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Receive Antennas (below 30MHz) - For all topics related to receive antennas used on HF, MW, LW, etc. For transmit antennas use the Amateur Radio Antennas forum.

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Old 08-20-2017, 4:22 PM
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Default QRM eliminator

After a lot of OT entries on the "The making of a Dipole" thread, I decided to start a new one in view of the interest.

Here's a copy of my last entry...

Quote:
The "SEM QRM Eliminator", the "WiMo" and the "DK9NL X-Phase" are so similar in the circuit that I suspect that they are direct crib of the SEM - the SEM is about 10 years before the DK9NL - only the connections of the phasing transformer are different - even the component values are the same. I'm in the process of removing all the Tx/Rx switching from the schematics to make just a 'receive only' version - just a handful of components - watch this space!
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So here's the schematic of the proposed build - time to hit the junk box! After I drew it out I see that the only difference between the SEM and the X-Phase is a ground on the wiper of the "Phase" control. I changed the FET's to something available to me with slightly better specs than the original J310's.
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Old 09-01-2017, 4:03 AM
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Slowly getting there - got all the components and assembled them on a bit of perforated board that fortunately fitted the box - a couple of hours more work and it should be ready for the initial testing although I have not yet put up a 'auxiliary' antenna to pick up the noise!
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Old 09-01-2017, 4:15 AM
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Looking good! Thanks for the picture and keep us updated.
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Old 09-02-2017, 10:36 PM
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Too bad they're not very effective for high RFI locations...
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Old 09-03-2017, 12:43 AM
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A bit of a setback here! I've completed all the wiring and double checked everything but it don't go! I can't even get a signal through on the "main ant in - receiver out". The original schematics used a J310 JFet but they are obsolete now so I subbed the 2N5485 which I though would be a good alternative but no go. Two more approaches - rewind the transformer on a different core and/or replace the 22ohm in the 12v supply with a choke.
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Too bad they're not very effective for high RFI locations
Agreed - but then nothing is! They're certainly not the magic bullet, but all I want to do is reduce the noise from one direction from a plasma or faulty TV - they certainly won't kill interference from another station as the path on HF is continually changing.
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Old 09-04-2017, 5:10 PM
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Hopefully Iv'e found a source locally for some J310's. Subbing the resistor for a choke made a small increase and I proved it does work by eliminating some local noise but can't get my original signal strength through on the Main Ant line which makes me suspect the FET's.
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Old 09-04-2017, 5:19 PM
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Looks like you're on the right track. Hopefully you can find a better replacement for the FETs!

Jim
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Old 09-08-2017, 8:37 PM
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A bit of research and found how to test JFet's with an ohmeter which showed they were both good and also found that the 2N5485's were very close in specs to the J310 so that was a blind alley. On a whim I modified the circuit to the SEM variety instead of the X-Phase and - hey, it works! A temporary connection of my discone as the Aux antenna and I can definitely enhance or reduce an interfering signal and the Main Antenna gain is about the same as the original signal - yippee! When it stops raining and blowing I'll rig up a proper Aux antenna.

New schematic and a picture of the final article attached.
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Old 09-10-2017, 7:49 PM
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I don't know what you have for a main antenna but I've had good luck with a noise antenna running 90 degrees away from the main antenna. The feed points of the two antennas are only a few feet apart.

Jim
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Old 09-11-2017, 4:34 PM
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It doesn't show up very well in the pic, but the near mast is one end of the OCFD, the mast with the discone is the middle of the OCFD and there's another mast right down the other end that's the other leg. There's another redundant TV mast that you can't see over the garage roof where I'll take the noise antenna to, from the discone mast again. The centre mast is right over the room where all the radios are. This is quite an old pic, there's other antenna hanging from everywhere now! I must take a new pic sometime with better detail. Unfortunately that tree is not on my property!
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Old 01-11-2018, 9:50 PM
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Nice work, wish I had some technical skills like you guys!

I have an old S.E.M. QRM Eliminator Mk-2 but no wall adapter for it. Since it uses an RCA jack how can I use it, I can't find a wall adapter that will plug into it? Thanks for any help!
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Old 01-12-2018, 7:10 PM
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Quote:
Originally Posted by pjxii View Post
Nice work, wish I had some technical skills like you guys!

I have an old S.E.M. QRM Eliminator Mk-2 but no wall adapter for it. Since it uses an RCA jack how can I use it, I can't find a wall adapter that will plug into it? Thanks for any help!



Doing a little research, that unit was produced in early 1990's. You probably won't be able to find an adaptor for it.

I found a schematic for it and it looks like it needs a 12VDC supply for it.

If you are handy with a soldering iron you could build one for it using a spare 12V 500ma wallwart power supply. With a RCA audio cable and some heat shrink tubing or electrical tape and a DC voltmeter it can be easily made.

Many of us here could step you through it if you are interested.

Jim
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Old 01-12-2018, 11:12 PM
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Default QRM Eliminator

Thanks, Jim. I actually have soldered a few PL-259s to coax before so I'm up for giving it a try. If anyone could give me step by step instructions so I don't mess up the positive/negative I'd appreciate it.
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Old 01-14-2018, 11:34 AM
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Quote:
Originally Posted by pjxii View Post
Thanks, Jim. I actually have soldered a few PL-259s to coax before so I'm up for giving it a try. If anyone could give me step by step instructions so I don't mess up the positive/negative I'd appreciate it.

I've looked at pictures of the inside of your unit here online and it looks like the positive wire(red)comes from the center post of the RCA receptacle and goes to the ON/OFF switch. You may want to open up your unit to verify this.

If this is the case you will need to find the positive wire from whatever you are using as a power source(wall wart or stand alone power supply). If using a wall wart cut off the DC end of it, separate and strip the two wires. Using a voltmeter in the DC setting mode get a positive reading on the red prod of your meter when touching the two prods across the two wires. On some wall wart cables a white tracer is on one of the wires to indicate which one is positive. Make sure the wall wart is rated 12VDC(make sure it is DC as some wall warts will supply 12VAC) and at least 350mA(a larger amp reading will work fine).

On the RCA cable you will need to find which wire goes to the center pin. This may be obvious if it is a coaxial type cable. If not use the continuity mode of your meter to find out which one goes to the center pin after cutting off one end and stripping the wires.

Once this is completed solder the positive lead of the power supply to the center pin of your RCA connector. Then the negative lead to the other wire. Make sure the solder joints are separated and covered with electrical tape or heat shrink tubing when you are finished. Then double check your RCA connector with your voltmeter to verify positive voltage on your center pin before plugging it in.

Good luck and feel free to ask any more questions.

Jim

Last edited by a29zuk; 01-14-2018 at 11:39 AM..
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Old 01-14-2018, 2:29 PM
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Everything that Jim said plus:

Make sure you have a transformer type wall-wart NOT a switch-mode supply - the weight is the give-away - the transformer model is far heavier size-for-size.
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Old 01-14-2018, 5:54 PM
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Quote:
Originally Posted by majoco View Post
Everything that Jim said plus:

Make sure you have a transformer type wall-wart NOT a switch-mode supply - the weight is the give-away - the transformer model is far heavier size-for-size.

Thanks Martin......forgot about that!

Jim
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