radio wave propagation

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 LeeWood 18 Nov 2018

Can someone explain to me how/why phone signals propagate, given that mast signals are v strong and mobiles not very; if the phone was merely a receiver (as radio) all is apparent - the strong signal is delivered far away - how does the weaker signal return?

I want to know because I'm trying to set up a repeater/amplifier to send wifi to a building 100m from the house. I've researched and obtained a device which might do the job - but now wondering if I need two such - one at each end.

In reply to LeeWood:

Can’t answer that question, but I have had some success with the devices which communicate with your router by piggy backing on the mains electricity waveform as a carrier. This does assume that the other building is on the same circuit as your house. I’ve been successful at 50m so it’s worth doing some homework. The bandwidth is significantly higher than WiFi too. 

1
OP LeeWood 18 Nov 2018
In reply to paul_in_cumbria:

Thanks but no I don't think this will work - different circuits from the meter feed, a wireless connection seems to be more practical

 David Riley 18 Nov 2018
In reply to LeeWood:

You can calculate the path loss between two aerials. The loss will be the same in either direction. Then divide the power of the transmitter by the loss and make sure the receiver is sensitive enough to decode the signal remaining.

Post edited at 11:13
 David Riley 18 Nov 2018
In reply to LeeWood:

The weaker return signal is generally usable because the main receiver is more sensitive.

 Connorh 18 Nov 2018
In reply to LeeWood:

If you have a clear line of sight that nothing is likely to obscure I'd suggest a point to point wifi link.

If you do not have a clear line of sight then you could run a armored cat 6 cable underground. Although the distance is nearly pushing it, you might see some signal degradation. Suppose you could do a fiber run but it could get very expensive. 

 

I wouldn't get bogged down with the details of how signals propagate unless you are actually interested in that, you don't need to know to get this working (there are many point-to-point solutions available) .  I do a bit of radio stuff at work and its really complicated, being without physics education it goes way over my head. 

See  youtube.com/watch?v=LG-AZz_nm5E&  - for an idea, Point to point links can be really powerful. 

Post edited at 11:52
 sbc23 18 Nov 2018
In reply to LeeWood:

Ubiquiti / Unifi do a wide range of Wi-fi links for situations like this. Some have very high bandwidth and also long range (25km+). The cheaper end units are well built and are easy to set up. Their Wi-fi access points are superb.

Post edited at 12:12
 HardenClimber 18 Nov 2018
In reply to LeeWood:

(as per Steve)

http://www.broadbandbuyer.com/store/wifi-links/

might do.

You might manage with a simple extender rather than a link.

In reply to LeeWood:

You want a directional wifi antenna at each end. There are plenty of DIY designs for these, or you can buy one.

As David says, base station antennas are bigger, so have more gain than your tiny phone antenna. They are also directional, usually covering 120 degrees.

 Jim Fraser 19 Nov 2018
In reply to LeeWood:

Your phone does not have a fixed transmitting power. It is calculating all the time how much power it needs to transmit to stay on the network and which part of the network it should engage with. If the base station is 10m away then it transmits next to damn all and if it can barely reach it then the transmitter is full on. That's why your battery dies quickly in not-spots: transmit power is high and repeated attempts are made to re-register on the network.

The antenna at the base station is usually a large plate antenna these days (or a high gain tuned antenna at older base stations) compared to the very tiny one on your phone and that plate is backed up by an extremely sensitive receiver that has a range of tricks up its sleeve to make sense of the incoming traffic. Everything in a digital transmission stream is error-checked, so everything works until at the limits, when a number of packets repeatedly fail, the entire call or message fails.

The base station transmits in sections, of usually one third of a circle par antenna and then the signal quickly degrades in accordance with steradian geometry.

Your phone probably wants to receive at least 150 femtoWatts for a decent signal but can still operate when receiving far less. Maybe even 5 to 10 fW for some services. Once you are into the microWatt range then that's a really excellent signal. 

A fW is a a quadrillionth of a Watt (thousandth of a million-millionth or 10 to the minus 15th power). 

With a typical wifi link using two antennae, it is the same as the microwave links used between some base stations or between oil platforms. The two antennae are to a greater or lesser extent focussed in the direction of each other. You are avoiding the losses of the steradian geometry of normal radio transmission by trying to create a straight 'pipe' of radio energy aimed at the opposite antenna. That way you can transmit considerable distances with far smaller losses of power density due to the geometry. 

It might be that you can use one wifi antenna that is less focussed and transmits across the whole building. At 100m, it probably depends what the kit is designed for whether you use one antenna, or use two and bring the signal into the building then retransmit inside. The stuff I have looked at is for 3 to 10km. You need to ask any proposed supplier specific questions about their equipment. 

 

Post edited at 01:43
OP LeeWood 19 Nov 2018
In reply to Jim Fraser:

Many thanks for this detailed summary - I hope the appropriate technology has been built into the device I've acquired. Before I penned this thread I really was hesitating - but its obviously worth a trial  

And for the future I won't forget that femto is 10 to the -15 !

Thanks also to other responants - I'll post my progress in due course

 Snyggapa 19 Nov 2018
In reply to LeeWood:

Powerline extenders don't need to be on the same circuit - only the same phase - so if your outbuilding is on a slave supply fed from the house then it'll almost certainly work. Even if it has its own feed from the street or pole it has a 3/1 chance of working since there are only three phases so that's the odds of it being on the same one as your house. The only issue would be distance. 

Worth trying a set on sale or return (or cadge from a mate) before ruling it out.  We have a powerline extender in a shed which has it's own consumer unit fed from an external garage , which has it's own consumer unit fed from the house. The "main" part is in the house the other in the shed - and gives WIFI to the garden  

 Toerag 19 Nov 2018
In reply to LeeWood:

As Jim said, directional antennae are your friends because they are the equivalent of a spotlight rather than a beacon radiating in all directions.

 wercat 19 Nov 2018
In reply to Snyggapa:

a lot of them cause massive radio interference


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