Another $%&*(&^%$%!!!! Digital TV Phenominabubble

[landtuna]

I posted this in the Phoenix Board because my specific issue is with a Phoenix station but it might apply to digital TV anywhere.

I have a digital converter box attached to my PC feeding an analog TV card. The antenna for this box is a combo rabbit ears/UHF loop which sits in the casement of a window nearby pointing directly at South Mountain (the antenna farm). The antenna sits between the glass and a venetian blind which is metal (probably aluminum).

While watching virtual 12/RF 36 I get a lot of pixelation if the blinds are down (behind the antenna) and an excellent picture if I raise the blinds. I have noticed the same effect if someone walks behind other antennas in boxes in other rooms.

In analog-land any degradation of signal would be caused by walking in between the antenna and the transmitter, not behind the receiving antenna.

Whyizzat?

 

[KeithE4]

I posted this in the Phoenix Board because my specific issue is with a Phoenix station but it might apply to digital TV anywhere.

I have a digital converter box attached to my PC feeding an analog TV card. The antenna for this box is a combo rabbit ears/UHF loop which sits in the casement of a window nearby pointing directly at South Mountain (the antenna farm). The antenna sits between the glass and a venetian blind which is metal (probably aluminum).

While watching virtual 12/RF 36 I get a lot of pixelation if the blinds are down (behind the antenna) and an excellent picture if I raise the blinds. I have noticed the same effect if someone walks behind other antennas in boxes in other rooms.

In analog-land any degradation of signal would be caused by walking in between the antenna and the transmitter, not behind the receiving antenna.

Whyizzat?

First of all, how long ago was this? KAZT/7 is on RF 36, not KPNX, which went back to 12 last June.

Second, try to get something other than a loop for UHF. They're bidirectional, not unidirectional. Not good for analog, worse for digital in my experience.

Also, your blinds might be close enough to the antenna to cause some kind of phase cancellation, hurting the reception. If they are about 4-6" behind the loop, they act as a reflector, which would help if the antenna/blind combo is pointed in the right direction. Too little spacing between the driven element (the loop) and reflector (blinds) can cause problems.

 

[landtuna]

First of all, how long ago was this? KAZT/7 is on RF 36, not KPNX, which went back to 12 last June.

First correction: This comes from doubting my own memory and relying on KPNX's web site. The correction should be: Virtual 12 & RF 12. The station web site has apparently not been updated since last June.

Then - I noticed this yesterday, Sunday, Jan 31st.

Second, try to get something other than a loop for UHF. They're bidirectional, not unidirectional. Not good for analog, worse for digital in my experience.

This may not apply since we're dealing with RF 12

Also, your blinds might be close enough to the antenna to cause some kind of phase cancellation, hurting the reception. If they are about 4-6" behind the loop, they act as a reflector, which would help if the antenna/blind combo is pointed in the right direction. Too little spacing between the driven element (the loop) and reflector (blinds) can cause problems.

I thought this might be the case but why would the problem appear on digital and not analog? I've had this same setup for several years and never had a problem on analog.

The blinds are immediately behind the antenna (less than 1 inch) although they do not touch. I've tried them open and closed with identical results. Removing them from behind the antenna seems the only fix.

 

[KeithE4]

Second, try to get something other than a loop for UHF. They're bidirectional, not unidirectional. Not good for analog, worse for digital in my experience.

This may not apply since we're dealing with RF 12

No, but since all but 3 of our stations are UHF, it does matter. I go nuts adjusting the $10 rabbit ears (with a UHF loop) connected to my bedroom TV to be able to get KPHO on 17. And I'm only 5 miles from South Mountain, although I am inside a stucco-infested/RF shielded apartment building, where my apartment faces south.

Also, your blinds might be close enough to the antenna to cause some kind of phase cancellation, hurting the reception. If they are about 4-6" behind the loop, they act as a reflector, which would help if the antenna/blind combo is pointed in the right direction. Too little spacing between the driven element (the loop) and reflector (blinds) can cause problems.

I thought this might be the case but why would the problem appear on digital and not analog? I've had this same setup for several years and never had a problem on analog.

It probably has more to do with the transmission method (8VSB vs AM). With AM you might get ghosts. With digital, any multipath either will either work (apparently it's possible to lock on what we used to call the ghost) or it will kill the picture as a result of phase problems. AM is more forgiving; the worst thing that happens is a noisy or ghosty picture. That is the only advantage to the old analog system, though.

The blinds are immediately behind the antenna (less than 1 inch) although they do not touch. I've tried them open and closed with identical results. Removing them from behind the antenna seems the only fix.

Rabbit ears are a disaster for VHF, just like a loop is for UHF. Just like with analog, each channel requires a different positioning of the elements and/or the switch if there is one. The blinds probably make matters worse since they are too close to the antenna for either band.

 

[tripinva]

My spectrum analyzer shows that I lose at least 10 dB just by closing my blinds in my apartment on VHF. I say "at least" because the signal is at 10 dB with them open (not enough to decode) and it vanishes into the noise floor with them closed.

Blinds are bad news.

- Trip

 

[landtuna]

My spectrum analyzer shows that I lose at least 10 dB just by closing my blinds in my apartment on VHF. I say "at least" because the signal is at 10 dB with them open (not enough to decode) and it vanishes into the noise floor with them closed.

Blinds are bad news.

- Trip

But are the blinds in front of the antenna or behind it?

Mine are behind and still affect the signal badly.

 

[tripinva]

Doesn't matter. VHF signals cannot make it through the walls due to wiring and other metal in the walls, so the only hope is to catch a reflection through a window. You put blinds over the window, you murder the signal.

- Trip

 

[fybush]

That's something of a generalization, Trip. I spent much of last week in the San Francisco market - in Palo Alto, to be specific, which is 27 or so miles over fairly rugged terrain from the Sutro Tower site in the city and perhaps 23 miles from the Mount San Bruno site. Much to my surprise, I was able to get clean locks on both Vs in the market, KGO-7 from Sutro and KNTV-12 from San Bruno, from inside the building where I was staying (second story of a residential structure)...and even more to my surprise, the lock held when I collapsed the VHF elements of the HDTVi, which suggests to me that the UHF part of the antenna is better on high-V than I'd anticipated.

Given how tricky the Bay Area terrain is, I'm loath to generalize too much; it may just be that the spot where I was staying had exceptionally good line of sight. (Indeed, I just ran it through TV Fool, which agrees that I was line-of-sight to all of the major TV sites in the Bay Area.)

And there is a certain amount of attenuation, to be sure; to get the clean readings I sent Trip for rabbitears, I had to go outside with the antenna. Inside was a little too noisy for that purpose, but enough signal to produce a perfectly usable picture and audio.

 

[tripinva]

That's just my experience.

- Trip