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March 1968 Radio-Electronics
 [Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
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The "In the Shop ... with
Jack" column appeared monthly in Radio-Electronics magazine, although the
title changed occasionally. In it, Mr. Darr, a celebrated circuits troubleshooter,
addressed reader questions on electronics, offering free troubleshooting help via
mail. One inquiry concerned microvolts per meter (μV/m), a measure of RF field strength.
The author clarified that μV/m represents the voltage induced across a 1-meter wire
in free space, aligned with the transmitter's polarization. Note that μV/m in
the case of RF field strength is NOT a scaling factor for distance form the
emitter; it refers to the field strength induced in a 1-meter length of wire.
Hence, a 2-meter length of wire will have twice the value of a 1-meter wire.
This assumes a far field measurement where the wavefront is planar and at a
right angle to the detection wire. Key misconceptions were
corrected: antenna height doesn't define μV/m (though it affects signal pickup),
frequency is irrelevant unless the wire is resonant, and polarization must match
for optimal reception. The article also explained RF signal propagation, noting
that energy spreads thinner over distance, reducing field strength. Transmitters
output power (watts), but receivers detect voltage (microvolts).
In the Shop ... With Jack - What Are Microvolts/Meter
 By Jack Darr
This column is for your service problems - TV, radio, audio or general and industrial
electronics. We answer all questions individually by mail, free of charge, and the
more interesting ones will be printed here.
If you're really stuck, write us. We'll do our best to help you. Don't forget
to enclose a stamped, self-addressed envelope. Write: Service Editor, Radio-Electronics,
200 Park Ave. S., New York 10003.
What Are Microvolts Per Meter?
That was the question. The reader who wrote in was confused. After looking through
my reference library, I was too. I found three or four ways of figuring μV/m, including
one in a very old book which said it meant "the number of meters an antenna is mounted
above the ground." Outside of some very specialized books, there is very little
about μV/m in "the literature." So, here's a digest of what I dug up.
The basic expression, microvolts per meter, is used in figuring the field strength
of an rf signal. What it means is the number of rf microvolts which will appear
across a piece of wire exactly 1.0 meter (39.37") long (Fig. 1). For this definition,
the wire is suspended in free space in a radio-frequency field, in a position exactly
at right angles to the line toward the transmitter, and with the same polarization.
 Now let's see what all this means, in practical
work. Also, let's clear up a few popular misconceptions. I ought to know about these;
I've been using them for years!
1. The height of the antenna wire above (earth) ground has nothing to do with
the definition. (I know - the higher an antenna the more signal it picks up. But
that's a different matter.) The expression is an indication of the rf field or voltage
at the point where the wire is.
2. Frequency also has nothing to do with this definition, except in the case
of a resonant wire. Because the wavelength of 300 MHz is 1 meter long, the signal
frequency must not be 300 MHz or any multiple or sub-multiple of it. If the wire
is resonant, the reading has a different meaning (μV/λ). The quantity μV/m
is a standard unit used to indicate the energy that an antenna will intercept in
a radiated-energy field at any frequency.
3. Polarization of the transmitting and receiving antennas must be in the same
plane; if one is vertical, the other should be, too. Usually, this is the configuration
for maximum energy pickup. TV signals (in the US and Canada) are horizontally polarized,
and so are the receiving antennas. (In Great Britain, they're vertical.) CB signals
are almost all vertically polarized, although there's no law that says they have
to be. It is easier for mobile units to use vertical whip antennas and for fixed
stations to obtain omni-directional coverage. So far as transmitting range goes,
there isn't any appreciable difference between the two.
In the same letter, the reader asked about the very rapid dropoff in the signal
energy as you go away from the transmitter. Let's see why this happens.
The Transmitted Field
Rf signals travel at the speed of light-300,000 meters per second. Let's turn
on a transmitter and feed one very sharp pulse of energy to it. (This is one of
those "ideal" antennas. I can use it if I want to; it's my antenna!) Fig. 2 shows
what happens. 1/300,000 second later, the rf energy radiating from the antenna has
gone off in all directions (above the ground surface), and the resulting energy
is in the shape of a hemisphere, with a radius of 1 meter, at (a). This is often
called a shell, or wavefront. My ideal transmitter is using 1 kilowatt of energy
to produce the rf field in the transmitted shell.
Now, the shell contains an rf field produced by 1 kilowatt. Look at Fig. 2 (b),
the same shell 1 second later. Still having the same total rf field, it's now 300,000
meters in radius. Considerably more area, eh? The actual amount of energy or field
at b is a heck of a lot less because of the tremendous expansion. If you want the
exact figures, you can work it out by figuring the surface area of a sphere with
a 2-meter diameter and then another of 600,000 meters diameter, then dividing by
two. That's not for me; I'll take your word for it. Just remember this one key fact:
No matter how big the shell is, it still has the same total rf field it had when
it left home! The bigger the shell, the thinner the field gets spread out. So, if
we spread a field produced by 1 kW over three miles, we have a fairly good signal;
spread the same amount over half of all creation, and it's not quite so powerful!
Watts or Volts?
One more point: The TV transmitter (like all transmitters) puts out power - so
many watts of rf. The rf current flowing through the antenna circuit produces a
field. And it's this field of energy that's intercepted by the antenna, where it
induces a voltage that we can measure in microvolts per meter. The receiving antenna
couldn't care less about power; all it wants is a little voltage, which is amplified
by the front end of the tuner.
Finally, my inquisitive friend asked, "Why are low-band TV stations allowed a
transmitted signal level of only 100-kW e.r.p., while high-band stations are allowed
316-kW e.r.p.?"
To clear up that abbreviation, e.r.p. means effective radiated power, and is
commonly used in FM and TV transmitting work. It's the power output of the final
amplifier stage, less transmission-line loss, multiplied by the power gain of the
antenna. (They get this power gain by reducing radiation in the vertical plane and
concentrating it in the horizontal plane.) To get 100 kW of e.r.p., a station could
use a 25-kW transmitter and an antenna with a gain of about 4 (depending on the
amount of line loss).
The reason for the allowable maximum power difference is simply that frequency
affects signal propagation. The higher the transmitting frequency, the less distance
the signal travels, power output being the same. Another way of putting it: Signal
attenuation increases with frequency.
So, low-band vhf stations (54-88 MHz) may use no more than 100 kW e.r.p. High-band
vhf stations (174-216 MHz) have a harder time pushing their signals out, and are
allowed up to 316 kW e.r.p. Uhf stations (470-890 MHz) have an even harder time
of it, and can use up to 5 megawatts!
But that's not the whole story. The FCC wants all TV stations to have equal service
areas when operating at maximum coverage. But vhf and uhf field strength depends,
not only on e.r.p., but also on transmitting antenna height AA T (above average
terrain). The higher the antenna, the less power needed to produce the same coverage.
The maximum power of 316 kW for a high-band vhf station can be used only if the
antenna is not over 1000 feet AA T. Increase the antenna height and you must crank
down the power. A high-vhf station with antenna height of 5000 feet AAT, for instance,
can have a maximum e.r.p. of roughly 1.5 kW.
Matter of fact, the above rules are also modified by geographical location. In
the high-population Northeast US (roughly New England to Illinois and Virginia)
the FCC limits on power are more severe than in the rest of the country. This was
done because more stations must "live with each other" in populous areas than in
sparsely settled regions like, say, Wyoming.
Brightness Trouble in Oscilloscope
I've got a faithful old DuMont 304 scope, and it's always done very well.
Now, though, it's got brightness-control troubles. When I turn the control full-on,
the trace gets very wide, as if blooming. Back down halfway, and it looks pretty
good. However, it won't put the spot out as it should.
Any ideas? -R. H., Houston, Tex.
Some. Check all resistors in the negative-voltage network. Also check the voltage
between the CRT's cathode and grid. Apparently the grid isn't going far enough negative
(with respect to the cathode) to extinguish the spot, when the brightness control
is turned all the way down. I seem to remember a similar trouble in this model once
before; check the Z-axis (or intensity-modulation) coupling capacitor. It's connected
to the CRT grid, but there is a 2.2-meg resistor shunted to ground on the jack side
(see drawing). If this capacitor is leaky, it will upset grid-to-cathode bias.
Lightning Arresters for Coax?
I can't find a good lightning arrester for use on a coaxial cable lead-in
system. Any recommendations? -P.K., Los Angeles, Calif.
From much experience in two-way radio antennas, and in TV, I'd say that a very
good lightning arrester for any kind of coax would be a simple ground on the outer
shield at the point where the line goes into the house. The easiest way is to use
a Blitz Bug lightning arrester made by Cush-Craft for coaxial cable.
You can clamp the line in any of the conventional arresters, and then "jump"
the normal gap with a wire, so that the outer braid of the coax is actually grounded.
You can also slit the outer vinyl jacket, and wrap about 6 to 8 turns of solid copper
wire around the braid. Don't try to solder it; you'll melt the inner insulation.
Spray this with Krylon clear plastic, then wrap it tightly with vinyl tape, to
keep it weatherpoof. Use a good ground rod. R-E
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