Exciters/transmitters (was Re: WHIL/WWEL)

[Shawn Mamros <mamros@MIT.EDU>]

>>I know what a transmitter is (I think!), but what's an exciter? 
>>
>I believe that it's actually a little transmitter with low power
>output--just enough to drive a final power amplifier that normally drives
>the antenna by way of a coupling network. When the final power amplifier is
>off, if the signal from the exciter leaks through to the antenna, you have a
>low-power radio station. Indeed, I believe that many of the Class-D (10W)
>noncommercial FMs and probably not a few of the FM translators use exciters
>as their transmitters.

My understanding is that an exciter (in FM, at least) is the unit that
actually modulates the audio signal (or, more properly, the combined
L+R, stereo pilot, and L-R signals, plus whatever other subcarriers
the station may be using) onto the station's assigned frequency.  The
output of the exciter is a signal that should be receivable on any FM
receiver, albeit at lower power.  Dan's right that low-powered stations
typically use just an exciter as their transmitter (as WTBS did, back in
its 10 watt days, with an old Gates unit that dated back to the FM sign-on
in 1961).  Most of the exciters I've seen on the market today can put out
as much as 50 watts; older exciters typically put out only 10-20 watts.

I was a bit loose with the terminology in my earlier message w.r.t. the
word "transmitter".  Strictly speaking, an FM transmitter consists of the
exciter plus one or more RF power amplifiers which amplify the exciter's
output to the required power.  Under WMBR's old setup, the Collins was
the exciter and the Cetec was effectively just being used as an RF amplifier,
so the two boxes taken together formed our transmitter.  But because they
were physically separate boxes (connected only by a piece of coax), and
because the Cetec was purchased with the intent of it being the entire
transmitter, we typically referred to it as such, even though it probably
wasn't entirely correct terminology.

- -Shawn Mamros
E-mail to: mamros@mit.edu

------------------------------