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FM Transmitter


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For special events, temporary installations, and remote broadcasts, KCHUNG will sometimes use an FM transmitter. Although still very low-powered, the output of this transmitter can possibly exceed the legal limit for un-licensed broadcast, and anyone using it should be trained in using it properly, as well as aware of the relevant legal issues (read here, or here, or here).

KCHUNG owns two types of FM transmitters: one is the Ramsey fm25b (a manual for this can be found here). KCHUNG owns 10 of this second kind of transmitter, which is very compact, has an internal antenna, is tuned by adjusting a screw on the circuit board, and is best-suited to extremely local (i.e., within 100-300 feet) broadcasts. The other is digitally-tuned, and requires an external antenna that needs to be adjusted manually.

To set up the digitally-tuned transmitter:

you will need:

transmitter unit
DC power supply / transformer and AC cable
SWR meter (note: at the moment our SWR meter is broken so for the moment we use less precise methods for adjusting the antenna--see below)
speaker stand
coaxial cable
antenna rod
3 ground rods
antenna / ground rod connector (metal tube with a ufo-shape that screws into the end)
mounting brackets
  1. select a location for installing the antenna. higher is better, away from walls / buildings / etc if possible.
  2. thread one end of the coaxial cable through the antenna / ground rod connector's metal tube, screw it onto the ufo-shape.
  3. screw each of 3 ground rods into the antenna / ground rod connector.
  4. screw the antenna into the connector
  5. using the mounting brackets, attach the antenna / ground rods / coaxial cable onto either the speaker stand, or some other mounting location (pipe sticking out of a roof / guard-rail, etc)
  6. normally, the other end of the coaxial cable would be connected to the SWR meter, and then a short jumper coaxial cable would connect to the transmitter. for now, skip to the next step.
  7. attach the other end of the coaxial cable to the transmitter unit. IMPORTANT: never connect power to the transmitter without first connecting the antenna. never disconnect the antenna without first removing power from the transmitter
  8. once the antenna and ground rods are connected to the transmitter, connect the transmitter to power. there is no on/off switch, plugging into the power supply turns the transmitter on.
  9. use the menu button to navigate the settings on the transmitter. volume should be set to 79, power set to 99, frequency set to whatever frequency you like (use a radio to find an open / un-used frequency first).
  10. to roughly adjust the antenna, pick a broadcast frequency at one end of the FM range (i.e. lower, like 87.9, 88.3 etc., or higher, like 107.9). the antenna is most efficient (i.e., maximum range) when its length is proportional to the wavelength of the broadcast frequency. for lower frequencies, start with the antenna all the way extended (low frequency = long wavelength). for higher frequencies, start with the antenna as short as possible (high frequency = short wavelength).
  11. connect audio to the transmitter from a mixer or computer using the 1/8" line input (not the mic input)
  12. listen on a radio to confirm that broadcast is working!

troubleshooting a broadcast that uses the digitally-tuned FM transmitter

the most common problem is audio distortion: make sure to not exceed a volume setting of 79 on the transmitter, and to keep input levels consistently in the middle / not peaking.

transmitter placement is the best way to get a decent broadcast range.

adjusting the antenna to get the most efficient output really needs an SWR meter.

ground rods can interfere with sensitive electronics like turntables or speakers, make sure to keep them separated.

Page last modified on May 20, 2017

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