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Practical Gain__

By VE1ZAC

No subject garners
more confusion to hams, new and old, than gain, and what it really means. (Thus
“Practical” gain)

Gain is often used
in the same sentence as transmitter or linear amplifier output, receivers, S
meters, or antennas. And, it is almost always described in terms of dB. If you
are a ham, you should, by now, have absorbed what the heck a dB is all about. If
not, here is a simple way to absorb what it is about. (My apologies to the
rigorous among you… this is not!)

__Practical
dB’s__

A db is nothing
other than a ratio. It has no dimensions, and isn’t a quantity of anything. In
fact, the easiest and really only thing you need to remember about dB is that
three of the little doobers ( 3 dB) is the same
as 2 X. ( 2 times, or twice as big). Also, since we are dealing with a number
that has to do with powers of a number, -3 of the things (-3dB) means ½. So, if we talk about something having a gain
of 3 dB, we mean it has twice the “Something” of one that doesn’t have a gain
of 3 dB. We can attribute the 3 dB with
something else, like direction. Especially when we talk about antennas further
on.

Fine, you say,
that one is easy. So how do I deal with other dB numbers, for example, 6 dB ? Well.. ask
yourself how many 3 dB’s there are in 6 dB ? There are two of them. Each 3 dB
is 2 X, so we now have 2x 2x which is 4 times. Ahhhh… 6 dB means something is 4 times as big. This works because we are talking about
arithmetic on powers of a base number. To multiply, we add the powers.

What about 10 dB ? There are 3 of
the 3 dB’s in there at 2 X each, for a total of 8 x and 1 dB left over. You know the “Times”
in this case has to be less than 2 and greater than 1..
so if you guess at something in between you will be
pretty close. Let’s use 1.25. In fact 10
dB is conveniently 10 x. Another good one to remember.

Minus
dB’s are simply dividing instead of multiplying. This -6 dB means ¼ and -10 dB means 1/10.

__Practical
transmitter power__

We often talk
about amplifiers as having gain. For example, my shiny new linear amplifier
might have a gain of 10 dB. That means
that if we drive it with our basic 100 watt transceiver output, it should
produce a power of 100 x 10 = 1000 watts, or 1 kW. (Assuming, of course, that
it is OK to drive it with a signal this large)

__Practical
antenna gain__

I have an antenna
that has a gain of 6 dB. What does that
mean? Well, first off, we need to know where that gain is, since it is an
antenna. It might be in one direction, it might be everywhere. We need to ask
where it is. If it is a beam, it likely has gain in it’s
forward direction. Then, 6 dB means any received signal will be 4 times as big,
and anything transmitted in that same direction will appear that it has an ERP,
or effective radiated power 4 times as large.
We could get the same result, at an observer’s receiver, by using a ‘no
gain’ antenna and using a 6 dB amplifier at our transmitter. The observer
wouldn’t know the difference. (Keep in
mind that these beam type directions are often VERY specific…. That is, a
certain direction, and certain elevation above the horizon… thus the need to be
able to point our beam in various directions)

Another number
that comes up with beams is the F/B (front to back) ratio. This is the
difference in forward and reverse gain from the same beam. So, our same beam we
used above might have a F/B ratio of 15 dB (not
unusual, by the way). That means we have 2x 2x 2x 2x 2x
gain = 32 times gain one way compared to the other. Or, if we look in the
backward direction, we reduce reception and transmission to 1/32. Pretty small. Great for eliminating a source of interference, by pointing away
from it.

Here are some
EZNEC model outputs of a short Beverage antenna recently undertaken at the
authors QTH. (This Beverage is a 200
feet long wire 6 feet high, and terminated with a resistor to ground. Slightly
different from the one built) Notice all the dB gain information and how much
effort is taken to demonstrate the direction of the gain numbers. The green
line is the particular direction and point we are describing. BTW, that point
can be moved around all through the model to probe gain in any direction in the
field. You can see that the gain changes all over the field. It is obvious that
any number we refer to needs some direction information.

Another term has
come up: dBi.
This means dB gain referenced to an isotropic antenna. Sometimes we use dBd, which refers the gain to a classic dipole. That one
means a lot to some people, because they understand dipoles well.

Here is a 3D view
of the same antenna. These views represent a macro or far field view of the
antenna field. Imagine it laying over the Earth.

__What is that S
meter telling us ?__

There are two
receiver issues dealing with gain. The first is the poorly understood and oft
quoted S meter. What the heck is an S meter, what is it really telling us anyway?

An S meter is a
fairly poorly engineered measurement device which is supposed to tell us, very
roughly, what the gain of a received signal is, compared to a shorted antenna
input. The scale is arbitrarily divided up into 9 S units (1 to 9) with the far
left needle position being 0. Beyond that, there are a few more divisions that
are marketed in dB, often in

units of 10 dB. These might go up to +50 or 60
dB.

S unit
calibrations are, unfortunately, not completely uniform. However, by and large,
receiver manufacturers try and have the value of one S unit = 6 dB. There are
some that go as low as 4 dB. In our
discussion we will use 6 dB, but be forewarned that this might not apply to
every transceiver o or receiver out there. There is also another loose standard
that says “S9 shall be equivalent to a 50 uVolt
(micro volt, or 50 X .000001 volt) signal on the antenna terminals with a 50
ohm load”

Remembering our ability
to add dB’s to get equivalent gain meanings, S9 means 9x6x, or 54db, or 2 to
the power of 18 times more signal than what would be present with a shorted
antenna input. All of us know in practical terms that if we have background
noise of 2 S units, and the signal we find very
clear is 4 S units, the difference is 2 S units,. This means our
received signal is 12 dB louder than the noise. If our signal is S9 plus 20 dB,
that would be 54 dB plus 20 dB = 74 dB, which now means 2 to the power of
almost 25. A really big number. (With numbers this awkward, you might now get
an inkling of why we want to use dB at all…. It’s because the numbers are much
easier for our brains to deal with, than say 2 to the power of 25!)

In practical
terms… if we are now at a receiver and watching a signal from our friend with
the two antennas, we might see a S meter reading of 4 on his signal with a ‘no
gain’ antenna, and when he switches to the 6 dB beam, we see the signal go up
by one S unit ( 6 dB) to S5. That’s all it means.

The same situation
might occur with an amplifier. Without the amp, the signal is S4. With the 10
dB amp in play, the signal goes up 10 dB, or maybe 1 ½ S units.

Here is another
example: QRP (5 watts) vs. QRO ( 100 watts). What does
it mean? Well, 5 watts times 20 = 100 watts, 10x is 10
dB and 2x is 3 dB, leaves us with 13 dB difference. What does that mean for the
listener and his receiver? 2+ S units…
that’s all. Puts things in perspective!

(I hope there is a
round of “Ah-hah’s” being heard out there.)

Remember, that
these S meters are seldom very accurate, and two different rigs with the same
input may or may not show the same S number. For that reason, it is a big
mistake to give signal reports with S units. Use the objective RST reporting system
and state what your ears are hearing, not what your eyes are seeing. It will
mean a lot more to the other ham.

__Practical
Receiver gain and dynamic range numbers__

Manufacturers
often report their receiver gain numbers in glowing terms, expecting the purchaser
to equate big numbers with better. This often is misleading. Remember that when we amplify our antenna
signal, we are also amplifying the noise that is attendant with the signal. So
cranking the whole thing up isn’t going to be much use unless we can figure out
how to pick our signal out of the noise. The ability of the receiver to do that
has less to do with gain and more to do with operator controls. (Take a look at
VE1RGB’s article on this website.) The gain number must refer to something, and
a quality manufacturer will refer to gain of a signal with a certain SNR or signal to
noise ratio. Remember our example of the signal 2 S units above the noise ?

__Caution:__ SNR calculations are different for power dBs and voltage level dBs. Another discussion subject.

Dynamic range is a
more useful number, and often tells a practical story about a receiver. Here
the dB number will refer to the change in signal strength “Range” that the
receiver is capable of dealing with under a set of good conditions. A high
number here is a good thing, and often difficult to obtain. Surprisingly, many
complicated receivers may have a worse value for dynamic range than a very
simple receiver. Something to keep in mind.

There are other
receiver and transmitter measurements that use dB as well. That may be the
subject of other articles in the future.

For now, let’s
recognize the ubiquitous “dB” as a practical hams friend from the math world.