[mythtv-users] Semi-OT: OTA, Comcast business HSI and the single coax

mw meatwad2021 at gmail.com
Fri Jul 25 20:16:32 UTC 2008

Allen Edwards wrote:


> I think he has this solved with the addition of a coax line but to be
> clear on what I was saying it was to filter out the cable co's main
> bands leaving only the low and high bands.  If the cable co's don't do
> it, he would need to.  However, if they are sufficiently attenuated,
> it might still be OK as the other sources of isolation will take care
> of it.

I caught up to the OP's last post and see that he has resigned to the 
additional run. However, I still have issue with mucking about with the 
cableco's plant by notching and combining. In a perfect world, this 
makes sense but we have an infrastructure which is far less than perfect 
and that cable wiring inside your home is connected back to the cable 
company. Whatever messiness makes it into the inside wiring will make 
it's way to the outside plant which may even include the aerial strung 
along the poles.

> On the inside wiring being a radiator, this should not be a problem as
> these were installed by the cable co and the levels of the non OTA
> 100-400 MHz signals are going to be less than they would have been had
> he had cable TV.

This is a dangerous assumption. Most wiring found /inside/ the home is 
done by the homeowner. The cableco's have a legacy of staying outside 
the home and punching through to where the outlet is, aka "giftwrapping" 
the home much like the OP describes. Now imagine all those older, 
ultraviolet soaked cables which are unlikely to have modern connectors, 
wrapped around the outside of a house. Not only does this configuration 
begin to resemble a LF/HF antenna, these are ripe conditions for leakage 
AND injection.

> Cable co's use (our should) RG-6U, which won't radiate.  The concern
> is the radiation out the antenna.  For that, the signal has three
> sources of isolation before it could get to the antenna.
> If perfect any one of these would prevent the signal from reaching the
> antenna 1) The filter on the cable signal.  2) The splitter isolation
> between the inputs 3) the reverse isolation of the amplifier.  They
> are not ideal, which is why I suggested that they all be there.
> If you want more isolation, use two amplifiers with an attenuator
> between them.  The OTA signal will be amplified, attenuated and
> amplified.  The Cable signal will be attenuated by the second
> amplifier, attenuated by the attenuator, and attenuated by the first
> amplifier.  I really don't  think this would be necessary, however.
> Also, the RFI example Brian gave was a clock signal, which is huge
> compared to these low level cable signals.

Regardless, the delay devices Brian mentions were not even part of the 
transmission chain, rather production. This means that RF leaked out of 
the unit and then picked up somewhere in the transmission side. The 
point is that even a device that works as designed for it's intended 
purpose (say, a cheap, imported OTA amplifier) can cause chaos depending 
upon it's application.

> You made a good point about coax being a radiator.  Cheap Radio Shack
> coax might as well be an antenna.  Don't use it.  If you have cable
> and use this kind of coax, one problem you will have it that the OTA
> signals will get into the coax and mess up the cable channels.  I only
> use RG-6U and professional grade connectors.
> Anyway, I just wanted to make clear that I am not proposing something
> that is going to cause RFI.

All it takes is one lousy (lossy) termination outside the premises and 
you'll likely end up with an excellent radiator illustrating yet again 
the dramatic difference between the ideal and real worlds. Sadly, even 
company cable techs (as opposed to the contractors) are less than 
diligent in this regard, especially in cold, wet environments. If you 
happen upon a pedestal that is left open or use binoculars to look at 
the aerial cable plant in your area, you'll see what I mean.

My personal feeling is that the cable plant should remain the cable 
plant and other signals should find another path into the system in 
question. As you add more and more digital devices as loads within the 
home (ATSC tuners, cable boxes, the cable modem, etc.) the effects of 
poor cabling become really apparent, really quickly.

Ideally, we would replace all of the wiring with RG-6QS using 
compression connections designed specifically for that manufacturer's 
cable. We would have it enter the home at one and only one location 
where it would have a proper ground (ideally, tied to the ground rod at 
the AC mains meter/panel, otherwise to it's own ground rod.) We'd pass 
it through a directional coupler (DC) using the drop tap for the cable 
modem and then use a splitter(s) for other runs. Possibly add a proper 
amplifier between the DC's pass-through tap and the splitters if you 
plan on more than four additional runs. Finally, we'd run each 
additional drop from the splitter via the basement/crawl/attic, fishing 
through the walls to get to the various outlets. For most homeowners, 
this would require bringing in a low-volt contractor and it sounds like 
this is outside of the OP's budget.

[Hit DELETE now to avoid hearing one of those "I remember when..." 

A perfect example of this is the 14th Street bank building in downtown 
Boulder, CO. In the eighties, the city placed a moratorium on rooftop 
antennae and grandfathered that building. It was a congested mess. 
Paging, trunked mobile, private point-to-point - you name it, it's up 
there and it was all rusted to hell, masts at strange angles, elements 
busted off or pointeing where they shouldn't.

We'd done a great deal of PCS, MMDS and LMDS testing there in the 
nineties and had to avoid downtown like the plague. After years of 
appealing to the FCC to get the various owners of the many antennae to 
repair their installs, the problems persisted and even worsened when the 
offending antennae and support structures were replaced/repaired and 
properly painted.

Imagine our surprise when we discovered the FM audio of a local TV 
station was found riding the carrier of an AMPS channel we'd borrowed 
from US Cellular while testing a phased array, five miles away, at the 
Qualcomm building. Sure enough, we tracked it back to the bank building. 
The same principles of unintentional radiation apply to the OTA/cable 

None of the leads nor terminations had been redressed.

A proper assumption for the technician in the field to make is that 
other technicians have fallen short of their duties ;) As the OP 
indicated the history of the wiring in and around the home, we can 
safely assume that the infrastructure is not RG-6QS and that at least 
some of the terminations are less than perfect.


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