[Elecraft] Additional Information about the Elecraft BL1 Balun

[Robert Friess]

Several subscribers have asked for additional technical information
regarding the Elecraft BL1 balun.  The purpose of this posting is to the
address the questions that have been asked.



The BL1 is a Guanella design often described as a "current balun".  It is
configured as a 4:1 balun designed around a 200-ohm load impedance.  When
terminated in 200 ohms it is a very wide bandwidth design providing a SWR of
less than 1.2:1 from 500 KHz to 55 MHz.  The BL1 continues to perform as a
4:1 transformer over a wide range of load impedances with only a gradual
reduction in bandwidth for deviations of 3 to 1 in load impedance.  Outside
of that range and depending on frequency, the transformation ratio deviates
from 4:1 but remains useful.



The most important factor contributing to this performance is the use of the
particular binocular core chosen for the BL1.  The core is constructed of
type 43 material which has a permeability of 850.  Only three turns on each
side of the balun are required to achieve the low-frequency response
characteristic.  Approximately eight inches of cable are required for the
three-turn winding.  The short length of this transmission line is
responsible for the high frequency response of the BL1.  The use of high
permeability material like type 43 has not been common practice except in
the case of so-called "bead baluns" which are in reality another realization
of a 1:1 Guanella balun.  I am not sure why type 43 has not been more
frequently used although some authors have referred to type 43 material as
"lossy".  In a well designed balun, there is very little flux in the core
and the loss characteristics of the BL1 is very good.  An insertion loss
measurement was made using a HP8505 Network Analyzer.  For this test two BL1
baluns were connected back-to-back and the loss measured between the 50 ohm
connectors.  The measured loss was then divided in two to obtain the loss of
each balun.  The baluns were swept from 500 KHz to 60 MHz. The insertion
loss was almost flat from a mid-band loss of about 0.12 dB reaching 0.25 dB
at each frequency extreme.  The measured insertion loss converts to an
efficiency of about 97% at mid-band, which includes all losses including the
core, the cable, the connectors, reflection loss, etc.  It is clear from
this measurement that the core losses, which probably account for no more
than half of the total, are very low.



The cable used for the BL1 has a characteristic impedance of close to 100
ohms.  This is the ideal value for a 4:1 balun.  The plastic sheathing of
this cable is very useful in controlling the twisted pair transmission line
and protecting it from distortion when the winding the transformer.  The
plastic sheathing also plays a secondary role in improving the insulation
between turns and between the winding and the core as added protection
against arcing.  This is a minor factor, however, as the potential
differences at the rated power levels are quite low.



George, W5YR, questioned the choice of a 4:1 configuration in favor of a 1:1
configuration.  He pointed out that several authors have favored the 1:1
configuration and that it is the preference of many amateurs.  I believe
that it is equally unlikely that a non-resonant antenna with a random length
transmission line will present 50 or 200 ohms at the feedpoint so there is
little reason to choose either configuration for that reason.  It is,
therefore, useful to consider other factors.  The primary factor in making
the choice for the 4:1 configuration was the ability of a 4:1 transformer to
extend the matching range of an antenna tuner beyond the range of impedances
that it was designed to operate over.  However, I also believe that some
users may have a clear preference for the 1:1 configuration.  The BL1 is
easily re-configured as a 1:1 balun by connecting the load-side windings in
parallel rather than in series.  Information on this alternate configuration
will soon be posted on the Elecraft website.



I want to thank George, W5YR, for his ideas and thoughts regarding the BL1.
His input has been very useful in identifying the need for additional
information.  Subscribers with other questions welcome to send them to me
and I will do my best to answer.



73,

Bob, N6CM