Measuring antenna gain

Practical ways to get accurate results

    Computer modeling has revolutionized the way radio amateurs look at antennas.  Armed with one of the powerful software packages that have come along in recent years, it is possible to design more antennas in a day than could be designed in a lifetime on an antenna range.  Consequently, actual field measurement of antennas--using the classic scientific method of experimental research--has gone out of fashion.
    However, computer modeling has its limitations.  It is not always possible to model all of the variables that come into play with real-world antennas.  And the modeling process has pitfalls even for the experts.  Well-known software producer Brian Beezley, K6STI, published an article in Vol. 4 of the ARRL Antenna Compendium called "An Adventure in Antenna Modeling," in which he described his own frustrating attempt to design an antenna with exceptional low-angle radiation.  Concluding, he said this:  "In the end I decided to write up this fiasco for several reasons.  First, I wanted to demonstrate how foolish it's possible to become when you get carried away with computer modeling.  Powerful software is no substitute for common sense.  Second, I wanted to point out how easy it is to draw invalid conclusions when you ignore the limitations of antenna-modeling algorithms."
    Roy Lewallen, W7EL, another well-known modeling software author, said much the same thing in a February, 1991, QST article, "MININEC:  The Other Edge of the Sword."  He cited an example of an amateur whose computer modeling showed that a dipole less than a foot above a poor ground yielded 45 decibels gain over a dipole.  Of that amateur, Lewallen said, "...he recognized that the answer was ridiculous, but sometimes we're not so lucky and the errors are tougher to spot."
    Well then, how can a radio amateur who wants first-rate antenna performance be certain an antenna is really working as it should?
    One answer is to measure the antenna's gain against a known reference.  Antenna gain measuring sessions have been conducted at VHF/UHF conferences since the 1960s.  Often these sessions are conducted by antenna experts using professional quality signal sources and measuring instruments.  But any amateur willing to invest some time can set up an antenna range somewhere and obtain accurate antenna gain measurements with nothing more sophisticated than a low-power transmitter, a receiver and an audio VU meter.   I published an article in QST in October, 1977, called "Measuring Antenna Gain with Amateur Methods" to describe these methods.
    Because so few amateurs do actual gain measurements today, it seemed worthwhile to summarize what that article said on this web page.
    The article said that any clear area can be an antenna range.  The trick is to avoid obstructions and reflections:  if the received signal is louder when the antenna is pointed away from the source, there is a problem.
    To conduct comparison tests, two antennas are placed side by side on masts of the same height, using equal length feedlines.  A steady signal (a carrier) is generated perhaps 40 wavelengths away, and it is detected on a CW/SSB receiver that is not overloaded but has its AGC disabled.  Then a VU meter can be used to indicate the difference in received signal strength of the two antennas.  As a precaution, the two antennas are swapped so that antenna #1 goes on the mast and uses the feedline formerly used by antenna #2.  Given some care in measurements and a stable path, it is possible to determine the difference in the gain of the two antennas down to a fraction of a decibel.
    Although this test setup is most practical with VHF/UHF antennas, the same principles work at HF as well if one can obtain the right hardware (e.g., a tower trailer to support a reference antenna beside each antenna being tested).  Also, new antennas can be designed using these antenna range principles.  A variety of element length and spacing combinations can be tried until the best results are achieved.  While this is far more tedious than computer modeling, it does produce repeatable, practical real-world results.  The Quagi antenna was designed in this fashion in 1972.  The accompanying photographs illustrate how an antenna range can be set up and used.

A seaside antenna range...

...designing a new antenna by substituting elements while watching a meter.
A closeup view of the design process...
A simple meter gives a readout in dB.
Measuring the gain of a large HF quad...
Conducting a similar measurement of a large Yagi array...

    A final note:  after measuring dozens of HF antennas against the reference antenna shown here, I published an article in Ham Radio magazine called, "Quads and Yagis Revisited" (May, 1979).  It concluded that the oft-claimed superiority of the quad design was not observable in these field measurements--at low radiation angles, high angles or anywhere in between.

-Wayne Overbeck, N6NB

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