I been puzzled by the most frequent and most common uses of the 4:1 balun.
It seems that hams believe necessary to use a balanced feed system, open
wire line and such that they must employee a 4:1 balun. Most oversight is
that we are not matching the line but matching the load. The transmission
line has the task of transferring the energy from the source to the load.
However at certain wavelengths the transmission line can serve as some form
of impedance transformation network.
The best example where I see correctly used a 4:1 balun is for feeding a
folded dipole. In this case, depending on height above ground, a HF folded
dipole presents about a 200 to 275 ohm load. Thus a 4:1 balun at the end of
the feedline presents a 50 to 70 ohm load to the transmitter. The SWR on
the line, being 300 ohm line is very close to ~1.2:1 and for a 450 ohm line
very close to ~2:1 and for a 600 ohm line about ~3:1.
A horizontal 1/2 wave dipole at about 0.25 {1/4}wavelength above average
earth will have a feed impedance of 75 ohms while with it a bit higher near
0.34 {1/3}wavelength the center Z rises to near 100 ohms. Closer to ground
at about 0.125 {1/8} wavelength the Z drops to about 30 ohms. The effect of
height on the radiation resistance of a horizontal 1/2 wave antenna is not
drastic so long as the height of the antenna is greater the 0.2 wavelength.
Do keep in mind that a 4:1 balun is acting much like a transformer and the
above Z values will be thusly divided by 4. Many tuners become quite lossy
at impedances near 5 to 10 ohms which could be the Z presented to the tuner
after the 4:1 balun. i.e. 30 ohms / 4 = 7.5 ohms
And yes a balun can have a ratio of 1:1 thus balanced on side and unbalanced
on the other. Or a 4:1 balun can be balanced on one side and unbalanced on
the other in this case, using a 50 ohm source and a 4:1 balun we could have
200 ohm on the {4} side and 50 ohms on the {1} or working as a 1:4 balun 50
ohms on {4} side and 12.5 ohms on the {1} side. A balun doesn't care which
is input and which is output.
I have encountered antenna conditions where a 4:1 ratio was needed on one
band and a 1:4 ratio needed on another band. The solution was simple, just
turn the balun around.
73
Bob, K4TAX
----- Original Message -----
From: "Augie "Gus" Hansen" <augie.hansen@comcast.net>
To: <tentec@contesting.com>
Sent: Saturday, September 13, 2014 8:12 PM
Subject: Re: [TenTec] Zepp?
On 9/13/2014 3:38 PM, Rick - DJ0IP / NJ0IP wrote:
...
Final note: is it 4:1 or 1:4? Gosh I don't know.
Sevick defined it one way, but the vendors selling these devices call it
the
other way. Let's forget the semantics and specify exactly what we are
talking about: the 50 Ohm side is towards the TX and the 200 Ohm side is
towards the antenna.
BALanced-to-UNbalanced
200-to-50
therefore, in this case, 4:1 balun in anything I write down or say about
the device.
Cheers,
Gus Hansen
KB0YH
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