[TowerTalk] Coax, conduit and toroids
K8RI-on-TowerTalk at tm.net
Wed Jun 27 14:46:46 PDT 2012
On 6/27/2012 3:03 PM, Steve Hunt wrote:
> I challenge a couple of those comments:
> 1) The voltage on the inside of the shield wrt ground is half the
> differential mode load voltage when the antenna is BALANCED, not when it
> is MATCHED as you said.
I should have said and meant to say balanced, you are correct although
it is the system that must be balanced
> 2) Would you like to explain to me the difference between a 1:1 Guanella
> current balun and a common-mode choke?
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.
> 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
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