On Wed, 18 Nov 2009 07:00:26 -0600, "amdx" wrote:

The CENTER of the driven element (the plastic screw hole) to the
CENTER of the last director is 36.7cm (14-7/16")

Thanks. My numbers came out to 36.52 cm which is close enough.

ALL directors or the same length (within .003")
They are 46.6mm in length.

Amazing...

Also, measure the coax balun cable dimensions. Mostly, I'm interested
in the:

Nice. I'll see if I guess(tm) the coax type. It doesn't seem like a
good fit for any of the common cables as the center conductor is
somewhat larger diameter than any of these listed. See if you can
find an exposed center conductor without any tinning or soldering.
http://www.epanorama.net/documents/wiring/coaxcable.html
One more dimension... the approximate outer jacket diameter of the
coax (not including the shrink tube).

My guess is solid poyethylene, if you have a test other than poking it
with a pin to get feel of it, which I did.

Weird(tm). Low loss coax would need to be foam or teflon. Solid
polyethylene is easier to work with, cheaper, but not the best.
However, a short piece like this balun would not have much loss, so I
guess it doesn't matter what flavor is used:
http://en.wikipedia.org/wiki/Coaxial_cable

Gotta do paying work today. I'll play with this more in the next day
or two (so I don't forget what I'm doing).

--
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 Wed, 18 Nov 2009 07:00:26 -0600, "amdx" wrote:

The CENTER of the driven element (the plastic screw hole) to the
CENTER of the last director is 36.7cm (14-7/16")

Thanks. My numbers came out to 36.52 cm which is close enough.

ALL directors or the same length (within .003")
They are 46.6mm in length.

Amazing...

Also, measure the coax balun cable dimensions. Mostly, I'm interested
in the:

Nice. I'll see if I guess(tm) the coax type. It doesn't seem like a
good fit for any of the common cables as the center conductor is
somewhat larger diameter than any of these listed. See if you can
find an exposed center conductor without any tinning or soldering.
http://www.epanorama.net/documents/wiring/coaxcable.html
One more dimension... the approximate outer jacket diameter of the
coax (not including the shrink tube).

My guess is solid poyethylene, if you have a test other than poking it
with a pin to get feel of it, which I did.

Weird(tm). Low loss coax would need to be foam or teflon. Solid
polyethylene is easier to work with, cheaper, but not the best.
However, a short piece like this balun would not have much loss, so I
guess it doesn't matter what flavor is used:
http://en.wikipedia.org/wiki/Coaxial_cable

Gotta do paying work today. I'll play with this more in the next day
or two (so I don't forget what I'm doing).
Jeff Liebermann

I have found the need to work for a living, gets in the way of a lot of
fun!

I took the coax loose on the MFJ-1800 and and removed the toroids, I found
the letters found M1Z/111-RG and then the insulation ended. Argh!
Oh, I have a second antenna, so I took that one apart, Eureka! RGS-303
http://wireandcable.thermaxcdt.com/i...d-70-?&seo=110
50 ohm coax. PTFE center insulator, FEP jacket.
Mike

On Thu, 19 Nov 2009 06:05:20 -0600, "amdx" wrote:

I have found the need to work for a living, gets in the way of a lot of
fun!

5 hours on the phone on a slow motion conference call this morning.
Absolutely nothing useful accomplished. Such online meetings should
be banned, taxed, or both as a threat to national productivity.

I took the coax loose on the MFJ-1800 and and removed the toroids, I found
the letters found M1Z/111-RG and then the insulation ended. Argh!
Oh, I have a second antenna, so I took that one apart, Eureka! RGS-303
http://wireandcable.thermaxcdt.com/i...d-70-?&seo=110
50 ohm coax. PTFE center insulator, FEP jacket.
Mike

Thanks. 50 ohm coax does not make it a matching section to a 200 ohm
folded dipole. However, the ferrite beads are a good way to simply
block the reflected power from the folded dipole so that it looks like
it's matched. In any case, that reflected power is lost (converted to
heat) in the ferrite beads. So much for efficiency. When I change
the characteristic impedance of the model from 200 ohms to 50 ohms,
the VSWR climbs to 5.5:1. Yech. (Note that the radiation efficiency
is 75% with or without the mismatch).

I suppose the antenna could be made to function by replacing the coax
section with a real 1/4 wave 4:1 balun, but I'll leave that to MFJ to
figure out.

If you need some more entertainment value, it would be interesting to
actually measure the gain of the antenna. Find a known good reputable
antenna with similar gain. A panel or patch will work. Find a signal
source that isn't infested with reflections (including ground
reflections), Fresnel Zone issues, and is fairly stable (i.e. doesn't
physically move). Use Netstumbler, WirelessMon, or Kismet to compare
the signal strengths. For additional accuracy, use a step attenuator
to adjust the signal levels to a common reference level. Better yet,
use a spectrum analyzer.


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

amdx wrote:
Hi All,
My original thread seems to have died, still wondering how the folded loop
is matched to 50 ohms. (probably isn't? cheap, but works?)
I've added a dimensional drawing and some more pics.
If more info is needed let me know.


Here is a drawing and some more pics.
http://i395.photobucket.com/albums/p...intFileJPG.jpg
http://i395.photobucket.com/albums/p...connection.jpg
http://i395.photobucket.com/albums/p...MFJRuledDE.jpg
http://i395.photobucket.com/albums/p...Jruledcoax.jpg
Hope I covered everything,
Mike


Hi,
I used to measure impedance of antenna feed point with home made noise
bridge. I know there is a 300 Ohm to 75 Ohm trnasformer balun.

On Nov 13, 8:28*am, "amdx" wrote:
*Hi All,
*My original thread seems to have died, still wondering how the folded loop
is matched to 50 ohms. (probably isn't? cheap, but works?)
*I've added a dimensional drawing and some more pics.
If more info is needed let me know.

Here is a drawing and some more pics.http://i395.photobucket.com/albums/p...Jruledcoax.jpg
* Hope I covered everything,
* * * * * * * * * * * * * * * * *Mike

The folded dipole is going to have about 4 times the impedance of a
plain old dipole.(POD) In this case if you used a POD with your yagi
antenna the feedpoint impedance would be in the neighborhood of 10
ohms
with a folded dipole it will be more like 40. Not a bad match for 50
ohm coax

Jimmie