At 8:18 PM -0300 9/24/02, Carlos Augusto S. Pereira wrote:
>...the only place I have to bring the coax and rotor cables from the
>roof to the second floor (where is located my apartment) is the same
>place where the 75 ohms antenna tv cable passes. Does anyone have
>experienced the same situation? TVI will happen for sure?
1. It is important to put good common-mode-current RF chokes on both
your coax and your rotor cables. First, right at your Yagi feedpoint
put an air-wound choke coil in your coax, by winding six turns of the
coax closely, in a single layer (not bunched or overlapped), on a
4-inch-diameter form such as PVC drain pipe. This choke will have
low power-dissipation but will not have enough choking impedance by
itself. Then you should have two good ferrite-core chokes, spaced at
intervals of about 2.5 meters (a quarter-wave at 10 m). A good
ferrite choke can be made by winding about 12 turns of your cable on
an Amidon type FT-240-43 or FT-240-77 toroidal core. ("FT" means
ferrite toroid. "240" means outside diameter = 2.4 inches, inside
diameter = 2.0 inches, and thickness about 0.4 inches IIRC. "43"
means type 43 ferrite, which has mu/mu0 of about 850 and is fairly
lossy at HF. Palomar is another source of such toroids. They're
made by Fair-Rite. I'm sure there are other sources outside the US.)
A toroidal-core choke like this has an inductance of around 35
microH, IIRC. Ferrite "beads" slipped or snapped over a cable are
_not_ good enough at HF. They don't have nearly enough impedance.
The chokes on your coax should be co-located with the chokes on your
rotor cable, so that one cable cannot carry RF current past the choke
on the other cable.
2. In addition, put good ferrite common-mode-current chokes on the
TV-antenna cable(s). If the TV cable(s) run(s) parallel to your
ham-antenna cable, then the common-mode-current chokes on the
TV-antenna cable(s) should be co-located with the chokes on your
rotor and ham-coax cables, so that one cable cannot carry RF current
past the choke on another cable.
3. If a TV antenna is near your Yagi, you should also insert a
high-pass filter in its feedline, to keep HF RF out of the
preamp/distribution amplifier.
5. If the TV preamp or distribution amplifier on your roof has a
separate cable for power, then put a ferrite choke on this pawer
cable, too. Same for the IF and power cables of any satellite-TV
downconverter.
6. Put a good low-pass filter at the output of your HF TX or xcvr.
If you do all these things you should have no trouble with TVI. I
transmit 1.5 kW on all HF bands, 80 through 10 meters (except 30 m);
the coaxial feedline of my ham antenna runs right by my TV antenna
and very close to its coaxial feedline, yet I have no TVI on any
channel. In my case I also inserted a second high-pass filter at the
downstairs end of the TV cable, and I also inserted common-mode
chokes at the downstairs ends of all cables. This may have been
overkill, but chokes are cheap and you may as well do all of them at
once.
An important other benefit of common-mode chokes is that they stop
RFI from the electric power wiring inside your building, and from the
sweep circuits of the TV sets in your building, from being conducted
up to your ham antenna. This makes a huge difference on the
lower-frequency ham bands.
73 de Chuck, W1HIS
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