[mythtv-users] Coax splitters - how painful are they?

Brian Wood beww at beww.org
Tue Sep 25 19:38:43 UTC 2007

Jay R. Ashworth wrote:
> On Tue, Sep 25, 2007 at 08:01:41AM -0600, Brian Wood wrote:
>> A "perfect" two-way splitter would have 3db. loss. Since nothing is
>> perfect in this world, most splitters actually lose 3.5db. per leg for a
>> two-way split, and 7db. for a four-way.
> The Ideal 1GHz 4-port I just installed says 7.4db.

I've seen units that measured out at 7.0db, but they topped out at
around 300 mhz.

Such devices tend to have more loss at higher frequencies, so they
usually just throw away signal at the lower frequencies in order to meet
the "flatness" spec.

>> Note that a dual tuner has to have a splitting device in it to feed both
>> tuners, so you are really asking whether it is better to have the
>> splitter inside or outside your computer case. Since the splitter on the
>> dual-tuner card was probably chosen to be as small and cheap as
>> possible, it's probably better to use an external split.
>> (I think some dual-tuner cards have what they call an LNA (low-noise
>> amplifier) to make up the split loss, I suspect that the term
>> "low-noise" is used pretty loosely in this case, a true LNA would cost
>> more than the card.
> Yeah, I know; right?

'LNA" is a term usually associated with satellite receiver amplifiers
(or LNB (low-noise block converter) these days. Even they have got
pretty cheap for home receivers, they do this by cheaping out on the
flatness spec., also a converter does not have to be as well designed
since the output is not at the same frequency as the input, when it is
the same you have to be careful about feedback in such a high-gain device.

A true LNA for satellite use, with good flatness and with the noise
figure flat across the band, still costs a LOT from an outfit like
Watkins Johnson or Hughes.

These are all GasFET devices today. I remember the early days of
satellite receivers when we used cryogenically-cooled parameteric
amplifiers. Guess I'm getting old :-)


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