Making Antenna Gain Measurements
Measuring antenna gain is much more difficult than first meets the eye.
dBi (dB relative to an isotropic source in free space) is, to my mind,
the
best reference for computer modeling. Conversion to dBd (dB relative
to a Dipole in FREE SPACE) merely involves subtracting 2.2 dB.
The problem is that many, if not most, people then ASS-U-ME that they
can measure the difference between a dipole over gound and another
antenna at the same height and use that number as the gain in dBd.
Unfortunately, this is NOT the case (except perhaps at some specific
height where this relationship just happens to hold true).
The reason that gain relative to a dipole at some height above earth
is NOT equal to the actual antenna gain in dBd is that the gain of a
dipole over earth is NOT constant with height.
The radiation resistance of a dipole varies with height above ground
due to the reflection from the earth coming back and combining in
phase or out of phase, depending on the specific antenna height
in wavelengths. This variation in radiation resistance is plotted in
many antenna books, including the ARRL antenna books. The
range of variation is from approximately 45 to 97 Ohms. Also,
plotted in many of these books is the GAIN VARIATION due to this
effect, which amounts to about 2 dB. Unfortuantely, the gain
variation curve has been OMITTED in the latest (18th edition)
of the ARRL ANTENNA BOOK. To my mind, it is crucial to
understand this dipole GAIN variation as well as the impedance
variation when making antenna comparisons. I hope that ARRL
will see fit to reinstate this curve in future editions of the ANTENNA
BOOK.
Neglecting this important variable obfuscates the truth in measuring
antenna gain. (Yagi's are less susceptible to gain and impedance
variations with height due to the pattern rejections below the antenna.)
To put this 2 db gain variation in perspective, recall that 2 dB is the
usual difference between a 3 and 5 element Yagi (assuming adequate
boom length in both cases). Also, 2 dB is usually the net gain obtained
by properly stacking two identical Yagi antennas. A 2 dB variation in
reference antenna gain can artificially inflate (or deflate) the
"measured"
gain of the antenna under test.
The CURRENT in a dipole is maximized when the radiation resistance
is minimized (P = I^2*R, for constant power I is inversely proportional
to
the square root of R). One such point occurs at approximately 5/8
wavelength.
This corresponds to 80 ft on 40 meters (40 ft on 20 meters) which
indeed have proven to be very effective heights for dipoles. While
increasing height above these levels lowers the angle of radiation,
the gain of a DIPOLE will fall, thus minimizing the benefit of additonal
height (for a dipole) until over 1 wavelength is reached. Again, this
imformation was available in older antenna books but has been
dropped from the latest ARRL ANTENNA BOOK. It is important
to have a complete understanding of the fundamentals to see the
whole picture in making antenna comparisons.
Another potential problem with using a dipole as the reference antenna
is that it radiates equally in the forward and backward directions. If
there
are any conductive objects BEHIND the dipole (such as power lines,
house wiring, gutters, metal backed insulation, fences, vehicles,
water towers, hills or mountains, etc.) then reflections from these
objects can also alter the apparent gain from the dipole. A Yagi of
KNOWN gain vs. frequency is probably a better reference antenna
since it will minize the effects of reflections from below or behind
the reference antenna.
de Tom N4KG
--
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|