Jim,
I frequently talk to folk who question the utility of CM chokes at the
antenna feedpoint. I find that one or more of the following questions
usually helps them "see the light":
1. Would you intentionally connect an additional, random length, antenna
wire to one side of your dipole and ground the other end?
2. Might you expect Tx RFI problems if the other end of that additional
antenna wire comes close to your TV/Computer/'Phone, and is connected to
a common ground?
3. What would you expect to happen to Rx noise levels if the other end
of that additional antenna wire comes close to your TV/Computer/'Phone,
and is connected to a common ground?
4. What would happen to the radiation pattern of a beam if you connected
an additional wire to one side of the feedpoint, made the wire a
multiple of a half-wave long, and connected the other end to a good ground?
It only requires the realisation that the outer surface of the coax
braid can - and does - act as an "unintentional" antenna wire to see the
wisdom of using a choke.
Have a happy and peaceful 2012,
Steve G3TXQ
On 02/01/2012 08:59, Jim Brown wrote:
> On 1/2/2012 12:32 AM, Steve Hunt wrote:
>> All that I've ever measured lie in the range 0.95 to 0.99.
>>
>> Did you notice that I said*COMMON-MODE* velocity factor? Signals
>> flowing on the outside of the braid are Common-Mode signals - signals on
>> the inside are Differential-Mode; they can have totally different
>> velocity factors.
> Yes, on all counts. A common mode signal can be thought of as using the
> coax braid as a long wire antenna. Thus, the common mode VF for coax is
> simply that which corresponds to an insulated wire having the diameter
> of the shield. It is well known that using a fatter conductor moves the
> resonant frequency down a few percent as compared to a small diameter
> conductor, and so does adding insulation. How much it moves depends on
> the diameter of the wire, the thickness of the insulation (the jacket),
> and the dielectric constant of the insulation (the jacket). A thin wire
> with no insulation would be close to 1.0 Vf.
>
> 73, Jim K9YC
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