On 8/6/2010 12:02 PM, Jim Brown K9YC wrote:
> On Fri, 06 Aug 2010 11:48:22 -0500, Dr. Gerald N. Johnson wrote:
>
>> The purpose of these "baluns" is to attenuate any RF current on the
>> outside of the shield. That helps cut feed line radiation when the
> coax
>> is hooked to an antenna. Sometimes that helps cut TVI and received
>> interference from local sources. Sometimes that hurts working
> stations
>> that need the vertically polarized component from feedline radiation.
>
> First, let's call these things by their real name so that we
> understand what they do. They are COMMON MODE CHOKES, NOT BALUNS.
When at the antenna, they do decouple the shield outside current and so
act as a 1:1 balun the same as a quarter wave long decoupling sleeve or
stub, common parts of balun construction. These balun constructions do
not force a balance, but allow the antenna to set the balance.
>
> As Jerry notes, the function of a common mode choke is to kill current
> on the feedline -- that is, to prevent the feedline from acting as
> part of the antenna. That's good for at least four reasons. First, it
> preserves the directional properties of the antenna. Second, it
> prevents the feedline from receiving noise and coupling it to the
> antenna. Third, it prevents the feedline from radiating RF into your
> living room (and your neighbor's living room). Fourth, it prevents the
> feedline from coupling RF into your shack.
>
> Also as Jerry notes, there is no benefit from using a choke on jumpers
> in the shack. The ideal place for a common mode choke is at the
> feedpoint of the antenna (that is, up in the air where the coax
> connects to the antenna). Chokes are also useful as "egg insulators"
> to break up feedlines into non-resonant lengths (like an egg insulator
> in a guy wire) so that they can not act as parasitic elements to other
> antennas (like a vertical nearby).
>
> Strings of beads are next to useless at HF frequencies, both because
> their impedance is far too low to be effective, and because they are
> inductive, not resistive at HF. A common mode choke must be resistive
> so that it cannot resonate with the line. If it resonates, it is
> useless. A string of beads IS quite useful at VHF and UHF.
That depends a great deal on the ferrite mix. There are beads suitable
for HF and beads suitable for VHF and up, generally when used for this
purpose they are chosen to be lossy (e.g. resistive) at the operating
frequency which is considerably higher than the frequency where they'd
be used as conventionally wound transformer cores or there are many used
to get a large core cross section with only one turn.
>
> There is a lot of detail and a tutorial discussion about this in
>
> http://audiosystemsgroup.com/RFI-Ham.pdf
>
> 73, Jim Brown K9YC
>
73, Jerry, K0CQ
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|