[Elecraft] Corrected Elecraft BL1 balun description $ price]

[George, W5YR]

Hi, Bob

Well, thanks for the detailed response, but I fear we remain in
disagreement - hi!

The point of my concern is that you are describing the performance and
operation of this BL-1 as if it were a conventional transformer in which the
primary/secondary turns ratio determines the impedance ratio. (I am assuming
that you are speaking of a true balun, however.)

As you know, a transmission-line transformer does not operate like that.
Turns ratio has no meaning in the balun world.

Each "winding" is a short transmission line operating in transmission-line
mode. The number of turns the line makes around a core is determined by the
impedance to be presented in the common-mode operation of the unit, not a
desired impedance transformation.

The conventional 4:1 current balun used today comprises two 100-ohm Zo
transmission-lines wound around a common ferrite core with the line inputs
connected in parallel to present a 50 ohm input impedance and the outputs
connected in series for a 200 ohm load.

Such devices operate with a 4:1 transformation *only* when loaded with a 200
ohm resistive load. Then the input looks like 50 ohms resistive.

However, try placing a 300 ohm resistor on the output and measure the input
impedance . . . don't expect to see 75 ohms, as the unit is NOT a 4:1
impedance transformer.

I use a number of non-resonant antennas fed with ladderline and several
tuners in the shack. Measurements and calculations of the input impedance of
the ladderline and of the coax to which it transitions via a W2DU bead-balun
rarely show an impedance anywhere near 200 ohms on any band. Impedances with
real parts as low as 10 ohms are common, however. Obviously, a 4:1 balun
would result in an unworkable load.

I trust that this honest criticism of the design as described is taken in
the spirit in which it is offered. I have a K2 and dearly love it. I have no
brief against Elecraft products. However, in this case, I hope that Joe Ham
is well served by this design.

Bob, I have found that the K2 has the capability to match a remarkably large
set of impedances. However, I have seen instances in the field where a 4:1
balun would permit a match with a K2 when a 1:1 would not, but here there is
no indication of how much of the output power is being passed to the antenna
and how much remains with the tuner. Again, when a 4:1 is operated with
arbitrary impedance loads, there is no telling what the input impedance
looks like and that the K2 is trying to match.

Finally, if the circuit is rewired as a 1:1 balun is it a current balun or a
voltage balun?

73/72, George
Amateur Radio W5YR -  the Yellow Rose of Texas
Fairview, TX 30 mi NE of Dallas in Collin county EM13QE
"Starting the 58th year and it just keeps getting better!"
w5yr@att.net




----- Original Message ----- 
From: "Robert Friess" <rfriess@comcast.net>
To: <w5yr@att.net>; <elecraft@mailman.qth.net>
Cc: "Eric Swartz" <eric@elecraft.com>
Sent: Wednesday, October 15, 2003 3:57 PM
Subject: Re: Re: [Elecraft] Corrected Elecraft BL1 balun description $
price]


> Hi George,
>
> While it is certainly true that the probability of finding exactly 200
ohms,
> 50 ohms or any other fixed impedance at the end of a unmatched
transmission
> line is low, the BL1, like all low loss transformers, depends on a
> particular impedance presented to the secondary in order to produce 50
ohms
> at the primary.   Termininating the BL1 in 200 ohms allows us to measure
the
> performance of the transformer.  The BL1 frequency response when
terminated
> in 200 ohms is much wider than most baluns and should allow reasonable
> frequency
> response and balance over a wider range of load impedances than is usually
> found.
>
> We decided that there were many cases where matching to a higher impedance
> balanced line would be desirable.  Folded dipoles and other resonant loop
> antennas are examples.  Furthermore, most antenna tuners have limited
> matching capability, so even in the unmatched case, a step-up to higher
> impedance may well extend the capability of an antenna tuner to match an
> arbitrary load.
>
> That said, it is a simple matter to rewire the secondary connections of
the
> BL1 to 1:1 if that is your preference.
>
> 73,
> Bob, N6CM