Antenna physical size
I can`t find it now, but I believe Mark, NM5K on this thread wrote that
my quotation from page 929 of Terman`s 1955 opus raised flags. I said I
was surprised in my original posting. I was aware at the time that Kraus
shows identical field paterns for a small loop an a short dipole, but
the E and H fields are exchaged between the two antenna types. The Kraus
diagrams are on page 58 of the 3rd edition of "Antennas".
I`ve learned not to quarrel with Terman. So, I reread the page 929
quotation. I posted it correctly. He did say a loop antenna responds
much less to the electric induction field than does a simple wire
antenna of comparable intercept area.
So I looked for a similar statement in Terman`s "Radio Engineering" in
an edition published in 1947. On page 664, I find:
"In the case of a doublet antenna, the electrostatic induction field
becomes proportionally stronger than the magnetic induction field as one
comes closer to the antenna. With the loop antenna the reverse is true."
We know that the induction, or near, field is equal to the radiation
field at about 1/6 wavelength. Closer to the radiator, the induction
field is much stronger. Farther away, the radiation field predominates
Our intrest is usually in the radiation, or far field. Measurements are
usually made several wavelengths away from an antenna to be sure the
induction field has become insignificant.
I`ve thought how and why my experience confirms Terman`s statements
but I won`t bore anyone with these thoughts.
Best regards, Richard Harrison, KB5WZI
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