Having some RFI issues in my Audio

[W5EFR]

Here is what I did last night:
- Checked it out by running the audio into the Mic Jack, using just the Mic, Mixer, and Rolls Isolation Box... Distortion is still there

- Tore the entire $^#& system apart
- New ground system, single point connection (Audio Rack, TX, Tuner, AC Power Strip), and pounded in a new ground rod
- Cleaned up the power runs, AC and DC
- Cleaned up the Coax runs
- Rearranged the system to get the TX/Antenna Tuner further away from the Audio Rack

Then I started testing....

The system was about as simple as I could get it:
Mic-Mixer-Isolation Box-TX

The RF is still in the audio, and no Computer interfaced to the system.

I Scoped the signal coming out of the Isolation box, and it looks like the signal I was putting into the mixer, I can't see any distortion in it, it looks like a 1khz sine wave... maybe my 20Mhz O'scope isn't sensitive enough to actually see it...

[w3jn]

Plug a hand mike into it (without disconnecting anything else) and see if you still have the problem.  If you still hav ethe problem, disconnect the audio chain.  If you *still* have the problem, focus on the power supply, coax interconnects, etc.

You mentioned a tuner in your last post that you didn't mention before.  You are running a vertical with the tuner in the shack?  Fed with coax?    Kind of a recipe for disaster there, OM, depending on what you're doing - ie is this a multiband vertical, say a R-7, and you're just trying to tune out a bit of SWR - or is this a base-loaded vertical where you're trying to match a 10:1 SWR?

[W5EFR]

Plug a hand mike into it (without disconnecting anything else) and see if you still have the problem.  If you still hav ethe problem, disconnect the audio chain.  If you *still* have the problem, focus on the power supply, coax interconnects, etc.

Ok, I'll try that tonight... (where the hell is that hand Mic ...)

You mentioned a tuner in your last post that you didn't mention before.  You are running a vertical with the tuner in the shack?  Fed with coax?    Kind of a recipe for disaster there, OM, depending on what you're doing - ie is this a multiband vertical, say a R-7, and you're just trying to tune out a bit of SWR - or is this a base-loaded vertical where you're trying to match a 10:1 SWR?

Actually, I run the tuner (MFJ-949E) as more of a Antenna switch and dummy load. My Antennas are a 10 meter vert (base 25' off the ground) and a fan dipole (10, 20, 40, 75 meters). It is located next to the transmitter, and fed with about 2' of RG-8 from the TX.

I have never had to use the tuner as a tuner, my SWR's are pretty good (under 2:1) with the antennas I have.

[N3DRB The Derb]

did you say your ant is 2 ft away from our rig?

[w4bfs]

I just wanna see Derb stuff them in a 34b ...  You could sell tickets fer thatun ... beefus

[Ralph W3GL]

Guess he never heard of "NEAR FIELD"... (feeder is 2 ft of coax?)

Okay, I stand corrected...  How far from the actual antenna is the gear set-up? 

It still sounds like near field hetting into his system.  Works semi-okay on SSB but craps with steady carrier.

[Mike/W8BAC]

Read it again guys. The tuner is 2' from the TX. Try to be constructive here. Did your knowhow come naturally? Man!

[W3RSW]

Once I found that the mike cable shield wasn't working effectively for some reason.  -spiral coil type of cable.   
 
When I ran a wire from the mike shell directly to the input connector shell, the hum and RFI disappeared.

I think that an RF ground loop might also be the culpert. 
Looks like Star grounding has been already tried but might try it again with very little equipment.  The mike only needs the pre-amp. And is the balanced feed fed into a pre-amp stage on the transmitter that usually takes a mike input directly? i.e. is the audio fed into one amp stage too many? 

All kinds of things to try.  Sounds like if it happens into a dummy load then the preamp circuitry might have an open ground or open shielding inside the case.  Stranger things have happened. 

So as everyone says, isolate by removing everything. Then try another preamp directly into the transmitter without any other processing equipment.

My shack has 5 or 6 ft. of coax to a LC tuner into a single wire feeder to a windom; the tuner is right above the op. desk.  The single wire feed line radiates on some bands but no RF occurs unless something is not matched properly. ... this after finding the "no shield" shield mentioned above on the mike cable.

You've run into the bug-a-boo of a lot of hams having antennae too close to the rf generators.  We live in houses, not remote stations feeding a whole ground planed hill with five miles away.   One or more of you equpment internally may have an open ground or leaky shield from something as simple as a star washer on a mike input jack going open.  Hence the RF near field, even from signal going to a dummy load gets in the audio line.   

So get a different mike, a different pre-amp and see if the rf is still there. Then add back the components, (not so damn many ;you really don't need all of them) one at a time to see which is defective.  If not any then go for the wiring again.  Something will jump out in the middle of the night and you'll go "Eureka!"

[W5EFR]

Tonights Update.. With Audio!

OK, I picked up a Tripp Lite Ultra Isobar Surge Suppressor, as it has internal HF and VHF filtering.. just in case... Plugged in the Audio Rack and Power Supply for the rig...

No Joy, still RF'ing

So, I hook the radio to the dummy load 4 feet away from my rig (not grounded to the star ground), through the RF sampler. Sampler audio output to my sound card, O'Scope output to my O'scope. Put the rig down on 40 meters (well within the range of my 20Mhz O'scope) unplugged everything else, found the hand mic (brand new in the box) and gave it a shot...

http://riley-music.com/BowsStuff/718AudioProb/HandMicAccDisCont.mp3

No Joy...

I'm starting to think my audio rack isn't the problem....

Any ideas, oh Gurus of the AM?

[ka3zlr]

OK, lotsa hands in soup..Listen to what Rick is saying he's right on the money... Good job on cleaning things up..Now start at the beginning...

No RF sampler No Computer...and no computer connections...Turn the computer off....On air testing with basic rig antenna coupler, antenna, jumper wire 3' or 6' then add components..find somebody on the air then add system in...at the point when distortion is found run a connector wire from the rack to rear of the rig....assuming rack is connected case to case...Ground run from station point to exit 2 inch copper flashing...inline Rf sampler shud be basic inductive type directly to scope..such as found in handbook..or on Garys main site here...I think it's there...or on huzmans..I forget where that's at now...it's real simple to build but try a different sampler...Hard ground the Scope directly...

Then Plug and Play...and it's VSWR Not swr's...LOL.....That cracks me up....

[w3jn]

I listened to that audio clip and it sounded fine to me, albiet a bit pinched.

Let's start at the beginning.  I guess we all presumed your concern was correct: that you're getting RF in your audio.

Your first post sez "I heard RF in the audio".  What makes you think this - on the air reports, another receiver in the shack, a modulation monitor, something funky going on with the transciever?

RF in the audio, on AM, will generally manifest itself as feedback and/or copious distortion.  I don't see that on the audio clip you posted.

Particularly with your last test - hand mike, dummy load - there's really no chance for RF to get into the audio unless the patch cable to the dummy load is bad.  I think the problem is something else.  Let's explore what you're noticing, and go from there.

[W5EFR]

I listened to that audio clip and it sounded fine to me, albiet a bit pinched.

Let's start at the beginning.  I guess we all presumed your concern was correct: that you're getting RF in your audio.

Your first post sez "I heard RF in the audio".  What makes you think this - on the air reports, another receiver in the shack, a modulation monitor, something funky going on with the transciever?

RF in the audio, on AM, will generally manifest itself as feedback and/or copious distortion.  I don't see that on the audio clip you posted.

Particularly with your last test - hand mike, dummy load - there's really no chance for RF to get into the audio unless the patch cable to the dummy load is bad.  I think the problem is something else.  Let's explore what you're noticing, and go from there.

Hello John,

Thanks for all the suggestions with chasing my problem.

I am starting to think my problem is not RF related to my transmitter. It seems that I can get this "beating frequency" or "motorboating" under my transmit audio even using a 4 watt CB I had kicking around here.

I ran a quick test last night with my Mic/Mixer/Rolls Isolation Box and piped it into a Uniden CB with my RF Sampler on the back, then into the dummy load.

I got the noise there as well.

Testing the Icom Hand Mic with the Icom, and getting the noise rules out the Audio Rack as the point of the noise source.

the only real thing these to radios had in common, besides the dummy load and sampler, is the Power Supply. I am thinking my NewMar power supply is suspect...


 

[w3jn]

OK, where is the beating effect or noise coming from?  You say it's in your transmit audio.  What are you using to listen to your transmit audio?

[W5EFR]

OK, where is the beating effect or noise coming from?  You say it's in your transmit audio.  What are you using to listen to your transmit audio?

I use this RF Sampler:
http://amfone.net/ECSound/RFsampler.htm

hooked to the back of the transmitter, and run the audio output to my Computer to record with Cool Edit Pro.

[W3RSW]

The audio sounded clip grainy to me and yes, maybe not too much rf afterall. Hard to tell.

what does it sound like as audio (granted, from the possibly tinny hand mike) before it goes to the transmitter and rf pick-off detector?.   how do the two compare?

Simple diode detectors without adequate f/b signal ratios 'to get over the knee into a quasi -linear region' tend to distort audio quite noticebly, e.g. a crystal set into an audio amp never sounds as good as the same crystal rectifying a larger signal with adequate resistive loading in an IF amp, thence to the audio amplifier. 

Your audio sounds very similar as that received in near field on a simple crystal set. 

So get a tape recorder going at a more remote site, say about half a mile from the house, then run your lashup and then playback the audio received there.  Might be a real eye-opener.  Might sound pretty good. 

Have you talked on-air to any of the guys on this site? 
I love problems like this, way fun and we all learn a lot. 

[W5EFR]

Hello Rick

Thanks for the reply....

The audio sounded clip grainy to me and yes, maybe not too much rf afterall. Hard to tell.

what does it sound like as audio (granted, from the possibly tinny hand mike) before it goes to the transmitter and rf pick-off detector?.   how do the two compare?

This recording was just the hand mic, so the "before the TX" Audio is a bit hard to do. Tonight, I am hooking the audio rack back up and I am going to record it straight off the mixer, and then off the Detector. I'm am starting to wonder if I have been chasing a "non-issue" caused by my RF Sampler/Detector, making me think that my TX audio is bad, when actually, the bad audio is caused by the detector that is being used to input to the computer for recording...

Simple diode detectors without adequate f/b signal ratios 'to get over the knee into a quasi -linear region' tend to distort audio quite noticebly, e.g. a crystal set into an audio amp never sounds as good as the same crystal rectifying a larger signal with adequate resistive loading in an IF amp, thence to the audio amplifier. 

Your audio sounds very similar as that received in near field on a simple crystal set.

Hmmm, that is pretty much the whole sampler setup!

Is there a better way to monitor your TX audio live?

So get a tape recorder going at a more remote site, say about half a mile from the house, then run your lashup and then playback the audio received there.  Might be a real eye-opener.  Might sound pretty good. 

That is also on the agenda for this evening... I just need to figure out how to set it up.

Have you talked on-air to any of the guys on this site? 
I love problems like this, way fun and we all learn a lot. 

Nope, I have never talked "on-Air" in AM mode, with my little radio and low, subdivision requirement dipoles, I probably wouldn't even be a bump in the noise floor.

[w3jn]

You're getting RF into your computer, not RF into your audio.  That RF tap is really meant to drive a set of headphones, not directly drive a computer sound board.  The computer, with all its millions of PN junctions, is gonna be quite sensitive to even modest RF levels injected into it.  If you want to drive the computer, you should insert a 600 :600 ohm audio transformer between the sound card input and the RF tap's audio putput.

I'm not really a fan of this sort of RF tap because it can cause ground loops and damage if you transmit thru it with no antenna at high power.  Dumb, yeah, but who amongst us hasn't done it.  Notice also that this design has NO capacitor isolation between the RF and the detector diode.  You're gonna build up a fair DC voltage at the audio output - for 20 watts at 50 ohms, lets figure P=V^2/R, 20=V^2/50, V=10V, half wave rectified, so about 7VDC, just the carrier!

Better would be a small transformer ie as what's in a SWR bridge - that way you're isolated from the RF.  For my scope pickoff I just use 1 turn of wire wrapped around the coax coming from the xmitter, then fed directly into the vertical amp of the scope (at my lake xmitter) or the RF sample output from a Bird wattmeter (my home QTH xmitter).

My mod monitors are fed with a 1000:1 capacitive tap on the transmitter.  This is good for a bit of ground loop action once in a while as well.

Your next step is to get that bad boy on the air, make some contacts, and get a few reports!  If you wanna do much AM at all you *really* need a scope, and not one of those junky Heathkit "signal monitors" either.  For 75 meters a cheapo 10 MHz scope is fine; for up to 10 meters you'll want a 50 MHz (at least) model.  A $5 hamfest special is fine.

The added benefit of having a REAL scope around the shack, besides being able to see exactly what's going on with your signal, is you can use it for troubleshooting as well!  Ifn' ya had a scope, you wouldn't have had to go thru all this torture 

[W5EFR]

You're getting RF into your computer, not RF into your audio.  That RF tap is really meant to drive a set of headphones, not directly drive a computer sound board.  The computer, with all its millions of PN junctions, is gonna be quite sensitive to even modest RF levels injected into it.  If you want to drive the computer, you should insert a 600 :600 ohm audio transformer between the sound card input and the RF tap's audio output.

Thanks, I would like to try something else for the "On Air" recording input to the Computer... and cap isolate it... any schematic suggestions?

I'm not really a fan of this sort of RF tap because it can cause ground loops and damage if you transmit thru it with no antenna at high power.  Dumb, yeah, but who amongst us hasn't done it.  Notice also that this design has NO capacitor isolation between the RF and the detector diode.  You're gonna build up a fair DC voltage at the audio output - for 20 watts at 50 ohms, lets figure P=V^2/R, 20=V^2/50, V=10V, half wave rectified, so about 7VDC, just the carrier!

Yikes! I never thought of that...

Better would be a small transformer ie as what's in a SWR bridge - that way you're isolated from the RF.  For my scope pickoff I just use 1 turn of wire wrapped around the coax coming from the xmitter, then fed directly into the vertical amp of the scope (at my lake xmitter) or the RF sample output from a Bird wattmeter (my home QTH xmitter).

My mod monitors are fed with a 1000:1 capacitive tap on the transmitter.  This is good for a bit of ground loop action once in a while as well.

Your next step is to get that bad boy on the air, make some contacts, and get a few reports!  If you wanna do much AM at all you *really* need a scope, and not one of those junky Heathkit "signal monitors" either.  For 75 meters a cheapo 10 MHz scope is fine; for up to 10 meters you'll want a 50 MHz (at least) model.  A $5 hamfest special is fine.

The added benefit of having a REAL scope around the shack, besides being able to see exactly what's going on with your signal, is you can use it for troubleshooting as well!  Ifn' ya had a scope, you wouldn't have had to go thru all this torture 

Yes Sir, I do believe you are correct! I am going to put it on the air tonight, throw it against the wall, and see if anything sticks.. If I can find anyone that can hear my PW signal...

I HAVE a scope, a 20Mhz dual trace, but I never did see any noise on my Audio... because there wasn't any...

Thanks John

[w3jn]

Instead of feeding the diode with a resistor, try a capacitive divider.  Perhaps 10 pF/500V from the RF to a 1000 pF, which would be grounded.  Connect the diode to the junction of the 2 capactors.  Put a 100K resistor to ground on either end of the diode, a .01 to ground in parallel with the 100K resistor at the audio end of the diode, and feed the audio with a 1 uF or so cap.

RF in --------------------------------------  RF out
                                        |
                                       ---
               1                       ---   10 pf
 Audio   ---)|----|<----------|
                    |--       |        ---
                   /    |   /          ---   1000 pF
          100K  \    --- \    100K  |
          .001   /   ---   /           |
                   \    |    \            |
                    ---|--------------   <gnd

[N3DRB The Derb]

damn, johnny, you be the ASCII master. 

[w3jn]

 

[ka3zlr]

Yessum, them computers do make life interesting.....

[W5EFR]

Thanks for all the help, suggestions and other comments.

It looks as if I was chasing a red herring, caused by the diode detector I was monitoring my audio with. and ON-Air check I did confirmed that my audio was free and clear of RF in AM and SSB modes...

But I learned new things, got a new surge suppressor/RF filter for my AC power and got a new compressor out of the deal...

Thanks a lot to  everyone.

Not I need to find a quality way to monitor my on-air signal... I should build John's detector and try that...

[N3DRB The Derb]

don't forget the "Hammer Down and Happy" lighted on-air sign. 

[w3jn]

Great, Bow!  Here's a case where the test equipment was causing the problem    Been there done that.

In any event that RF tap is a seat of the pants design, don't take it as gospel.  There are other ones out there that are probably better.  You could even slightly modify some old CB SWR meter and get good results - those things bypass the detected audio to ground, just reduce the bypass cap to .001 or so, and put in a series dc blocking cap to the audio.