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Author Topic: Reports of garbled audio  (Read 2405 times)
pmraiders

Posts: 47




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« on: June 17, 2013, 08:55:47 PM »

I have a fairly new setup, so its hard for me to single out any one point of failure.

Here is my setup.

KX3 -> Amplifier -> MFJ-993B Auto Tuner -> g5rv

I suspect that my amp is having a problem. I see no reason why an antenna tuner (newest addition along with the g5rv) would cause garbled audio.

So far I haven't been able to make a QRP contact with the KX3 since trying to single the amp out.

Thanks for your reply,
Jordan
KG7DBM
« Last Edit: June 17, 2013, 09:14:24 PM by pmraiders » Logged
KA4POL
Member

Posts: 1864




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« Reply #1 on: June 17, 2013, 09:39:47 PM »

You might be getting RF into the audio amp. So your idea to eliminate the RF amp is a good approach. Of course it is difficult to really judge the situation without hearing the audio. Try to locate a ham close by.
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pmraiders

Posts: 47




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« Reply #2 on: June 17, 2013, 10:32:31 PM »

What do you mean getting RF into the audio amp?
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KA4POL
Member

Posts: 1864




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« Reply #3 on: June 18, 2013, 01:04:06 AM »

This is similar to RFI in TV or radios, only this is getting into your own rig via the microphone line working as antenna. That's only one possibility. You also could have a problem inside the KX3 which you'll find out about when trying low power.
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KE3WD
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Posts: 5694




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« Reply #4 on: June 18, 2013, 07:42:51 AM »

If you didn't at least put an "ugly balun" loop at the connection point of the coax to the G5, RF could be riding back into the shack on the shield of the coax and causing this problem. 

Bing search should bring up pointers on how to do the Ugly Balun by wrapping a few turns of coax. 


73
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W8JX
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Posts: 5365




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« Reply #5 on: June 18, 2013, 08:31:08 AM »

What do you mean getting RF into the audio amp?

Were is antenna in relation to rig? Also non resonant antennas like G5RV tend to couple RF back to rig through feed line without a choke/balun.
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pmraiders

Posts: 47




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« Reply #6 on: June 18, 2013, 01:25:58 PM »

This is similar to RFI in TV or radios, only this is getting into your own rig via the microphone line working as antenna. That's only one possibility. You also could have a problem inside the KX3 which you'll find out about when trying low power.
The KX3 is known to be good. I have made QRP contacts in the past with just the radio and a 40m dipole. Up until saturday I was getting good audio reports while using the amp. I am going to go re-hang a tuned antenna and eliminate the tuner and the G5 for the moment.

If you didn't at least put an "ugly balun" loop at the connection point of the coax to the G5, RF could be riding back into the shack on the shield of the coax and causing this problem. 

Bing search should bring up pointers on how to do the Ugly Balun by wrapping a few turns of coax. 


73

I didn't do an ugly balun... I may need another length of coax to make one.

The antenna is at the other end of 75ft of coax.
« Last Edit: June 18, 2013, 01:55:35 PM by pmraiders » Logged
N2MG
Administrator

Posts: 121



« Reply #7 on: June 18, 2013, 01:40:26 PM »

The balun your tuner has is NOT used when driving coax. If you check the schematic it's clear it's meant to drive balanced line right from the tuner.

You need to try a balun at the antenna (between the end of the coax and the feed point of antenna).

RF coming back into the shack along cables is a prime source of "funny business"

Mike N2MG
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KE4JOY
Member

Posts: 1316




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« Reply #8 on: June 18, 2013, 03:29:12 PM »

Grounding grounding grounding. A tuner needs a good reference ground.  Wink
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W0BTU
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Posts: 1553


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« Reply #9 on: June 18, 2013, 05:45:07 PM »

I think it's RF feedback, too.

Grounding might fix it, but maybe not. An ugly balun might fix it, but keep in mind that they are narrow-band devices. They are for single- or dual-band antennas, not for multiband antennas like G5RVs. Even for single-band antennas, you need the correct number of turns of the right type of coax on the correct diameter for them to be effective.

You should use a ferrite-core broadband 1:1 current balun (choke) right at the junction of the window line and coax to keep RF off the outside of the coax shield. I used one of K9YC's designs (Jim Brown, http://audiosystemsgroup.com/RFI-Ham.pdf) using about 6 turns of the coax feedline wound through 4 or 5 stacked Fair-Rite 2.4" diameter #31 ferrite cores. The choke balun should be right where the coax joins to the open-wire line. I'll bet that fixes your problem.

What band are you experiencing the problem on? You may need a different mix than #31, which worked for me on 75 meters.

An "ugly" balun made from coax wound on an air core is not recommended, as it does not span near as wide a frequency range (see http://www.karinya.net/g3txq/chokes/).

http://www.w0btu.com/g5rv_antenna.html
« Last Edit: June 18, 2013, 05:47:37 PM by W0BTU » Logged

KQ6Q
Member

Posts: 963




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« Reply #10 on: June 18, 2013, 08:53:15 PM »

what make/model of amplifier are you using after your KX3 ?
Why did you do that instead of getting a K2 or K3 ?
Do you have a problem when working CW on the same bands with your current setup ?
As others have suggested, you're very likely getting RF from the amplifier output back into
either the Mike input on the KX3, or the cable between the KX3 and the amplifier.
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pmraiders

Posts: 47




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« Reply #11 on: June 18, 2013, 10:14:30 PM »

The entire station is grounded to an 8ft ground rod driven into the ground. I purchased a KX3 and an amp to satisfy my desire for portable operation with power to make things a little nicer at home.

I don't operate CW yet, I am in the process of learning morse code.

The setup was working fine before I rotated the ends of my antenna. I have since moved them back a bit and ensured that the degree angle between the poles is within specifications as I am operating the antenna in an inverted V configuration. Also, the amp worked fine on my 40m dipole prior to the G5RV setup with no audio problems, so I believe that we can safely rule out the mic cable as a problem.

Thanks for the replies. I will get a 1:1 choke balun for my antenna and see how that works. I have only operated 40m and 20m, both of which were working and I got bad audio reports on 20m.
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WA3SKN
Member

Posts: 5420




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« Reply #12 on: June 19, 2013, 03:08:17 AM »

So what does it sound like when working into a known good dummy load?
First test the KX3 into the dummy load.
Then the KX3 and amp into the dummy load.
Then the KX3 and amp and tuner (it should be able to load into a 50 ohm dummy load!).

I am sure that there is RF feed back causing your problem, but the first thing to do is test with a dummy load.  If it works OK, then you can determine how the RF is getting in and move the antenna, shield the rig, or filter... as required.
Good luck with the project!
73s.

-Mike.
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M6GOM
Member

Posts: 864




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« Reply #13 on: June 19, 2013, 04:45:04 AM »

Grounding grounding grounding. A tuner needs a good reference ground.  Wink

If grounding your equipment at the shack cures a problem, lowers noise, gets rid of RF on the mic etc you've got issues with your antenna system long before you get into the shack.

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KE3WD
Member

Posts: 5694




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« Reply #14 on: June 19, 2013, 05:25:33 AM »

Any Balanced Antenna does not need any sort of Earth Grounding to work properly. 

The Balun is your best bet to cure your problem as stated here. 

For the G5, the better balun would consist of the type where you slide the large ferrites over the coax in a row and cover with heatshrink tubing, this placed right behind the SO connector at antenna feedpoint. 

I only mentioned the Ugly Balun because it is relatively easy to try, coiling up about six turns of the coax in about a 10" dia circle and using some wireties to keep it in that shape.  That would serve as a way to see if the problem really is RF on the shield, which it most likely is, without a lot of time wasted or money expenditure.  Of course, it would work better on some bands than others, but the idea here is to PROOF the thing with the ugly balun.  If it eliminates the problem on one or more bands but not all, that's enough proof for me, time to Bing up the G5RV ferrite balun kit and order and install one.  I used one for several years, worked good enuff.


73
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