Channel 3 In San Francisco

[larrykenney]

A few weeks ago a new low power translator station, K03HY appeared for the San Francisco Bay Area. They are transmitting from Mt. Tamalpais up in Marin County with 2.5 kW.

I expected to see a lot of interference from electrical noise since it's in the low VHF segment, but it's been solid as a rock. I haven't seen any break up or interruptions to their signal at all. We don't have many thunderstorms here (maybe one or two a year), so I haven't been able to see what happens when lightning strikes.

Another low power station, KTVJ, is supposed to come on the air on channel 4 from the same transmitter site in the next few weeks. It's only going to be running 300 watts so it'll be interesting to see how it holds up.

Larry
SF

 

[GaryPen]

I thought that the FCC didn't allow both ch 3 and 4 to be used in the same area.

 

[Trip]

There's no actual rule to that effect, to my knowledge. But that has been taken into account before.

Of course, low-VHF digital signals tend to have such poor indoor reception that it probably doesn't matter.

- Trip

 

[Mr Tony]

I thought that the FCC didn't allow both ch 3 and 4 to be used in the same area.

with digital the rarity of both RF3 & RF4 being used in the same area is pretty non existant

 

[larrykenney]

With digital, adjacent channels transmitting from the same location don't cause any problems. On Sutro Tower here in San Francisco we have several adjacent channels: 29, 30; 33, 34; 38, 39; and 43, 44, 45. On Mt. San Bruno we have 27, 28; and 40, 41. Down in the South Bay we have 49, 50 and 51. The problems occur when adjacent channels are not transmitting from the same location.

Larry

 

[Mr Tony]

The problems occur when adjacent channels are not transmitting from the same location.

Larry

I assume you mean same vicinity
In Minneapolis there are stations on RF31,32,33,34 and 35. But 32,34 & 35 are at the Shoreview "Antenna Farm" (suburb of Minneapolis) and 31 & 33 (low powered) are on the IDS building which is maybe 10 miles away (downtown Minneapolis)

 

[larrykenney]

The FCC rules specify something like 14.5 miles as being the same location. No... they don't all have to be on the same tower.

 

[Jim5506]

I'm having a problem resolving why location would have any thing to do with adjacent channel interference, assuming the transmitters are operating up to spec.

 

[Trip]

It's the receiver, not the transmitter, that has the issue. A receiver can only discriminate an adjacent signal up to a certain amount, which turns out to be about 30 dB. Here's a picture of my spectrum analyzer showing WEBR-CD 17 (which was not decoding) next to WMBC 18:

http://www.rabbitears.info/specan/nnj/b17.gif

Ignore the fact that it has multipath. Now, WMBC is perfectly in spec, but the receiver in my relatively cheap spectrum analyzer couldn't separate the channel 17 signal from the must stronger channel 18 signal. By contrast, here are WFME-29, WFUT-30, and WPXN-31:

http://www.rabbitears.info/specan/nnj/b30.gif

You can clearly see the separations between the signals because they have similar received signal strengths. Even though WFME is 10-15 dB weaker, it's still usable and decodes cleanly.

- Trip

 

[Jim5506]

So, its the relative signal strengths, not just the location of the transmitters (which may or may not have a bearing on signal strength).

 

[Trip]

Yes, but stations which are co-located are more likely to have similar received signal strengths.

- Trip