DTV UHF help

[markopolo]

Not a Class B specific question but there are so many knowledgeable forum members here that I figured I'd ask.

I used to receive these two channels (with amplifier) but they would break up due to weather or position of sun. I moved the antenna 3' higher and 3' to the east. Maybe 12' above ground level.

DTV 29.1 563 MHz
Max ERP: 390.000 kW
Eff. pwr: 191.630 kW
Dist: 41.7 mi Path: 2Edge
Rx: -74.0 dBm NM: 17.0 dB
Az: 185.3° (true)
Az: 202.4° (compass)

DTV 27.1 551 MHz
Max ERP: 86.000 kW
Eff. pwr: 85.656 kW
Dist: 44.5 mi Path: 2Edge
Rx: -83.5 dBm NM: 7.5 dB
Az: 181.0° (true)
Az: 198.1° (compass)

Now 29.1 is very good, no amp actually needed. Weather, sun doesn't affect it. No reception of 27.1 now even with amp. It's there but too weak to show a picture. I've spent a fair bit of time aiming for best signal strength, tried multiple TV's in the house etc.

Could 29.1 interfere with 27.1 at all?

I'm using a now 9 year old Channel Master CM 4221HD Antenna - https://www.channelmaster.com/Digita.../cm-4221hd.htm

Any suggestions from the RF pros on the forum would be appreciated.

[Winston]

We don't understand your question "could 29.1 interfere with 27.1"? Also, please define "2Edge". The short answer to your question is "no" . . . we see no possibility that two weak signals on substantially spaced frequencies could interfere with one-another.

Unless you have line-of-sight and are above the "Fresnel Zone", at these extremely short wave frequencies (less than a meter in wavelength), you are in 'propagation never-never land'. Your results are not surprising nor unexpected. Moving just a few inches one way or the other (in three dimensional space) can take a non-existent signal to a very serviceable level (and, of course, visa versa). You may have noticed this phenomenon listing to your car FM radio while at a stop light . . . let the vehicle roll (but don't hit the car in front of you!) and the signal quality (for anything other than very strong signals) can change dramatically.

If you have the patience, you will be able to find a spot where both stations are satisfactory.

[markopolo]

Thanks Winston. That answers my question. I didn't know if the strong signal (in the new position) for 29.1 was somehow affecting 27.1.



2 Edge comes from TVFool. It's double edge diffraction.

[Winston]

Channel 29.1 is now strong enough to 'see', but we wouldn't call it a 'strong signal' particularly in the sense of causing interference . . . . it is, after all, 41 miles away. If you were sitting under their transmit antenna, then maybe.

[DAWg134]

"2Edge" means that there is an obstacle of some sort, usually terrain but possibly a tall building, in the line-of-sight between your antenna and the broadcast antenna, resulting in the signal being refracted not just once, but twice in order to reach you. Consequently, atmospherics will play a big part in signal strength at your set. That said, I chased a similar problem for a couple of weeks before I discovered that the local television station was performing maintenance on its transmitter and had temporarily switched to a backup setup. The problem was resolved when the maintenance was completed and they reverted to their primary transmitter and antenna. I can't recall how I discovered that there was ongoing maintenance - only that I had Googled the specifics of the local station.

[JohnnyFry]

My first thought on this is that the wavelength of these UHF signals in on the order of 54 Cm and any reflecting surface close by could cause cancellation issues if the effective path length difference was 1/2 wavelength (27 Cm) or a multiple of that. Suspects could include, but are not limited to: metal building siding, a metal chimney pipe, a parked car. Rejection of this type of multipath signal is what highly directional antennas are intended to minimize.You may have encountered this effect while listening to an FM signal in a car while waiting at a stop light, moving the vehicle very slowely can cause the radio signal to fade in and out almost completely. As you have observed, moving the antenna slightly will cause a change in reception.

[avanti]

The kind of effects described above are good illustrations of why MiMo antennas are a good idea and why "signal amplifiers" such as WeBoost are pretty much garbage. The former provide alternative signal paths in order to avoid such interference. Signal boosters simply give you a more powerful crappy signal.

There are just enough situations in which amplifiers are helpful in order to delude people into giving them 5-star reviews. Most of the time, though, people simply look at the number of "bars", which reflects the amplitude of both the signal and the noise. If you want to really evaluate such products, you need to look at Signal/Noise ratios. If you do it right, you will discover that the situations in which amplifiers are helpful are rare. MiMo rules!

[markopolo]

Originally Posted by JohnnyFry

My first thought on this is that the wavelength of these UHF signals in on the order of 54 Cm and any reflecting surface close by could cause cancellation issues if the effective path length difference was 1/2 wavelength (27 Cm) or a multiple of that. Suspects could include, but are not limited to: metal building siding, a metal chimney pipe, a parked car. Rejection of this type of multipath signal is what highly directional antennas are intended to minimize.You may have encountered this effect while listening to an FM signal in a car while waiting at a stop light, moving the vehicle very slowly can cause the radio signal to fade in and out almost completely. As you have observed, moving the antenna slightly will cause a change in reception.

The house is close behind the antenna and there's a gutter and fascia etc. I have tried moving it up & down the existing mounting but haven't raised it above the roof. I know higher is better & I might have to do that but next up will be to try a Yagi style antenna.