Why Does High Pressure Affect Tv Reception?

[Bert45]

We are getting poor TV reception right now. It's a lovely day, blue sky no wind. According to worldweatheronline the pressure now is over 1020, which is considered high. But why is TV reception affected? Radio waves travel through a vaccuum, and I wouldn't think that a few more air molecules in the way would have any effect. But I'm wrong. Why?

[goodgoalie]

You receive a weaker signal as the signal from your local transmitter is lifted higher into the sky. Also, the signal from a distant transmitter might interfere with your reception.

[Buenchico]

Normally the higher up you go, the cooler the air becomes. However during periods of high pressure an atmospheric temperature inversion can occur, where a layer of warmer air rests upon a cooler one. The interface between the two serves as a reflector for radio waves, so that they can travel further than usual. That means that signals from different transmitters broadcasting the same signals can arrive at a TV aerial. If those signals aren't in phase, the end result is a weakened or distorted signal. Similarly signals from transmitters using the same frequency but broadcasting different content (such as those from TV stations on the continent) can interfere with each other.


In this diagram, the houses on the right should only be receiving signals from the transmitter on the right but, due to 'bounce reception', they're also receiving signals from the transmitter on the left, resulting in a degradation of their 'own' signals:
https://www.bbc.co.uk/reception/sites/default/files/inline-images/High%20Pressure%20JUN2020%20Simple.jpg

[Bert45]

Thanks, goodgoalie. But why should the signal be lifted? And why should the signal from a distant transmitter travel further? It has always amazed me that, no matter how many gigawatts of power is put out by a transmitter, the signal must weaken with the inverse cube of the distance, so a few miles from the transmitter the energy must be in microwatts, and yet that's enough to give me 200 channels. Wow! Mind boggling!

[Buenchico]

>>> no matter how many gigawatts of power is put out by a transmitter, the signal must weaken with the inverse cube of the distance, so a few miles from the transmitter the energy must be in microwatts

The Astra transmitters that provide Freesat and Sky signals are 22,000 miles away, with transmitted power generally below 10 kW (although at least one of the group now use 13 kW and there are plans to eventually use up to 100 kW of power for replacement satellites). So it's remarkable just how strong a signal can be picked up by a very small dish on the side of a house!

[Bert45]

That makes it clearer, Buenchico. So it's not so much the high pressure in itself that affects reception, it's that inversion layer. I know it's only a diagram, made to make things easier to understand, but I live in the Peterborough area, where there are no hills for many miles. I suppose the hills don't matter, the point being that you don't get a signal from the distant transmitter most of the time because the hills are in the way. No, wait! If the hills weren't there, we would get the signal from the distant transmitter, and it would be stronger because it hasn't travelled so far, the straight line route being shorter than the bounced route. It's very flat around here.

[Buenchico]

I've recently been thinking of getting myself a 'QRP' (= low power) amateur radio transmitter, having passed the Radio Amateur Examination half a century ago but, up until now, never having taken out a licence.

The current record for a 'QRP' transmission by a radio amateur is for a distance of 1,650 miles using a power of just 1 microwatt!

[Buenchico]

You simply get signals from lots of places that you shouldn't be getting them from, Bert. It's even possible for signals to bounce several times, using both the inversion layer and the ground. Here in Suffolk, for example, FM reception of Classic FM tends to get completely swamped by signals from radio stations in Belgium and sometimes even from Eastern Europe!

[Bert45]

Mind boggling even more! Thanks, Buenchico. Just incredible!