RX Common Mode Filter Choke / Noise Suppressor / RF Isolator

[nanZor]

I am using a mix of two products also:

1) the myantennas CMC-0510-R. Being rx-only, Danny was able to get really really good choking impedance down low to 500khz. Most amateur transmit versions drop off rapidly below 3.8 or perhaps 1.8 mhz, so for rx-only, I think the myantennas rx version is ideal up to about 15 mhz or so. Still usable higher up, but I did this too:

2) I followed that up with about 5 MFJ-700-A4 snap-ons to my RG8X / lmr 240 just for a little extension higher up. And an MFJ 915 at the shack end. But yeah, winding multiple turns through a core is the most effective.

Sure I could have followed up with another choke with a different mix for higher frequencies, but this seems sufficient for now. I might replace the shack end with another 0510-R, but for now I'm happy.

 

[db_gain]

For cores that are good at vlf, don't toss out that pc psu that fried, those toroids are optimised for vlf.

 

[nanZor]

All this time I never realized that putting an rf-choke / isolator up at the feedpoint was only HALF the story until I started to visualize it. Despite excellent tech advice from W8JI and others, it is now plain as day as to why you need TWO - one up at the feedpoint and one near the receiver (or better yet your service panel bulkhead)

What I finally saw:

1) You have dutifully obtained the million-dollar choke / isolator and duly placed it near the feedpoint. Peachy. Now your antenna is no longer influenced by the common mode of the feedline, maintaining the desired pattern, and noise ingress from the shack itself is stopped from entering the feedpoint that way.

2) You STILL have what is effectively a random ground wire attached to your receiver's ground lug, and snake it around the coax and tape it off just before the choke. Sure it's isolated from the feedpoint, but consider it like taping some wire all up your feedline.

If you have a perfect RF-ground, well ok. But if not, you have a nice long extended ground wire attached to your house elevated well up into the air right next to your coax. You get the idea without actually doing this.

Still hearing little gremlins after you placed the million-dollar choke up near the feedpoint? Are you assuming those gremlins are being directly received by the antenna itself and just shrug it off?

Or could your elevated random ground wire wrapped around your coax going nowhere be a sneaky entry point - even though it is not connected to the center conductor of anything, finding it's way in?

The test is simple - disconnect your antenna up at the feedpoint, leaving the choke connected even, and listen for the gremlins across the bands. Now install another choke near the receiver and try again. If you have a perfect ground, congratulations. If not, well there you go.

Only NOW do I truly understand why W8JI and a slew of others recommend grounding your coax *between* two chokes!

I can't explain why it took DECADES for me to see it in my mind. Thanks guys, this thread made me visually see it mentally.

 

[iMONITOR]

All this time I never realized that putting an rf-choke / isolator up at the feedpoint was only HALF the story until I started to visualize it. Despite excellent tech advice from W8JI and others, it is now plain as day as to why you need TWO - one up at the feedpoint and one near the receiver (or better yet your service panel bulkhead)

What I finally saw:

Aliens!

 

[nanZor]

What's mind-boggling is that I incorporate the common mode in a number of antennas I use on purpose - and choke the heck out of where necessary.

But now, the light bulb went on - it's like attaching a long random wire to your ground terminal and placing it up in the air. Probably not the best of ideas, and nobody would do that on purpose, but that's exactly what's going on when you choke only one side of the transmission line. (ie, feedpoint isolation vs grounded "random wire" isolation.)

This has some pretty big implications for me and may start testing *indoor* antennas again.

Example - I hang a nice little loop near the window and choke the coax near the feedpoint there. BUT, even with a small run of say 15 feet of coax to a receiver on a nearby desk, that could still be considered 15 feet of unruly random *ground* wire passing through my indoor environment.

Thus, even for that small a run, being indoors I'd surely contemplate using TWO common mode chokes at opposite ends of the coax run. Especially if one is in an apartment or other dwelling where we don't have the luxury of a real nice rf ground in the first place.

I've got yet another choke on the way, so I'll let you know when I whip up something indoors again.

Thanks again iMonitor - you got my juices flowing on this one....

 

[iMONITOR]

Thanks again iMonitor - you got my juices flowing on this one....

You're very welcome, but I must confess I had no idea what I was doing, just trying stuff out. I hear ya, It's been interesting. I never used these devices. Mine is indoors also and my coax is only about 15 feet long. It's really worked well for me, but I confess, I'm not sure if I fully understand what's happening, as it seems different situations call for different placement and give different results.

 

[nanZor]

INDOOR test of second choke proves successful!

Previously years ago I had strung an indoor loop in a bay window with the bottom at about 3 feet high. About 16 feet in circumference to an LDG 1:1 indoor choke balun. And about 16 feet of coax to the receiver. Thought that was all I needed. No rotation possible of course.

Results were iffy, but I knew that being indoors presents it's own problems with coupling.

Thanks to iMonitors prodding, I tried again but this time I used an inline choke near the rig as well. Coax is randomly strewn on the floor the way it was before. Wires hung with strips of blue painter's tape for support away from the window a few inches. The "real" owner of the house says that this antenna, even with it's white wiring from an unused extension cord is a limited-time offer. Time to hide it behind drapes.

Well, what a difference that makes. Previously I had fooled myself by using a quiet linear supply, but this time used the switcher with it's little growlies on the low bands. Yep - choke made the difference - oh sure, I can still hear the growlies, with the actual antenna closer to the supply, but at a much reduced level with less interaction with the coax when I swap the receiver-side choke in and out. Not as good as my outdoor antenna, but the difference was noticeable.

No "tuning", other than having to run my receiver's 10db preamp.

Zheesh - all these years man, thinking that just a feedpoint choke, even with a short run was that all was needed. In my situation, I needed one at each end especially on the indoor antenna. Gonna' be fun tonight!

 

[prcguy]

Did you ever get anything from W6LVP? His loop experimenters kit allows you to make a small loop from wire, maybe 1ft dia to real big and attach to his balanced preamp. A 2 or 3ft active loop like this might outperform your passive loop and its easier to hide.

INDOOR test of second choke proves successful!

Previously years ago I had strung an indoor loop in a bay window with the bottom at about 3 feet high. About 16 feet in circumference to an LDG 1:1 indoor choke balun. And about 16 feet of coax to the receiver. Thought that was all I needed. No rotation possible of course.

Results were iffy, but I knew that being indoors presents it's own problems with coupling.

Thanks to iMonitors prodding, I tried again but this time I used an inline choke near the rig as well. Coax is randomly strewn on the floor the way it was before. Wires hung with strips of blue painter's tape for support away from the window a few inches. The "real" owner of the house says that this antenna, even with it's white wiring from an unused extension cord is a limited-time offer. Time to hide it behind drapes.

Well, what a difference that makes. Previously I had fooled myself by using a quiet linear supply, but this time used the switcher with it's little growlies on the low bands. Yep - choke made the difference - oh sure, I can still hear the growlies, with the actual antenna closer to the supply, but at a much reduced level with less interaction with the coax when I swap the receiver-side choke in and out. Not as good as my outdoor antenna, but the difference was noticeable.

No "tuning", other than having to run my receiver's 10db preamp.

Zheesh - all these years man, thinking that just a feedpoint choke, even with a short run was that all was needed. In my situation, I needed one at each end especially on the indoor antenna. Gonna' be fun tonight!

 

[nanZor]

Didn't get the LVP experimenters kit yet. But, what I know it will do, is it will allow one to use a smaller circumference loop, and get sharper nulls than the 4-square footer non-rotatable one I just whipped up near a window. Very important indoors especially.

I'll tell you this much - always being a step behind KK5JY, my fixed-position lash-up indoors was *good* compared to the on-ground loop, BUT the little LDG feedpoint balun just couldn't deal with anything below about 2mhz. So 160m was a total washout, along with BCB and VLF with nothing but horrendous noise that no filter in the receiver could tame. But good on higher bands - once I put another choke near the rig since my grounding is pretty awful. Or for when I run solely portable on batteries too.

BUT, I noticed that he also switched his in-air loop to an isolated transformer, rather than a feedpoint choke. I did the same with the PAR EF-SWL transformer (jumper between 1&2 disconnected, and loop wires going to 2&3).

THAT is the best combo I have ever used indoors! The PAR / lnr precision transformer and the Myantennas CMC-0510-r at the receiver end got me BCB and 160 meters back! Indoors! Incredible.

One can always build their own, but this off the shelf combo has me blown away. Not a "kinda' , sorta' , close your eyes" kind of thing, but BLAM. That isolated transformer at the loop feedpoint was key.

I'm doing A/B comparisons between two radios (Kenwood 590s on the LOG, and the 4-foot square sided window lashup on an Icom 718), and fortunately they have the same pattern in the same direction. (The log having the same directional pattern as a small vertical loop)

I should put the window-loop outdoors but didn't have time. But I can already hear the similarities, so it will be a blast later.

Not to take away anything from LVP - I may still get it in order to obtain sharp nulls, much sharper than these larger types can provide.

 

[nanZor]

Indoor loop vs outdoor log test:

The indoor loop, having the same pattern as the log compared favorably to the log, but there are important differences:

Indoor loop transformer - like the outdoor log, the small vertical loop benefitted from an isolated transformer. However for the small indoor loop, impedance transformation was not important. Whether a 1:1 or 9:1 didn't really matter - but what DID was the PAR EF-SWL binocular core isolated transformer being able to go down to .5 Mhz. That brought back BCB and 160m, whereas other methods of feedline attachment to most amateur products started to degrade below 2 mhz.

Preamp - Although there is roughly 20db of "gain" as compared to the on-ground LOG or DOG by virtue of not being on the ground, the small size and impedance mismatch meant that the 10db preamp in either transceiver was still nearly mandatory all the time. The log or dog did not need any preamp.

Indoors vs outdoors - at sunrise, I took the small vertical loop just on the other side of the window. Of course the situation improved. VERY favorably, and depending on conditions, the preamp was turned on or off. Getting it further away from the house made bigger improvements. Of course.

What's the point of the comparison? - Sure enough, being outdoors is preferable to being indoors. And here is where the 60-foot circumference on-ground log, or 30-foot dog shines: For whatever reason you simply can't put an antenna in the air outdoors, the on-ground antenna is a GREAT solution for getting away from the house and outdoors. Compared to the small vertical loop, it is NOT penalized for being on ground, since it has the same pattern as a small vertical loop, and even with all the ground loss, the match is still good enough for rx purposes with most quality feedlines. Thus you hear more, even being on the ground, as compared to a small vertical loop outside. You can get similar performance by making the small vertical loop well clear like the loop on ground is, but again you may have *reasons* for choosing the stealthy on ground option. HOA, legal, aesthetic, mechanical, theft or vandalism and such that would take down your LVP, MFJ, or Wellbrook.

RF choking - indoors can be even tougher than outdoors. Isolated transformers are ideal - but they should have specs that go as low as your listening needs are. If that includes BCB or 160m, then the PAR/LNR precision EF-SWL transformer, or the Myantennas products like the CMC-0510-r choke are good off the shelf items to incorporate. If you can build your own, then great!

That was a great test, even though the bands were in pretty rough shape. It was still enough to make a good comparison with with my listening targets.

 

[nanZor]

Indoor VERTICAL polarization note:

Fun fact - electrically, both the small vertical loop and the on-ground loop are vertically polarized.

Near-field effects about interference being either vertically or horizontally polarized have been discussed ad-nauseum elsewhere, so I won't go into that.

But what indoor users who have noise problems should really look into is coax choking / isolating issue first with good products before throwing in the towel and making antenna assumptions.

I was about to give up on the indoor small vertical loop, when presented with horrendous noise rendering BCB and 160m totally unusable. A switch to an isolated transformer fixed that issue completely, whereas years ago I would have immediately thought that "oh, the noise is vertically polarized like my antenna."

This may give hope to those using hamsticks and portable whatnot indoors with radial wires running around the baseboards etc etc. Serious choking / isolating at both ends, even with short indoor coax runs may make a BIG difference, even if you *think* you have an RF ground.

 

[nanZor]

Honorable mention for amateurs:

If you are using a transceiver for your rx-only antennas, one wrong antenna switch or mistaken tune-up away may smoke your nice homebrew $6 transformer, PAR EF-SWL or CMC-0510-r choke.

The RadioWavz B19TX is a galvanically isolated 9:1 transformer rated to take a bit of power. With the small vertical receiving loop, the 9:1 is not a big deal, but the isolation is. I have it in place now just in case. BCB and 160m doing just fine. While I certainly won't use it for transmit, it's there in case I biff it and initiate a tune accidentally.

 

[nanZor]

Too much late night dx'ing - let's get my dimensions right!

My first windowsill-height small vertical loop is 16 feet in circumference, 4 feet per side as a square. 2 feet above ground near windowsill.

Tonight's version near a larger window is 24 feet in circumference, in a rectangle, with 8 feet top and bottom, and 4 foot sides. In wanted just a little more than the square version could provide indoors. For the most part, the 10db preamp in the rig is still necessary unlike the log in the backyard.

 

[db_gain]

I had a corner window antenna farm, one window was a loop fed at the corner, meaning it had vertical polarity, the other window was fed in the bottom center and was horizontal. Of course, being at the corner of the house the antennas had different azimuth of reception too. Worked pretty fair for a few feet of wire on hf. Worked Michigan from Indiana on 40m with a IC703 qrp rig on ssb with it.

 

[nanZor]

The good thing is that with these small loops being so close to ground (2-8 feet high), it really doesn't matter much whether you feed it in a corner, middle, or side - they have the same pattern being so close to ground, so that make it very convenient indoors to hide a feedpoint out of the way behind a curtain etc.

Like everyone else, I just ferrite choked the heck out of it - and assumed I had done my best. Taking the hint from the KK5JY project pages, I finally tried an *isolated* transformer, and bingo, the low bands stopped coupling so badly into the indoor antenna.

I still use ferrite chokes near the rig. That combo, especially the isolated choke (ala Radiowavz B19tx, and not an autotransformer) was significant. Not too many off the shelf *galvanically isolated* 9:1's out there these days.

Palomar Engineers does a great job explaining ferrite choking, and general reading highly recommended:

Home - Palomar Engineers®

But man, that physically isolated transformer makes all the difference in the world at the feedpoint. Not sure if Palomar has galvanically isolated versions, but I suppose one could wind one that way with a kit version if they like.