dB

Antenna Gain

AF5NP Antenna, dB, Decibel, Dipole, Efficiency, Gain

Newer amateur radio operators may read or hear some discussion of antenna gain.  The gist of it usually involves how much stronger signals are with a particular type of antenna.

This concept of antenna gain can be confusing or misleading to new hams because it sort of suggests that some antennas actually amplify received or transmitted RF signals.  Not so.

Like the decibel (dB), antenna ‘gain’ is always relative.  So when we speak of antenna gain it refers to a performance improvement compared to a different type of antenna.

More commonly, gain is used to compare highly directional antennas (beams, dishes) to monopoles or dipoles.

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While antenna gain usually emphasizes directivity it may also be used to compare two similar types with different characteristics, as efficiency is another contributing factor.  For example, we can observe and measure the relative gain of a stubby “rubber duck” handheld radio antenna (pathetic) vs. a 5/8-wave whip (much better) even though they are both vertical monopoles with the same directivity.

The two types of antennas usually referenced against are isotropic and the simple dipole.  When measuring performance of other antennas, they will often be compared to one of these two.

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If the gain improvement is relative to the theoretical isotropic antenna, the units should specify the gain as dBi.  If measured against a dipole antenna, the gain should indicate dBd.  Any other comparisons should mention the reference antenna in the text.

Some good references for antenna gain are given below: Continue reading →

dB, or not dB…

AF5NP dB, Decibel

…what was the question?

Whether ’tis nobler in the mind to understand dB or just plod along wondering what the heck other hams, publications and spec sheets are talking about is up to you.  With all due respect to Shakespeare’s Hamlet, decibels (dB) are a frequent subject in ham radio.  All three US license class exams have questions involving dB in their pools.

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Hams new and old don’t have to be technical experts but all should at least be familiar with the decibel.  To that end we will give a simplified explanation here.

Decibels (dB) are a convenient and standardized way of measuring a change or difference between two conditions.  In audio,  radio work and electronics in general, we are often dealing with very large or very small numbers and the difference between them can be many digits long.  To make the numbers more manageable, expressing ratios of large and small values is better done using a non-linear logarithmic scale.  Logarithms are based on orders of magnitude (10,100,1000,10000… and 1/10, 1/100, 1/1000, 1/10000…).

The human ear responds to sound logarithmically so decibels are a natural fit to measuring sound levels.  Similarly, radio work behaves non-linearly in some ways so the logarithmic approach works well here.  We’re stuck with dBs, like it or not.  In amateur radio the dB is commonly used in context of amplification, feedline loss, antenna gain, filter bandwidth and RF signal strength.

Technically the decibel is a ratio between one state and another; it’s not an absolute measurement, it’s relative.  Always ask, dB relative to what?, because this is a ctitical factor.

More practically it often comes down to the amount of amplification or attenuation.  In amateur radio it almost always involves power changes so we will focus on this aspect.  Decibels can also apply to other units such as voltage but this gets a little more complicated and not as widely used in ham radio.

A power ratio is simply comparing two Continue reading →