READER-TO-READER TECHFORUM
characteristics and a higher withstand
voltage which is critical to
accommodate the elevated voltages at
full power output. Most film type
resistors such as RLR or RN series are
not suitable for high frequencies
above approximately 10-20 MHz
(value is also a factor) due to the high
parasitic reactance and skin effect of
the resistive film. You can use the RLR
series resistors if you are willing to
accept reduced accuracy. Carbon
composition resistors are much better
for HF, but they are not very common
these days. For simplicity and easy
availability, resistors R1 and R2 are 5%
values, therefore the attenuation will
have a slight error from the ideal
100:1.
For higher accuracy, use 1%
resistors, R1 = 5.11K and R2 = 51.1
ohms. Diodes D1 though D6 are used
to protect the scopemeter's input and
limit the input voltage to
approximately ± 4. 2 p-p. Make sure to
use short wires for all interconnects.
This attenuator is suitable for the full
frequency response and input levels of
your instrument. Figure 1a shows a
coax between the transceiver and the
UHF T adapter.
If convenient, connect the T
adapter directly to the transceiver and
skip the coax at point 1. The
connection between R1 and R2 to the
T adapter at points 3 and 4 should
be as short as possible, or better yet,
use suitable coaxial adapters. The
transceiver power output will be:
Power output = (Vrms2 X 100) /
Rload or (Vp-p2 /2) / Rload
for a 50 ohm resistive load
Power output = (Vrms2 X 100) / 50
or (Vp-p2 /2) / 50
Note: If you are using the antenna
as a load, the equations above may not
yield correct results because the
antenna may not be an exact 50 ohm
resistive match. Please see the ARRL
handbook for more details on this
subject. Do not use wire-wound
resistors; these resistors have high self-inductance and at high frequency, the
actual value is higher and reactive.
76 February 2011
If you wish to experiment and use
an RF probe, Figure 1b is a classic
basic probe. The minimum detectable
input is approximately 0.3 volts and
the maximum 30 volts. The common
1N34A germanium diode is preferred
for such applications because it has
lower forward voltage drop than
silicon, has good high frequency
characteristics, and is readily available.
The probe should be good to 30-
40 MHz. Keep the leads between the
probe and the RF source as short as
possible. C1 should be a ceramic type
and keep all the leads on the RF
region as short as possible. Do not use
a film type capacitor because it will
have high parasitic inductance which
will reduce the frequency response.
The resistor R3 value and type is not
critical because it is on the DC
section.
Good luck with your amateur
radio hobby.
Mort Arditti, NA6MA
Los Angeles, CA
[#11105 - November 2010]
Using Up Batteries
With so many devices we use
today having a power regulator in
them, our batteries become ineffective
before they really run out of power.
What can be done with these batteries
that have plenty of life in them, but not
enough to get past the power
regulator? Is it possible to blink an LED
until there is like half a volt or less in
each battery? What interesting things
can these "dead" batteries do until they
are truly dead?
#1 Once a set of batteries is "used
up" for widgets that need a certain
minimum voltage, I simply toss them
in a box and use them for flashlights.
They are perfect for bedside
flashlights, as they may not be as
bright as one with fresh batteries, but
perfectly adequate for waking up in
the middle of night for nature calls or
whatever. Once a flashlight with these
batteries starts to go dim, they are
truly ready for the recycling bin.
Larry Supremo
Escondido, CA
#2 I have used 'dead' batteries in a
cordless headset and battery-operated
clocks. A company I worked for used
AA batts in their products, so I had
more than I could use in the headset.
They usually lasted less than two
weeks but only took a moment to
replace.
The batts I have used in clocks
usually last two months or more. Just
put them in a clock you can get to
easily.
Judd H.
near Reno, NV
#3 I have an LED flashlight that
provides more light with a fresh cell
than I need. I use used cells, and the
flashlight gives enough light for many
hours and really runs the cells flat. This
particular flashlight uses a voltage
boosting circuit to enable it to use just
a single 1.5 volt cell.
Michael La Moreaux
Ann Arbor, MI
[#12102 - December 2010]
Garage Door Remote Control
My garage door opener remote
operates using a carrier frequency
designated W, with modulating
frequency 52. Since it is now
inoperative (with a fresh nine volt
battery), how may I begin to repair or
replace either the transmitter or
receiver? The manual control
connected to the receiver works fine.
I've done a fair amount of
electronic circuit design and repair, but
nothing in this frequency range which
I believe is up in the MHz range.
#1 You didn't state the brand, but
your mentioning of carrier W and
modulating frequency of 52 Hz makes
me think you have a Sears remote and
receiver like the ones I've been using,
and fixing for some 30 years. The first
thing to go wrong with the remote
was the pushbutton switch
mechanism. It's made of thin spring
metal that eventually breaks. As a
workaround, I cut two pieces of wire
long enough to reach outside the
case, soldered them to the board
switch terminals, and soldered a
