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Phasing CP (was:Re: A question about 75 Ohm connectors)



on 2/12/03 5:26 PM, Doug Faunt N6TQS +1-510-655-8604 at faunt@panix.com
wrote:

> I think I was unclear: One element would immediately at the feedline
> tee, the other element would be 1/4 wavelength of coax away from that
> point.

Doug,

I looked up some info in the Satellite Expermenter's handbook on feeding CP
yagi arrays.  There are basically two choices.  In one choice the antenna
elements are physically located next to each other at roughly a common feed
point.  In the second, the feed points are physically separated by a 1/4
wavelength.

I'll deal with the first case since that's what you have.

OK, each antenna's impedance is 50 Ohms.  You'll need to join these two
impedances at a tee and then feed that into a common 50 Ohm transmission
line.  Therefore, the impedance of each branch of coax feeding each antenna
needs to look like 100 Ohms so that when combined it makes 50.  You probably
understand that already (I'm explaining in detail for those who don't.).

We can create an impedance transformation from 100 Ohms to 50 Ohms by using
a 1/4 wave transformer of 75 Ohm coax.  So working out from the tee, we have
100 Ohms.  We then run through a 1/4 wavelength of 75 Ohm coax on each
branch and we are now transformed on each branch down to 50 Ohms.

But we have one more problem.  We now need to feed the arrays 90 degrees out
of phase.  We do this by adding an additional 1/4 wavelength of 50 Ohm coax
to one of the branches before feeding the antenna.


This will give us the necessary impedance and the 90 degree phase shift.

Now you may need to add extra coax in order to be able to physically and
practically reach the feed points.  You do that by adding equal lengths of
50 Ohm coax to each branch.  But remember from transmission line theory that
if you have a 1/2 wavelength piece of coax, it does not matter what it's
impedance is as long as the impedances on each end are the same.  This way,
the side of the branch that is fed only with the 1/4 wavelength of 75 Ohm
coax can be lengthened by adding an additional 1/2 wavelength of 75 Ohm
coax.  The side of the branch that has both the 1/4 wavelength of 75 Ohm and
the 1/4 wavelength of 50 Ohm can be lengthened by adding an additional 1/2
wavelength of 50 Ohm coax.  By using the same characteristic impedance for
the extension that is at the end of each branch it makes assembly easier.

Remember though that you need to make sure that it is a 1/2 wavelength
ELECTRICALLY not via physical measurement.  Be sure to take into account the
velocity of propagation of the coax when creating the 1/x wavelength
sections in each case.

Does this make sense?  I hope I worded it well enough!

73,

Jon
NA9D

-------------------------------------
Jon Ogden
NA9D (ex: KE9NA)

Citizen of the People's Democratic Republik of Illinois

Life Member: ARRL, NRA
Member:  AMSAT, DXCC

http://www.qsl.net/na9d   <- Updated on 1/22/03!!!

"A life lived in fear is a life half lived."


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