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rickman · October 14th 14, 09:10 AM posted to rec.radio.amateur.antenna

On 10/14/2014 1:21 AM, wrote:
rickman wrote:
On 10/13/2014 11:45 PM, wrote:
rickman wrote:
On 10/13/2014 1:36 PM, wrote:
gareth wrote:
Quoting from Electromagnetism
By F.N.H.Robinson
in the Oxford Physics Series
1973 edition
ISBN 0 19 8518913
Chapter 11, Radiation,
page 102
Formula 11.11

Has in the equation for radiated power the term

(2*PI*L/LAMBDA)**2

where L is the antenna length and LAMBDA the wavelength,
thereby showing that the radiated power decreases when the
antenna length decreases.

I will read up further and report further...

You do that and while you are at it take note of the fact that the
expression you give is unitless and can not be power.

You will also find that the total power radiated by an antenna is the
surface integral of the average Poynting vector over a surface enclosing
the antenna. The surface usually chosen is a sphere in the far field to
keep the equations "simple".

He is taking a portion of the equation and presenting it out of context
assuming that this is a valid way to consider what he wishes to show. I
would like to see the full equation. The devil is in the details.

Actually, there is no "the" equation for the power radiated by an
antenna other than the surface integral of the average Poynting vector
over a surface enclosing the antenna.

There are some approximate rules for specific cases and limiting
conditions, but this isn't one of them.

What he presented is for a 1/2 wavelegth antenna 9.87 and a full wave
antenna 39.48.

WTF is that??

I have no idea what you are talking about. Where did you get these
numbers? 9.87 what?

(2 * 3.14 * 5 meters / 10 meters) ^ 2

9.87 nothing; the expression is unitless, i.e. a pure number without
units.

In the expression you have a length divided by a length, which cancels
into a unitles number.

As my old physics professor used to say, always check the units of your
answer; the arithmatic may be correct but it is meaningless unless the
units are correct.

Sample problem:

You drove 100 miles and used 5 gallons of gas. What was your mileage?

5 gallons * 100 miles = 500 gallon-miles --- wrong units.

5 gallons / 100 miles = .05 gal/mile --- wrong units.

100 miles / 5 gallons = 20 miles/gal --- correct units and correct answer.

I have no idea what you are going on about. Ok, 9.87 is a unitless
number. So is 33.043. Now what?

--

Rick