4G Field Trials

Chris Green

Member
My mum and dad have received a card from at800.tv informing them that their area is to take part in field trials of the 800mhz band for 4g. They live in the Isleworth and Brentford area of west London and have been told to expect a filter in advance of the start of the trial on 29th April. Am I right in guessing that since Crystal Palace's channels are from 22 to 30, i.e. nowhere near the old 61 and 62, there's not likely to be a problem? I'll report back.
 

Black Hole

May contain traces of nut
It depends on signal strength. If the 800MHz signal is strong enough, it can overwhelm the broadband input stages prior to the tuning filters. An extra input filter will probably be a passive tuned circuit to notch out the 4G signal before it gets to any non-linear components that could be overwhelmed.
 

gomezz

Well-Known Member
Depends on what kind of aerial she has. If it it a wideband aerial then it could be affected.
 

Black Hole

May contain traces of nut
Any kind of aerial can be affected if the signal is strong enough. The filter should be fitted before any kind of amplification or distribution.
 
OP
Chris Green

Chris Green

Member
Thinking ahead to when this applies to me too, do we only need one filter to protect 'daisy-chained' kit, i.e. fitted anywhere in the lead before the first item, say my Hummy for example?

Taking on board what BH just posted, how do people with a masthead amp get round this or do I take it he meant internal amps and distribution systems?
 

Black Hole

May contain traces of nut
It applies to any wideband amp in the signal chain prior to the channel tuning - if the 4G signal causes it to saturate, all other signals will be wiped out.

I am aware that the aerial itself applies a degree of band-pass filtering, but the cut-off is not very sharp so there will be some kind of pick-up even from a Group A aerial (the A band goes up to 600MHz, 800Mz is less than half an octave above that) depending on the direction of the 4G signal relative to the TV signal and the polar (ie directional) response of the aerial at the frequency in question, and any break-through in the cable etc. It must be understood just how strong a mobile phone signal is: compare the size of aerial used to receive TV with the size of aerial on a mobile device, and of course the aerial on a mobile cannot be directional which means it needs a stronger signal still.

The problem with amplification is two-fold: the easiest to understand is clipping. The amplifier has a certain maximum output voltage excursion, above or below the average output ("zero"). If the input signal is so great that the output signal would exceed the maximum excursion, the signal peaks get clipped at the maximum. However, if the signal that is being clipped is the unwanted 800MHz signal and the signal you actually want is a superimposed ripple on that, the clipping wipes out the wanted signal.

Problems occur before the amplifier reaches clipping though. Amplification theory relies on the amp being "linear" - that is, the output is exactly proportional to the input. Unfortunately the linear region is much smaller than the full output range of the amplifier (limited by clipping). I won't go into the maths, but as soon as any non-linearity is introduced into the amplifier input-output curve, signals start interfering with each other instead of passing through essentially separately. If you have signals going in at (say) 600MHz and 800MHz, 600MHz and 800MHz come out but also 200MHz and 1400MHz (the difference and the sum).

This is called intermodulation distortion (and is how the heterodyne stage in a receiver works to reduce the "tuned" frequency to a much lower fixed frequency into the tuning and detector stages). The problem is that there are not just two signal frequencies going in, there's a plethora, and some of the intermodulation products will be at the same frequency as your desired signal. A strong local signal, even if out-of-band, could over-drive the amplifier so that it goes non-linear and then the intermodulation produces a mush to try to pick the signal out of (ie makes the signal-to-noise ratio worse).

This shows what the problems are. To what extent they will affect anyone in particular depends very much on the local conditions. The solution is to eliminate the 4G signal before it passes through any amplifier, including the mast-head. Eliminating it after the mast-head amp or other distribution might work if the problem is occurring at the receiver, if so - fine. If a 4G operator causes anyone's TV reception to fail, they are responsible for sorting it out so don't be afraid to complain - first to the operator and then to the regulatory authority if necessary. Solutions might include simple filtering, replacing the aerial with a more selective one, pointing it at a different transmitter, or in the worst cases offering a cable or satellite alternative to Freeview.

Having thought about it, provided the 4G signal is sufficiently off-axis, my preferred solution is to link two aerials in a phased array positioned precisely to reinforce the Freeview signal and cancel out the 4G frequency/direction, with no mast-head amp, then kill any residual signal with a notch filter at the bottom of the downlink prior to distribution. This isn't going to work if there are two 4G signal sources locally!
 
D

Deleted member 473

My mum and dad have received a card from at800.tv informing them that their area is to take part in field trials of the 800mhz band for 4g. They live in the Isleworth and Brentford area of west London and have been told to expect a filter in advance of the start of the trial on 29th April. Am I right in guessing that since Crystal Palace's channels are from 22 to 30, i.e. nowhere near the old 61 and 62, there's not likely to be a problem? I'll report back.


Do we have any democratic control over this sort of thing?
 
OP
Chris Green

Chris Green

Member
My Mum's filter turned up today so I fitted it for her, prior to where the aerial goes into her STB, and thence to the telly. They supply the filter and a (very) short male-female jumper cable, plus velcro pad should you need it. Pity it had to be supplied as having input and output at 90 degrees to eachother. IMHO, straight-through would have been a lot neater.

I noticed it's marked as a channel 57 filter. I assume they mean by this that it's a low-pass filter for anything including and below channel 57. Her neighbour was a bit incensed as they have two aerials, one serving the front and the other the back of the house. It looks like they'll have to buy an extra filter themselves. The first impression is that at least it has had no effect on signal strength, although the actual trial in the 800mhz band isn't pencilled-in to start till the 29th April.
 
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Chris Green

Chris Green

Member
Quick update: Mum and Dad report no problems several days into the 'Isleworth and Brentford' 800mhz test. I haven't tried it without the filter though. It did occur to me that since this is all about the proximity of a mast, causing swamping of other frequencies, whether this test is a rigourous as it should be. After all, if someone used a 4g phone in their lounge - OK it's a very low strength transmitter, but boy is it close - whether this also would be capable of 'swamping' their TV signal, since it would effectively be capable of affecting their RF input AFTER the filter.
 

Black Hole

May contain traces of nut
This would be a different situation, as it involves break-through into the screened system.

In electronics product testing circles this is called Electromagnetic Susceptibilty, and all CE-marked products (legal requirement for sale in the EU) are tested (or at the very least a strong case made by analysis) for an adequate imperviousness to ES. However, the CE tests are for safety, so the test is still a pass if the unit malfunctions, as long as it malfunctions in a safe way. The manufacturers own test specification would probably not allow malfunction in a situation as common as using a mobile phone in the room, but they may not have envisaged the use of a mobile phone right in the frequency band.
 
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