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Posted: Tue May 24, 2016 9:15 pm
by JMG
Numerous articles and YouTube videos have been published on how to deal with RFI issues. Most proposals end up proposing creative shielding designs or by asking for a metallic casing. With limited reliable results.

RF interference manifests itself as a higher noise floor and as spurious signals. To analyze these issues, the RSP has been taken to an EMC test range.

The measured results highly depend on the test frequency and the setup of the test configuration. All tests were performed at a frequency of 149MHz. The length of the cables connected to the antenna port was 80cm. Different results may be obtained for other frequencies and setups, e.g. in SW bands.

How do unwanted signals enter the receiver?

The most common way how spurious signals may appear is by loading the receiver with strong signals from the antenna. Less efficient antennas, filtering or even a resistive attenuator may help. But this is a topic for a different discussion.

The cables connected to the antenna and the USB port form a dipole antenna. The signal picked up between the ground terminals of these cables spreads over the board and finds its way to the receiver. This received signal has been measured to be >40dB lower compared to a real antenna placed at the same position.

A good and proven practice to keep this antenna signal from entering the receiver is to add a choke (ferrite) to the antenna, to the USB cable or to both. The choke can be placed quite elegantly within the casing over the cable connecting the antenna port and the board.
The chokes do not keep the USB signals from entering or leaving the board, however. These signals are bound within the shielding of the cable.

A really efficient solution (to be considered for future designs) is to short the ground terminals of the antenna and the USB connector with as low an impedance as possible. Both the connectors should be close together and not at opposite sides of the casing. Connecting the ground of the two ports results in a signal attenuation of >15dB.

The board itself may also receive signals. The parasitic antennas on the board are quite inefficient. The received power is >50dB below that of a real antenna. Hence, this effect is of less importance.

Due to the limited shielding effect of coax cables, the antenna cable itself may pick up RF signals. These signals will not be suppressed by the chokes placed over the cable. The signal received via this path has been measured to be >30dB lower compared to a real antenna. These tests have been run with a well-terminated RG58U cable of length 80cm. This effect may appear for strong close-by signals entering the antenna cable. The shielding effect of coax cables has not been measured yet at lower frequencies, such as those of noise emitted by switching regulators. The effect could be worse there.

In summary, the most important impact will be from signals picked up by the dipole antenna formed by the cables connected to the antenna and the USB input port of the receiver. These signals have to be blocked from entering the receiver. Signals penetrating the coax shield may also result in RFI. The effect of signals entering directly into the board is of minor influence. For the given test, setup no effect on the noise floor has been observed.


Posted: Sat Mar 18, 2017 11:12 am
by JMG
A common mode rejection filter has been added to the USB data lines. This will give some additional improvement in noise suppression. Addding the filter to a RSP2 is not easy at all. The traces on the board are VERY small.
In conjunction with a very short connection between the ground ports of the USB- and the antenna connector all the ferrites may be omitted.


Posted: Mon Mar 20, 2017 10:32 am
by CaptainNemo
Thanks JMG for sharing with us this summary.
There is a lot of post on this issue, I find your summary/report is well structured an full of interesting insights.
I look forward to hear more hints from you.


What about the PC connected?
I ever used a laptop, since I started this hobby.
There are particular hints when using a laptop vs a desktop or a deskside?



Posted: Mon Mar 20, 2017 5:31 pm
by DanubeBCL
The major problem is not RFI created in our own house or created by the construction of the receiver itself (shielding, computer noise a.s.o). We BCLs all normally have the technical skill to overcome such noise with shielding, choke filters, common mode chokes, grounding, galvanic isolation, whatever. Most of us have listening and technical experience for more than 50 years and know how to deal with electromagnetic noise.

The major problem is noise coming from badly filtered devices in the neighbourhood (like switched PS, lawn mowing robots and all the rest) and devices specially designed to distribute high level broadband noise (like homeplug powerline PLC/PLT modems). We cannot do anything against such noise which comes in through our antennas for these reasons:
- The neighbours are stubborn and do not co-operate.
- The authorities do not take any action.
- The noise levels are legally allowed thanks to intensive lobbyist work of the manufacturers.
- Courts often judge to the detriment of the complainant with the argumentation that noisefree old style radio reception is no longer necessary because there are other sources of information available.
I once had a talk with a lawyer who said: "It is merely impossible to convince a judge that listening to a weak radio station 10000 miles away transmitting most of the time in a language you do not understand is vital for your personal lifestyle."

The "noise" created in our own devices is the least problem.

73, Heinrich


Posted: Fri Nov 17, 2017 9:46 am
by JMG
Why have the modifications not been applied to the RSP1A?
Consequent grounding of the antenna- and the USB-port and/or internal decoupling result in a dramatic improvement in interference immunity.


Posted: Fri Nov 17, 2017 9:26 pm
by vk7jj
DanubeBCL wrote:The major problem is noise coming from badly filtered devices in the neighbourhood (like switched PS, lawn mowing robots and all the rest) and devices specially designed to distribute high level broadband noise (like homeplug powerline PLC/PLT modems). We cannot do anything against such noise which comes in through our antennas for these reasons:

Hi Heinrich,

For many years I operated under a similar unspoken assumption:

- that the noise from those devices was radiated from somewhere near their physical location.
- because it was remotely radiated RF I could do nothing about it.

I've only recently personally challenged that assumption and found it to be only partly true.

My experimentation shows that a major vector for delivery of noise from those sources is in fact *conduction* via your mains wiring.

Think of your mains wires as a very high quality parallel [pair/trio] of conductors connecting your shack *directly* to every actively radiating device in your neighbourhood. Earth is irrelevant as an RF sink at the point of generation.

Additionally, when the wires bearing that conducted RF arrive at your shack, from the point where your coax braid parts company with your receiver, your coax braid starts radiating the RF signals which are present as a potential against "ground" and against "mains active and neutral".

Any doubt about the ability of a nice copper mains pair of wires to carry signals far should be dispelled when we consider that ADSL and ADSL2+ carry RF very successfully using a bucket of frequencies up to and including 1.1 MHz to 2.2 MHz respectively and do so at very much lower power levels for many kilometres underground.

The points above are quite easy to prove to your own satisfaction if you set about it the right way.

And for others reading this and looking fondly at the ferrite choke on their USB cables, a little research will show you the ferrite grade in use is probably only good down to a couple of MHz and only presents an impedance in the order of a 100 ohms or so to the nasty RF present on the "single turn" of USB wires passing through it.

Happily, the impedance offered by such a ferrite device increases in proportion to the square of the number of turns passing through it, so relatively inexpensive ferrite tubes can provide in the order of a 1K ohm of impedance when multiple turns of coax or USB cable are passed through two or three tubes bunched together, or two sets of three tubes. I've done a great deal of work on this topic over the last twelve months and reduced my noise levels by well over 20dB. The satisfaction is immense :-)


Posted: Sat Nov 18, 2017 8:06 am
by Rdube123
You are right


Posted: Sat Nov 18, 2017 12:43 pm
by DanubeBCL
Hallo Phil,
although mains lines of course carry a lot of noise from the neighbouring houses into my own house they are not the problem.
1. The power lines entering my house run through strong RF filters.
2. I use intensive grounding with additional earth rods at multiple places in my antenna installation.
3. All my coaxial cables go through common mode chokes made of 40 turns RG 174 on an Epcos N30/42 (6.6 mH inductance) in series with 30 turns on an FT140-77 (2.5 mH inductance).
4. All my coaxial cables are buried in the ground by appr. 90% of their lengths.
5. All my PCs have additional extra line filtering.
5. USB cables (when necessary) are wound through big ferrite cores. But USB is not a problem. May antennae are appr. 50 m away from my house.

The RF noise goes directly into the antennae! The noise comes from
1. (Most evil source): Homeplug PLC modems
2. Flatscreens TVs and all sorts of switched power supplies
3. Lawn mowing robots (mainly their charching docks)
4. So called "smart" homes with carrier based powerline communication
5. Pedelec charching stations
6. Photovoltaic inverters
7. ...?
These devices are all in the neighbourhood, not in my house! I can receive the noise from surrounding houses within a distance of more than 400 meters. Noise level (depending on distance, source and activity) between S4 and S9+20dB! Believe me: You cannot do anything against that noise except for trying to bring your antennae more than 1000 metres away from these sources (remote receiver). But this is a big fincancial problem for me. I cannot expect help from the national authorities here because either they don't care or the noise is fully legal (lobbyyist work by the manufacturers)!

73, Heinrich


Posted: Sun Nov 19, 2017 8:52 am
by vk7jj
DanubeBCL wrote:Hallo Phil,

Hi again Heinrich, thanks for taking the trouble to reply in detail, what a comprehensive arrangement you've put in place, a genuine pleasure to read about.

My location before this - which is 40+ bush acres in semi-rural Tasmania, with the radio shack inside a large Colorbond metal shed AKA Faraday cage - was right on the edge of the sea, a wonderful take-off point but abysmally noisy from DC to well over 2m owing to the effects of salt causing arcing across power line insulators. The short drop down interconnections where one set of wires passes over another at right angles were close to resonance on 2m resulting in a storm of noise on that band, so the noise there was almost totally radiated RF.

Having experienced both scenarios, ie lots of radiated noise and lots of conducted noise via power lines I now see the situation as the classic piece of string where each individual has to battle her own RF environmental demons and the first step in that battle IMHO is as ever knowledge and information.

The simple assumption that we can't do anything about our noise because it's all radiated somewhere else and our antennae are nothing more than powerless victims is the saddest of self fulfilling prophesies.

One way to carry out a quick check for those interested is wandering around with a hand-held like a Yaesu VX7 or 8 that does HF on AM, which is good for a noise sniffer. The rubber duck is fine for finding local noise sources that correlate with visible problems on the SDR waterfall and a dozen turns of wire around a ferrite rod (a loopstick to those in the US) is great for more distant noise sources as well as helpful directivity.

The lightbulb moment occurred to me when I grounded the handheld's antenna socket to mains earth, causing the noise level to increase dramatically without any commensurate increase in the distant weak short wave radio station I was listening to. That was the start of a long journey of testing and evaluating solutions.

I've gone the opposite way to you, I have no earth in my radio shack, everything floats. Sure, my several masts are grounded well at their base but that's irrelevant. I have connected my coaxial braid feed-ins to the steel shed at the point where they exit resulting in a good noise reduction for my VHF and UHF systems where coax is the easy solution, and moved all my HF to a combination of either home made teflon twisted pair feeders where the lower impedance is a better match to the relatively low impedance of my 160m loops, and otherwise use 450 ohm twin feed.

I have given up any hope of getting rid of the noise on end fed wires and my off-centre-fed HF doublet.

My radio shack is full of extremely noisy devices, several Raspberry Pi's, a DVR video surveillance system, two large solar inverters, Windows boxes x 2, Macintosh x 2, WiFi router, a gigabit switch, all sorts of other digital stuff - yet all noise into my antennae has been eliminated with the right feed lines, feed line chokes and strategic connections between braids and metal shed.

After many years of enthusing over a large range of ham radio activities it's a bit amusing that "noise" has turned out to be one of the most interesting things to learn about and experiment with.

73 Phil.