QAMP20 • 14
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 another way, we assume you know that accurate,
commercially built RF wattmeters cost much more than what 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 don’t 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 difference 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, though, 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
"flakey" when tuning up into a light bulb, and by all means you should not
consider a light bulb an accurate indicator of the QAMP-20'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
Summary of Contents for QAMP20
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