Jeff,
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First, the type of antenna you need to use is dependent upon your frequency range of interest.? And from that if you are looking for accurate field strength measurements you have to use ?an antenna with a known antenna factor (AF) or to put it basically, a known gain figure to know what signal level it is giving you.? This is usually obtained by comparing your subject unknown antenna under test (AUT) against a known gain antenna on either an open test range or in an anechoic chamber.
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There can be more simpler approaches (somewhat including theoretical) determining the antenna factor but all depends upon what you are using for your test environment, reference antenna or other means to obtain a known field strength measurement at your AUT location.? Any of these approaches with loose field strength data will give you loose results when trying to obtain the antenna factor.
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If you are planning on using a specific antenna for relative (comparison) measurements I wouldn¡¯t be as worried about an accurate antenna factor.? But if you are doing absolute signal level measurements then you ?need to have reasonable accuracy in order to determine the field strength you are measuring.
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Aside from this the gain of an antenna is not necessarily flat over its usable frequency range.? Manufacturers of EMC (Electro Magnetic Compatibility) test range antennas will usually provide a curve indicating the AF over that range of frequencies.
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I have basically leaned on the side of very accurate antennas in this note.? And from looking at some of the antennas in the links below you will find that they are manufactured to very high standards.? This is primarily because these antennas are required to take measurements that are used to provide test results in accordance to very stringent government and industry standards (i.e. FCC, IEC, VDE, CSA, etc.)
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Below are few reference links regarding antenna calibration plus some examples of manufacturers and distributors who provide calibrated antennas for precision measurements that are sometimes very esoteric.? Don¡¯t be shocked by what you see in the way of antennas.? These are utilized for highly accurate measurements but will give you an idea of the style that are used for various frequency ranges when employed in a precision test environment.
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I hope this helps.
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Greg
Dr. David Kirkby, Kirkby Microwave Ltd