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.

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.

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.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

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

"Jeff Liebermann" wrote in message
...
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

I remeasured the coax, (shield to shield) it is 2.135" long.
The length of the Loop is 4.85"
Got a little hurricane coming our way, need to take care
of the boat and business today. Need to drive 9 hours Tuesday,
then again on Wednesday. I hope to get dimensional pictures
posted on Thursday or Friday.
Mike