Andy writes:

I have a need for a quarter-wavelength transmission line at 25 ohms
impedance.

My SPICE simulator tells me that I can connect two 50 ohm, quarter-
wavelength
transmission lines in parallel and get the exact same performance
as the needed
25 ohm line.

However, I have not seen anyone try this, nor seen it used in
equipment.

Does anyone here have any experience with doing this, or have any
learned
opinions as to why it might, or might not, be a good idea ??

Thanks for your informed advice..

Andy W4OAH

AndyS Wrote:

I have a need for a quarter-wavelength transmission line at 25 ohms
impedance. My SPICE simulator tells me that I can connect two 50
ohm, quarter- wavelength transmission lines in parallel and get the
exact same performance as the needed 25 ohm line.

However, I have not seen anyone try this, nor seen it used in
equipment. Does anyone here have any experience with doing this, or
have any learned opinions as to why it might, or might not, be a
good idea ??


No problem. It's a standard technique for matching yagis, for example:

http://www.mydarc.de/dk7zb/start1.htm
(click the third item in menu list)


--

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek

It works well.

I needed some Zo=25 ohm for a 1:4 balun (12.5ohms50ohms) and simply
used parallelled lengths of 50 ohm for the windings.

73,
Steve G3TXQ

steveeh131047 wrote in news:0a13ea69-ccea-4555-a5c8-
:

It works well.

I needed some Zo=25 ohm for a 1:4 balun (12.5ohms50ohms) and simply
used parallelled lengths of 50 ohm for the windings.

If you are talking about a balun made with an electrical half wave of coax,
you may be hard put to observe the difference between using 50 ohm or 25
ohm line. If the load was an ideal resistor, using the 'matched' Zo gives
least loss and best bandwidth, but on a narrowband antenna, the balun
bandwidth is less an issue, and may even compensate the antenna (depending
on the chosen Zo).

The OP's application to a quarter wave line is sensitive to Zo though, and
it works fine, but it is quite important to bond the shields effectively at
both ends of the line.

Owen

Ian White GM3SEK wrote in
:

....

No problem. It's a standard technique for matching yagis, for example:

http://www.mydarc.de/dk7zb/start1.htm
(click the third item in menu list)

Ian,

The DK7ZB match (http://www.mydarc.de/dk7zb/images/match-12+28.gif) is
interesting. Ideally, the transformation sections (the inside of the coax
TL) are an electrical quarter wave, and the balun (the TL formed by the
outside of the coaxes in parallel and the boom etc) is a quarter wave.

I would be surprised if constructions typically shown achieve that
optimisation, due at least to the use of solid PE dielectric inside the
line, and a air / vinyl dilectric outside.

The construction at
http://www.mydarc.de/dk7zb/DK7ZB-Mat..._Isolation.jpg is quite
complex, containing what might be seen as as an electrical quarter wave
transformer and the balun formed of a U shaped stub of physical length
0.5*0.25*0.66=0.08 wavelengths shunting one side of the DE, but this stub
lies with its hot side against the cold boom forming another o/c TL
element. If the balun shunts the DE with a high impedance, it will be due
to not just the loop length, nor just the s/c stub length, but parallel
resonance with the boom / half loop o/c stub.

If that is the case, the construction is not simply explained by the
first diagram that I referenced above.

I am not suggesting that this implementation doesn't work, but that there
are design parameters beyond those implied by the electrical circuit
given.

Thoughts?

Owen

In article
,
AndyS wrote:

Andy writes:

I have a need for a quarter-wavelength transmission line at 25 ohms
impedance.

My SPICE simulator tells me that I can connect two 50 ohm, quarter-
wavelength
transmission lines in parallel and get the exact same performance
as the needed
25 ohm line.

However, I have not seen anyone try this, nor seen it used in
equipment.

Does anyone here have any experience with doing this, or have any
learned
opinions as to why it might, or might not, be a good idea ??


In article
,
steveeh131047 wrote:

It works well.

It has to work well.

Suppose first that you connect a load with impedance Z to the far end of a
quarter wave transmission line of 25 ohm surge impedance. At the near end of
the quarter wave transmission line this will look like 25^2/Z ohms, or 625/Z
ohms.

Now start over, but instead of the load with impedance Z suppose you have
two identical loads, each of impedance 2*Z, which if they had been connected
in parallel would have had an impedance Z. Take two different quarter wave
transmission lines, each of surge impedance 50 ohms, and connect the two
loads (separately) to the far ends of the two transmission lines. At the
near ends of each of the transmission lines this will look like 50^2/(2*Z)
ohms, which simplifies to 1250/Z ohms. Connect the near ends of the two
transmission lines in parallel. You will then see 625/Z ohms, the same as
you saw with the Z ohm load at the end of the quarter wave 25 ohm line.

By symmetry the voltages across, and the currents through, the two 2*Z ohm
loads have to be the same. So nothing will change if they are connected in
parallel. Do that and you will have a single Z ohm load at the far end of
the two paralleled quarter wave 50 ohms lines.

David, ex-W8EZE

--
David Ryeburn

To send e-mail, use "ca" instead of "caz".

Owen Duffy wrote:
Ian White GM3SEK wrote in
:

...

No problem. It's a standard technique for matching yagis, for example:

http://www.mydarc.de/dk7zb/start1.htm
(click the third item in menu list)

Ian,

The DK7ZB match (http://www.mydarc.de/dk7zb/images/match-12+28.gif) is
interesting. Ideally, the transformation sections (the inside of the coax
TL) are an electrical quarter wave, and the balun (the TL formed by the
outside of the coaxes in parallel and the boom etc) is a quarter wave.

I would be surprised if constructions typically shown achieve that
optimisation, due at least to the use of solid PE dielectric inside the
line, and a air / vinyl dilectric outside.

The construction at
http://www.mydarc.de/dk7zb/DK7ZB-Mat..._Isolation.jpg is quite
complex, containing what might be seen as as an electrical quarter wave
transformer and the balun formed of a U shaped stub of physical length
0.5*0.25*0.66=0.08 wavelengths shunting one side of the DE, but this stub
lies with its hot side against the cold boom forming another o/c TL
element. If the balun shunts the DE with a high impedance, it will be due
to not just the loop length, nor just the s/c stub length, but parallel
resonance with the boom / half loop o/c stub.

If that is the case, the construction is not simply explained by the
first diagram that I referenced above.

I am not suggesting that this implementation doesn't work, but that there
are design parameters beyond those implied by the electrical circuit
given.

Thoughts?

There are two separate issues here. The impedance transformation with
paralleled coax is no problem (which answers the original question).

The other issue is the way the feedline is looped to form a balun... or
so it is claimed. Like you, I am quite dubious about that.

It is highly reminiscent of a system used 20-30 years ago on the yagis
made by Antennes Tonna (F9FT). The coax exited vertically downward from
the feedpoint, and the instructions were to leave a quarter-wavelength
hanging loop before the coax came back up to the boom. There was no
electrical connection to the boom at this point - the coax was taped
tightly down, and then continued along the boom as normal.

The quarter-wavelength loop was about 19in for 144MHz; but for 432MHz it
should only have been one-third of that, which was an impossibly tight
bend for heavy low-loss coax.

No doubt this arrangement would *somewhat* increase the common-mode
impedance at the feedpoint, and thus act as a sort of balun, but it
never seemed like a properly engineered solution. Many people replaced
it with a home-made sleeve (bazooka) balun, and eventually the Tonna
company did the same (though again, only sort of).

I completely agree that the velocity factor for the common-mode current
on the outside of the coax will be different from the velocity factor
inside the cable. This applies to both single and paralleled cable. Once
again this makes a "quarter-wave" loop a less than satisfactory solution
for a balun.

50MHz is much easier. For that band I have built several yagis using
paralleled 75-ohm satellite TV coax to match a nominal 28-ohm feedpoint
to 50 ohms [1]. An electrical quarter-wavelength is about 1.0m so there
is no problem about coiling the paralleled coax to make a reasonably
effective choke balun (the problem with velocity factors does not arise
because the choking impedance is predominantly inductive). First tape
the two cables together along their entire length, and then coil them
into three turns.

However, the effect of any choke balun can be spoiled by the capacitance
between the coax shield and the boom, which appears in parallel with
only one side of the feedpoint. To minimise this effect, the entire coil
should be spaced away from the boom by a *much* wider insulating gap
than DK7ZB recommends. If I'm feeling cautious, I sometimes add a few
ferrite beads at the start of the main 50 ohm feedline.



[1] All credit to DK7ZB for making the point that it's much easier to
design high-performance yagis with a feedpoint impedance in the region
of 25-30ohms. He feels that increasing the feedpoint impedance to 50
ohms (eg by adding a closely-spaced first director) makes optimization
more difficult, especially when 28 ohms is such an easy impedance to
match. But that is yet another separate issue...


--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Owen Duffy wrote in
:

....
The construction at
http://www.mydarc.de/dk7zb/DK7ZB-Mat..._Isolation.jpg is quite
complex, containing what might be seen as as an electrical quarter
wave transformer and the balun formed of a U shaped stub of physical
length 0.5*0.25*0.66=0.08 wavelengths shunting one side of the DE, but
this stub lies with its hot side against the cold boom forming another
o/c TL element. If the balun shunts the DE with a high impedance, it
will be due to not just the loop length, nor just the s/c stub length,
but parallel resonance with the boom / half loop o/c stub.

I should have explained that looking at the reference picture, and
considering the insulation between cable and boom, I made the assumption
that the braid nearest the boom was connected to the DE, and other braid
end was effectively grounded.

It may actually be the other way round.

In any event, the physical construction is not IMHO well approximated by
the electrical equivalent shown, nor the description "This line is a
simplified coaxial sleeve balun to avoid sleeve-waves on the braid of the
cable running to the station". Making this balun adequately effective may
involve more than just cutting the line sections to the dimensions
indicated.

Owen.

Ian White GM3SEK wrote in news:6O1lIpHoLZzKFAM3
@ifwtech.co.uk:

....
[1] All credit to DK7ZB for making the point that it's much easier to
design high-performance yagis with a feedpoint impedance in the region
of 25-30ohms. He feels that increasing the feedpoint impedance to 50
ohms (eg by adding a closely-spaced first director) makes optimization
more difficult, ...

Absolutely, and designing an effective, efficient, reproduceable matching
arrangement is an essential part of system design.

Whilst I have reservations about the design pics etc referenced, the
fundamentals of the Yagi itself look good, and with an impoved matching /
balun element, it looks like a practical high performance antenna. They
are certainly becoming more popular in this part of the world where most
home brew designs are DL6WU based (for good reasons, but peformance isn't
the principle goal).

An improved design could persue a more reproduceable balun that had an
adequately high shunt impedance on one side of the DE, or better, a
symmetric balun, and the shunt impedance was tuned out as part of the DE
tuning adjustment. The latter, if done properly, should result in more
symmetric current distribution on the DE, and pattern closer to that
modelled.

On that point, I am often somewhat amused at the out of hand dismissal of
gamma matches because of pattern distortion, and recommendation of
asymmetric baluns in their place. Symmetric baluns are just not very
popular with hams on VHF and UHF.

Owen

David Ryeburn wrote in
:

....

It has to work well.


And, one is not restricted to two identical cables in parallel, it is
possible to use more than two cables, and they do not need to be of the
same Zo, but they must all be of the same electrical length for the simple
explanation to hold.

Owen