Ramsey Electronics QRP20 - Manual (Page 18)

QRP-20 • 18
14 to 15 volts DC. The easiest method is to place two fresh "D cells" in series
with your power source, if a full 13.6-15 volts DC is not available.
If your supply voltage is in the 11-12 volt range, you can expect a 200 to 250
ma current flow and about one-half watt of the RF output power. With a solid 14
to 15 volt supply, you can expect about 400 ma current draw and up to 3/4 watt
of RF output power.
VERIFYING TRANSMITTER RF OUTPUT:
The most important thing to know is whether your transmitter is delivering some
measurable and reassuring level of RF power to the antenna. The sound of the
transmitter's keying in a receiver is of some help, but even the simplest crystal
oscillator can send a fine signal into your neighbor's receiver, as we have
already discussed.
Ideally, you have an small RF wattmeter, already inserted in the antenna line,
capable of accurately measuring low output power in watts and even milliwatts!
And it cost you less than what you paid for the transmitter kit. Right?
If not, we have a few other ideas for you, OK?
Saying the same thing one more way, we assume you already know that
accurate, commercially built RF wattmeters cost much more than what you paid
for this Ramsey transmitter kit.
Since this solid-state transmitter does not require tuning or adjustments, a
periodic power output check-up should suffice. If you do not own or have
access to a low-level RF power meter, use a trick that is decades old, the
common flashlight or panel bulb. All you need to know is the basic differences
between bright, superbright, dim, unlit and burned out! Using such a bulb to
check power output is also a satisfying way to put Ohm's Law to work. Your
Radio Shack catalog specifies operating voltage and current in milliamperes for
a variety of small replacement lamps. It could be worth your time to make up a
simple plug-in "output tester" for your transmitter, a male RCA plug connected
to a socket for the bulb of your choice or even soldered directly to the bulb.
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!