[Towertalk] Testing Equipment?

[Chuck Counselman]

At 3:52 PM -0800 12/12/02, Dino Darling k6rix wrote:
>When a manufacturer tests antennas on a range, what equipment is 
>used?  A field strength meter?  A spectrum analyzer?


I don't know about many manufacturers, but I've seen it done, and 
participated, at a few.  There's no universal answer, but typically 
the electronic equipment is pretty basic and the physical and 
mechanical setup is pretty elaborate.  The electronic equipment 
includes a signal (usually sinusoidal) generator which may be 
fixed-frequency, stepped, or swept, and a matching receiver. 
Typically the receiver tracks the generator or vice versa.  An 
ordinary "network analyzer" is commonly used.  For some antennas one 
cares about phase shift, in which case the network analyzer is likely 
to be a "vector network analyzer."

The antenna under test may be used either to transmit or to receive. 
I've seen both.  Whichever, the other antenna is usually directional, 
for the purpose of rejecting waves reflected from the ground, walls, 
whatever.  antenna under test is typically rotated while the other 
antenna remains stationary.

The "other" antenna has polarization appropriate to the situation. 
E.g., if the antenna under test is supposed to be circularly 
polarized, then the other antenna is often a helix.  (Helices have 
good circularity.)  Usually helices of both senses (RCP & LCP) are 
used in order to measure the polarization "purity" of the antenna 
under test.  Often a linearly polarized antenna is used and is spun 
about the line of sight to determine the circularity of the antenna 
under test.  For a ham antenna, cross-polarized response is probably 
not often measured.

In addition to towers and rotators (or turntables), the physical 
setup usually includes measures for the suppression of unwanted 
reflections.  In an indoor range, usable only for short wavelengths, 
usually every surface is studded with long pointy pyramids of 
RF-absorbent, e.g., carbon-loaded, low-density plastic foam.  Outdoor 
ranges may use baffles that look like tipped-over fences.

There are "far-field" ranges and "near-field" ranges.  In a 
"far-field" range, the sensor is in the far field, in other words the 
radiation field, of the antenna under test.  In a "near-field" range 
it is not, and the radiation field is calculated from near-field 
measurements.

73 de Chuck, W1HIS