tech stuff – Page 35 – Engineering Radio

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.

Burk Autopilot

The old version of the software, that is. I like the graphical interface, just one glance is all that is needed:

I have not had a chance to fool around with the newer version, the screen shots on the Burk website look a little bit different.

The setup and programming of macros is pretty easy; power/pattern change times, Pre-sunrise, and post-sunset functions, automatic tower light monitoring, AM Directional Antenna readings, and automatic transmitter restoration routines. If programmed correctly, the software can eliminate many of those late-night/early-morning phone calls, which is always a good goal.