[TowerTalk] TA-33 feed

[Eric Gustafson]

>Date: 30 Jul 1998 00:04:26 -0000
>From: w7ni@teleport.com (Stan Griffiths)
>

Snip...

>>
>>	That would only be true if your feedline has VERY poor
>>	shielding.  This is true for ladderline in the near
>>	field.  Ideal COAXIAL cable keeps all of the field
>>	between the center conductor and the INSIDE of the braid.
>>	Only a small amount of leakage should radiate from the
>>	line.  (BTW, I was only thinking of a neighbor's
>>	TVI....when I am operating, MY TV is OFF :-) N4KG
>
>I agree that is true for a normal matched coax application.  I
>don't really know what happens when the VSWR is high.  I am not
>sure the field is really confined to the dielectric between the
>shield and center conductor in that case.  (I'm not sure it's
>not either, for that matter!)
>

For the purpose of discussing fields large enough to be a TVI
problem, fields which leak through the shielding of a coaxial
system are insignificant pretty much regardless of how lousy the
coax is.  Even Tandy RG-58 "type" line is good enough to meet
this criteria.

Radiation from the shield of coaxial feedline is due to antenna
currents flowing on the outside of the feedline.  Not due to
transmission line currents flowing inside the feedline.

Once RF current is flowing on the outside surface of the
transmission line shield, the coupling mechanism for TVI is not
limited to radiation.  Since these RF currents are also flowing
on the cabinet of the transmitter, they are easily conducted by
the TX power cord into the house wiring system that the TV is
also plugged into.  So, the TVI can be due to either conducted
EMI, radiated EMI, or both.

Any nonlinear devices connected to the same house power system as
the transmitter can cause harmonic energy to be generated outside
any filtering in place between the transmitter and the antenna.
A lot of modern appliances (including the TV itself) place
nonlinear elements on the power line with minimal or no RF
bypassing.  These harmonics, once generated, can be radiated by
your house wiring and interfere with your neighbor's TV
reception.  The offending appliances don't even have to be turned
on at the time to create a problem.  Your TV could possibly be
causing the neighbor's TVI even when your TV is not in use.

However, even though there are several legitimate mechanisms
whereby radiation from the transmission line can cause observable
TVI, none have anything at all to do with the SWR on the line.
If the line is properly decoupled from the antenna so that no RF
current is flowing on the outside of the shield, then the line
won't be radiating (or conducting RF into the house power system)
regardless of the degree of mismatch between the antenna and the
transmission line.




Snip...


>>	SWR at the antenna WILL cause higher currents to flow in
>>	the feedline and so will tend to exacerbate whatever
>>	leakage there is from the coax.

Possibly, but they will STILL be insignificant.




>>	I don't know the attenuation of a signal that leaks from
>>	a cable vs. what is radiated from the antenna, but I
>>	expect it is a LOT.  I would expect poor cable might be
>>	as bad as 10 dB down and good cable (like hardline) could
>>	reach 50 dB or more.  N4KG

Actually, it is more like 40 to 50 dB for really crummy coax and
in excess of 100 dB for really good line and very nearly
unmeasurably high for hard line with high quality, correctly
applied connectors.

>
>Yep, I agree, but all this assumes there is no radiation from
>the outside of the coaxial shield.  I'm out of my league here
>since I don't really know what radiation really occurs from coax
>with high VSWR.
>
>Stan  w7ni@teleport.com
>

As I said above, the radiation from the outside of the coax is
independent of the SWR on the line.  It depends only on the
degree of decoupling of the coax outer shield from the antenna
system's radiation currents.

73, Eric  N7CL

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