Showing posts with label mhz. Show all posts
Showing posts with label mhz. Show all posts
Sunday, September 1, 2013
FM Beacon Broadcast Transmitter 88 108 MHz
This circuit will transmit a continuous audio tone on the FM broadcast band (88-108 MHz) which could used for remote control or security purposes. Circuit draws about 30 mA from a 6-9 volt battery and can be received to about 100 yards. A 555 timer is used to produce the tone (about 600 Hz) which frequency modulates a Hartley oscillator. A second JFET transistor buffer stage is used to isolate the oscillator from the antenna so that the antenna position and length has less effect on the frequency.
Fine frequency adjustment can be made by adjusting the 200 ohm resistor in series with the battery. Oscillator frequency is set by a 5 turn tapped inductor and 13 pF capacitor. The inductor was wound around a #8 X 32 bolt (about 3/16 diameter) and then removed by unscrewing the bolt. The inductor was then streached to about a 3/8 inch length and tapped near the center.
FM Beacon Broadcast Transmitter Circuit Diagram
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The oscillator frequency should come out somewhere near the center of the band (98 MHz) and can be shifted higher or lower by slightly expanding or compressing the inductor. A small signal diode (1N914 or 1N4148) is used as a varactor diode so that the total capacity in parallel with the inductor varies slightly at the audio rate thus causing the oscillator frequency to change at the audio rate (600 Hz).
The ramping waveform at pins 2 and 6 of the timer is applied to the reversed biased diode through a large (1 Meg) resistor so that the capacitance of the diode changes as the ramping voltage changes thus altering the frequency of the tank circuit. Alternately, an audio signal could be applied to the 1 Meg resistor to modulate the oscillator but it may require an additional pullup resistor to reverse bias the diode. The N channel JFET transistors used should be high frequency VHF or UHF types (Radio Shack #276-2062 MPF102) or similar.
Fine frequency adjustment can be made by adjusting the 200 ohm resistor in series with the battery. Oscillator frequency is set by a 5 turn tapped inductor and 13 pF capacitor. The inductor was wound around a #8 X 32 bolt (about 3/16 diameter) and then removed by unscrewing the bolt. The inductor was then streached to about a 3/8 inch length and tapped near the center.
FM Beacon Broadcast Transmitter Circuit Diagram
The ramping waveform at pins 2 and 6 of the timer is applied to the reversed biased diode through a large (1 Meg) resistor so that the capacitance of the diode changes as the ramping voltage changes thus altering the frequency of the tank circuit. Alternately, an audio signal could be applied to the 1 Meg resistor to modulate the oscillator but it may require an additional pullup resistor to reverse bias the diode. The N channel JFET transistors used should be high frequency VHF or UHF types (Radio Shack #276-2062 MPF102) or similar.
Thursday, April 11, 2013
Antenna Tuning Unit ATU For 27 MHz CB Radios
This antenna
tuning unit (ATU) enables half-wavelength or longer wire antennas to be
matched to the 50-? antenna input of 27-MHz Citizens’ Band (CB) rigs.
The ATU is useful in those cases where a wire antenna is less obtrusive
than a roof-mounted ‘vertical’ or ground-plane. It is also great for
‘improvised’ antennas used by active CB users on camping sites and the
like because it allows a length of wire to be used as a fairly
effective antenna hung between, say, a tree branch at one side and a
tent post, at the other. Obviously, the wire ends then have to be
isolated using, for example, short lengths of nylon wire. It is even
possible to use the ATU to tune a length of barbed wire to 27 MHz. The
coil in the circuit consists of 11 turns of silver-plated copper wire
with a diameter of about 1 mm (SWG20).
The internal diameter of
the coil is 15 mm, and it is stretched to a length of about 4 cm. The
tap for the antenna cable to the CB radio is made at about 2 turns from
the cold (ground) side. Two trimmer capacitors are available for tuning
the ATU. The smaller one, C1, for fine tuning, and the larger one, C2,
for coarse tuning. The trimmers are adjusted with the aid of an
in-line SWR (standing-wave ratio) meter which most CB enthusiasts will
have, or should be able to obtain on loan. Select channel 20 on the CB
rig and set C1 and C3 to mid-travel. Press the PTT button and adjust C2
for the best (that is, lowest) SWR reading. Next, alternately adjust
C3 and C2 until you get as close as possible to a 1:1 SWR reading.
C1
may then be tweaked for an even better value. No need to re-adjust the
ATU until another antenna is used. In case the length of the wire
antenna is exactly 5.5 metres, then C3 is set to maximum capacitance.
Although the ATU is designed for half-wavelength or longer antennas, it
may also be used for physically shorter antennas. For example, if
antenna has a physical length of only 3 metres, then the remaining 2.5
metres has to be wound on a length of PVC tubing. This creates a
so-called BLC (base-loaded coil) electrically shortened antenna. In
practice, the added coil can be made somewhat shorter than the
theoretical value, so the actual length is best determined by trial and
error. Finally, the ATU has to be built in an all-metal case to prevent
unwanted radiation. The trimmers are than accessed through small
holes. The connection to the CB radio is best made using an SO239
(‘UHF’) or BNC style socket on the ATU box and a short 50-W coax cable
with matching plugs.
tuning unit (ATU) enables half-wavelength or longer wire antennas to be
matched to the 50-? antenna input of 27-MHz Citizens’ Band (CB) rigs.
The ATU is useful in those cases where a wire antenna is less obtrusive
than a roof-mounted ‘vertical’ or ground-plane. It is also great for
‘improvised’ antennas used by active CB users on camping sites and the
like because it allows a length of wire to be used as a fairly
effective antenna hung between, say, a tree branch at one side and a
tent post, at the other. Obviously, the wire ends then have to be
isolated using, for example, short lengths of nylon wire. It is even
possible to use the ATU to tune a length of barbed wire to 27 MHz. The
coil in the circuit consists of 11 turns of silver-plated copper wire
with a diameter of about 1 mm (SWG20).
The internal diameter of
the coil is 15 mm, and it is stretched to a length of about 4 cm. The
tap for the antenna cable to the CB radio is made at about 2 turns from
the cold (ground) side. Two trimmer capacitors are available for tuning
the ATU. The smaller one, C1, for fine tuning, and the larger one, C2,
for coarse tuning. The trimmers are adjusted with the aid of an
in-line SWR (standing-wave ratio) meter which most CB enthusiasts will
have, or should be able to obtain on loan. Select channel 20 on the CB
rig and set C1 and C3 to mid-travel. Press the PTT button and adjust C2
for the best (that is, lowest) SWR reading. Next, alternately adjust
C3 and C2 until you get as close as possible to a 1:1 SWR reading.
C1
may then be tweaked for an even better value. No need to re-adjust the
ATU until another antenna is used. In case the length of the wire
antenna is exactly 5.5 metres, then C3 is set to maximum capacitance.
Although the ATU is designed for half-wavelength or longer antennas, it
may also be used for physically shorter antennas. For example, if
antenna has a physical length of only 3 metres, then the remaining 2.5
metres has to be wound on a length of PVC tubing. This creates a
so-called BLC (base-loaded coil) electrically shortened antenna. In
practice, the added coil can be made somewhat shorter than the
theoretical value, so the actual length is best determined by trial and
error. Finally, the ATU has to be built in an all-metal case to prevent
unwanted radiation. The trimmers are than accessed through small
holes. The connection to the CB radio is best made using an SO239
(‘UHF’) or BNC style socket on the ATU box and a short 50-W coax cable
with matching plugs.
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