W0BTU Broadband Preamps
for Low-Noise Receiving Antennas
Covers approx. 100 kHz to 30 MHz
Broadband preamp using bipolar transistor
Original amplifier - still in use. Modifications (one resistor and a heat sink on the transistor) have been made since this photo was taken. See below.
Transformer uses two Amidon FB-73-801 ferrite beads bifilar wound with # 24 or #26 enameled wire
The TRW LT1001A is no longer available; it was given to me years ago. I recently found a letter in my files dated 1994 where Motorola said that the replacement for the LT1001A was the MRF5812. That's why I used the MRF5812 in the 2 other preamps below.
The 2N5109 and some similar transistors will also work.
+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 1N914/1N4148 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 so that I can match the signal levels while comparing different antennas. It's also in series with the input. Approx. 2000 ohms. I use it to reduce the preamp gain so that the signals from the RX antennas are similar in strength to the signals from the TX antenna.
- 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 usually necessary) and feeding 12 VDC through the coax to power the preamp with a bias-tee arrangement (two broadband RF chokes and two .1 uF disc ceramics).
- Since I created this page, this preamp now has the W7IUV emitter resistor modification which further improves the 3rd order intercept and compression points. W7IUV gives the following performance specifications on his site:
"P1dB (1 dB output compression power) should be greater than +21 dBm. Usually runs about +24 to +27 depending on parts used.
"OIP3 (output third order intercept point) should be +41 dBm or greater. I’ve seen +44, which is as high as I can accurately measure with my test setup."
Somewhere, he implied that the noise figure is about 6 dB. Likely, the NF is lower without his emitter resistor mod; however, the IM3 is not as good.
The increased current required a heat sink on the transistor, not shown here.
- There are advantages to using a bipolar transistor over an FET or JFET. See the above-referenced ARRL publication for more information.
- I have only ever used a TRW LT1001A in the original amplifier at the top of this page. It has slightly better specs than the 2N5109. The LT1001A (now unobtanium) was offered for use in CATV applications. The preamps below use the surface-mount (SO-8) MRF5812.
Other W0BTU broadband preamps based on same design, using MRF5812 NPN bipolar transistors. I probably built these sometime before 1995. The relays are to remotely switch the preamp in and out of the signal path.
I've modified the one on the right since these photos were taken. They do not (yet) have the W7IUV circuit mod, and out-of-band signals overload it when connected directly to the feedpoint of my Beverage "pointed" NW. That may be partly due to front-end protection diodes conducting. Stay tuned.
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.
Transistor data sheets
Low noise receiving antenna info
I haven't tried this or studied it much, but for the price...
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- Page created July, 2010 -
Last edited 10/11/2015