Rich,
Those are very nice numbers, and I'm sure they are based on
some factual information in a certain test. Now here it
comes!!
DB is a ratio. To actually use numbers that are expressed in
dB, you have to know what it is a ratio between and what
the measurement describes. Without a lot more information
than simple "dB amounts", they are just meaningless numbers.
Let me give you a few examples:
1.) All of the VHF repeater stuff here uses UHF connectors.
The undesired signals are limited by the depth of notch in
the three cavities in each line at -120dB, rather than by
cables (RG-214) or connectors!
2.) I can run 30 ft of parallel very poor quality Radio
Shack cables (maybe 30-50% shield coverage or whatever it
is) to a dummy load and a power level meter, and the
crosstalk is negligible at HF. Radiation from antennas 500ft
away would be worse than the field a few inches from the
cables.
3.) My station is wired with single shield cable, UHF
connectors, and has open shields (groundplane style one-side
shielding) in switching systems. The preamps and switching
relays aren't fully "boxed", but they are close to a
groundplane. Despite that and the liberal use of phono
plugs, the coupling between antennas a thousand feet apart
is much greater than crosstalk in boxes, relays, and wiring.
4.) I measure significantly more radiation from common mode
currents on feedlines of my VHF vertical antennas than I do
egress through shield and from UHF style connectors.
5.) Using lab grade network analyzers I can measure filters
all day, and I almost never see a case where connectors or
cables limit measurements.
One thing being better than another is nice, but something
can be a thousand times better and still the effect is
negligible. Without knowing how something is measured and
what it actually means, any ratio answer is useless.
73, Tom W8JI
> Isolation of typical braided shield coax (in the HF
range): 70 to 90 dB
> (varies with construction and type of braid)
-70 to -90 below what and at what distance and cable length?
> Isolation of commercial shielding "boxes" or enclosures:
100+ dB, except
> for any penetrations
100dB or more below what, and at what distance?
> All interconnects BETWEEN the filters and receivers done
using hard
> line and nothing but Type-N connectors. (Use regular coax
between the
> relays and the filters, and between the relays and the
antennas. Again,
> the antenna isolation is the lowest denominator, so put
the bucks where
> they are needed.)
> Isolation of hard line: 100 to 120+ dB (except for
leakage at connectors)
Below what and at what distance?
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