return loss coincides exactly with the drop in received signal strength at the other end of the path. Thus, the tower crew took apart the splice and water poured out of it. I would estimate at least 4-6 ounces of water (180 ml), perhaps more.

7/8 coax cable splice connector, opened up

We then began to take in the details:

The 7/8 coax coming out of the building was Cablewave FLC78-50J
The 7/8 coax going up the tower was Andrew LDF4-50A
The splice connector was Andrew L45Z
The center conductor threaded connector did not fit properly into the Cablewave cable, it was too loose.
The cable was chaffing on a tower leg, about 50 feet above the splice because it was not properly secured to the tower
The 7/8 splice connector was missing an O ring on the backnut of the Cablewave cable

Thus, water ingress causes the high return loss. Problems with this system began immediately after Hurricane Irene, at the end of last August. We were able to make a temporary fix using two type N connectors of the proper manufacturer for each type of cable. The radio station returned to air just before noon, about 45 minutes after turn off. After the repair, the return loss dropped to about 20 dB, which is good.

The permanent fix is for the entire run of cable from the transmitter room to the STL antenna to be replaced. That type of line splice should have never been used on a 950 MHz STL, and it was certainly wrong to mix cable types with an Andrew connector. Those little details will always manifest themselves eventually.

Tower light malfunction

We were notified that the WFAS-AM tower lights were out, thus, it was time to investigate. This problem was easy to find. Upon removing the waterproof cover on the tower light flasher box, I found this:

SSAC melted tower light flasher, damaged by lightning — melted SSAC B-KON tower light flasher, damaged by lightning

As soon as loosened the screws on the cover, I smelled the unmistakable odor of burned electronics and plastic. I disconnected the flasher and covered the photocell, which turned the side markers on. Of course, the top flashing beacon was dark, therefore, it was time to report the outage to the FAA. The nationwide number to report tower light outages is (877) 487-6867. That number is for an automated system, however, eventually, it leads to a live person. Since the new reporting system was established, the only required information is the tower ASRN. From that information, the operator will access a database and have all the required information to issue a NOTAM. In the past, many questions were usually asked; what is the nearest airport, how far away is the airport, how tall is the obstruction, what is the position, etc? Therefore, things have become slightly easier than before.

Once the outage is reported and a NOTAM is issued, the tower owner generally has fifteen days to correct the problem.

Summing to Mono

It is surprising to me how many times I have seen this done incorrectly in the field. Summing a stereo source, whether it is balanced or unbalanced is not simply twisting a couple of wires together. This will effectively reduce the impedance of the outputs by one-half. With newer, active balanced outputs, this may cause damage to the output amplifiers.

The parallel resistance formula is thus:

Therefore a 600-ohm stereo output tied together would look like this:

Rt = 1/(1/600+1/600) or 300 ohms.

It also creates an impedance mismatch with the next piece of gear, which will affect the common mode noise rejection of the circuit.

The best way to sum is through a resistive network. That way stereo separation is maintained, the impedance of the output circuits is maintained and the output amplifier will not current cycle. That looks like this:

Pretty easy to fabricate in the field. It is good to do things the right way, it sounds better on the air too.

The Broadcast Electronics FW-30 automatic exciter switcher

This is a neat piece of kit, designed to save those late-night/early morning callouts, which is the ultimate goal of all broadcast engineers, or at least it should be. This seems like a really good idea, however, BE has discontinued the product line, and the last manual update is from 2000.

Broadcast Electronics FW-30 exciter switcher innards

A small four-port coax switch is located next to the power supply transformer. This is controlled by the circuit board. The circuit board senses a loss of excitation by detecting a forward power level below the threshold set on the board. The power sample comes from the exciter forward power remote metering terminals. Thus, it can be used with any exciter(s) that have a remote forward power sample.

The idea is to use the RF fault function output of the FX-30 (later FX-50) exciter to automatically switch from a faulted exciter to one that is working. Finally, it can be hooked to a remote control for manual switching. The unused exciter is muted and routed to a dummy load mounted on the back of the unit.

Broadcast Electronics FW-30 front, mid 1980's BE blue — Broadcast Electronics FW-30 front, mid 1980’s BE blue

An alternate configuration would be to route the backup exciter to the backup transmitter instead of the dummy load. This would create the best redundancy on a limited equipment budget. It also has a battery bank designed to hold the last state of the unit through a power outage. As we have a good-sized UPS powering the remote control, STLs, and satellite receivers, the batteries are not needed.

On the face of it, a pretty good idea. I have had a few exciters fail over the years, which normally means the backup transmitter is placed in service by remote.

I did download the manual, but since it is currently listed on the BE website, it’s probably not a good idea to post the schematic. Suffice to say, it is a tad bit complicated what with all the CMOS logic and that. It is very possible to duplicate the functions of this equipment with a simple RF forward power sample and set a failure threshold with a comparator circuit. Hook that to a small four-port coax switch and a couple of RF mute/un-mute commands to each exciter and: Viola! Automatic exciter switching!

Perhaps a good rainy day project.