On 13/10/14 12:34, Jeff wrote:
On 13/10/2014 09:15, 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...



That makes no sense, at least quoted out of context, as it would imply
that the power radiated was independent of the power applied. So an
infinitely long antenna would radiate infinite power !!!!

Jeff

His problem is he is not considering the Radiation Resistance, Loss
Resistance, and reactive element which determine the eff., and Zo.
(The reactive element represents the energy 'stored' in the field around
the antenna- just like the energy store in an inductor or capacitor,
both reactive components.)

A short dipole, for example, will be a poor match but RRLR. Provided
the feeder loss is low, either by good matching or the use of low loss
feeder (assuming the PA is 'happy') then the overall losses are low and
the RF only has one place to go, to be radiated.

A short dipole has other issues, in particular if matching is used to
overcome the issue of the Zo, then the matching network plus antenna
will have a very narrow bandwidth (compared to a full sized dipole) and
adjustment will be essential to maintain efficiency if the frequency of
operation is changed.

Remember, the use of 'standard' Zo of 50 or 75 ohm is not essential, nor
is maintaining a feeder SWR of 1.5, provided the PA can cope and feeder
loss can be tolerated/reduced (eg by using open wire feeder).


No doubt he will dismiss this with his usual tirade of abuse etc, but
that is his normal response when corrected.