QRM Eliminator

Hi All,

In the past we've had members ask about the usability of a typical QRM eliminator for their radio operations...

The responses (including my own) weren't all that conclusive with some finding them invaluable and others next to useless. Of course there's always the issue of the PTT (push to talk) line connection and all sorts of ingenious methods including RF switching have been used by many often requiring some sort of modification.

I'd built a unit similar to the many types out there (schematics/circuit diagrams are everywhere) but as I didn't have much luck with it the whole shebang hit the 'trash' pile. (Probably later stripped for parts as I can't find it again. )

However, more recently I found myself hankering for some sort of local QRM control and looked back at them but this time decided to purchase a ready made unit rather than build another. The unit I bought was this one :-



They vary in price (you'll have to shop around) but this one took my eye due to the extras that came with it. I wasn't going to use the little antenna or the SMA/PL259 adapters on my setup but have found them invaluable for utilising with my NanoVNA. (Cheers again Alan mate. )

The other leads were added to my box of leads I keep handy and of course as with everything radio I had to buy a bespoke PTT lead to use with my Yaesu FT450. (Why do manufacturers do such things as unusual leads? I had hoped an old mouse Min-DIN would do the job but pin 2 used for the PTT is not connected on an old mouse lead! You just have to suck it up but again shop around as the 'official' lead is expensive and can be obtained cheaper from other suppliers.)

I will tell you now that this isn't all simply plug-and-play, you will have to experiment a bit....

For instance, the first thing that you have to remember is that this is designed to 'eliminate' local QRM/Interference so don't expect results if you're experiencing typical atmospheric noises you'll find on some bands. Unfortunately some videos showing the use of such items falter at this stage.

The other important thing is you'll have to experiment with is the 'noise' antenna you use. If you utilise anything too small you won't pick up much of your local QRM unless of course it's coming from your laptop/PC in your shack, in which case I'd advise you look at ferrite solutions instead. If the 'noise' antenna is too large you'll pick up the signal you actually want to receive and it'll be attenuated heavily along with any noise. Again I've witnessed this mistake in typical demonstration videos.

I had to play a bit to get things 'just right' and landed up with a 20 foot wire that runs vertically for a bit up my neighbour's adjoining wall (where a lot of QRM from modern devices comes from!) as well as horizontally across my ceiling/loft floor space where my antennas are mounted. This seemed to work out just about right for my use.

The QRM eliminator is actually fairly simple to use once you get used to it and I found that if I tuned to my signal of interest, turned on the unit with 'Gain 1' turned down, I could twiddle the 'Phase' & 'Gain2' controls until I'd cancelled out the noise. Turning back up 'Gain1' brings in the signal you want. Sometimes the results are very surprising and a signal buried in the noise jumps right out!!

Don't be disappointed if it doesn't seem to work as well as you expect as sometimes signals are so weak you'll never dig them out.

Also worthwhile to utilise old tricks first such as turning on any attenuation controls or reducing that RF Gain control. (Most seem to turn that one all the way to 11!)


So have I found it of any use?

Absolutely! From a spot of SWL activity pulling in Far East SW signals I couldn't hear otherwise to popping in a bit of clarity to QRP stations it's all been on the whole a very good experience. Just remember the caveat that you're subsiding local QRM not atmospheric noises. (It also gets rid of the 'clicks' when someone turns on a light or socket as well as when my wife turns the washing machine to full speed spin!!)

In fact I found it so useful that as well as becoming a permanent attachment to my station I modified it somewhat to my own tastes.
(That's any warranty out of the window!)

For starters the 'blue' LED switch on the front is bright enough to burn out your eyeballs! A 10K resistor added to the LED side of it was enough to tame that annoyance down. (Three middle connectors are the actual switch contacts, the two outer are the LED, just add the 10K to the positive side after disconnecting the 12 volt lead from it but not the switch contacts.)

There are as ever talks about 'upgrading' the relays in such units but the ones that came with mine could easily handle 100W of power if required. As I'm still a Foundation holder (by choice thanks very much ) that won't be an issue for me. Up to you if you undertake such a thing.

I did however add some TX/RX LED lights so I know what the damn thing is doing as along with the relay clicks on the radio it was hard to discern by ear.

The circuitry for such is shown below and very simple, (not complicated like some units I've seen and a bit of stray RF will render them useless!) :-



Nice & simple, just the way I like it.  

The circuitry to the left of the image is the original relay transistor switching on the QRM eliminator, just add the circuitry to the right. I simply drilled a couple of holes above the power switch to take 3mm LED's, soldered the resistors to them and then three fly leads for the ground, positive and signal wire from the collector/relay diode arrangement.

Great seeing a green RX LED and switching to red TX upon transmitting akin to CB radios of old.

As usual take on any such modification at your own risk and if you can't identify the cathode end of a LED or diode then don't bother.
(Same goes for knowing which end of the soldering iron is the hot end....if you smell BBQ, you're holding it wrong! )


So there you go, more of my 'waffle' and hopefully of use to at least one person.

How about you? Have you purchased and utilised such a unit to your satisfaction or did you think it was a waste of money?

Let us know in the comments.


(BTW, this isn't of much use utilising with FT8 as you'd think, the software behind such operations already does a great job of digging out signals from the noise. It did however get rid of an annoying SMPS signal that strays & wanders across my 20m reception every so often. Worth it for that alone.)

All the best,
Victor

A great writeup as always Victor. So happy for you that you got the unit to perform, and some nice little and useful mods you've made to it that the rest of us should be able to follow and copy if we desire to.

I haven't played with one myself to date, so can't and shouldn't offer any opinions on the factory standard unit. I did see the below short video a while ago which I thought very interesting as an alternative use for it with ultimately the same desired result of less QRM...
https://youtu.be/vJ3RT4UfUY0?si=Yi5HDoEmRh3va3oU

Cheers Neal mate.  

I know it's all a bit of 'old-hat' now (everyone chats about the latest thing....currently the Quansheng handies) but thought if it works it doesn't matter how old the 'tech' is.

I'm happy with how it's performing and did also think about utilising it such as you've pointed out with the video link but recall reading about it from a Dutch Ham regards HF diversity reception. I might have to pull out my old active loop and have a play.

As ever there's always something to learn, experience and appreciate with this radio hobby and just nice to share some of the adventures in case it's handy for anyone else. 

Been a little while since I wrote up anything but I've been enjoying a revisit to the VHF/UHF airwaves recently and getting my station sorted up there. Certainly makes a change from the stream of call sign applications we see on the forum and very little of anything else......

(Hint, hint - get your writing gear on and make some postings all of you. )

As ever, always appreciated there Neal and hope you're keeping well.


All the best,
Victor

Hello Victor,

A great review and modification of this device. Great technical knowledge from you as always.

73s John.

I bought a second hand one (old obscure make from yesteryear) from my local ham club's junk sale night for a Fiver but when I set it all up and twiddled, zilch effect, just like it wasn't in line. Got consigned to my junk box. May have to consider buying a new modern unit to killl my HF QRM.

Have you got a link from where you got yours Victor ? Thanks

Hi David,

They are easily blown unfortunately if the PTT line isn't used properly and then they're rendered useless. I didn't fancy buying a second hand one for those reasons even if I could repair it myself.

I don't really like giving out eBay links and such like due to obvious commercialism or even comebacks should anyone have a problem with a particular seller. Let's just say I did the usual of checking feedback scores and it was only a tenner more to purchase from a UK vendor so arrived two days later. (It was less than £50 but worth it when you count the connector/adapters etc. that I utilised elsewhere. )

As I mention it isn't really a plug-and-play bit of kit and did take a few days to get the 'noise' antenna just right and learn how to operate it effectively. (Sometimes it works brilliantly and other times it won't dig those signals out but when it does it's invaluable!)

Sorry to not be more helpful but my intentions are usually honourable ones.

All the best,
Victor

Was it a kit or ready assembled Victor ?

That was fully assembled David.  

Despite eventually modifying it the old eyes and hands aren't quite as dexterous as they once were.
(Especially with any surface mount stuff!)

Thanks for bringing up the subject of the QRM Eliminator, Victor.

I tried the QRM Eliminator to get rid of my severe Powerline (PLC modem) QRM at both my locations in 13 and 19 div. with very little success. I was able to reduce the (PLC) noise to signal ratio somewhat but not to my full satisfaction. I think the reason is that the PLC noise doesn't come from just one direction, but is all over the place because it is radiated from the mains wiring in the walls.

The principle of the QRM Eliminator is to feed the interfering signal, ideally picked up by a second antenna that only receives the interference,  into the signal path of the "wanted" signal, but inverted by 180 degrees, so that it will null out the interference. The problem with PLC seems to be that there is no defined phase of the interfering PLC signal. It comes from everywhere in the house and there is no defined "anti phase" (180 degrees) to be obtained by the "noise antenna".

Luckily at both locations my problem has been solved more or less in the meantime, but not by the QRM Eliminator. In 13 div. the neighbour seems to have realised that the pre-installed network cable in his house is there for a good reason. In 19 div. the PLC interference is not more than S2 to S3 and only at frequencies higher than 27.600. Probably some new PLC equipment in the neighbourhood.

I think the Eliminator can solve certain issues with QRM, but for the present-day PLC it is not the ideal solution. Wish you all a clean frequency.

Zintus

Hi Zintus,

It definitely won't cure everyone's QRM woes and especially for those unfortunate enough to suffer with the effects of power line communication technology. (What a terrible only-for-profit driven idea!)

Along with my efforts at cleaning up my mains and a plethora of well placed ferrites my QRM eliminator seems to be doing the job for me. I hope that everyone can clean up their station reception just as effectively.

All the best,
Victor