Prev: NEWS: Hotspot sniffer eavesdrops on iPhone [VoIP & video] inreal-time
Next: Time machine & SuperDuper!
From: Tony Hwang on 14 Nov 2009 18:21
amdx wrote:
> "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
>
>
>
Hi,
Feed point impedance becomes near that when it is just a dipole(not
folded). On a haf wave element at center feed point the current and
voltage phase is such that the impedance is near that figure.
If it is vertical uater wave whip, the other half mirror image is
prjected into the ground forming dipole and in that case feed point
impedance at the bottom is around 50 Ohm. I am old dinosaur EE(class of
60), my brain is not as bright as it used to, LOL!
From: Tony Hwang on 14 Nov 2009 18:22
Tim Wescott wrote:
> 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?
>
Hi,
Also folded dipole tends to have broader b/w compared to dipole.
From: Tony Hwang on 14 Nov 2009 18:24
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
>
>
Hi,
Even now Yagi-Uda antenna design is empirical business tinkering with it
in the antenna range based on theory plus actual field result.
From: Baron on 14 Nov 2009 18:44
Tony Hwang wrote:

> Tim Wescott wrote:
>> 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?
>>
> Hi,
> Also folded dipole tends to have broader b/w compared to dipole.

Due to the transformer effect of folding it.

--
Best Regards:
Baron.
First  |  Prev  | 
Pages: 1 2 3
Prev: NEWS: Hotspot sniffer eavesdrops on iPhone [VoIP & video] inreal-time
Next: Time machine & SuperDuper!