Vectronics VEC-1290K - Manual (Page 17)

1'EC-1290K Instruction Manual AM Radio Transmitter
clip on JMP 1. Capacitor C 12 will now tune the transmitter frequency from
870 to 1420 kHz.
A frequency counter is the quickest method to set the transmitter precisely on
channel. Attach the counter input to either side of resistor R3. The signal is
strong enough at this point to drive any 50-ohm or high-impedance counter
without problem.
A portable radio can also be used. Set the radio to the clear frequency, and
apply an audio source to the transmitter. (This is discussed in the OPERATING
INSTRUCTIONS section.) Slowly tune C12 until the transmitter is heard on the
radio.
Hint: Store unused shorting clips where they won't become lost. Place unused
shorting clips over one pin of open junipers.
Antenna Matching:
Important Note: For reasons given later, the transmitter is designed to be
operated only with the supplied 6-foot antenna wire. Attempting to use other
antennas will result in a severe mismatched condition, reduced range and
distorted audio. The antenna wire should be run out straight, avoiding
proximity to nearby metal objects. If more range is needed, try relocating the
transmitter to a different location.
Once a clear channel has been found, and the oscillator set to frequency, the
antenna matching network must be tuned. Refer to Table 2. Note that some
frequencies, such as 800 kHz, can be matched by one of two possible jumper
configurations. Try both, and use the one which works best. Using the previous
example where the oscillator was set to 910 kHz, Table 2 shows the proper
antenna match range is selected by placing a shorting clip across the pins of
JMP4. (910 kHz falls between 730 kHz and 1400 kHz.)
When the proper antenna matching range has been set, capacitor CI I is tuned
for maximum radiated signal. This can be done by carefully adjusting CII while
increasing the distance between the transmitter and a portable radio. As the
signal grows weaker with distance, adjust CI I for the loudest signal.
If you have access to test equipment, the following steps may also be
Using an oscilloscope: If your oscilloscope has a vertical amplifier with
useable bandwidth to at least 2 MHz, you may attach the probe to the junction
of ANT 1, C 1 I and L6 as shown on the schematic. C 11 should be peaked for
maximum scope amplitude. A low-capacitance 10:1 probe is recommended to
avoid detuning.
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