Re: Antenna Length Calculation...was: "your mail"

["Kurt Wesseling" <kwessel@enm.maine.edu>]

Dan wrote:
> 
> Kurt, what was wrong with Phil's calculation?  He calculated,
> correctly, that 1/4 wavelength of a 93Mhz signal, in free space,
> is about 20 inches.  In real life, we'd slow down the propagation
> speed a tad below 300Mm/s, but that would only serve to show the
> effective wavelength as a bit *less* than 20". Where does the number
> that you use, that gives us 30", come from?
> 

Dan:

My apologies to the list.  We should be doing this in 
"alt.audi.antenna.d" I think...

Anyway,  the problem is we don't live in the theoretical "free space" 
that antenna theory is built on.  Most text book antenna discussions 
are describing an infinitely thin isotropic radiator.  One that is
perfectly conductive, has a perfect radiation pattern, and is not
affected by earth ground.  It works well theoretically, but is
impossible to build.  

Real antennae do have diameter, resistance, impedance, and ground to 
deal with.  Feedline velocity factor (the speed at which the RF 
travels thru the feedline) has to be figured in as well.

I don't think it will serve our purpose to get into all those factors 
in much detail.  Here's how I got the numbers: 

Standard real-world 1/2 wave antenna formula is 468/FmHz.
The result of that formula will give you a half wavelength antenna in 
feet.  E.G. 468/93.0 mHz =  5.03 feet.  If you divide that result by 
2  (to get the quarter wave length)  you get: 2.51 feet.  But what is 
2.51 feet in inches ?  2.51 * 12 = 30.12

To avoid all that, simply take 2808/FmHz and you will get the actual 
1/4 wave length in inches.  That's where I get the 30.12 inches.

I've built enough MF, HF, VHF & UHF antennae to know that these 
formulae work in actual practice.  A 20 inch radiator will resonate 
around 140.00 mHz NOT 93.0 megs.

We can also discuss height above ground, feedline impedance and 
velocity factor, and host of other _wicked_ exciting stuff.  All of 
these factors affect antenna length and VSWR as well...but do we 
REALLY want to ???  8-]

Hope this helps a little.

--Kurt

______________________________
Kurt Wesseling
Technical Support Center
The Education Network of Maine
800-696-4357
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