Low noise, high IP preamplifier for 10Mhz to 500MHz.
Two of these pre-amps serve already more than 10 years for both 2m and 70cm repeaters in Rotterdam. Without damage, acting as sensitivity optimizing components in very critical and difficult circumstances with an antenna height of 105m above streetlevel.
Uncritical construction. Just solder.
Download the accurately printable PCB mask by <right-clicking> on "PDF" and click "Save as". Open the PDF in Adobe Reader and print.
Zin/out: 50 ohms (PCB signal tracks are 50 Ohms).
Uin(max) 40mVeff @ Ub=13,6V and f=145MHz.
Sensitivity: 12dB sinad <= 0,125uV 1/2 EMF.
Voltage gain: 18dB @ 145 MHz.
This amplifier unit has no selectivity and is universally usable. The necessary selectivity should
be connected directly onto the input connector.
The amplifier is sturdy, insensitive to power voltage changes and proved to overcome severe static's. It is tested during more than 10 years of service on two VHF/UHF repeaters.
The diode prevents damage in case of reversed power or nearby lightning.
The 3W wire wound resistor serves as filter coil, lowers the supply voltage to 12V, and limits peak currents when switched-on.
At a supply voltage of 13,6V is has highest IP, and is most tolerant to very strong unwanted signals. Transistor dissipation is then 200 mW.
At lower supply voltages it has a bit lower noise figure, but lower IP.
For absolute isolation from the transmitters RF signal, the box is divided in five screened compartments to create well filtered power.
In case of fault, the amplifier can be changed easily for another amplifier, or simply omitted, and is therefore service friendly.
As in- and output impedances are 50 ohms, standard 50 ohms 145MHz or 435 MHz helix band pass filters (Toko), can easily be connected. This should be
done directly to the INPUT connector of the amplifier, without an connecting cable.
The very selective DIY helix band pass filter described elsewhere on his site is highly recommended for 145MHz use.
This PCB MUST be DOUBLE sided.
Download this PDF file , and print it on inkjet foil at size 100% or "Original size", in COLOR, max contrast, max resolution, best photo paper. Dimensions 44mm x 25 mm (resolution 200 DPcm). The PCB is double sided, with the non etched back plane acting as "ground" surface.
Drill a 5 mm hole for the transistor.
Drill 1.5 mm holes at the markers "+". Solder copper rivets in them on both sides of the PCB.
All eight SMD parts are soldered onto the top surface of the PCB.
The PCB must NOT make contact with the housing, only at the four places near the input- an output connectors. (See photo).
The housing is a TEKO steel box 44mm x 102mm x 25 mm inside (see photo).
REM: Inside is no room for "screw in" connectors.
When this amplifier is used in a repeater system, do not use BNC connectors. BNC connectors have poorer mass connections than N, and limit the total screening to only 80dB.
For use in a repeater system, in- and output connections must be silver plated or stainless steel N-connectors with flat flanges and PFTE insulation.
They can directly be soldered onto the outside of the steel box.
The flanges of the N-connectors have to be trimmed in order to let room for the sides of the lid.
Remove the (nickel) plating on the outside of the box prior to tinning the solder places there.
At the places where the mass connections of the PCB will be soldered in contact with the mass of the input and output connectors, the connectors should be tinned also before they are soldered in place. See photo.
The power connection can be any coaxial type. BNC is handy, because standard coax cables can then be used for connecting power, and are screened for
Before installing the screening walls, solder the 1000pF feed through capacitors first.
Solder each of the four screening walls at one point onto the bottom of the box. After that solder them to two more places onto the box sides. See photo.
"Fb" are VHF ferrite beads.
When finished, check the current drawn, it should be 28 mA for best IMD performance.
You can correct it by changing the value of the emitter resistor of the BFR91.
Then check the voltage gain at 145MHz, it should be 18 dB.
The preamp was developed for, and tested with our 2m and 70cm relay stations (pi3rtd and pi2rtd). Their duplex filters had over 90dB separation, and very good selectivity due to the application of extra cavity band pass filters.
With pre-amplifiers connected, the repeater receivers showed no sign of desensitization nor IMD when the repeaters transmitter output power was varied from QRP (< 1W) to QRO (>40W!).
During testing it became clear, that the preamplifier had to be connected directly onto the output of the band pass filter.
If the amplifier input was connected to the input band pass filter through a coax cable, the amplifier could become instable at certain cable lengths, due to extreme mismatches at certain frequencies.
The best solution therefore is to screw the amplifier input directly onto the output of the receiving band pass filter, as shown in the drawing above.
Why installing a preamp outside a repeater casing.
Due to installing a preamp outside a repeater transmitter cabinet :
- best results will be obtained, as no direct radiation from the transmitter into the preamplifier housing is possible
- the construction is simpler
- if an amplifier should become defective, it easily can be exchanged, or just be omitted in case of emergency.
- one type wide band preamplifier is suitable for both 145MHz and 435MHz repeaters.