[RFI] Common Mode Noise?

[Jim Brown]

Again, thanks!  I remember reading about that military project, forget 
where. Great example!

The first times I encountered open wire line were either on a tour of 
the Crosley VOA site by our senior EE class (U of Cincy), or maybe at AM 
broadcast stations, where it is widely used and coaxial to feed 
verticals. I've also seen that coaxial open wire line at commercial HF 
communications RX and TX sites north of San Francisco.

73, Jim K9YC

On 1/22/2025 1:54 PM, David Eckhardt wrote:
> Jim, far too many hams solidly believe open wire, parallel conductor 
> XMSN lines, radiates just because it isn't shielded.  Absolutely wrong 
> if care is taken to keep the currents balanced and the phases opposite. 
> They simply do not understand nor want to take the time to understand 
> the rank basics of a transmission line, coax or open wire, or for that 
> matter, G-line.
> 
> Nor do they understand that ANY amateur antenna like a dipole is 
> UNBALANCED.  Period!  Even Uncle most times can not assure that.  As you 
> commented, the local environment dictates unbalance between the two 
> sides of a dipole, unless it's in free space.....  How many of us 
> absolutely assure homogeneity for a couple of lambda below our 
> antennas?  I'll bet none of us hams do, myself included.  But I know how 
> to build, measure, and install common mode chokes and use them 
> intelligently.  Further, I have never, except for the first three or so 
> years I was licensed, "grounded" (earthed) my station.  Three kinds of 
> earth connection:  1)  safety (the "green" wire), 2) ESD bleed dump, and 
> 3) lightning remediation (not direct strikes, but for radiated fields 
> from near-by strikes).  Also, realize the typical amateur budget does 
> not allow for protection against direct strikes!
> 
> When Uncle installed the ported coax around the fissile material storage 
> facility on Kirtland AFB in Albuquerque, N.M., he spent millions and 
> millions of your and my tax dollars in preparing the surrounding soil.  
> Even then it wasn't perfect and quite problematic.
> 
> Yes, I use parallel conductor XMSN line for my HF wires (doublet), 
> exclusively, except for my vertical which is by design, unbalanced.  
> Hams don't get that, either!  How many times have I read the question on 
> a number of sites, "Do I need a balun or common mode choke on my 
> vertical"?   DDaaaaahhhhh......
> 
> Dave - WØLEV
> 
> On Wed, Jan 22, 2025 at 7:35 PM Jim Brown <jim at audiosystemsgroup.com 
> <mailto:jim at audiosystemsgroup.com>> wrote:
> 
>     On 1/22/2025 10:43 AM, K9MA wrote:
>      > The whole point of using coax is to prevent radiation until the
>     RF gets
>      > to the antenna. There's no need to further complicate the issue.
> 
>     There is no more or less radiation from a 2-wire line than from coax.
>     What causes radiation is common mode current, and 2-wire can carry just
>     as much common mode as coax of the total antenna system is unbalanced.
>     Just as in coax, common mode current in 2-wire line shows up as a
>     difference between current in the two wires, taking both magnitude and
>     phase into account.
> 
>     Antennas can be unbalanced by their surroundings, like variable height,
>     sloping ground, nearby conductive objects, imbalance in the termination
>     of 2-wire line in the shack, including a poorly balanced tuner or
>     matching network.
> 
>     73, Jim K9YC
> 
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> 
> -- 
> *Dave - WØLEV
> *
> 
>