On 31 jul, 19:33, Steve Reinhardt
wrote:
Gentlemen,

If a man was of a mind to try to get some approximate antenna gain
comparisons, how many wavelengths distant might you like to separate the
antennas?

The proposed scenario is this: make a pair of 2M dipoles, one for
reference, one for receive. I was planning on using the local high
school football field, which is on the order of 50 wl, give or take.
Transmit a few mW at the design frequency, measure the signal strength,
then repeat with an alternate antenna, say a j-pole, collinear, or
something else.

Now, this leaves out a whole bunch of useful information, that would be
tough for me to measure, like spherical gain distribution, etc. I'm
hoping for a figure of merit for the actual implementation of the tested
antenna. (Which, as you can imagine, I could model and save myself the
aggravation.)

I was pondering all this, when it occurred to me that I could not
easily determine when I get to the point where the square law behavior
dominates. I've seen a couple of equations relating the antenna
dimension to wavelength, but I must be really stupid today, because it's
just not sinking in.

Anyone care to comment?

73,
Steve
W1KF

Hello Steve,

When your "Antennas Under Test" are moderate gain devices, I would go
for several wavelengths. For low to moderate gain (up to 10 dBi), you
are in the far field within about 4 WL.

The reason for the short distance is that the direct signal is strong,
hence influence of reflections is less. You can reduce the effect of
reflections by taking a receive antenna with some directivity.

You can be sure that you are in the far field distance when
D 2*B^2/lambda, where B = overall size of the antenna (from one
extremity to another). For several antenna types (like yagis), you can
halve this distance when you are interested in main beam gain only.

Best regards,

Wim
PA3DJS
www.tetech.nl