One of my preamps built on a circuit board
Guido, IK2BCP built his preamp "dead bug" style in a diecast box
Longer Beverages seldom require amplification; however, on those shorter than 550 ft and the K9AY and DO loops, a preamp in the line means the difference between copying a station well enough for a QSO and just barely hearing it. The preamp is virtually foolproof in construction and operation, provided that you get the phasing correct on the toroid! While primarily used on 80 and 160 meters, this unit performs well on 40M as well and would most likely work well right up to about 30 mhz.
If you visit W7IUV's web page via the link above you will find a lot of very useful information on building this preamp as well as some mods which the designer has implemented to make this preamp more resistant to IMD. I built my first preamp to the original design, which supposedly provides more gain. The second was built using the original emitter resistor values but changing values as per IK2BCP (change the .01 capacitors to .1 and add by-passing and filtering on the +VDC line as Larry has done in his modified unit). As both of my stations are in rural areas far from BC interference, improving the IMD was thought not to be of great importance.
If you would like to build the preamp on an etched board, check out Kits and Parts dot Com; this vendor offers a complete kit of parts including a 2N5109. While the parts values are slightly different, the design is pretty well the same, yielding a 17-20db preamp which is somewhat easier to build than etching your own board or building "dead bug" style.
One of my preamps built on a circuit
board
Guido, IK2BCP built his preamp "dead
bug" style in a diecast box
There is no need for a printed circuit board in constructing this preamp. Perf board and point-to-point wiring will work just fine. As there is no tuned input, you might wish to consider employing a couple of stages of filtering at the input to keep nearby BC band signals out. At the very least, a good tight metal enclosure, proper coaxial connectors and a well-filtered power supply are all "musts" if this preamp is to perform satisfactorily. The preamp shown above is mounted in my "Bevbox", a metal enclosure housing my RX antenna switching as well as a relay in the TX line to the amplifier to remove power and so ground all RX antennas when in TX mode.
On December 30 2008, Larry posted some comments on the Topband Reflector regarding building his preamp. These are well worth reading and are reproduced below.
"Recent posts regarding the preamp circuit I have on my web page and which
has appeared in ON4UN's book seem to
indicate a lot more problems building the circuit recently. I hope to update
the preamp info on my web page to minimize
problems folks have duplicating the circuit but in the meantime maybe this
post will help.
The schematic presented in ON4UN's 4th edition on page 7-98 is incorrect.
The 47 ohm resistor shown in the emitter
should be a 4.7 ohm resistor. The schematic shown on my web page
under the "Rotatable Flag" topic is correct but is
optimized for my application. If the circuit is built **EXACTLY** like
the schematic shows, it will work as specified in
the text. Parts substitutions are possible but unless you have a good background
in RF design, don't try it. I hope to
update the web site soon with more parts options.
The power supply is specified as 12 volts. That means 12.0 volts +/- 0.1
volt. It does not mean 14 volts or 13.6 volts
or anything else. The part values specified on the web page are chosen
to maximize IMD performance with 12.0 volts
applied. If you run more than 12.0 volts, you run the risk of damaging
the transistor which is running near it's maximum
ratings. USE A GOOD HEAT SINK!
Some of the parts are critical. Use disk ceramic caps or other caps
that are known to have a low ESR and no HF
parasitic resonances. If the cap has a plastic case, it is probably not
the right part to use! Use 1% metal film or carbon
film resistors. Most metal film resistors have low enough parasitic
inductance to work fine in this application. Be aware
that this circuit has a usable bandwidth of over 100 MHz so build it accordingly.
The 4.7 ohm resistor in the emitter lead
is critical, make sure it is not less than 4.7 ohms! 5 ohms is OK, 4.5
ohms is not.
The current drain should be 89 ma. +/- 5 ma. If it is not, you screwed
something up. The transistor should be hot to the
touch, but if the heat sink is adequate, you will be able to just barely
keep your finger on it. If it is hotter than that, you
either need a bigger heat sink or you are drawing too much current.
The 0.01 uF ceramic coupling caps that are specified may be increased in
value to 0.1 uF if, and only if, the preamp is
preceded by a very good high pass filter. Using a good 7th order or better
high pass in front of the preamp will allow the
use of the 0.1 uF caps. The higher value caps should be used if VLF operation
is contemplated. The 0.01 uF caps have
been specified to provide a small amount of BC band filtering for those
applications which need a bit more BC rejection.
I suggest using the 0.01 uF caps unless you really know what you are doing.
(Design note: if this preamp follows a
complex filter such as an elliptic function filter, you will probably
need to use the 0.1 uF caps in order to present the
filter with a proper termination impedance).
The 2N5109 and 2N3866 transistors have been tested in this circuit and
will provide performance specified. Other
transistors might be used but I have not tested any nor can I suggest any
substitutes. Both of the above transistors
are readily available from several mail order sources. Substitute
devices need to be compatible with the Ft, hfe, NF,
and power dissipation of the specified devices."
Have fun!
73, Larry, W7IUV