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Jeff Liebermann[_2_]

On Sun, 8 Nov 2009 18:17:57 -0800, (Dave Platt)
wrote:

Odd multiples of 1/4 wavelength will neatly transform the endpoint
impedances according to:
Zcoax = sqrt (Zin * Zout)
or
Zcoax^2 = Zin * Zout
So, with a 50 ohm load, 75 ohm coax, and 3/4 wavelengths of coax:
Zout = 112.5 ohms
which is a bit closer to what I would expect to see with a folded
dipole antenna.

Another thing to note: based on the pictures posted today, the DE
isn't all that close to being a classic folded dipole, with
close-spaced segments. The segments are much more widely spaced... it
looks to be about half-way between being a folded dipole, and a
one-wavelength loop such as might be used in a Quagi design.

Good point. It does look a little on the short size for a folded
dipole. I also noticed that there's a plastic insulator at the
midpoint of the driven element. The midpoint can be at ground
potential with either a folded dipole or full wave loop, but this
design goes out of its way to use an insulated spacer. The only
reason I could think it would be necessary is if the balun isn't quite
balanced and grounding the midpoint sorta fixes half the driven
elements mismatch.

This is how a typical 1 wavelength loop Yagi driven element is usually
built:
http://www.directivesystems.com/loopyagi.htm
Notice the lack of a balun, exposed wires and ferrite beads.

This is going to significantly change its free-space impedance, I
would think. An FD would be around 300 ohms, a one-wavelength
circular or square loop would be somewhere in the general neighborhood
of 100 ohms.

Well, the wire length of a full wave loop and a folded dipole are
roughly the same. The way a folded dipole works is that you start
with a 1/2 wave 72 ohm dipole. Adding the extra wire creates a 4:1
transformer, resulting in 4*72 = 288 ohms.
http://www.qsl.net/w4sat/fdipole.htm
Take the same folded dipole and spread the 4ea 1/4 wave sides into a
square or circle, and the impedance changes to about 100 ohms. Off
hand, I would guess that the MFJ-1800 DE is about half way in between
a folded dipole and a loop at perhaps 150-175 ohms.

This DE may not need as much impedance transformation (from coax) or
proximity reduction (e.g. from a reflector and one or more directors)
than a classic FD would, to achieve a decent match to a 50 ohm coax.

Agreed. The question of the moment is whether the MFJ-1800 balun is
50, 75, or 93 ohm coax and its length (shield to shield).

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

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