Help needed -Spurious emissions

[SteviegtrParticipant @steviegtr]

Hi guys & gals i guess. This is not engineering as such. But I recently got my radio ham Licence. (RAE) after taking the exam back in 1989. I never applied for the Licence , but recently contacted Ofcom. They said a City & guild pass was for life & gave me a licence. So now i have the creds to transmit & talk around the world,, which i do & enjoy. My problem is my table lamp at the side of my bed. So if you have time to watch my video, i would appreciate your input. Knowing there are a lot of very clever guys on the forum, i will except any help you can give me.

Not from the usual you are a chimpanzee brigade guys, but from anyone with knowledge of electronics & harmonics.

Steve.

[SteviegtrParticipant @steviegtr]

Spurious emissions

[John HaineParticipant @johnhaine32865]

You might think you're in a Faraday cage but it's remarkably hard to make a fully screened box! But anyway presumably there is an antenna outside for your transmitter? I doubt this is a spurious emission problem but just poorly designed capacitive sensors on the light switches picking up a high rf field. Best way to deal with it probably to have lights with proper switches.

[Robert Atkinson 2Participant @robertatkinson2]

These lamps are well known for being sensitive to RF.
First easy thing to try is a ferrite clamp or core on the mains lead. A few turns if you can manage it.
Othewise you are into modification of the lamp. I recommend you DON'T modify the lamp or even take it apart. It has significant safety implications. A lot of these lamps are marginal on safety anyway and even opening them can cause damage that makes them unsafe. As we can't know the original safety design changing or adding components can also affect the safety.

Best solution is get rid of the lamp.

Robert G8RPI.

[SillyOldDufferModerator @sillyoldduffer]

Good news first: the touch screen lamp is at fault, not the transmitter. Touch screen lamps aren't supposed to respond to RF in any form, whether the fundamental signal, spurious or harmonics!

Most likely, the lamp's proximity detector circuit isn't bomb-proof (the necessary components are often omitted), and is being activated by a strong electric field. A different make of lamp might resist better.

The interfering signal could be direct or indirect via the house wiring which can act as an efficient receive antenna and pick up energy from the near field by induction. The near field is a zone of high intensity close to the antenna, weakening rapidly out to about 2 wavelengths, but allowing strong inductive pick up in any wires within 0.159 of a wavelength. So, an antenna transmitting on 14.2MHz (21.1 metres), should ideally be at least 3.4 metres distant. Antennas are best positioned as far and high from nearby structures as practical.

Most likely, the house wiring picking up the near field and is feeding a whopping RF signal into the lamp, which can't cope. House wiring can also respond to the far field. In this case maximum pick up occurs when wire lengths are at quarter waves multiples. This is quite likely in house wiring runs: 5metres, 10metres, 15metres. Might be possible to fix the lamp by shortening or lengthening the lead, or – more practically – by fitting a choke near the lamp. This could be home-made by wrapping several turns of mains cable around a suitable toroid, or commercial items are available. Water and gas pipes, telephone cables and similar can also cause trouble.

The nature of the antenna may be relevant. Not familiar with the sigma, but an antenna matched at the base with a balun is almost certainly unbalanced, which can cause local pick-up. There's a high-risk that despite the balun the outside of the coaxial feeder will become part of the antenna and radiate all the way back to the transmitter. They're noisy on receive too. May be worth choking the down-lead near the top by winding 20 or 30 turns of coax around a 4 inch plastic drainpipe.

Spurious and harmonics are significantly reduced by an ATU even if it isn't tuned optimally. However, although ATUs can usually get a low SWR at many different settings, some settings are better at nailing harmonics than others. Depends on the circuit inside, but best rejection is usually the highest inductance and lowest capacitance combination that achieves an acceptable match. I don't think this problem is due to spurii or harmonics though.

Given the best antenna that can be managed must be close to the house, low power is a good way of reducing trouble, and data modes are popular because steady signals are less likely interfere than peaky intermittent ones like SSB. Balanced antenna cause fewer problems if one can be fitted in: it's hard to beat a 1/2 wave dipole or a centre fed tuned-line antenna performance wise too. End fed antenna are easier to accommodate but more likely to cause RFI.

Probably worth improving the earth. Earth rods aren't good for this purpose because RF doesn't penetrate far into the ground. Only the first 300mm of an earth rod is likely to be effective, so current flow is restricted by the surface area of the thin rod and the conductive quality of the top soil, which is probably very poor. Good radio earths are close to the surface and consist of multiple copper wires radiating out over a large area. A quarter wave of wire, sometimes called a counterpoise, (about 5 metres for the 20m band) laid close to the surface is often a better radio earth than an earth rod. (But note a counterpoise is useless as a safety earth!)

Faraday cage – good idea, but the shield needs to be complete and well-bonded. Windows can be meshed over, but doors need special attention.

I have a different problem: my home is on a corner plot and surrounded by telephone and power cables on poles. The noise level is so high it's hard to make sense of anything other than strong signals.

Word of warning – radios don't like swarf, so keep them well protected from mucky metal working!

Dave

[SteviegtrParticipant @steviegtr]

SOD. Plenty to read there. I'll try the ferrite rings on the mains lead to the lamps 1st. As not sure of the response if I tell the Landlady she needs to buy a new lamp. That would probably not go down well.

Will persevere with it.

Steve

[SillyOldDufferModerator @sillyoldduffer]

Posted by Steviegtr on 18/09/2022 14:49:42:

SOD. Plenty to read there. I'll try the ferrite rings on the mains lead to the lamps 1st. As not sure of the response if I tell the Landlady she needs to buy a new lamp. That would probably not go down well.

Will persevere with it.

Steve

I'm impressed you can share a house with your ex! I'm the loveliest person I know and I couldn't do that.

Could you buy her a new lamp as a present?

RFI problems can be difficult to fix but persist. Quite often necessary to apply a few different countermeasures, each takes a bite out of the problem until the interference disappears.

A compromise arrangement that worked for me is an auto-tuner under a pole at the end of the garden:

This is useful for the lower bands because it gets more wire in the air and a loading coil can be added. Ideal if a chimney stack is available because of the extra height, and it might also allow the wire to be carried to the front of the house or further. But with loss of height, some versions support the antenna with a line over the ridge line, or attach it to the gutter. The length of the wire isn't critical, but a loading coil improves efficiency on the lower bands if they're a particular interest.

The main advantage is most of the radiation occurs away from the house and the visual impact can be kept low. The disadvantage is the cost of an waterproofed automatic tuner. I bought an expensive pair of MFJ Bias T power injectors, which allow the ATU to be powered via DC on the coax; with hindsight bell-wire would gave been cheaper.

A useful trick when experimenting with antennas is to use the SDR websites to listen to yourself (I send callsign in CW plus a few dots to reduce the nuisance). Quite interesting to see how strong, or weak I am in the Netherlands or Moscow. WSPR is wonderful for automatically testing long range performance, which vary with time of day and 'conditions'. It showed my lack of contacts is because I can't hear replies due to high local noise – the antenna is OK on transmit.

Dave

[Robert Atkinson 2Participant @robertatkinson2]

You need to be VERY careful when adding "earths" including radio antenna mountings and counterpoises that are in any way (including via DC power supply connections) to the mains supply protective conductor "earth".
This is because most modern domestic mains istallations reley on the neutral connection (back to "ground" at the substation) for the protecive connection. Connecting an "extra" earth is not an issue under normal conditions, but if thare is a break in the neutral you earth can carry the load current of yuor house AND others on the same connection. This is all unfused and can result in a fire or electric shock.
Take professonal advice.

Robert.

[SteviegtrParticipant @steviegtr]

The supply to the garage / workshop is fed with a SWA cable which i ran underground when building the structure. The house mains is a pair of conductors which run across 5 terrace houses on the brick wall. Each house has a drop from this & into the cupboard where it is through a 60A cutout. There is a ground connection which disappears under the concrete. It is bonded to Gas & water mains. Which i know the gas is now ran in plastic so no earth there.

The SWA has a 45A RCD which i fitted at the Henley block. The earth sheath & 1 conductor are connected to the house earth. In the workshop the SWA is glanded to a steel consumer unit. Earth bonded to that & the garage wall by a earth tag. The earth grounding rod i fitted outside was to ground the Tranceiver & other bits. The Coax sheath must be grounded to the garage as it passes through the wall & has a connection .

I am going to do as others have suggested 1st , which is ferites on the mains leads to the lamps. If that does not work then i will have to look further. No point in doing anything else just yet.

Steve.

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