SPECIFICATIONS:
EME157B2
KIT
Frequency Range:
Tuneable 120 to 150MHz
Bandwidth:
5MHz @ 3dB, ( depends on input tuning )
Gain:
Typically 16dB Gain with –6dB onboard attenuator fitted
Noise Figure:
<1.5dB Typically 1.15dB ( Includes Relays )
Tested with a HP8970B / HP346B
Through Loss:
0.05dB maximum at 145MHz ( Bypass Mode )
Power Handling:
100Watts Carrier ( +50dBm )
Power Supply
+11 to 15vdc @ 100mA
Size: PC
Board
66.7
x
83
x
25mm
Kit
Webpage:
www.minikits.com.au/eme157.htm
DESCRIPTION:
The Pre-amplifier has been designed to be
easy to construct, and can be centre frequency tuned for use
between 120 and 150MHz. The pre-amplifier is suitable for
the 144-148MHz Amateur Band, and also suitable for weather
satellite reception on 137MHz. The pre-amplifier uses a low
cost BF998R MesFET that has very good characteristics on
the VHF and UHF bands. Devices like the ATF54143 were
considered but they are generally unstable on 144MHz, and
require the use of SMD components and additional circuitry
for biasing. The pre-amplifier is essentially a tuned input
BF998R MesFET, with a wideband 50ohm matching
transformer on the output. The pre-amplifier is best mounted
close to the antenna and incorporates RF changeover Relays
and power supply decoupling so that power can be fed via the
coaxial cable to power the pre-amplifier. The design is simple
to tune, and does not have any instability problems due to the
wideband output matching circuit.
AMPLIFIER DESCRIPTION:
Refer to the circuit diagram. The
input filter consisting of a 2-10pF trimmer and L1, is a series
tuned resonant circuit that helps to reduce the 3dB bandwidth
of the amplifier to around 5MHz. This helps with overload
from signals outside the 144MHz band, especially strong Ham
50 and 430MHz transmissions. L1 connects to the 50ohm
tapping point on L2 and with another 2-10pF trimmer in
parallel forms the main input tuned circuit for the amplifier.
The noise figure is determined by the matching and the tuning
of the input filter trimmer capacitors. Back to back 1N5711
diodes, are used on both the input and output of the amplifier
to bypass any stray RF from destroying the MesFET when the
pre-amplifier is in TX mode. The BF998R is biased for 10mA
with 4 volts on Gate 2. The output impedance is matched to
50ohms using a high frequency F29 material ferrite balun
transformer. A 6dB attenuator on the output of the reduces
the gain to around 16dB and also helps with 50ohm matching
and RF protection.
RF RELAY SWITCHING:
The Pre-amplifier has an RF
sensing circuit to ensure that the amplifier is bypassed in
Transmit mode. In Receive mode, the Relays are switched on
so that any loss of voltage to the amplifier drops the relays out
into bypass Transmit mode. Transmit RF is sensed via a 22k
resistor
( R1 ), a
nd rectified by two 1N5711 diodes. The DC
voltage produced switches the BC547 transistor on, which
discharges the 10uF capacitor and pulls the 2N7000 Fet’s
gate input low. The Fet’s drain voltage goes high releasing
the relays into TX mode. When going back to Receive mode,
the 10uF capacitor discharges slowly to allow for SSB
operation inbetween voice peaks chattering the relays. The
relays however are best switched with DC from the
Transceiver, or from a Transmit sequencer with the RF
sensing being used as a safety circuit only. For manual DC
switching, the +12vdc supply to the pre-amplifier is controlled
by swi12vdc to the pre-amplifier in RX mode, and dis-
connecting the power in TX mode. This can either be done
by feeding a se12vdc DC feed cable up to the pre-
amplifier, or by using the RF decoupling circuit L4 on the PC
board, and using the coaxial cable to feed +12vdc to the pre-
amplifier..
POWER SUPPLY:
Refer to the circuit diagram. The
preamplifier uses a 78L08 regulator to provide the primary
voltage for the BF998R MesFET. Power supply filtering
capacitors are used to lower the regulator noise and decouple
RF from entering the power supply voltage rails.
CONSTRUCTION:
1
. The PCB supplied is a professionally made plated through
hole board to make construction easier. It comes as a set of
two boards that can be broken apart easily, and filed flat to
clean up the boards edges Plated through earth holes are
shown as dark colored pads on the PCB overlay diagram.
Earth connections do not have to be soldered on both sides of
.COM.AU
2M VHF PRE-AMPLIFIER
Rev4
Mini
kits
®
Copyright
©
Mini-Kits
www.minikits.com.au
Revised
29/12/2014
Summary of Contents for EME157B2
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