[TenTec] Antenna measurements

[Stuart Rohre]

Yes, indeed, start with Maxwell's equations, but you can understand 
antenna action from the basics in Chapter 2 of John Kraus's "Antennas" 
2nd Ed.  Not sure how much he changed the book order by the 3rd ed., but 
look for an illustration of point source of charge, a straight wire, a 
curved wire and the accompanying discussion.  It is a multi part 
illustration.

In Chapter 2, radiation results from accelerated charges, and for steady 
state harmonic oscillation in an antenna, we speak of current.  (Charges 
are used with speaking of a pulse applied to an antenna).

Stationary charge does not radiate, but when charge accelerates, its 
velocity changes over time, and it radiates as the electromagnetic wave.

One thing that causes charge to accelerate, is when it reverses 
direction  at the end of the antenna element, (a discontinuity).
The charge is reflected there where the conductor stops.  Not much 
radiation happens from the reflected charge per Kraus, but it has to 
stop and attain velocity again, and thus accelerates.

An illustration of the above in Chap. 2 summarizers the cases for 
radiation and includes that a charge moving at uniform velocity along a 
curved or bent wire, accelerates when it goes around the curve, and thus 
radiates.

Also, an electric charge oscillating back and forth in simple harmonic 
motion along a wire undergoes periodic acceleration, and thus radiates.
(Our usual radio signal is this simple harmonic oscillation case.)

A non oscillating charge moving at uniform velocity along a straight 
wire does not radiate.  (No acceleration).

We are most comfortable thinking of simple harmonic oscillation motion\ 
in our antennas, and thus we more often speak of antenna current rather 
than charges.

In other chapters, Kraus starts with a coaxial feedline and spreads it 
out like a flower blossom to turn it into verious types of antennas; a 
very good graphic which helped a lot in distinguishing feedline "action" 
from an antenna.

Years ago, I looked at Kraus's first edition but found the 2nd edition 
much more clear, better illustrated, and with some guest authors on 
chapters such as Method of Moments analysis.

-Stuart Rohre
K5KVH