On Mon, 26 Dec 2005 20:15:41 -0800, Roy Lewallen
wrote:

hat a thin half wave dipole in free space
has a gain of about 2.15 dBi. If you make the dipole shorter, the gain
decreases slightly ......
...has dropped to 1.76 dBi. ...
The gain of a lossless quarter wave dipole in free space is about 1.84 dBi....

Over a perfect ground plane of infinite extent, the gains of antennas
typically increase by about 3 dB compared to free space. In the case of
a free space dipole vs. a quarter wavelength antenna over a perfect
ground plane, the difference is exactly 3 dB. This is because they have
exactly the same radiation pattern (except of course, that the half
pattern of the vertical below the ground plane is missing), but the same
amount of power is being concentrated in half the volume. Consequently,
the gain of a 1/4 wave vertical over a perfect and infinite ground plane
is about 5.15 dBi. This also can't be achieved in practice because there
isn't any such thing as a perfect ground plane of infinite extent.

Roy, thanks for detailed explanation :-) I got now some stuff
clarified even more than enough! :-)

(btw, the old book I mentioned was written 20y ago from a "local"
radioamateur [prof.& dr.] one, I use it much. & there I saw the
comparision chart of different type of basic antennas; so must be
kind of a mistake there like it looks, the reference of 0dB was set as
halfwave dipole there...)

Roy I have also another 2 quick questions for short answers:

- if I connect to a 50ohm TX out a cable of 75ohms (w/o "perfect"
Z transform add_on there), I can aspect approx. 1: 1,5 SWR [50:75]
& because of Z miss-match I will loose no more than 0,5dB (that same
my book states for that SWR in some diagram); yes I know that is in
"ideal" conditions & TX is only 50mW ... could you confirm me
briefly that?

- next : twin-lead/symetric cable of only half wavelenght (or 3x,5x
etc. half wavelenghts, shorted by a velocity factor sure) mirrors the
input Z to its end; does a coaksial cable do the same at the same
lenght or for that is a different simplified formula?

TIA!



--
Seasons Greetings & Regards , SPAJKY ®
mail addr. @ my site @ http://www.spajky.vze.com
more than 3y - "Tualatin OC-ed / BX-Slot1 / inaudible setup!"