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Radio transmissions and ADSL2+

Octal
Dabbler
Posts: 16
Registered: 03-02-2014

Radio transmissions and ADSL2+

This might be one for the technical guys and gals to answer.
I understand ADSL2+ bandwidth extends from 140KHz to 2.2MHz. That presents me with a problem, because at the top end of that band sits the amateur band of 1.81MHz to 2MHz and if I run, even a very low power it kills my connection stone dead.
I can get around that for my own connection by fitting a low pass filter to restrict top end to 1.5MHz, that reduces my speed from the present 16M down to about 12M which I could live with and continue to transmit, but this doesn't help everyone else in the district who I might also knock off line. The main radiating part of the antenna is about 20 metres away from the house and I have calculated the field strength from it to be very low, well within acceptable limits, so that other electronic equipment will not be affected by my transmissions.
Also in the ADSL2+ bandwidth there is the Medium wave broadcast band from 0.5MHz to 1.6MHz with some very high power transmitting stations putting out enormous field strengths, so I don't think this problem is that uncommon.
I would be interested to know if this is a problem encountered before? If so, how was it got around?
Thanks.
25 REPLIES
Community Veteran
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Registered: 21-01-2013

Re: Radio transmissions and ADSL2+

I don't have any recent experience with amateur radio but I don't think things should be as bad as you're saying.
A high resistance fault or earth fault on one of the phone wires could be adding to this issue.
It would be interesting to get BT to measure the characteristic of your line, but I think you run a big risk of them charging you (£130 for a visit) if they fail to find a fault with the line.
May be worth asking in the kitz forum, they're more in to thing like this.
Octal
Dabbler
Posts: 16
Registered: 03-02-2014

Re: Radio transmissions and ADSL2+

Thanks for replying npr.
It isn't totally bad, all the other bands I can operate on with as much power as I want without a problem, so I'm sure if there was a problem with the line then it would have shown up on the other bands as well.
It isn't a totally unknown problem because the low pass filter I use is this one:
http://qso365.co.uk/wp-content/uploads/2012/02/ADSL-filter.pdf
It is very effective, but it just slows my speed a bit.
If I get no joy here I will ask the kitz forum, I didn't know about that one, so thanks for that.
Community Veteran
Posts: 1,841
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Registered: 21-01-2013

Re: Radio transmissions and ADSL2+

If it's not a exacerbated by a line fault I feel sorry for neighbours of radio hams.  Shocked
They're unlikely to know how to knock up a low pass filter.
Just a wild thought, do you have a sky box or similar connected to the phone line -- something which could picking up the RF and injecting it in to the phone line.
Please let us know if you make any progress on this issue.
Octal
Dabbler
Posts: 16
Registered: 03-02-2014

Re: Radio transmissions and ADSL2+

Nothing else connected to the phone line and the router is pugged in directly to the master socket with a very short connection. The telephone is a cordless phone connected directly to the master socket with its microfilter.
Just one other thing, the phone line comes in overhead, so I've put some ferrite clamps around the phone line to remove any common mode RF currents coming down the phone line, also put lots of ferrite clamps on all the incoming and out going circuits on the router and the power supply lead just to make sure there was no RF getting in by those routes, it didn't cure the problem of knocking out the router on that one particular band, but as an aside I have noticed a reduction in noise level on my receiver, so it has done some good anyway.
I have built some other filters, but I don't want to rock the boat too much by using the band and asking the neighbours if they would mind fitting filters, so at the moment I don't use that band. It is only a small community here and some of the neighbours are ex Royal Signals, so they know all about my aerials and what they are for  Smiley
I will update if I get any further, but I suspect that I won't because of the type of problem it is, I might be being cynical though.
Thanks once again.
Community Veteran
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Registered: 31-08-2007

Re: Radio transmissions and ADSL2+

It was quite a while ago that I did a bit of investigation into filtering to overcome this problem. That pdf document of the ADSL filter may have been one that I came across Octal, I certainly remember several bits of information I dug up at the time with some extensive searching.
It may be worth doing some further searching. If I can find any further info on what I located before I'll post back with it, but it won't be until next week I expect due to other domestic priorities atm.
As I expect you are aware, twisted pair wiring is important for all phone line connections to reduce the amount of stray pickup of RF in the ADSL spectrum. As implied by npr, line balance is also important. In severe cases within your own property a screened twisted pair modem lead might help. But also as far a common mode filtering goes, try some ferrite rings with 12 turns instead of clamp-ons, they've been effective in some cases of reducing the impact of MW broadcast interference. Don't know what type of Master socket you have, but if it's a fairly recent NTE5A you could considered fitting a Mk3 SSFP as well, as that has an "improved" common mode filter, but you'll still probably need the ferrite rings.
As you are no doubt also aware, the licensed amateur is responsible for resolving any interference problems that they may cause. That doesn't sound like it would be a huge issue with some of your neighbours and their backgrounds, I would have thought they would be very co-operative over any problems.
Now as far as line balance goes, I'm sure one of the more technical CRT guys would be able to run a full copper test on your line and post the results which would show if you had any faults or if the line wasn't well balanced (this is on the assumption that you pay line rental to Plusnet - BT Retail if you are with them are unlikely to be interested or will want to charge you).
Community Veteran
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Registered: 06-04-2007

Re: Radio transmissions and ADSL2+

The 'line balance' is the most important thing here, if the line is not balanced then the purpose of the twisted pair will not work (it works to cancel common mode noise, look up the term CMRR), your HF transmitter will induce common mode noise onto any nearby wires depending a little on their length.
--
3Mb FTTC
https://portal.plus.net/my.html?action=data_transfer_speed
Community Veteran
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Registered: 31-08-2007

Re: Radio transmissions and ADSL2+

I suspect most of us posting here so far are aware of all that. However I would say that any lack of twisted pair wiring is only likely to make matters worse.
Community Veteran
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Registered: 21-01-2013

Re: Radio transmissions and ADSL2+

Quote from: Anotherone
In severe cases within your own property a screened twisted pair modem lead might help.

On the couple of occasions I've tried screened modem leads it's made rf pickup worse.
I once tried running a twisted pair phone cable along a earthed cold water pipe. That made a noticeable reduction in sync speed -- won't be doing that again.
Just realised no one has mentioned removing the bell wire from the internal phone line. I'm sure the OP will be aware of that trick but may as well mention it for completeness.
Community Veteran
Posts: 19,101
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Registered: 31-08-2007

Re: Radio transmissions and ADSL2+

Damn, yes I meant to mention that, but something side-tracked me and I ended up forgetting Embarrassed - the bell-wire being one of the best things for unbalancing the pair Shocked
Octal
Dabbler
Posts: 16
Registered: 03-02-2014

Re: Radio transmissions and ADSL2+

Thanks for the replies so far.
I must admit I have been on the Kitz forum and they have been very helpful, as you are here I might add, which I'm grateful for.
They do have an article on their web page that specifically address my issue, it isn't an unknown problem by any means, particularly if you live near one of the high power Medium wave broadcast stations around the country.
Interesting about balanced line, I use balanced line all the time to feed my antennas and if the line if perfectly balance there is no radiation from the feeders and the none is received by them. We go to great lengths to make sure the line is balanced using either the correct matching networks or a 1:1 current transformer.
I have been experimenting with a 1:1 current transformer so far without success just before the router, I suspect I'm using the wrong ferrite material because most of mine are intended for high frequency, but I'll keep trying. So far it has had no affect on the broadband speed, but the common mode pickup is still getting though.
I notice the bell wire has been disconnected in the master socket, I suspect the BT engineer might have done that when we had a problem with the phone line and he replaced the socket because it was an old tatty one originally. Just a thought, if the bell wire is disconnected it effectively an open ended un-terminated wire that can act as an antenna an at certain frequencies this could have quite a high RF voltage on the end of it which could capacitively enter one of the parallel pair, it then is not a common mode signal but the router will read it as a signal.
OK, question, what does the bell wire do and does anyone know how its terminate at the exchange end? And what sort of signal comes down it if any? I don't want to start messing with that just to have an angry BT engineer knocking at my door Smiley
Community Veteran
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Registered: 21-01-2013

Re: Radio transmissions and ADSL2+

The bell wire circuit starts at your BT master socket, there's only the two signal wires coming from the exchange. In the master socket there's a capacitor and resistor in series between the two wires, the bell wire comes from the junction between the two components.
As you can imagine any rf on the bell wire will pass through the capacitor better than through the resistor putting the twisted pair out of balance at higher frequencies. Any capacitive coupling should in theory be the same on both wires of the twisted pair and therefore be cancelled out.
Note:
Most modern phones don't use the bell wire, but there's no problem if you have one that does need the bell wire --- the broadband filter contains the bell circuit and recreates the bell connection at it's phone socket.
Octal
Dabbler
Posts: 16
Registered: 03-02-2014

Re: Radio transmissions and ADSL2+

npr, thanks for that, I'll leave the bell wire alone then. Incidentally, I've got no phone extensions the main phone is connected to the master socket and the other phone is connected wirelessly, that's all I need in our little bungalow.
Octal
Dabbler
Posts: 16
Registered: 03-02-2014

Re: Radio transmissions and ADSL2+

I have been doing some experimenting and had some positive results, not a full cure, but better than before.
I built a little ferrite transformer, well two actually because one didn't have sufficient turns on one because of the low frequencies involved. here is a picture of the finished project:

What happens now is instead of the internet completely cutting off when I transmit, it just slows down for the duration that I'm transmitting, it seems to hover around 7-8M but resumes back to its normal speed very quickly as soon as I stop, so as I say it isn't a complete cure, but a darn sight better than what I was getting before.
I have noticed something interesting when I'm doing speed tests, I now have a slight increase in download speed, it isn't much, the only thing I can think of it's cutting out some other common mode interference that might be on the line. Anyway, I'm going to leave in in circuit for the time being and see how it goes, I might even look for another ferrite of the same mix, only slightly larger so that I'll able to get more turns on the core.
I will post back at a later date with the results.
Community Veteran
Posts: 1,841
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Registered: 21-01-2013

Re: Radio transmissions and ADSL2+

Thanks for the update, it's good to know you're making progress.  Cheesy
Just a few thoughts:
a) would it be better in a metal container?
b) the modem cables look to be the flat non-twisted pair type. Being so close to the transmitter it may be better to ensure everything is twisted pair including the length between socket and your DECT phone -- bet that's flat non-twisted pair..
c) have you looked at the router stats while transmitting. Depending on the router you're using it may be interesting to graph the various connection errors eg SNR Margin, CRC errors, FEC errors etc.
DSLstats  http://www.s446074245.websitehome.co.uk/