FM transmitters – Page 12 – Engineering Radio

WRKI WINE transmitter move, update 2

Man, this is taking longer than I thought it would. We moved the Harris FM25K last week, all went well. The only hangup, as you can see, is the harmonic filter and the height of the racks next to the transmitter. The transmitter had to go on a 4×4 to get the filter up over the racks. The output from the transmitter to the harmonic filter cannot be changed in any way, shape, or form (e.g. adding a little bit of line section to the top of the transmitter), or else the transmitter will not run. So, up on 4×4’s it is.

There we were, all ready to turn the transmitter on. Press the high voltage on button, lots of volts but no current and no power output. Seems something is wrong with the outboard IPA driver (over in the bottom of the rack, that thing pulled out with the manual on it).

The IPA is a Silicon Valley Power Amplifier 500-watt unit, which replaced the internal IPA driver about ten years ago. The tube in the Harris FM25K needs at least 390 watts to drive the transmitter to full power. Unfortunately, this particular amplifier was not in the best environment prior to the recent move. It was sitting in an unconditioned building on top of the backup transmitter in high heat and humidity. According to the manufacturer, such abuse is bound to take its toll sooner or later. The latter being, of course, the night we want to turn the thing back on and go home.

Time to drop back and punt. I found an old RVR 250-watt amp at a sister station nearby, which was also in pretty bad shape but repairable. That unit was pressed into service temporarily and with 200 watts drive, the old 25K put out about 11 KW. We need to affect permanent repairs to the RVR power amp before we place it into temporary service. I don’t want any 2 am phone calls. The Silicon Valley Power Amp needs to have the amplifier module sent back to the manufacturer and rebuilt. They will refurbish the entire thing for something like $900.00 plus shipping. Considering what it does, that is worth it.

This is a little short cellphone video of the turn-on at half power. This is a very loud transmitter, as such, I think the audio is a little distorted.

When this beast gets up to full power, I will update this, again.

Low Pass Filter design

Every good transmitter, tube transmitters, in particular, require harmonic filtering. The last thing any good engineer or broadcaster wants is to cause interference, especially out-of-band interference to public safety or aviation frequencies. All modern transmitters are required to have spurious emissions attenuated by 80 dB or greater >75 kHz from the carrier frequency. In reality, 80 dB is still quite high these days, especially in the VHF/UHF band, where receivers are much more sensitive than they used to be. A good receiver noise floor can be -110 dB depending on local conditions.

The principle behind a low pass filter is pretty easy to understand. The desired frequency is passed to the antenna, while anything above the cut-off frequency is restricted and shunted to ground via a capacitor.

In this case, the resistor is actually an inductor with high reactance above the cut-off frequency. Often, these filters are lumped together to give better performance. This is a picture of an RVR three-stage low pass filter:

RVR three stage low pass filter — RVR three-stage low pass filter

RVR is an Italian transmitter maker that sells many transmitters and exciters in this country under names like Bext, Armstrong, etc. The inductors are obvious, the capacitors consist of a copper strip sandwiched between teflon insulators held down by the dividers in between the inductors.

Schematically, it looks like this:

For the FM broadcast band, a good design cutoff frequency would be about 160 MHz. This will give the filter a steep skirt at the first possible harmonic frequency of 176 MHz (88.1 x 2 = 176.2).

Values for components:

Capacitors	Value	Inductors	Value
C1	20 pf	L1	74.7 nf
C2	54 pf	L2	75.1 nf
C3	54 pf	L3	73.9 nf
C4	20 pf

The inductors are wire, or in this case copper strap, with an air core. It is important to keep the transmitter power output in mind when designing and building these things. Higher carrier powers require greater spacing between coil windings and larger coil diameters. This particular filter is rated for 1 KW at 100 MHz.

The Gates BFE-50C Amplifier

Found in a pile of junk in the corner of an older transmitter site, this Gates BFE-50C or otherwise known as an M5675 Amplifier. This was used as an IPA in a Gates FM 1C transmitter installed around 1960 or so. The rest of the transmitter has long since departed, likely to the scrap yard, however, somebody thought to remove this and set it aside.

Gates BFE-50C 50 watt VHF amplifier — Gates BFE-50C 50 Watt VHF amplifier

This unit is missing it’s grid tune knob. The grid tune capacitor is still there, however. There is also some evidence of heating on R403 and R407/408 likely due to a prolonged overdrive condition. Otherwise, it is in good shape.

The design is pretty simple, a pair of 6146’s in push pull, three watts in nets about 50-60 watts out, according to the manual, which can be found here (.pdf). The power supply voltages are fairly tame, 500 volts plate, 300 volts screen. The one thing that this design does not have is any type of harmonic filtering. When used with a larger transmitter, this makes sense because the transmitter output will have overall harmonic filters. If this was to be used on it’s own for any reason, a good harmonic filter would need to be designed and installed.

Gates BFE-50C or M5675 50 watt VHF amplifier — Gates BFE-50C or M5675 50 Watt VHF amplifier

The schematic is straight forward. Gates, the old Gates Radio of Parker Gates, designed good equipment. Click on image for higher resolution.

It is a bit hard to see in this picture; the input section consists of three turns of #14 gauge wire coupled to two 4 turn sections of 14 gauge wire on either side of it. This is matched to the ~~grids~~ Screen1 of the 6146s with C401. L412, C411 and L413 form a low pass filter. L412 consist of one turn #14 gauge wire, L413 is five turns of #14 gauge wire. All coils are 3/4 inch in diameter.

The output section is even simpler, using just one loop of small diameter copper tubing. The plate tuning is accomplished by C407, loading is C406. Power output is adjusted by varying the screen voltage using R405.

Advantages of this design:

The 6146 tube is fairly rugged, at class AB the 50 to 60 watt output range is well within the plate dissipation for a push pull configuration.
No special parts are needed, everything can be found or fabricated by hand
The 500 volt supply is fairly tame, maximum PA current should be less than 0.2 amps for 50 watt output and 50% PA efficiency.
Output tuning and load allow for tuning into less than ideal loads, if required.
If operated as a stand-alone unit, some type of plate current meter should be used to aid tuning. A harmonic Filter would need to be designed and built for the output.

All in all, a pretty cool little FM amp.

The BE FM20T transmitter

This is the main transmitter for WYJB in Albany, NY. The backup is the Harris FM20H3 on the right. I haven’t turned that unit on lately, but it normally makes quite a fuss the first time the Plate On button is pushed. The FM 20T on the other hand, is mellow and even-tempered.

WYJB 95.5 Mhz, class B, transmitter Albany, NY

One other thing of note; The FM20T is still on its original tube. I looked up the maintenance records for this transmitter, it was installed in December of 2000. Eleven years later, the ~~4CX15000A~~ (ed note; 4CX12000A) is still cranking out 15 KW TPO, which is impressive. I found that high-power ceramic vacuum tubes actually seem to last longer when run closer to their limits than those that are running at half power.

Judicious management of filament voltage is required to achieve this type of longevity. There is a set procedure for installing a large ceramic vacuum tube:

After the tube is in the transmitter, run it at a full filament voltage for at least an hour or so before turning on the plate voltage. This allows the getter to absorb any stray gases in the tube.
Once the plate voltage is applied, proper tuning should be completed as quickly as possible. Tuning procedures vary from transmitter to transmitter, however, the general idea is to obtain the maximum power output for the least amount of plate current while keeping the PA bandwidth within acceptable limits. Some transmitters can get narrow-banded at high efficiencies, which manifests itself as higher AM noise.
After the tube has been in use for 90-100 hours, the filament voltage should be reduced gradually until a drop in the transmitter output power is noticed, then increased by 0.1 volts.

This maximizes the filament life for that particular transmitter and power output. Once the filament can no longer boil off enough electrons, the tube power output drops and it is time to replace it.

This site also has two other radio stations, WZMR, 104.9 and WAJZ 96.3 , both class A using solid-state transmitters of less than 1,000 watts:

WAJZ and WZMR Energy Onix solid state transmitters — WAJZ and WZMR Energy Onix solid-state transmitters

Not the prettiest sight in the world, but it does stay on the air. There is no money to go back and neaten up this work, unfortunately.

The tower supports all three antennas. There was some discussion of a common antenna for all three stations, however, WZMR is a directional station, thus it would require its own antenna. Doing a common antenna for the other two stations was cost prohibitive, so the tower supports three two bay antennas.

WYJB, WZMR, WAJZ FM antennas, New Scotland, NY

The stations are all located in the New Scotland, NY tower farm. WYJB is licensed to Albany, WZMR is licensed to Altamont and WAJZ is licensed to Voorheesville.