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RECEIVE
MULTICOUPLER #2
If you have a
number of shortwave, broadcast and LF receivers and like to run more
than one at a time, either for receiver comparisons or tuning different
signals, then a multicoupler is a virtual necessity. It
allows several receivers to share the same antenna with no
interaction between them. That is, tuning one or changing the
band on one receiver will have no effect on the other sets.
Neither will you lose signal strength, which will happen when you tie
multiple receivers to a common antenna.
This is the
second multicoupler that I've designed and built. Although
the circuit has been simplified and draws considerably less power from
the 13.6VDC power supply (about 33mA), it has much better performance
than the original. This is due to the fact that instead of 4
power RF transistors and an RF driver transistor, it uses ONE premium
transistor (2N5109) designed for CATV service and the splitting action
is accomplished by the use of an MCL 4 way power divider.
The simplicity
and performance comes at a price though. That MCL splitter
costs roughly 40.00 new. IF you don't need LF
response down to <13KC, you can get by with one of the cheaper
MCL units that covers a smaller frequency range. It should be mentioned
that this power divider is the heart of the unit, insuring isolation of
about 35dB between receivers and a power balance between outputs of
about .5dB.
The basic circuit
is a one stage, low noise amplifier with high Intercept Points and 19dB
of gain. Since a multicoupler should have little or no gain,
the preamp is followed by a 6dB, 50 Ohm attenuator. That
insures a good 50 Ohm impedance match into the power divider.
The power divider has 6dB of insertion loss, giving an overall
input/output gain of 6-7dB (roughly 1 "S" unit).
Since the
amplifier stage does have 19dB gain over a wide bandwidth it is
ESSENTIAL that you follow good RF construction practices in it's
construction. Short leads and a good ground is a
must. "Dead Bug" construction on a copper clad PCB board is
good. I built mine on a small Universal PCB board from Radio
Shack and securely mounted it on a piece of copper clad with many
grounding staps between the boards. Keep the output leads
away from the input. Having said that, I had virtually NO
stability problems with the unit I built.
A few
construction notes:
1. The
transistor bias is adjustable via R3. To properly adjust it,
adjust the bias control so that the collector (case) of the transistor
has EXACTLY HALF as much Voltage as the junction of resistors R6
& R7 has. I.E., if there is 12.6VDC at the junction,
adjust the bias for 6.3VDC on the case of the transistor.
2. In normal operation the transistor runs quite warm and "should" have a small, finned heatsink attached. (remember, the case is at collector potential, DON'T ground it). 3.
Diode D1 isn't essential, it is there for reverse polarity protection
of the transistor. Diode type isn't critical
either. Any small rectifier type diode will suffice.
4. Unit should be mounted in a shielded metal box, and you can use any type of RF connectors you desire. (BNC, SO-239, RCA etc). I used BNC connectors because of their high quality and ease of installing (single hole mount). 5. Resistor R4 is there for RF stability and should be mounted right AT the base of the transistor (short leads). 6. Resistor R5 is there for temperature stability and to provide some degenerative feedback, DON'T bypass it for more gain. 7. Resistor R6 actually consists of TWO 390 Ohm, 1/2 Watt resistors in parallel. They run warm, don't use smaller Wattages (you MAY use a single 220 Ohm, 1 Watt resistor, I didn't have any). I asked Steve
Ratzlaff, a friend of mine who has the equipment to run some tests on
this circuit design. Here is what he had to say after running
the tests.
"I've
breadboarded and tested the preamp on the bench.
Using a 2N5109, at 13.0VCC, 36mA (runs hot but can hold finger on for several seconds = conservative rule of thumb for need for heat sink, OK as is). The two 390 resistors do run warm, but I used 1/4 watters. Below about 2MHz,
gain is +19.6db. -1dB rolloff at 14.9Mc and 20kc; -2db rolloff at
31.4Mc and 13.5kc. All these measurements are into well-defined 50 ohms
in/out.
Running intermod tests, I get +28dBm 3rd order output intercept (OIP3); +36dBm 2nd order output intercept (OIP2). This is average to above average performance, especially for so much gain, and is much better than most radios, so should be adequate for your usage." OK, here's the schematic and
parts list.
 COMPONENT VALUES
4X MCL PSC4-6 4 Way power divider C1, C2, C3 .1uF 50V Mylar C4 10uF 25V Electrolytic D1 1N4001 polarity protect diode Q1 2N5109 Transistor R1 3.3K 1/2W R2 180 Ohms 1/2W R3 470 Ohms Variable R4 10 Ohms 1/2W R5 4.7 Ohms 1/2W R6 2ea 390 Ohms 1/2W resistors in parallel R7 10 Ohms 1/2W R8, R10 150 Ohms 1/2W R11 39 Ohms 1/2W Melt
Some Solder!
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