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

On Fri, 20 Nov 2009 10:46:40 -0800, Richard Clark
wrote:

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.

No problem. Just post or email me the .EZ file. 4NEC2 will import it
or I'll just switch to EZNEC. (I promise not to complain about your
using inches).

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?

Unfortunately, yes. I got lazy and planted the folded dipole centered
about the axis of the other elements. I don't think (guess) it would
make much difference, but I'll move it to the correct position this
weekend.

Got any theory as to why the vertical and horizontal patterns are so
different? That only appeared when I switched to the flat wire folded
dipole. They were symmetrical with the wire rod driven element.

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?).

Yes, something is different. In past tinkering, I've found that 3
elements is sufficient to characterize the feed impedance, where the
additional elements just improve the gain and pattern.

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.

Well, I used a rod with the same circumference as the flat
(asymetrical) elements. Methinks the element diameter would have an
effect on the bandwidth of the antenna, but not on its characteristic
impedance.

73's
Richard Clark, KB7QHC

thanks much...

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

"Richard Clark" wrote in message
...
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 - mine do.
73's
Richard Clark, KB7QHC

Hi Richard,
What do you mean by "lack of driven element symmetry" ?
Mike

On Fri, 20 Nov 2009 18:26:17 -0600, "amdx" wrote:

Hi Richard,
What do you mean by "lack of driven element symmetry" ?
Mike

Hi Mike,

The driven loop is not symettrical to the plane of the directors and
reflector, thus it peers down (or up, or to the side - depending on
deployment).

Look boresight down the boom. The loop is off-center.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Fri, 20 Nov 2009 18:26:17 -0600, "amdx" wrote:

Hi Richard,
What do you mean by "lack of driven element symmetry" ?
Mike
Hi Mike,

The driven loop is not symettrical to the plane of the directors and
reflector, thus it peers down (or up, or to the side - depending on
deployment).
Look boresight down the boom. The loop is off-center.

73's
Richard Clark, KB7QHC

Good, I thought that might be the answer but wanted to check.
Glad I added that detail to the drawing, do you think that is
engineered to control output impedance?
What happens to the impedance if you center it?

Could you post your model or send it to me, I have a fellow
ham that tried to model the FD and could not get it to work.
He would like to see one that does work and find out what he did wrong.
Eznec if you have it.
Thanks, Mike

On Sat, 21 Nov 2009 06:24:49 -0600, "amdx" wrote:

Could you post your model or send it to me, I have a fellow
ham that tried to model the FD and could not get it to work.
He would like to see one that does work and find out what he did wrong.
Eznec if you have it.
Thanks, Mike


Hi Mike,

It has been posted to you as you asked. Jeff also has a copy.

Can you tell us what you mean about your buddy's model and how he
"could not get it to work?" Did he follow your dimensions?

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Sat, 21 Nov 2009 06:24:49 -0600, "amdx" wrote:

Could you post your model or send it to me, I have a fellow
ham that tried to model the FD and could not get it to work.
He would like to see one that does work and find out what he did wrong.
Eznec if you have it.
Thanks, Mike


Hi Mike,

It has been posted to you as you asked. Jeff also has a copy.

Can you tell us what you mean about your buddy's model and how he
"could not get it to work?" Did he follow your dimensions?

73's
Richard Clark, KB7QHC

He's out of town for a couple of days, when he returns I'll quiz him.
Mike