Maximizing Rf Power Output - Ramsey Electronics QRP-80 Instruction Manual

Rf voltage levels in this transmitter can vary from 2 to 10 volts RMS depending
on various factors. Typically, 1 watt power levels are achieved in 5-7 volts RMS
volts range. A good test bulb for this level is the PR-4 flange-style flashlight
bulb or the type 243 bulb with screw-in body. Both are rated to give normal
brilliance at 2.33 volts, drawing 270 milliamps of current. Using Ohm's Law
P=IE, we see that normal brilliance requires 2.33 volts x .270 amperes for .62
watts of DC power consumption. We can conclude that even 1/2 watt or so of
RF should light this bulb reasonably well. A type PR-12 bulb is suitable for
checking RF outputs in the 1-3 watt range. Try it out!
Please remember, though, that a flashlight bulb does NOT present the proper
load impedance to the transmitter output, so theoretical calculations based on
the bulb`s rating can only be approximate. For example, the PR-4 at full
brilliance presents only an 8.2 ohm load to the transmitter.
If ANY flashlight bulb lights up when connected to the antenna jack of this
transmitter, you can be satisfied that you have RF output power ROUGHLY
comparable to the DC power rating of the bulb you are using. If you burn out
your bulb, rejoice and put your rig on the air!
Amateur radio magazines and handbooks provide a variety of circuits for RF
wattmeters and relative field-strength indicators, including methods of using
your VOM as an indicating device. CQ magazine for March 1990 offers an
article by KB4ZGC on how to make a highly accurate yet inexpensive dummy
load and wattmeter capable of showing 1/10-watt differences in RF power.

MAXIMIZING RF POWER OUTPUT:

Hopefully, the introductory discussion about the amount of power increase
needed for significant boosts in RST reports will satisfy most users that good
communication opportunities are afforded by the transmitter's QRP output. The
simplest way to ensure maximum reasonable power output without component
damage is to run the DC voltage in the 14 to 15 volt range, observing a
maximum limit of +18VDC.
IMPORTANT NOTE: If you are experimenting with this transmitter and see a
sudden and massive increase in power output and DC current, you have not
reached the promised land or created a 25 watt transmitter! Sudden surges like
that are a sure sign of amplifier self-oscillation. Kill the DC power supply
immediately, because your Q3 RF power transistor has headed to self-
destruction while probably interfering with every TV set in the neighborhood. A
poorly matched antenna along with higher supply voltages is usually
responsible for this occuring. Any prolonged "parasitic" emissions will also
overheat and destroy the components in the Q4 keying circuit.
Please remember that modified PC boards are not eligible for factory repair
QRP-80 • 19