Variable TV Reception

Originally posted by JS357
We have two tv sets, and no cable or satellite. So we receive much programming OTA (over the air) and some via broadband. One is upstairs with its full size antenna in the attic. The other is downstairs with 'rabbit ears' of the amplified variety.

The downstairs tv reception is spotty at times, always when it is blustery or windy outside. I have put this d ...[text shortened]... ions for their very existence. I have researched this without good results.

Any ideas on why?

Air contains water which is a polarized molecule. What's happening with the water in the air affects reception of radio-waves.

Originally posted by JS357
We have two tv sets, and no cable or satellite. So we receive much programming OTA (over the air) and some via broadband. One is upstairs with its full size antenna in the attic. The other is downstairs with 'rabbit ears' of the amplified variety.

The downstairs tv reception is spotty at times, always when it is blustery or windy outside. I have put this d ...[text shortened]... ions for their very existence. I have researched this without good results.

Any ideas on why?

In the atmosphere, there are ionized blobs of air floating around that effect the diffraction of RF.

One job I used to have was a communications specialist on what is called 'space diversity tropo-scatter microwave communications'.

At the frequency used ATT, about 5 Ghz, most communications is done line of sight, if you can see the antenna you can communicate.,

But you can also aim the antenna, in this case, a parabolic dish about 10 feet in diameter, up slightly aiming above the horizon.

With a single dish, you will get signals through at around 100 miles away but the S/N ratio will be up and down like a fast elevator because of these ionized blobs in the atmosphere which can destructively interfere with the scattered signal or constructively add to the signal, a random event that changes second by second.

The space diversity part means you don't use just one parabolic dish but two, separated by about 30 feet. It turns out the atmospheric blobs average about 20 feet in diameter or so due to some esoteric atmospheric physics above MY pay grade to explain🙂

But when you have the identical signal going into two antennae like that and two identical dishes at the other end, there can be two way communications because of a device that combines the two signal paths. This results in a S/N ratio that is very good 99% of the time because when one signal goes down the other signal goes up so when you combine signals one or the other will contribute to the overall signal to noise ratio and you have a working communications system.

All that has been superseded by satellite communications now but it was a viable short range comm link, up to a couple hundred miles or so back in the 60's and 70's.

So in a storm or windy conditions, those blobs effect UHF tv signals as well and since you only have one antenna and no signal combiner you are left to the mercy of destructive and constructive interference of the signal.

If you could have two antenna's some distance apart and a way to combine the signals in a UHF mixer you could get much better reception or go modern and get a satellite dish like millions of other people do.

Alternatively, you could put up a high gain yagi antenna on the roof with a rotor to aim the thing for best signal and use a signal splitter to send two signals to the tv's, one upstairs and the other downstairs.

Of course that requires putting up some kind of tower, the higher the better. I remember some folks putting up towers 50 or 60 feet high with big Yagi antennas and rotors back in the day when there was no cable or dish. You probably would not need to go to that extreme but a simpler tri-mount support on the roof would do the job almost as well.

Originally posted by sonhouse
In the atmosphere, there are ionized blobs of air floating around that effect the diffraction of RF.

One job I used to have was a communications specialist on what is called 'space diversity tropo-scatter microwave communications'.

At the frequency used ATT, about 5 Ghz, most communications is done line of sight, if you can see the antenna you can co ...[text shortened]... go to that extreme but a simpler tri-mount support on the roof would do the job almost as well.

Thanks for educating me. I hadn't thought about blobs of ionization. Our main TV is upstairs in our tv room (converted bedroom) and I put a large antenna in the attic above it. We have line of sight of the transmission towers except "through" one wooden building across the street, and our reception is great.

Originally posted by JS357
Thanks for educating me. I hadn't thought about blobs of ionization. Our main TV is upstairs in our tv room (converted bedroom) and I put a large antenna in the attic above it. We have line of sight of the transmission towers except "through" one wooden building across the street, and our reception is great.

So get a signal splitter, you won't lose much for the upstairs tv but it will definitely help the downstairs one. Just a hundred feet of tv lead, whether coax or twinlead. Twinlead is the flat stuff and it works pretty well, 300 ohm impedance. Coax has the advantage of rejecting RF noise in the house if you have a lot of electronics but the impedance is usually 75 ohms which will cause you to lose some signal if you run a coax directly in to the normal old fashioned twinlead RF input. If you already have a coax plug on your TV you have it made there, use a decent brand of coax, you can get it at Radio shack if you have one around or online, probably Amazon. Get a coax signal spitter that is also 75 ohm with the cable connection. If you have a twinlead going to the TV upstairs, and twinlead downstairs you can get a 300 ohm splitter as well.

What happens if you have an impedance mismatch is the signal goes down the line and if it encounters the exact same impedance, the signal checks in to whatever that device is and doesn't check out, like the roach motel. If there is a mismatch, say you have a 300 ohm source and you stick it directly into a 75 ohm device, 300/75 is a 4 to 1 VSWR (Voltage standing wave ratio) and a significant portion of that signal is bounced back up to the source like a mirror. You aim for a 1 to 1 VSWR which is 75 into a 75 or 300 into a 300. If you did 75 into a 300 or vs versa you could end up with ghosts in the image, wider lines the longer the coax or twinlead. Anyway if you get the same impedance at the source and splitter and downstairs TV, whether its all 75 or all 300 ohm you will get a much better signal to the downstairs TV than an old rabbit ear antenna.