[TowerTalk] Coax, conduit and toroids

Steve Hunt steve at karinya.net
Thu Jun 28 00:14:04 PDT 2012


Take a look at the 1:1 Guanella current balun - it's identical to one of 
Jim's CM chokes. It achieves current balance by impeding common-mode 
currents. They are the same thing!

Steve G3TXQ

On 27/06/2012 22:46, K8RI wrote:
> Typical baluns of this type are transformers be they 1:1 or 4:1 or 
> what ever. They are capable of an impedance transformation, but Jim 
> said it much better than I and his tutorials are used world wide. I 
> believe you will find the choke is incapable of doing an impedance 
> transformation and serving as a choke at the same time. Roger (K8RI)
>> 73,
>> Steve G3TXQ
>> On 27/06/2012 19:40, K8RI wrote:
>>> In the case of a perfectly matched antenna the voltage on the inside
>>> of the shield is half the "differential mode" voltage, not common
>>> mode. Common mode voltage on the outside of the coax indicates the
>>> coax is also serving as an antenna. As Jim says, the antenna "system"
>>> can be very complicated particularly when unbalanced. It would be
>>> extremely rare to have an antenna that is perfectly balanced, so the
>>> choke at the feed point of the antenna to eliminate, "or reduce" the
>>> common mode voltage is a much better idea than the commonly used 1:1
>>> balun. This can also reduce received noise and in some cases present a
>>> much quieter band(s) Also there are some problems with the air core
>>> coil at the base of the tower. Air core coils are relatively narrow
>>> band with the optimum number of turns varying with the band in use.
>>> Even when resonant they do not present the resistance/isolation of a
>>> ferrite core choke to common mode voltages. I know they are expensive
>>> for good ones, but when you spend all that money on the rig, (amp?)
>>> coax, tower(s), and antennas, the extra for the chokes is a very small
>>> percent of the totals. I consider the ones at the antennas essential.
>>> The others are easily installed later if proven necessary. 73 Roger
>>> (K8RI)
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