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!"

Spajky wrote:

- 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?

No, you would lose 0.18 dB if your source is something like a signal
generator with fixed 50 ohm output impedance. If you match the impedance
so the source sees 50 ohms, you'll regain the 0.18 dB.


- 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?

All transmission lines transform impedances the same, regardless of
construction. However, the impedance seen at the input of an integral
number of half wavelengths of real transmission line won't equal the
load impedance, because of line loss. The error will be small if the
load impedance is about the same as the line Z0. But if they're quite
different, even a small amount of loss will change the input impedance a
significant amount.

Roy Lewallen, W7EL

On Tue, 27 Dec 2005 03:45:37 -0800, Roy Lewallen
wrote:

No, you would lose 0.18 dB if your source is something like a signal
generator with fixed 50 ohm output impedance. If you match the impedance
so the source sees 50 ohms, you'll regain the 0.18 dB.

nice not to have much mismatch Z loss :-) (RX+TX sum = 2x 0.18
dB=0,36dB signal loss) on LAN wireless 2,45GHz PCI card using
directly connected 75ohm quality cheaper & flexible Sat [d=6mm] cable
for only few meters to outside 3D_corner_reflector (75ohm) 2,45GHz
15dBi homemade antenna I am starting to build now ...

All transmission lines transform impedances the same, regardless of
construction. ...

Bye Telecom & Dial-Up expences & its speeds [have nice signal
strenght from wireless ISPs 100mW/12dBi antenna 1km away on clear
sight for a month already .. :-)

Thanks for clarifications Ron &
Merry Hollidays+Happy new year! :-)
--
Seasons Greetings & Regards , SPAJKY ®
mail addr. @ my site @ http://www.spajky.vze.com
more than 3y - "Tualatin OC-ed / BX-Slot1 / inaudible setup!"