What is the difference between "reflected" and "forward" on a FM transmitter?

What is the difference between "reflected" and "forward" on a FM transmitter?

I recall there being two other settings, but I was not sure what they were. I was told that "forward" should be used for standard transmission. Can anyone enlighten me on what the settings are for, and what kind of effect they have?

If we're talking about meter settings, it switches the meter between showing how much signal is getting put out (forward) and how much is coming back (reflected) down the line into the transmitter. It's basically used to match the transmitter to transmission line and the antenna array. You want to minimize how much of the signal is being reflected back into the transmitter by the resistance of the line and antenna bays.

It's always a good idea to keep an eye on that "Reflected" power reading.....I'd keep a chart of the two readings right on the front of the transmitter. Check the readings every day, if possible, and record them occasionally, so you get a feel for how they vary over the seasons.
If you see the reflected power coming up, especially after a storm (or, during the ice and snow season), you've got a problem with the antenna or transmission line.

And especially newer solid-state transmitters don't like reflected power. It can gobble up an output stage. Transmitters have protective circuitry to shut down or lower power when reflected power (sometimes called VSWR) is too high.

Thanks for the information guys! I recall there also being some sort of dial to switch between settings on the transmitter. This is probably a long shot, but would anyone know what settings the dial controls? I will try to get back up to the site one day to better describe what I am referring to.

A make and model of the transmitter in question would make it easier to give less general answers to general questions.

Transmitters used to have a Plate Voltage meter, a Plate Current meter (although some manufacturers put in a Cathode Current meter, so you had to take a Grid Current reading and subtract it from the Ik to get Ip.) There would also be tuning and loading controls, usually a set to match the IPA to the exciter and another set to match the output to the antenna. Some transmitters controlled power by varying drive (usually triode PAs), others by varying screen voltage (tetrode PAs.) Changing the power by changing the screen voltage would necessitate a touch-up on the PA loading, as one affected the other.

As manufacturers tried so save $$$, they replaced individual meters with a "multi-meter", which, in conjunction with a switch, would allow monitoring of various cirrcuit parameters. I have a BE FM10T and the multimeter will allow monitoring of Grid Voltage, Grid Current, Screen Voltage, Screen Current, Exciter forward power and Exciter reflected power. At one time, each of these functions would have had their own full-time meter. This transmitter has the aforementioned Plate Voltage, Plate Current, Output Power (calibrated in % for forward, and VSWR for reflected, as selected by a switch), the multimeter, a Filament Voltage meter, a filament hour meter, and a 3-phase line voltage meter.

If you have the manual for the transmitter, I would suggest curling up with a cup of hot chocolate and reading the Operation section first, then, if you're still awake, the Theory of Operation secton.

Hope this helps somewhat.

For that info, a brand and model number of the rig would be best, a photo could be helpful too.

Generically speaking the “Reflected” and the “Forward” on any transmitter's metering refers to the flow of RF power somewhere in the transmitter. Both values are obtained from a calibrated reflectometer, basically a calibrated directional coupler with appropriate signal sampling and displays. This metering function is almost always found at the output of the final amplification stage(s) of FM and television transmitters and often on AM transmitters these days, especially solid-state rigs. (It's actually required on many transmitters by the Rules.) Such monitoring will also be available at the inputs and outputs of RF power amplifier stages in transmitters that use vacuum tubes as they are needed to “tune-up” the tube (and yes, there are still a number of FM and television rigs out there that use “valves”, at least in the final stage).

In many transmitters referring to the samples as an RF power indication is a bit of a misnomer. They are actually RF voltage samples taken as near to the exact same place in a transmission line as practical. The voltage samples are usually “detected” at the reflectometer sample points with cartridge-type RF diodes (1N21 is typical). The Forward (sometimes called the Incident) power sample is fed to the front panel display (analog meter) and usually also to a VSWR monitoring circuit as a “reference”. In older FM and television transmitters that front panel display is usually calibrated to read 100% when the transmitter is operating at licensed power into a proper load (and that 100% reading is the ONLY point that the meter will read accurately.) But the reading can also be calibrated in watts, if desired. (The percentage scale is preferred as it is then common to all transmitters, regardless of actual output power.)

The Reflected sample follows much the same path as does the Forward sample, with a few differences. It's actually monitoring a parameter called voltage standing wave. The display will typically be calibrated in VSWR units, which stands for voltage standing wave ratio, but it could also be in watts as many intra-stage meters are calibrated that way. This meter shows how much power is actually being reflected back from the load (transmission line and antenna). It also feeds the VSWR monitoring circuitry.

Why are these two values of importance, particularly at the transmitter output? The Forward power is monitored to insure that the station is operating within its assigned output power range. As to the VSWR indication . . . it's important to insure that all the power generated in the final amplifier stage(s) actually gets transferred to the antenna load and is radiated. Any power that doesn't get radiated will be reflected back to the final amplifier(s). That's bad because it tends to reduce coverage area and it also can do serious damage to the transmitter and transmission line components. No system is perfect, typical VSWR is somewhere less than 1.1:1 And this parameter is monitored by circuitry in the transmitter that is designed to either reduce the transmitter's output power if the VSWR increases (fold-back) or to turn off the RF output completely if the VSWR exceeds a preset value to protect the RF system and the transmitter.

A very common cause of VSWR is ice accumulation on the antenna. But barring that, the VSWR should not “wonder around” day-to-day. If it does then your system has a problem.