folded dipole balun voltage loss?
Anonymous
bandwidth. Then there is a 300-75 ohm balun. This is a
2:1 stepdown transformer.
What if I want one channel? Would I be better off with
a simple dipole and a 1:1 balun?
This should give me twice the voltage?
charles
Anonymous <cri...@ecn.org> wrote:
> most TV antennae used folded dipole as it gives a wider
> bandwidth. Then there is a 300-75 ohm balun.
err, no. Adding elements reduces the impedance and it becomes something
like 75 ohms.
> This is a 2:1 stepdown transformer.
no it isn't
> What if I want one channel? Would I be better off with a simple dipole
> and a 1:1 balun? This should give me twice the voltage?
why?
--
From KT24
Using a RISC OS computer running v5.11
Wimpie
Hello,
When properly matched, there is no difference in performance (output
POWER) between folded dipole and HW dipole.
Mechanical advantage of folded dipole is that you do not need to make
an insulated support. For narrow band operation (no parasitic
elements), you can use an electrically half wave piece of 75 Ohms
coaxial cable to do the matching and bal. to unbal. conversion.
When making yagis, the purpose of the folded dipole is to get
impedance close to 75 Ohms. Addition of the parasitic radiators
reduces the impedance of the driven element. When using a HW center
fed dipole in a long yagi, the impedance of the yagi will be far below
75 Ohms.
Best regards,
Wim
PA3DJS
www.tetech.nl
please remove the obvious character combination in case of PM.
christofire
"Wimpie" <wima...@tetech.nl> wrote in message
news:dc781b91-94d7-4794...@e27g2000yqm.googlegroups.com...
Before accepting statements like this, and that of the previous poster, I
would encourage those who can to take a look at the terminal impedance of a
good-quality (i.e. professional) Yagi such as one of those manufactured by
J-Beam - which use a folded dipole as the driven element and use a half-wave
balun (insulated wire wound around a metal rod) to interface with the
co-axial tail. Those I have looked at have had terminal impedances close to
the specified cable impedance at their design frequency ... they have
certainly not been much lower than 50 or 75 ohms.
My own understanding is that they use a folded dipole for its advantageous
bandwidth, so they can manufacture, and their dealers stock, a smaller
number of models to cover a specified wide bandwidth (i.e. the bandwidth of
the antenna is determined by pattern and impedance considerations on account
of the array more than the driven element). I have also noticed that the
physical length of the folded dipole they use is related to the cable
impedance (i.e. between 50 and 75-ohm versions for the same frequency band)
whilst a similar type of balun is used.
Some cheap television aerials are sold with co-axial connections directly to
a folded-dipole 'driven' element but this is likely a cost-cutting measure,
which impairs ruggedness of the installation against household RF noise.
Chris
GregS
True, but its not going to help drive a preamp which
is matched to the input Z.
greg
Cecil Moore
> My own understanding is that they use a folded dipole for its advantageous
This may be true but there are other advantages/benefits.
Given that parasitic elements lower the resonant feedpoint
impedance and the target feedpoint is 50 or 75 ohms:
Which driven element makes more sense? One that starts
out with a feedpoint impedance of 75 ohms or one that
starts out with a feedpoint impedance of 300 ohms?
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com
christofire
"Cecil Moore" <nos...@w5dxp.com> wrote in message
news:pribm.35520$YU5....@newsfe21.iad...
I think you've misunderstood what I wrote. A J-beam Yagi such as
http://www.amphenol-jaybeam.com/datasheet_files/7043xxx.pdf has a
folded-dipole driven element _and_ a half-wave line balun interfacing it to
coaxial cable. This type of balun, as I expect you know, transforms the
dipole's impedance down by a factor of four so a nominal 300 ohms is matched
to 75 ohm cable. When 50 ohm cable is specified, the same type of balun is
used but the length of the dipole appears to be slightly reduced ... but now
I'm repeating myself. In such a case the parasitic elements can't be
reducing the feed-point impedance of the dipole very much.
Incidentally, the cable 'tail' is conventional 50 or 75 ohm cable (i.e. it
doesn't act as a transformer).
As I understand it, feed-point impedance is one of the several variables
involved in Yagi design, along with aspects of the radiation pattern such as
main-beam width, first and other sidelobe levels, and front-to-back ratio,
the resulting directivity gain, and the frequency response of all these
factors. Professional antennas may not be designed for maximum gain.
Chris