[TowerTalk] Coax, conduit and toroids

Steve Hunt steve at karinya.net
Wed Jun 27 12:03:13 PDT 2012


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.
2) Would you like to explain to me the difference between a 1:1 Guanella 
current balun and a common-mode choke?

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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