Thanks for all Steve...
On 12/31/2011 3:11 PM, Steve Hunt wrote:
> If you happen to have a coax feedline which is an
> electrical half-wave long (or multiples), and which is well grounded at the
> shack end, the effect would be much much greater.
The total length of feed line to the rig is about 85 feet. It's grounded
at 70 feet where it then goes into the shack using a 15 foot jumper.
Which do you take into account as being a multiple of half-wave? The
total length of the coax or the length between the antenna and ground?
Tony
> Tony,
>
> I made some azimuth plot measurements on my hexbeam with various CM
> choke arrangements, including "none":
> http://www.karinya.net/g3txq/hexbeam/common_mode/
>
> The most sensitive indicator of CM issues was the shape of the rearward
> lobe. Although the effects on that web page look small, I should
> emphasise that they will vary massively depending on the impedance of
> the conducted CM path. If you happen to have a coax feedline which is an
> electrical half-wave long (or multiples), and which is well grounded at
> the shack end, the effect would be much much greater.
>
> On the other hand, the requirements on choke impedance in this
> application are not so demanding. The impedance looking into one half of
> the beam is just a few tens of Ohms, so for a choke to be effective we
> don't need the several thousand Ohms of CM impedance that we would in
> some other applications. Take a look at the table at the bottom of this
> page:
> http://www.karinya.net/g3txq/chokes/
>
> You'll see that with the worst-case feedline length, a choke resistance
> of just above 1kOhm was sufficient to reduce the CM current to 30dB less
> than that in a half-wave dipole leg; you'll also see that for some other
> feedline lengths, no choke at all is needed to achieve the same result.
> Finally, the same table also illustrates why reactive choke impedance is
> undesirable and can actually *increase* CM current with some feedline
> lengths.
>
> 73,
> Steve G3TXQ
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