MFJ MFJ-8504, Инструкция по эксплуатации

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When activating any Receiver port other than port 1, it is best to disable the internal termination for that
port. Removing the cover, you will find a conventional black jumper or “shunt” located near receiver
output ports 2 through 4. Removing the shunt for a port fully enables the port. For storage, the shunt can
be left hanging on one pin. The other pin must be left open to disable the port’s termination load. It does
not matter which of the two pins are left open. If you are not using a port, it is generally best to terminate
the port. See figure 1.
We suggest adding a second receiver on port 3 or 4 first, rather than port 2. This will increase the
isolation between receivers and lessen any effects of receiver mismatches or spurious signals. Receivers
sharing ports 1 and 2 and receivers sharing ports 3 and 4 have the least isolation.
Antenna connections are dependent on station antennas. When configured with optional band separation
or splitting filters, Antenna 2 port should be set as the VHF frequency port.
General Overview
All printed circuit antenna connector positions can fit either a BNC or SMA connector. The antenna input
jacks run to a series of jumpers allowing filters to be inserted either in series or parallel, with split inputs or
a single input. This allows you to use one antenna for all bands, separate antennas sorted by frequency
using optional filters, or no filters at all.
This goes into the input of a push-pull MMIC amplifier stage with approximately 10 dB gain and 43 dBm
output IP3. Push-pull cancels even order harmonic distortion products, and the use of two MMIC’s
greatly increases dynamic range. The final output IP3, after splitting to 4 ports, is around 35 dBm IP3.
Be aware that all signals amplified by the MMIC’s will combine to “use up” available MMIC power. Many
weaker signals can be just as bad as a few strong signals at using up the dynamic range. As the
amplifiers come close to power saturation, they become increasingly non-linear and start to act like
mixers. This can cause signals to combine, and make phantom signals appear. The only solution to this
problem is to reduce the input signal level for some or all signals. The ideal input level is one where
external antenna noise sets the system noise floor. It does no good at all to have signal levels from an
antenna beyond the point where external noise just sets the receiver noise floor.
If the receiver is ideal in gain and sensitivity, without any antenna connected, you should just perceptibly
hear the background noise hiss change turning the MFJ-8504 off and on. Receiver sensitivity beyond this
point will decrease dynamic range.
The amplifiers are powered through a voltage regulator. The voltage regulator makes this device less
sensitive to power supply voltage variations, and that includes hum and noise. Additional bypassing and
filtering also helps clean up the power supply. The only significant worries are use of an unfiltered supply,
voltages below 9 volts or above 15 volts, reversed voltage, or radio frequency noises from switching type
supplies. Because this is a series linear regulator stepping down in voltage, current is constant. Less
voltage is less wasted energy.
The amplifier output is split in a traditional magic-T splitter, each split going to a second magic T. The
second magic T’s, each fed by one branch of the first split, go to receiver ports 1 and 2 from one branch
and ports 3 and 4 from the other branch. This means highest receiver isolation, of approximately 15 dB, is
between groups 1 or 2 and the group of 3 or 4, since two splitters isolate those groups.
These magic T’s have to be properly terminated for maximum isolation. To do this, 50 ohm internal
resistors are connected by removable jumpers to receiver ports 2, 3, and 4. It is not absolutely necessary
to properly terminate, but it helps performance a small amount. Whether the change is important or not
depends on each particular installation, there is no set rule. In general though, it is best to remove the
shunt when a receiver is used on ports 2-4 and to re-install it if the port is not externally loaded. It is not,
however, absolutely necessary to move the jumper unless peak performance is critical.