W0BTU Broadband Preamps
for Low-Noise Receiving Antennas
Covers 100 kHz to 30 MHz
Broadband preamp using bipolar transistor
Original amplifier - still in use
Transformer uses two Amidon FB-73-801 ferrite beads bifilar wound with # 24 or #26 enameled wire
Please don't ask me where to get the TRW LT1001; they were given to me years ago. The 2N5109 will work just fine in place of it.
+12 volt DC feed through 2.5 mH RF choke - early design
Subsequent RF chokes are made using ~5 turns of enameled wire through single FB-73-801 bead
Two back-to-back 1N914 diodes from preamp input to ground also shown. Since this photo was taken, there are now four diodes, two in series in each direction.
There is now a small incandescent lamp in series with the input to prevent damage from nearby transmitting antennas in proximity to the receive antennas.
The pot shown is to reduce the input signal. It's also in series with the input. Approx 2K ohms.
Other W0BTU broadband preamps based on same design, but using different active device
Prototype uses relays to switch preamp in and out of circuit
- Based on a design in Solid State Design For the Radio Amateur, © 1977 ARRL - Pages 97, 98, 122, 123
- The RF choke shown in the schematic can be eliminated. It was for locating the preamp remotely near the receive antenna (not necessary) and feeding 12 VDC through the coax.
- There are advantages to using a bipolar transistor over an FET or JFET. See the above-referenced ARRL publication for more information.
Schematic of improved broadband preamp using 2N5109, based on the same ARRL design. Click on the appropriate link in the left column on Larry's site to view his preamp page.
Low noise receiving antenna info
I haven't tried this or studied it much, but for the price...
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Last edited 9/15/2012