I am building a 70 cm Yagi. I want to make a balun out of a loop of coax
(yes, I know there are other methods but I prefer this one). My question is,
will rg58 do. Also, comparing like losses to like losses, should it be 50
ohm or 75 ohm and why?

Thanks in advance.

Suzy wrote:
I am building a 70 cm Yagi. I want to make a balun out of a loop of coax
(yes, I know there are other methods but I prefer this one). My question is,
will rg58 do.

Yes, provided that you won't be exceeding its power rating.

Also, comparing like losses to like losses, should it be 50
ohm or 75 ohm and why?

Only the outside diameter is significant in determining the balun
operation, and the only real effect that will have is in determining the
number of turns and size you'll need to get effective balun operation.
You should use the same impedance of coax as you use for feeding your
Yagi, unless you intentionally want a mismatch for some reason.

Roy Lewallen, W7EL

Roy Lewallen wrote in
:

Suzy wrote:
I am building a 70 cm Yagi. I want to make a balun out of a loop of
coax (yes, I know there are other methods but I prefer this one). My
question is, will rg58 do.

Yes, provided that you won't be exceeding its power rating.

Also, comparing like losses to like losses, should it be 50
ohm or 75 ohm and why?

Only the outside diameter is significant in determining the balun
operation, and the only real effect that will have is in determining
the number of turns and size you'll need to get effective balun
operation. You should use the same impedance of coax as you use for
feeding your Yagi, unless you intentionally want a mismatch for some
reason.

Roy Lewallen, W7EL


Roy,

I think you have misread the context, Suzy's application is a 1:4 balun
for a 70cm Yagi, formed from a half wave balun to provide the phase
inverted output of the balun.

She asked this question in another place five weeks or so ago, and my
response was:

=== quote

If you have designed the dipole feed point to be 200 ohms so that the
balun you describe transforms it to 50 ohms, the load on the half wave
section is 100 ohms and it has a VSWR of 2:1 if it is 50 ohm coax.

The loss on the half wave section will be increased by the higher VSWR.
The loss on a half wave of RG213 (Zo=50) at 432MHz with a 100 ohm load is
0.136dB, whereas it is 0.104dB for RG11 (Zo=75).

Does this say 75 ohm line is best. No, it says that RG11 is better than
RG213.

You can explore these and other options with the loss calculator at
http://www.vk1od.net/tl/tllc.php .

If you can't talk about it in numbers, do you really understand it?

Be ware of using low loss cables with foam dielectric for the balun...
check their minimum bending radius.

Don't depend on published velocity factor in cutting the balun, measure
vf and cut to tune the balun for the desired frequency.

===end quote

Apparently this is not regarded as a recommendation with explanation.

Roy, you may have a better turn of phrase.

Owen

Suzy wrote:
I am building a 70 cm Yagi. I want to make a balun out of a loop of coax
(yes, I know there are other methods but I prefer this one). My question is,
will rg58 do. Also, comparing like losses to like losses, should it be 50
ohm or 75 ohm and why?

RG58 will do fine. Some constructors use small hardline such as UT141,
but mostly for mechanical convenience. At this wavelength, larger cables
are quite difficult to bend into the small loop this is required.

Since the balun loop is an electrical half-wave, any impedance will
reproduce the same voltage at the other end. (Minor details: cable loss
will have a small effect on voltage balance, but probably not enough to
be detectable; the optimum impedance for VSWR bandwidth would be 100
ohms, but that is unimportant because other parts of the yagi will
probably have a larger effect.)

In summary, choose whatever cable you can conveniently form into an
electrical half-wave loop.



--

73 from Ian GM3SEK

Ian White GM3SEK wrote in
:

....
RG58 will do fine. Some constructors use small hardline such as UT141,
but mostly for mechanical convenience. At this wavelength, larger
cables are quite difficult to bend into the small loop this is
required.

Ian, I offered that solution elsewhe

===quote
Obsessions about loss aside, try using the calculator to explore a halfwave
of UT141/RG405 semi rigid coax on a 100 ohm load. This is just 0.141" or
4mm diameter, and you need 245mm (as the calculator will tell you). It is a
quite neat solution to a half wave balun if you are not too obsessed about
0.0xdB of loss.
===equote

The loss for UT141 is 0.03dB worse than RG58C/U in this case.

Owen

Yes, I see I did misread the context. Please disregard my response.

Roy Lewallen, W7EL

Ian White GM3SEK wrote in
:

Since the balun loop is an electrical half-wave, any impedance will
reproduce the same voltage at the other end.

More correctly, same magnitue voltage but opposite polarity.

Owen

"Ian White GM3SEK" wrote in message
...
Suzy wrote:
I am building a 70 cm Yagi. I want to make a balun out of a loop of coax
(yes, I know there are other methods but I prefer this one). My question
is,
will rg58 do. Also, comparing like losses to like losses, should it be 50
ohm or 75 ohm and why?

RG58 will do fine. Some constructors use small hardline such as UT141, but
mostly for mechanical convenience. At this wavelength, larger cables are
quite difficult to bend into the small loop this is required.

Since the balun loop is an electrical half-wave, any impedance will
reproduce the same voltage at the other end. (Minor details: cable loss
will have a small effect on voltage balance, but probably not enough to be
detectable; the optimum impedance for VSWR bandwidth would be 100 ohms,
but that is unimportant because other parts of the yagi will probably have
a larger effect.)

In summary, choose whatever cable you can conveniently form into an
electrical half-wave loop.



--

73 from Ian GM3SEK

Thank you so much Ian. Definitive answer at last (yes maybe the others were
too, Owen, but I am too thick to fathom them. g)