Homebrew 6:1 balun?
Ken
that expects a 50 ohm unbalanced antenna. I, however, will be using
a resonant, balanced dipole and a 300 ohm twinlead feedline. And no
tuner. I believe I will need a 6:1 balun.
I see a diagram for a 4:1 voltage balun in Fig. 36 of Chapter 26 of
the ARRL Antenna Book. What makes this a 4:1 balun? Can it be
adapted to 6:1? Are there any diagrams for a 6:1 balun? Does anyone
sell a 6:1 balun for QRP backpacking? I see big ones on the web, but
no little ones. How come almost of the references to 6:1 and 9:1
baluns are in German? Is RF different in Germany?
Ken KC2JDY
Ken
(to reply via email
remove "zz" from address)
Ken
I found the old discussions on Google. 4:1 is good enough. But if
you really want 6:1 you have to cascade a 4:1 balun and a 1.5:1 unun
as per Sevick's book.
CAM
> I want to use a QRP CW transceiver (7.000-7.060) having an SO-239 jack
> that expects a 50 ohm unbalanced antenna. I, however, will be using
> a resonant, balanced dipole and a 300 ohm twinlead feedline. And no
> tuner. I believe I will need a 6:1 balun.
Nope, you won't. Make the twinlead 1/2WL on 40m (~63 ft) and the
impedance seen by the transmitter will be close to the same as the
antenna feedpoint impedance. Your SWR will be close to 1:1 requiring
no tuner and all you will need is a 1:1 balun/choke.
--
cheers, CAM http://www.qsl.net/w5dxp
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CAM
>
> Never mind.
>
> I found the old discussions on Google. 4:1 is good enough. But if
> you really want 6:1 you have to cascade a 4:1 balun and a 1.5:1 unun
> as per Sevick's book.
Ken, you don't need a 6:1 or 4:1 balun. You are confusing the
characteristic impedance of the twinlead with the impedance
that will be seen by the transmitter. When feeding a resonant
dipole with 300 ohm twinlead, the transmitter will NEVER see
300 ohms. If it was a resonant folded dipole, the transmitter
would see about 300 ohms and you would need a 6:1 balun. You're
not talking about a folded dipole are you?
Tarmo Tammaru
Assuming you are using a folded dipole, its actual impedance is closer to
250 rather than 300 ohms. A 4:1 balun will be just fine. If it is a
straight, rather than folded dipole, why bother with the 300 ohm twin lead?
Just use a 1:1 balun, or no balun and 50 or 75 ohm coax. If you use 75 ohm
coax, an SWR meter will say your WSR is 1.5:1.
That's because it is calibrated for 50 ohms. The actual SWR will be near
1:1.
With QRP you are not going to be greatly concerned about RF in the shack;
so, I would go with a conventional wire dipole, no balun, and RG59, RG8X, or
RG58.
Tam/WB2TT
"Ken" <cprst...@att.net> wrote in message
news:3d96eaf8...@news-server.optonline.net...
Reg Edwards
> Ken, you don't need a 6:1 or 4:1 balun. You are confusing the
> characteristic impedance of the twinlead with the impedance
> that will be seen by the transmitter.
==============================
Hello CAM,
- and from where does this and similar confusions arise ?
They arise from the ancient but still popular mis-conception the common or
garden SWR meter, in its usual location immediately between transmitter and
tuner, actually indicates an SWR anywhere on the transmitting site. Its
name is a horribly misleading misnomer. We are are now in the 21st Century.
The meter's function is better described as a "Magnitude of Reflection
Coefficient Relative to 50-ohms Indicator". But this implies all
transmitters require a 50-ohm load resistance - which they don't. This is
a far too long-winded title anyway.
Since it doesn't actually measure anything, on this Great
ever-to-be-remembered Day of 29th September 2002, let it be announced that
this undoubtedly indispensible instrument will henceforth be referred to as
the TLI - the Transmitter Loading Indicator.
Unless someone can think of a better name before tomorrow. ;o)
How about marking the indicator face with green and red bands - good and
bad ?
If you MUST have some numbers on it, mark the scale 0 to 9.
It can continue to serve the double purpose of TLI and S-meter (the other
indispensible indicator) for transceivers.
Equipment manufacturers and handbook editors please force yourselves to
consider.
---
Yours, Reg, G4FGQ
Richard Harrison
"---will be using a resonant, balanced dipole and a 300-ohm twinlead
feedline."
Ken also wrote: "What makes this (Fig 36) a 4:1 balun?"
This is a centertapped choke or a 2:1 turns ratio autotransformer. The
impedance ratio is the turns ratio squared or 4:1.
For a 6:1 impedance ratio to match 300 ohms to 50 ohms requires a turns
ratio of about 2.5:1.
The pictorial in Fig 26 shows 5 bifilar turns. About 12 turns would
produce 300 ohms impedance when using 5 turns for 50 ohms impedance. You
could tap a 12-turn coil on the core at 5 turns or wind separate coils
on the core. Separate coils are likely to improve balance but may have a
little more loss. In an autotransformer, primary and secondary currents
flow in opposite directions in the shared coil turns. Another coupling
problem is reduced permeability with core materials for higher
frequencies.
Were a folded dipole made osf the same 300-ohm twinlead used for the
feedline, by shorting the ends od a 1/2-wave section and breaking one
side in its middle to attach the feedline, the antenna could match the
feedline. For this to happen, the dipole would need to be erected about
50 feet above the earth (1/2-wave).
A single-wire dipole has the same radiation as a folded dipole. Its
drivepoint impedance is about 72 ohms in free space or at 1/2-wave
elevation when the usual wire is used for its construction.
It can often be fed directly from an unbalanced
50-ohm coax at the usual too-low height used by amateurs and a fair
match is obtained. If the radio has a good ground and the feedline is
around 1/4 or 3/4-wavelength, 25 or 75 feet in this case, a high
impedance is presented to the antenna by the outside of the coax and
radiation from the coax should not be a problem.
Best regards, Richard Harrison, KB5WZI
Richard Harrison
"Its drivepoint impedance is about 72 ohms in free space or at 1/2-wave
elevation when the usual wire is used for its construction."
I was perhaps remiss not to say that the lowest elevation to find free
space impedance in an ordinary dipole is about 1/4-wave. These are cloud
warmers.
Richard Harrison
"You could tap a 12-turn coil on the core at 5 turns---."
I would center-tap the 12-turn coil for a ground connection and place
the 50-ohm connection at one turn from either end of the coil.
End-to-end, the center-tapped coil would present about 300 ohms if
loaded with 50 ohms across 5 turns.
CAM
>
> I wrote:
> "You could tap a 12-turn coil on the core at 5 turns---."
>
> I would center-tap the 12-turn coil for a ground connection and place
> the 50-ohm connection at one turn from either end of the coil.
> End-to-end, the center-tapped coil would present about 300 ohms if
> loaded with 50 ohms across 5 turns.
Why isn't this solution used more in practice? Are transformers made
from air-core coils a lot lossier than 4:1 baluns? Are ordinary
transformers made using toroids a lot lossier than 4:1 baluns?
Richard Harrison
"Why isn`t this solution (transformer) used more in practice? Are
ordinary transformers using toroids a lot lossier than 4:1 baluns?"
My speculation is that an ordinary transformer is no lossier than a 4:1
balun. Loss has to come from the core material or the copper coils wound
around that core. Some losses are functions of frequency and some are
functions of power handled by the transformer and these should be about
the same regardless the transformer application. This was for a QRP
application so we won`t worry about core saturation. The turns per volt
if good enough for the baluns in the example should serve as well for a
slightly different ratio. We really haven`t increased current in the
primary.
The original questioner referred to Fig 36 on page 26-25 of the 19th
edition of the "ARRL Antenna Book". The difference in a 2:1 turns ratio
and a 2.5:1 turns ratio only means adding one turn on either side of the
center-tapped coil in figure 36. As 1/2 the coil is the 5-turn primary
in the balun, and 5 turns of the 6-turn coil on one side of the
center-tap is proposed as the primary for the 2.5:1 turns ratio
transformer, not much has changed.
Power flow should be the same. Copper and iron losses should be about
the same. A slight difference in losses results from a transformation
ratio other than 1:1 in an autotransformer, but at the levels involved,
I doubt that it`s significant. I haven`t seen many 1:1 autotransformers,
but that doesn`t mean they wouldn`t be useful.
I seem to recall an air core r-f transformer problem caused by coupling
decline in longer coils. This may be a cause for Q decline and the
reason an optimum solenoid is kept fairly short. Is it no longer than 2x
the diameter? I forget. But I believe it is a reason coils get link
coupled in their mid-sections instead of at their ends.
Coils are not my speciality. Better gualified people might comment on
the disuse of air core r-f transformers. The toroids have better
permeability and coupling and this must be the reason they are used in
baluns.