QAMP-40 • 14
VERIFYING TRANSMITTER RF OUTPUT
The most important thing to know is whether your transmitter is delivering some
measurable and reassuring level of RF power. Then you can continue on to
adding the QRP amplifier and checking out the whole set-up.
Ideally, you have a small RF wattmeter, already inserted in the antenna line,
capable of accurately measuring low output power in watts. And it cost you less
than what you paid for the transmitter kit. Right? In the words of Wayne from
"Wayne's World"... Not! So here are a few other ideas for you to try.
Saying the same thing a new way, we assume you know that accurate,
commercially built RF wattmeters cost much more than you paid for this Ramsey
amplifier kit. Since this solid-state amplifier does not require lots of critical 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 a light
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 and a local automobile parts store is a
treasure trove of various other lamps. It may be worth your while to make up a
simple plug-in "output tester" for your amplifier - 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 amplifier can vary from 2 to 25 volts RMS depending on
various factors. Typically, 1 watt power levels are achieved in the 5 to 7 volts
RMS range, 5 watts at 12 to 15 volts, and 10 watts at 20 to 25 volts. A good test
bulb for this amplifier is the #93 automobile lamp bulb or the #1156 type bulb.
Both are 12.8 volt rated, with the #93 being specified at 1 amp and the #1156
being 2 amps for normal brilliance. Using some Ohm's law calculations shows
that the #93 is a 12 watt lamp and the #1156 is a 24 watt lamp. We can conclude
that 10 watts or so of RF should light a #93 bulb reasonably well, while 20 watts
should be about right for a #1156. Try it out!
Please remember that a flashlight bulb does NOT present the proper load
impedance to the amplifier output, so theoretical calculations based on the bulb`s
rating can only be approximate. For example, the #93 at full brilliance presents a
12 ohm load to the amplifier. Because of this, the amplifier may act "flaky" when
tuning up into a light bulb, and by all means you should not consider a light bulb
an accurate indicator of the QAMP-40's performance! If ANY light bulb lights up
when connected to the antenna jack of this amplifier, you can be satisfied that
you have RF output power at least equal 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
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