Ramsey Electronics HR40 Manual de Instruções Página 26

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FHT-1 26
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 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 PL259 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
Dot Input
Ground
Dash Input
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