Ramsey Electronics FT146 - Manual (Page 21)

FT146 • 21
transmitter keying in a receiver is of some help, but even the simplest crystal
oscillator can send a fine signal into your neighbor's receiver.
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 transmitter kit.
Since this solid-state transmitter 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. It may be worth your
while 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 25
volts RMS depending on various factors. Typically, 1 watt power levels are
achieved in 5 to 7 volts RMS volts range, and 5 watts at 15 to 20 volts. 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 a 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. Because of this, the
transmitter 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 FT146's
performance!
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 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!