Re: Circular versus linear polarization


"christofire" <christofire@xxxxxxxxxxxxxx> wrote in message
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<bradlington@xxxxxxxxx> wrote in message
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Ok all noted on the many ideas.

Here is one for you boffins:

After having a look at the circular turnstile type antenna with a
groundplane ,I sent a request to the main manufactureers regarding
using this in an inverted state for what I wanted to do on the
repeater.Still yet to get a reply and after a few months I guess they
are still thinking about it.

If they advertised it so great for horizontal to near vertical
satellite work then the same principle should apply if inverted.

Style of eggbeater looking at .
http://pagesperso-orange.fr/on6wg/Doc/Antenne%20Eggbeater-Engl-Part1-Full.pdf

What do the Boffins suggest.


1. How are you thinking of mounting it whilst upside down? Even if you
were to use a dielectric swan neck of some kind, you'd still have the feed
line passing through the volume into which the antenna is intentionally
radiating, so the radiation pattern would be upset to some extent. Of
course, if you were willing to modify the antenna you could probably pass
a co-axial feed line through an axial tube - but if you were going to that
much trouble you might as well build the thing yourself.

2. Their statement that: 'Each of the two loops forming the antenna has
an impedance of 100 ohms, and when coupled in parallel, they offer an
ideal 50 ohms impedance. We will use the properties of a quarter
wavelength coax line to achieve a 90 degree phase difference between the
two loops in order to obtain circular polarisation.' is a bit suspect.
This would be true for a half wavelength of line, indeed that is the basis
of a very common form of balun, but a quarter wavelength of transmission
line will cause an impedance transformation. If they use 50 ohm cable
then a quarter wavelength will transform 100 ohms down to 25 ohms, and
when this is connected in parallel with another 100 ohms the result will
be 20 ohms which is presented to the feed line. Not a particularly good
match!


OK - I've just noticed they specify RG62 for the quarter wavelength of line,
and that has 93 ohms characteristic impedance. It would transmform 100 ohms
down to 86.5 ohms which, in parallel with the other 100 ohms, would yield
46.4 ohms. Better.

Chris


.

Relevant Pages

  • Re: SWR Tells Me??
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    (rec.radio.amateur.antenna)
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