J
Jul
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Channel 6 did a story about their signal problem during the 6 PM news tonight: http://abclocal.go.com/wpvi/video?id=6863745
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tripinva said:
Not very good. I had it for a while, now it's a smear. WPVI is modern art on my screen.
Can anyone reference an article about VHF Lo band and propagation issues? Google is not helping me.
Julius May said:
The FCC rep cites 'power issues'. I think it's not the ideal set of frequencies to do DTV on.
Bill_W said:
Maybe a nudge up from the 7.56 kW will help. I think that's a little on the low side. That's from the FCC database on the CP for WPVI digital.
We'll see.
For comparison, WHYY is 20kW, and I have no issues.
Whoever wrote the sidebar to the WPVI Wiki listed the power as half a million watts.
WPVI knew this was going to be a problem and did NOTHING to advise the viewers about the specifics of receiving WPVI-DT versus all the other stations in Philly. Alot of people (like myself) only had a UHF band antenna to receive DTV, where all stations were broadcasting on. 6abc NEVER mentioned that they were moving from UHF 64 back to VHF 6 and your current antenna may not recieve 6abc after the transition, nor did they mention that VHF 6 is on a weaker band and there might be reception problems. 6abc should have given the viewers the full story instead of the same song and dance that all the other stations have for the last 2 years.
WPVI could have applied for another frequency on the UHF band. Other stations assigned to VHF channels 2-6 did, and got 'em.
tripinva said:
Really-what determines the max, exactly? Not quite the same thing, but aren't some non-comm FMs operating at more power than that?
Well, the hard limits for Zone I (which includes the whole northeast) are as follows:
Channels 2-6: 305m 10 kW
Channels 7-13: 305m 30 kW
Channels 14-51: 365m 1000 kW
Now, if you're above that height, the power has to be decreased as the height goes up, and is determined by a formula. (I have a calculator for it on my website: http://www.rabbitears.info/calc.php )
Since WHYY is below 305m in height, their power limit would be 30 kW assuming no reductions for interference. WPVI, however, is above that 305m height limit, and thus has to reduce power to compensate.
- Trip
Channels 2-6: 305m 10 kW
Channels 7-13: 305m 30 kW
Channels 14-51: 365m 1000 kW
Now, if you're above that height, the power has to be decreased as the height goes up, and is determined by a formula. (I have a calculator for it on my website: http://www.rabbitears.info/calc.php )
Since WHYY is below 305m in height, their power limit would be 30 kW assuming no reductions for interference. WPVI, however, is above that 305m height limit, and thus has to reduce power to compensate.
- Trip
tripinva said:
I know basically the ERP calculations (height, wattage, elements) but is this equation substantially lower for DTV? Their old analog did operate at or close to 75kW. So, what is the hard-and-fast rule vis-a-vis the digital standard? I think the health issues are far reduced in terms of EM signal saturation. I need a rationale.
softmachine said:
The ERP calculation doesn't change. I'm not sure how the health issues compare between analog and digital but I don't think that has anything to do with the limitations on WPVI's power.
Analog and digital powers cannot be compared directly.
In both cases the power changes constantly.
- In analog, there are "synchronizing pulses" which occur at predictable 63.5uS intervals. These pulses are (were?!) transmitted at maximum available transmitter power -- when we say (said) an analog station was running 74.1 kilowatts, we mean they were running 74.1 kilowatts during synchronizing pulses. The power is actually considerably lower most of the time.
- In digital, we can't predict when maximum power will happen. So we average the transmitted power over a period of time. When we say a digital station is running 7.6 kilowatts, we mean the power averaged over a period of time is 7.6 kilowatts. The power is actually considerably higher much of the time. (and also considerably lower much of the time) The peak-to-average ratio for 8VSB is roughly 4:1, so this station (and yes, I do mean WPVI(grin!)) is running about 25 kilowatts peak power.
So you're comparing 25kw to 74kw, not 7.6kw to 74kw.
Going back to Trip's table, it should be noted that applies to stations on channels added to the digital allotment table in the future. Channels in the initial table (i.e., those used by stations that are already on the air) are limited to facilities specified in an "Appendix B". (to a Public Notice) In practice it looks like WPVI's power is pretty close to the new-allotment maximum. (their antenna is 332m high, which as Trip said would require a reduction from the 10kw figure) But some stations may be limited to something considerably less.
In reality, UHF digitals will cover the same distance with less power. I had a snowy UHF that was almost never watchable at 50 kW over a distance of 79 miles, and a perfect lock on a digital off the same mountain at 55 kW. For a while, my strongest signal was at 17.9 kW from 53 miles away.
On VHF, it seems to be less true, but the FCC treated it that way. I'd venture to say that it CAN work with less power, assuming an outdoor setup. Indoor antennas, not so much.
- Trip
On VHF, it seems to be less true, but the FCC treated it that way. I'd venture to say that it CAN work with less power, assuming an outdoor setup. Indoor antennas, not so much.
- Trip
Antennaweb.org is still insisting that WPVI-DT 6 can be received in Cumberland County, South Jersey, with an unamplified directional antenna. Should I assume that's complete nonsense?
Neil Rattigan said:
The only channel I can find on which they'd stand any chance is 48.
- We'll presume channels 2-6 are out.
- Now, we'll calculate the distance between the WPVI tower and the service area of the nearest station on each other channel 7-51.
- Channels 9, 12, 14, 17, 22, 24, 25-29, 31, 32, 34, 35, 38, 39, 42, 43, 44, 46, and 50 are immediately eliminated. The distance is negative -- the WPVI tower is within the service area of stations on these channels.
- Except for the five channels listed below, each channel has at least one protected station within 150km. Channel 11 is the one on which the distance between the WPVI tower and the service area of the nearest protected station is greatest. That distance is 57km - about 35 miles. WPVI's coverage on channel 11 could not exceed 35 miles - and in fact would have to be quite a bit less, because the station would cause interference at a greater distance than that at which it would provide service.
- There are a handful of channels on which there is no station within 150km of the WPVI tower: 15, 16, 19, 20, and 48.
- Channels 14-20 are reserved for land-mobile. (WPHL gets to use 17 because they were there first...) WPVI can't move there.
Best I can tell the closest protected station on channel 48 is WRNN Kingston, New York, 194km away. The distance between their service area and the WPVI tower is a tad over 90km, 54 miles.
A quick unscientific estimate suggests WPVI could run up to 200kw on channel 48 at their existing 346m antenna height without their service area intersecting WRNN's.
HOWEVER..... stations cause interference at a greater distance than they provide service. It's quite likely WPVI would be limited to well under 100kw on channel 48 to avoid interference with WRNN.
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