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RE: Explain the RG62 thing to me, please...



Hello Jim

> You've got two 50-ohm antennas essentially in parallel (say, crossed
> dipoles in a turnstile arrangement), but one is fed with the RG62
> phasing section.  I understand that the phasing section also acts as an
> impedance transformer, but I don't see how it can transform the _whole
> system_ to a 50-ohm impedance.

I'm not sure exactly which design you're referring to, but I

A turnstile is two 75 ohm (not 50 ohm) dipoles assuming there's no external
effects, like ground or reflectors.

If you have two antennas wired in parallel, to make the 50 ohms total, each
antenna needs to be 100 ohms, just like putting two 100 ohm resistors in
parallel (the 90 degree CP phasing is irrelevent at this point).

Q. So how do you make 75 ohms into 100 ohms?

A. Stick in a 1/4 wave coax matching section...

Here's the 1/4 wavelength matching equation:

ZmatchingSection= SQRT ( Z1 * Z2 )

Where Z1 is the impedance of one of the ends your trying to match and Z2 is
the other end.

So Z1=100, Z2=75.

ZmatchingSection=86.6 Ohms.

Now last time I checked 86.6 ohms coax wasn't generally available, so go for
the nearest generally available option which is 93 ohm RG-62.

Changing the equation around, with 93 ohm you'll get an impedance of 115
ohm, giving 58 ohms when you parallel two of these up: close enough to 50
ohms not to worry about.

For CP phasing you'll need an additional 1/4 wavelength of 75 ohm between
one of the dipoles and one of the 1/4 wavelength RG-62 sections.

Working out which one is difficult to explain in text, but here goes.

Looking from underneath the antenna upward to the sky, lets say you have one
dipole #1 going 0 to 180 degrees and the other diploe #2 going 90 to 270
degrees. The centers of the coax will be going to the 0 degree limb and the
90 degree limb, the outers to the 180 degree and 270 degree limbs. You need
to feed the 90 degree limb 1/4 wavelength after the 0 degree limb for RHCP.
So the 1/4 wavelength 75 ohm phasing section goes in the 90 to 270 degree
diplole #2.

That's the theory.

RG-62 coax is being thrown out of buildings presently as its original use
for computer networks has now been surpassed with the use of structured
cabling systems using the CAT5 twisted pair standard. So next time you see
an office re-fit going on, check out the contents of the dumpsters... you
might get lucky.

73 Howard G6LVB

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