"Jeff Liebermann" wrote in message
news
On Fri, 13 Nov 2009 07:28:34 -0600, "amdx" wrote:

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.


I need an overall length, measured from the CENTER of the driven
element (the plastic screw hole) to the CENTER of the last director.
That's because a tiny error in the spacing between elements at 1.060"
(2.69cm) grows rather rapidly when multiplied by 14 elements.

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


Please check the length of the first and 2nd directors. I don't
believe MFJ would make them the same length as the other directors.

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

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

1. Shield to shield length.

54.2mm (2.134")

2. Center pin to start of folded dipole length. (i.e. exposed center wire
length).

61.75mm (2.431)

3. OD of center conductor wire.

0.94mm (0.037")

4. ID of shield.

3.3mm (.130)


5. A good guess as to the type of dielectric (foam, solid, or PTFE).

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.

6. Any markings that might identify the coax.

No help there.


That thing doesn't look anything like a
4:1 balun (unless there's something we missed under all that shrink tube).

Looks like 4 toroids that slide over the coax, you can see the outline of
the edges of each toroid.

Mike

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

On Thu, 19 Nov 2009 11:48:12 -0800, Jeff Liebermann
wrote:

Thanks. 50 ohm coax does not make it a matching section to a 200 ohm
folded dipole.

Perhaps not an optimal one (which would be the geometric mean of the
source/load) but an effective one - if it were designed so (I don't
think it was ever intended to be one).

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.

In fact the beads won't do that at all. They see only the common mode
circuit. Reflected power is going to reside in the transverse mode
circuit where the beads are invisible.

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).

My model shows a more benign mismatch to a 72 Ohm load.

73's
Richard Clark, KB7QHC

On Thu, 19 Nov 2009 13:20:01 -0800, Richard Clark
wrote:

My model shows a more benign mismatch to a 72 Ohm load.
Richard Clark, KB7QHC

I built several models of the antenna folded dipole assembly. The
simple rectangular rod folded dipole yielded about 300 ohms. A
slightly better simulation of the rounded ends, but still using a
round rod, was about 260 ohms. Converting it to a flat wire ended up
about 280 ohms. I never got anything even close to 72 ohms. It's my
http://802.11junk.com/jeffl/antennas/mfj1800/

Show my your NEC2 deck and tell me what I did wrong, and maybe I'll
believe that it's 72 ohms. Incidentally, the possibility that I
screwed up somewhere in the model is quite real:
http://802.11junk.com/jeffl/antennas/mfj1800/mfj1800.txt
(I'll convert this mess back to a macro form so it's easier to read
maybe this weekend).

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

On Fri, 20 Nov 2009 08:52:25 -0800, Jeff Liebermann
wrote:

On Thu, 19 Nov 2009 13:20:01 -0800, Richard Clark
wrote:

My model shows a more benign mismatch to a 72 Ohm load.
Richard Clark, KB7QHC

I built several models of the antenna folded dipole assembly. The
simple rectangular rod folded dipole yielded about 300 ohms. A
slightly better simulation of the rounded ends, but still using a
round rod, was about 260 ohms. Converting it to a flat wire ended up
about 280 ohms. I never got anything even close to 72 ohms. It's my
http://802.11junk.com/jeffl/antennas/mfj1800/

EZNEC shows:
Impedance = 73.13 + J 18.78 ohms
which, I suppose, could have the reactance driven out if I shift
frequency.

Show my your NEC2 deck and tell me what I did wrong, and maybe I'll
believe that it's 72 ohms. Incidentally, the possibility that I
screwed up somewhere in the model is quite real:
http://802.11junk.com/jeffl/antennas/mfj1800/mfj1800.txt
(I'll convert this mess back to a macro form so it's easier to read
maybe this weekend).

I don't have an option of NEC2 deck. One thing you might check, and
is something I reported about, is does your model have the loop
symmetrical to the plane of the directors/reflector? I followed all
of Mike's dimensions and I note that your lobe characteristics don't
show his lack of driven element symmetry - mine do. Again, I have
modeled only the three elements (Ref/Dr/Dir) as the additional
directors are unlikely to perturb drive point Z as much as to push it
from 73 Ohms up to your high 200s (triple?).

Another point, as I have described, I used 1/4 inch diameter wire in
place of larger flat sheet metal elements (which I note you try to
replecate, but only once). True, 1/4 inch is not as big as any flat
dimension, but as Roy reports on equivalence, flat is not the same as
diameter, but flat performance is closer to a smaller diameter round
wire. Hence the 1/4 inch.

73's
Richard Clark, KB7QHC

In article ,
Jeff Liebermann wrote:

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.

Ummm... I'd disagree (or at least quibble) on at least two grounds.

Ground #1: the ferrite beads would only block non-balanced current
flow back down the outside of the coax. They will have no effect at
all on power which is reflected back down the inside of the coax (the
center conductor and the inside of the shield) from any impedance
mismatch where the coax meets the folded dipole.

In effect, the presence of the beads (if they're choking the coax
properly) actually ensures that the transmitter *does* see the true
effect of any impedance mismatch. The transmitter is just as likely
to see a higher SWR than a lower one, when the beads are added.

Ground #2: the beads do not necessarily result in a significant loss
of power. If their RF impedance is high enough at the frequency of
use, then RF current flow through them will be negligible, and there
won't be a loss of power. Power loss in choke-ferrites tends to be
worst when the RF impedance is both resistive, and too low for the
application (i.e. still allows substantial current flow, which then
results in dissipation of power inside the choke).

--
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 Thu, 19 Nov 2009 16:20:21 -0800, (Dave Platt)
wrote:

In article ,
Jeff Liebermann wrote:

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.

Ummm... I'd disagree (or at least quibble) on at least two grounds.

Oh-oh.

Ground #1: the ferrite beads would only block non-balanced current
flow back down the outside of the coax. They will have no effect at
all on power which is reflected back down the inside of the coax (the
center conductor and the inside of the shield) from any impedance
mismatch where the coax meets the folded dipole.

Agreed, if there were a balun present. Without a balun, there's going
to be some radiation from the coax cable, presumably from the shield.
Disclaimer: I don't fully understand how baluns work, and certainly
don't understand what the piece of coax with ferrite beads on the
MFJ-1800 is suppose to do.

In effect, the presence of the beads (if they're choking the coax
properly) actually ensures that the transmitter *does* see the true
effect of any impedance mismatch. The transmitter is just as likely
to see a higher SWR than a lower one, when the beads are added.

You lost me here. Just what are the beads suppose to do if not block
the reflected signal? Presumably, they do serve some useful purpose.
Now, you're suggesting that they could make the VSWR worse?

Ground #2: the beads do not necessarily result in a significant loss
of power. If their RF impedance is high enough at the frequency of
use, then RF current flow through them will be negligible, and there
won't be a loss of power. Power loss in choke-ferrites tends to be
worst when the RF impedance is both resistive, and too low for the
application (i.e. still allows substantial current flow, which then
results in dissipation of power inside the choke).

This is beginning to sound like the discussion about whether it's
acceptable to use a high VSWR antenna. The standard answer is that if
the transmitter can take it without going into protection or
oscillation, then it's acceptable. I don't have a problem with that,
although I've never seen a Yagi-Uda antenna with a 50 ohm coax hung
directly onto a folded dipole because there are usually easy ways to
do the matching and balanced to unbalanced conversion.

Digging a deeper hole, I've been assuming that if the ferrite beads
were not there, the coax cable will radiate. After all, that's one
purpose of a balun, to prevent coax radiation from mangling the
pattern. That's still a dubious proposition due to the large length
of exposed center conductor at both ends of the coax piece, which
certainly will radiate some. I can add that to the model, but I don't
know how to model the ferrite beads.


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

On Fri, 20 Nov 2009 09:10:27 -0800, Jeff Liebermann
wrote:

You lost me here. Just what are the beads suppose to do if not block
the reflected signal?
Presumably, they do serve some useful purpose.

They block common mode current. That is their purpose as a CHOKE. A
good BalUn contains choking, but a choke is not always a BalUn. For a
quarterwave BalUn, the beads serve to isolate each end which is both a
choking AND a BalUn necessity. As the loop is isolated from the sheet
metal, isolation is preserved (allowing the section to "possibly"
serve as a quarter wave choke - neglecting the geometric mean of Zs).

Now, you're suggesting that they could make the VSWR worse?

That is only a possibility, not a necessity.

I've never seen a Yagi-Uda antenna with a 50 ohm coax hung
directly onto a folded dipole because there are usually easy ways to
do the matching and balanced to unbalanced conversion.

Yagis are often balanced by design. You should have seen 50 Ohm coax
coiled nearby the feedpoint as a choke (an alternative method to using
beads). If you have not, then you have probably seen an installation
that suffered degraded performance (and either to the operator's
unending grief, or to their total unawareness). You give some sense
of already knowing this with your comment that follows:

Digging a deeper hole, I've been assuming that if the ferrite beads
were not there, the coax cable will radiate. After all, that's one
purpose of a balun, to prevent coax radiation from mangling the
pattern. That's still a dubious proposition due to the large length
of exposed center conductor at both ends of the coax piece, which
certainly will radiate some. I can add that to the model, but I don't
know how to model the ferrite beads.

I've written to that where I took exception to you modeling the Z as
series, and I pointed out it was shunt. Apparently you didn't follow
that comment.

You model the beads as either creating an open circuit, or adding a
huge resistance in-line. The net result to current is the same. The
long and short of it is that to model the exterior of the coax, you
add a wire of same length back to the choke section and call it a day.

73's
Richard Clark, KB7QHC