I've posted a picture on alt.binaires.schematics.electronic
to an effort to help me get a better understanding of an antenna.
It is a 2.4Ghz antenna about 18" long.
The driven element is a copper ribbon 0.2" wide by 0.032" thick.
The copper ribbon forms a loop about 2" wide by 0.625" tall.
The gap in the ribbon would be 0.020", but note; in the pictures
a moon shaped piece has been cut from both ends. I think this
moon cut effectively make the gap look larger.
The total coax length is 2.350", the shield a bit shorter at 2.08".
There are 4 toroids on the coax. These are under a piece of heat
shrink, so I have no idea what material they would be.
All element spacings are all equal and element lengths are within
..030" of each other. I think they were designed to be the same but
manufacturing tolerances made the differences. The reflector is longer.
So now what can the group tell me about;
Driven element impedance
the gap
coax length impedance transformation
the 4 toroids
Thanks for the help, if you need any other details let me know.
Mike

"amdx"

I've posted a picture on alt.binaires.schematics.electronic
to an effort to help me get a better understanding of an antenna.
It is a 2.4Ghz antenna about 18" long.
The driven element is a copper ribbon 0.2" wide by 0.032" thick.
The copper ribbon forms a loop about 2" wide by 0.625" tall.
The gap in the ribbon would be 0.020", but note; in the pictures
a moon shaped piece has been cut from both ends. I think this
moon cut effectively make the gap look larger.
The total coax length is 2.350", the shield a bit shorter at 2.08".
There are 4 toroids on the coax. These are under a piece of heat
shrink, so I have no idea what material they would be.
All element spacings are all equal and element lengths are within
.030" of each other. I think they were designed to be the same but
manufacturing tolerances made the differences. The reflector is longer.
So now what can the group tell me about;


** Just like this one - ain't it ??

http://www.mfjenterprises.com/produc...rodid=MFJ-1800





.......... Phil

"amdx" wrote in message
...
The gap in the ribbon would be 0.020", but note; in the pictures
a moon shaped piece has been cut from both ends.

This decreases what's otherwise taken as a lumped capacitor right at the
feed point; this is generally undesirable because it lowers the impedance
from the theoretical calcuations.

There are 4 toroids on the coax. These are under a piece of heat
shrink, so I have no idea what material they would be.

Those toroid are a simple but effective balun: Without them, current that's
supposed to return to the inner side of the coax's shield instead has a
choice between that inner side or the outer side. Currents that flow on the
outer side of the coax's shield significantly distory the radiation pattern.
The toroids "work" because they look like inductors (hopefully big
impedances) to single-ended currents (current trying to go down the coax)
while the fields of current going down the center conductor of the coax in
addition to those from the same current going down the inside of the shield
cancel out to (almost) nothing outside the coax and therefore the ferrite
toroids aren't "seen."

Driven element impedance

Well, if you're driving it with 50 ohm coax, it should be realtively close
to 50 ohms at resonance!

---Joel

"Joel Kolstad" wrote in message
...
"amdx" wrote in message
...
The gap in the ribbon would be 0.020", but note; in the pictures
a moon shaped piece has been cut from both ends.

This decreases what's otherwise taken as a lumped capacitor right at the
feed point; this is generally undesirable because it lowers the impedance
from the theoretical calcuations.

There are 4 toroids on the coax. These are under a piece of heat
shrink, so I have no idea what material they would be.

Those toroid are a simple but effective balun: Without them, current
that's supposed to return to the inner side of the coax's shield instead
has a choice between that inner side or the outer side. Currents that
flow on the outer side of the coax's shield significantly distory the
radiation pattern. The toroids "work" because they look like inductors
(hopefully big impedances) to single-ended currents (current trying to go
down the coax) while the fields of current going down the center conductor
of the coax in addition to those from the same current going down the
inside of the shield cancel out to (almost) nothing outside the coax and
therefore the ferrite toroids aren't "seen."

Driven element impedance

Well, if you're driving it with 50 ohm coax, it should be realtively close
to 50 ohms at resonance!

I was surprised there was no matching section, I thought a folded dipole
would have a higher impedance than 50 ohms.

What type toroid material would be used at 2.4Ghz, and I'm thinking it would
look resistive at that frequency.

Thanks for the input,
Mike

"amdx"


I was surprised there was no matching section, I thought a folded dipole
would have a higher impedance than 50 ohms.



** A folded dipole, * all on its own * presents 300 ohms at resonance.

But when surrounded by directors and a reflector, the impedance changes
dramatically.

Simple to arrange things so the impedance drops to 50 ohms, to match
standard RF feeder co-ax.



What type toroid material would be used at 2.4Ghz, and I'm thinking it
would
look resistive at that frequency.


** Ferrites exist that work well at GHz.

http://www.univ.trieste.it/~carrato/.../murata_sf.pdf



....... Phil

"Fred Abse" wrote in message
newsan.2007.09.16.12.12.19.35340@cerebrumconfus. it...
On Sat, 15 Sep 2007 14:36:05 -0500, amdx wrote:

I was surprised there was no matching section, I thought a folded dipole
would have a higher impedance than 50 ohms.

It does. Around 300 ohms. However, adding parasitic elements reduces the
feedpoint impedance.

--
"Electricity is of two kinds, positive and negative. The difference
is, I presume, that one comes a little more expensive, but is more
durable; the other is a cheaper thing, but the moths get into it."
(Stephen Leacock)

.... is that so?

A folded dipole (thin, half-wave, symmetrical) has the same gain as a
non-folded one but a terminal resistance of about 300 ohms as opposed to
about 73 ohms for a non-folded dipole (two prongs, often written as 75
ohms). Doubling the number of closely-coupled conductors divides the input
current to half in each, and the radiation resistance of each conductor must
be half the radiation resistance of the whole antenna. When a pair of
terminals is applied at the mid point of only one of the conductors the
apparent terminal impedance is that of a non-folded dipole raised to the
power of 2.

The most common form of balun used with folded dipoles is a half-wavelength
of transmission line (of any characteristic impedance!) between the dipole
terminals, one of which is connected to the feeder (and all screens are
connected together if co-ax is used). This also acts as a transformer with
a turns-ratio of 2:1 (it produces the same voltage in antiphase at its far
end), so the 300 ohms of the folded dipole is transformed down to about 75
ohms.

We looked inside the balun of a J-Beam folded dipole from a 3-element Yagi,
for 50 ohms, and couldn't find any component or circuitry other than the
half-wave line between the dipole terminals, and it did look like 50 ohms on
a network analyser.

I've seen a 'genetic algorithm' program for Yagi design that took the
required terminal impedance as an input parameter, along with things like
gain and front/back ratio.

Hope this helps.

Chris

amdx wrote:
I've posted a picture on alt.binaires.schematics.electronic
to an effort to help me get a better understanding of an antenna.
It is a 2.4Ghz antenna about 18" long.
The driven element is a copper ribbon 0.2" wide by 0.032" thick.
The copper ribbon forms a loop about 2" wide by 0.625" tall.
The gap in the ribbon would be 0.020", but note; in the pictures
a moon shaped piece has been cut from both ends. I think this
moon cut effectively make the gap look larger.
The total coax length is 2.350", the shield a bit shorter at 2.08".
There are 4 toroids on the coax. These are under a piece of heat
shrink, so I have no idea what material they would be.
All element spacings are all equal and element lengths are within
.030" of each other. I think they were designed to be the same but
manufacturing tolerances made the differences. The reflector is longer.
So now what can the group tell me about;
Driven element impedance
the gap
coax length impedance transformation
the 4 toroids
Thanks for the help, if you need any other details let me know.
Mike




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