In article , Mike Coslo writes:


Maybe my brain works differently, but if you have a question pool that
has a certain number of questions regarding antennas, and you may be
called upon to answer a few of any of those questions, its a lot easier
for me to remember 468/[Frequency in mHz] for a dipole, 71.3/[Frequency
in mHz] for a 1/4 wave vertical, and a few other quick calcs, than it is
to remember the answer to a specific question.

Since U.S. amateur radio frequency allocations don't allow any
frequencies below the top of the AM BC band, I'd suggest you get
your brain to work with MegaHertz (MHz), not milliHertz (mHz).

The length of wire needed for a half-wave dipole is, in feet, equal
to about 468 divided by the frequency in MegaHertz. A quarter-
wave whip length in feet is 234 divided by the frequency in Mega-
Hertz. [lengths will be way too short if you use milliHertz...:-) ]

But, those values of 468 and 234 aren't correct for free space,
yet they are good for wire antennas. The Why of that is handy
knowledge that can be applied to other things.

BTW, just to make sure that old threads "never die," a DIPOLE
has just two elements (what the "di" part stands for). It doesn't
have to be exactly a half-wave in length...it can be 5/8 wave, a
full wave, most anything.. :-)

A person would have to be a little dense to try to do otherwise. And of

course, if you know the wavelength of the frequency in question, you can
ballpark the answer pretty closely almost without thought.

Constants like 2-3-4 or 4-6-8 have "built-in" mnemonics in the
form of consecutive digit increases.

The Boltzman Constant of 1.38 x 10^-23 doesn't have that digit
relationship, quite. Yet, if one works with it often enough (noise
voltage, noise figure, etc.) it becomes stamped in the synapses.

The resonance formula is F (Hz) = 1 / (2 pi Sqrt (L C)) with L in
Henries and C in Farads. Learned that many years ago.
Same with inductive reactance, X_L = 2 pi L and capacitive
reactance X_C = - 1 / (2 pi C).

Those with a programmable calculator can store the 2 pi constant
up to 10 digits for handy calculation recall...or try to remember the
definitely not-consecutive digit string...or use a transcendental
equality to get it or at least pi from a common value. :-)

[watch while the knowitalls jump in here with the latter equality...]

The lower end of "the bands" (on HF) for amateurs has the
progressive relationship of 3.5 - 7 - 14 - 21 - 28 way back in time
(AF or after Fessenden)...and for a reason less political and more
for convenience of the hams of 1954. [watch here for remarks
that aren't really pertinent about one of those bands and 1954...:-)]

Please excuse me. I have to study some "important" material,
all about the 1900 to 1906 period and how it "applies" to knoweldge
in radio of 2000 to 2006 period. That seems to be very necessary
to this venue and all must obey the newsgrope protocols... :-)

LHA / WMD