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From: amdx on 14 Nov 2009 13:57

"Tony Hwang" <dragon40(a)shaw.ca> wrote in message
news:hACLm.21858$de6.6577(a)newsfe21.iad...
> amdx wrote:
>> Hi All,
>> I'm having a disagreement about the use of this folded dipole
>> calculator
>> impedance calculator.
>> http://www.k7mem.150m.com/Electronic_Notebook/antennas/folded_dipole.html
>> We are using it to calculate the impedance of folded dipoles to drive a
>> yagi.
>> When entering data it asks for "Simple dipole feed impedance (ohms)"
>> Default is 72 ohms. This assumes a 1/2 wave dipole.
>> The fellow I'm disagreeing with say's,
>> "the reason that the variable in the calculator for the "dipole feed
>> impedance"
>> could be if you want to transform the z to something else, such as 50
>> ohms."
>> I don't even understand what he means.
>> I can only think of one reason to alter the 72 ohms, that would be if you
>> use a a different length folded dipole.
>> I'll readily admit the other fellow knows more about this than me, but
>> I need more convincing :-)
>> Can anyone explain this to me.
>> Thanks, Mike
>>
>>
>>
> Hi,
> The eq. seems right. Folded dipole is basically a loop.
> Usually we use 300 Ohm feeder.
> Tony
> VE6CGX

In what case would I alter the 72 ohms?
Mike



From: Tim Wescott on 14 Nov 2009 15:06
On Sat, 14 Nov 2009 12:05:21 -0600, amdx wrote:

> Hi All,
> I'm having a disagreement about the use of this folded dipole
> calculator
> impedance calculator.
> http://www.k7mem.150m.com/Electronic_Notebook/antennas/
folded_dipole.html
> We are using it to calculate the impedance of folded dipoles to drive a
> yagi.
> When entering data it asks for "Simple dipole feed impedance (ohms)"
> Default is 72 ohms. This assumes a 1/2 wave dipole.
> The fellow I'm disagreeing with say's,
> "the reason that the variable in the calculator for the "dipole feed
> impedance"
> could be if you want to transform the z to something else, such as 50
> ohms."
> I don't even understand what he means.
> I can only think of one reason to alter the 72 ohms, that would be if
> you use a a different length folded dipole.
> I'll readily admit the other fellow knows more about this than me, but
> I need more convincing :-)
> Can anyone explain this to me.
> Thanks, Mike

I've seen older Yagis that use folded dipoles, but the newer ones seem to
lean more toward gamma or T matches. You get more latitude for adjusting
impedance, in the gamma case you get a kinda sorta good match to coax
without a balun, and if you're employing plumber's delight construction
it's no more difficult to fabricate than a folded dipole.

So why a folded dipole, pray tell?

--
www.wescottdesign.com
From: Paul Keinanen on 14 Nov 2009 16:05
On Sat, 14 Nov 2009 12:05:21 -0600, "amdx" <amdx(a)knology.net> wrote:

> Hi All,
> I'm having a disagreement about the use of this folded dipole calculator
>impedance calculator.
>http://www.k7mem.150m.com/Electronic_Notebook/antennas/folded_dipole.html
> We are using it to calculate the impedance of folded dipoles to drive a
>yagi.

Excuse me, but what does such simple dipole calculator have to do with
the calculations for a Yagi feed element ?

Even assuming a single element feed element antenna, playing with the
tube diameter (upper and lower bars) or the number of bars in a folded
dipole will radically change the feed impedance.

Paul

From: amdx on 14 Nov 2009 16:39

"Paul Keinanen" <keinanen(a)sci.fi> wrote in message
news:qb6uf5djdffineb2u5eiuaov51hik3ti1v(a)4ax.com...
> On Sat, 14 Nov 2009 12:05:21 -0600, "amdx" <amdx(a)knology.net> wrote:
>
>> Hi All,
>> I'm having a disagreement about the use of this folded dipole calculator
>>impedance calculator.
>>http://www.k7mem.150m.com/Electronic_Notebook/antennas/folded_dipole.html
>> We are using it to calculate the impedance of folded dipoles to drive a
>>yagi.
>
> Excuse me, but what does such simple dipole calculator have to do with
> the calculations for a Yagi feed element ?

I think it all started with the MFJ-1800 that uses a folded dipole, but
there
are many yagis that use a folded dipole as the driven element.
But, I think (now) I understand enough to know why you ask the question.
I think your point is the impedance of dipole feed on a yagi is highly
modified
by the reflector and directors.

> Even assuming a single element feed element antenna, playing with the
> tube diameter (upper and lower bars) or the number of bars in a folded
> dipole will radically change the feed impedance.
>
> Paul
>

I excerpted this from my latest email to my friend I'm having the
disagreement with. (Might
not even be a disagreement, it could be a miscommunication.)
Excerpt;
"The online calculator takes the (Impedance of a dipole x Ratio). This
gives you the impedance
after folding the dipole.
We still have no idea what the impedance of a dipole is when surrounded by
the reflector and
director. We do know how to change the transformation ratio once we know
what the impedance
of dipole would be in that surrounding."
Mike


From: Baron on 14 Nov 2009 17:49
amdx wrote:

>
> "Paul Keinanen" <keinanen(a)sci.fi> wrote in message
> news:qb6uf5djdffineb2u5eiuaov51hik3ti1v(a)4ax.com...
>> On Sat, 14 Nov 2009 12:05:21 -0600, "amdx" <amdx(a)knology.net> wrote:
>>
>>> Hi All,
>>> I'm having a disagreement about the use of this folded dipole
>>> calculator
>>>impedance calculator.
>>>http://www.k7mem.150m.com/Electronic_Notebook/antennas/folded_dipole.html
>>> We are using it to calculate the impedance of folded dipoles to
>>> drive a
>>>yagi.
>>
>> Excuse me, but what does such simple dipole calculator have to do
>> with the calculations for a Yagi feed element ?
>
> I think it all started with the MFJ-1800 that uses a folded dipole,
> but
> there
> are many yagis that use a folded dipole as the driven element.
> But, I think (now) I understand enough to know why you ask the
> question.
> I think your point is the impedance of dipole feed on a yagi is highly
> modified
> by the reflector and directors.
>
>> Even assuming a single element feed element antenna, playing with the
>> tube diameter (upper and lower bars) or the number of bars in a
>> folded dipole will radically change the feed impedance.
>>
>> Paul
>>
>
> I excerpted this from my latest email to my friend I'm having the
> disagreement with. (Might
> not even be a disagreement, it could be a miscommunication.)
> Excerpt;
> "The online calculator takes the (Impedance of a dipole x Ratio).
> This
> gives you the impedance
> after folding the dipole.
> We still have no idea what the impedance of a dipole is when
> surrounded by
> the reflector and
> director. We do know how to change the transformation ratio once we
> know what the impedance
> of dipole would be in that surrounding."
> Mike

The nominal impedance of a dipole in free space is approximately 72
ohms. If it is part of a system its impedance will be influenced by
that system causing it to change from nominal.

Folding the dipole allows it to behave as a transformer ! Thus the
nominal dipole impedance can be altered to a value that can more easily
be matched to the feed line.

Commonly the impedance of a dipole that is part of a system. ie "Yagi"
falls to a much lower value. Under these conditions the folded dipole
can be used to raise the impedance seen at the feed point to a more
usable value.

Someone mentioned "Gama match". Whilst symmetrical gamma matches can be
used, single ended is popular because it is inherently unbalanced and
can be easily matched to a co-axial feedline.

--
Best Regards:
Baron.
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