Surprised by interference.
I know I am in an urban area, however with my loop aerial fixed on the chimney stack I am quite surprised that it is still prone to so much interference eg from SMPS's. I thought the offending SMPS would need to be quite close to the loop, but maybe that isn't the case.
I do have quite a lot of SMPS's in my own house, but I don't believe the problem lies with them. I have turned most of them off & the problem is still present. The only ones I can't turn off at the moment are the 2 for the router & the ONT box, if I unplug those I won't have any internet to feed to the Kiwi. I don't have any battery power packs or linear power supplies to use yet.
Obviously I can rotate the loop, this reduces the SMPS noise, but there seems to be 2 different sources of the noise, each at different frequencies & in different directions. The LW noise is worst in the N/S direction, whereas the noise from 400Khz to about 750Khz is worse E/W. I do have the VDSL noise much higher up as well. I have tried common mode noise chokes etc, they don't make much difference. I assume getting to the noise source is the only way to reduce it ?
Many thanks.
Trevor
Comments
Hi Trevor,
I don't think your situation is quite as bad as you think it is.
I'd suggest sorting out the main issue first.
Your loop is providing quite high signal levels, and the two strong medium wave stations on 693 & 1053kHz are almost certainly causing some intermodulation products over the rest of the MW band, and maybe beyond.
Try adding 10dB of attenuation and see if this improves things.
In fact, you could probably add as much as 20dB, without degrading the Signal To Noise Ratio.
The antenna only needs to raise the noise floor by 6 to 10dB on the quietest of frequencies, which are probably those above 25MHz.
Adding 10dB of attenuation should reduce all the signals by 10dB, if some reduce by a greater amount, or disappear completely, then they are likely to be false signals produced as intermodulation distortion.
If intermodulation products remain, then the loop amplifier is being overloaded, and a smaller loop may be required.
Regards,
Martin
this got rid of my MW gremlins
Also, what kind of design is the loop antenna? Having it up on the chimney may not be doing you any favors, where about 1.5-2mtrs off the ground is good for magnetic loops and being able to rotate out the noise is also the key feature of a loop too.
I have set the attenuator to 10db to see what difference it makes. I don't know how overload shows up on the waterfall ?
I had the loop in the garden for a good while, but there was just too much (smps?) interference, it has lessened now it's on the chimney
It is using a Well good amp, modelled on the Wellbrook I believe.
Your attenuator is currently set at 5dB, ideally you could do with a bit more.
If you look at a quieter section of the MW broadcast band, where there are very few actual signals, say between 1.15 & 1.25MHz, without any attenuation, I think you will see lots of carriers spaced at 9kHz intervals. These are intermodulation products, as a result of mixing between very strong signals, which are overloading the KiWi.
As you increase the attenuation, you will most likely see these rapidly reduce in level, until they become masked by the local noise floor. At night when propagation favours the LF bands, and signal levels increase, you will probably have to add a bit more attenuation. But looking at your KiWi, I think you have a sufficiently high signal (and noise) level, that you could easily add 10, 15, or even 20dB of attenuation, without degrading the overall Signal to Noise Ratio.
Massively high signal levels don't provide any additional advantage in terms of what you can hear. But they do "eat away" at the receiver's overall dynamic range, potentially causing overloading and generating spurious signals.
The best compromise, is to have a 6 to 10dB increase in the receiver noise floor, on the quietest frequencies, when the antenna is connected,
Regards,
Martin
Thanks for that, I will increase the attenuation for a day or two and see how that goes. Would a MW filter be a good idea or not ?
Hi Trevor,
I don't think you need a filter, you have more than enough gain ahead of the KiWi, that allows you to simply add broadband attenuation.
You don't need massively strong signals to achieve good reception, you just need to get the balance right.
Once the overload is sorted out, which I think confused the issue, you can then start dealing with the remaining interference.
The pulsing noise that can be heard using AM demodulation, between about 15 and 26MHz is a Ethernet data over mains extender, and hopefully you may be able to null that out to a certain extent.
There is some noise with 50/100Hz modulation around 3MHz and other frequencies, which is probably a mains powered charger or some other similar electrical item. However, that is at a fairly low level and isn't causing major problems.
Overall, I'd say it's pretty typical of reception in a UK urban environment, and I think you would have to live in a much more rural location to obtain significantly better results.
When I moved from a quiet rural location, back into a town, my noise floor was about 20dB worse using the same types of antennas.
Regards,
Martin
Points noted. Many thanks.