Well ive just finished of a 120watt transmitter has a 5 pole LPF along with a tuned 1/4 dipole with matching 50ohm RG6 been using it on special occasions using 2 meter RCA's to separate the audio equipment from the transmitter it works fine when a phone or other audio device is plugged in but when i plug in my mixer to it the vu meter is clipping and there's a hum... Anyoing as hell ive added ferrite chokes on fan wires etc so its pretty well made any tips on reducing humming and why it would make my mixer peak when there's no audio going into it
If you have a scope, see what the output is like. That will rule out or verify oscillations. Some equipment just doesn't play nice together.
Ground loop can be cured with an isolator (that Radio Shack thing isn't too bad for radio) or if we can get a block diagram of your setup and power routing, we may be able to suggest a cure.
Well ive just finished of a 120watt transmitter has a 5 pole LPF along with a tuned 1/4 dipole with matching 50ohm RG6 been using it on special occasions using 2 meter RCA's to separate the audio equipment from the transmitter it works fine when a phone or other audio device is plugged in but when i plug in my mixer to it the vu meter is clipping and there's a hum... Anyoing as hell ive added ferrite chokes on fan wires etc so its pretty well made any tips on reducing humming and why it would make my mixer peak when there's no audio going into it
First thing to double check is for any return VSWR from the antenna system, and to make sure there is a proper matching balun between the coax and the dipole. A 1/4 wave is not a 50 ohm load and has a high impedance at the feed point. If not properly fed you end up with RF traveling down the coax ground shield and onto anything else connected to the TX via the ground path...such as when connecting your mixer up to the TX..the ground path of the transmitter extends to the ground of the mixer via the audio cables and the mixer doesn't like that too much!
Below is an image of what happens when a dipole is not properly connected to an unbalanced feed line (coax).
What is needed is a way to transform the unbalanced coax into a balanced output feed for the dipole. The following images show a few examples of how this is done.
Current Balun
4:1 300ohm/75ohm coax balun
Sleeve balun
However the most simplest way is to coil the coax with 6 turns close wound about 1/4 wavelength away from the feed point, using the coax itself to form the coil. Though the following image shows only 4 turns and is for 50-70Mhz. An additional 2 turns will move it up in frequency and cover the FM band.
Coax coil balun
Once the proper terminating is completed to the dipole, then re-check VSWR and trim the dipole elements (both upper and lower elements evenly) to minimize VSWR.
At 120 watts, doing both proper terminating at the feed point of the antenna AND minimizing VSWR is very crucial just as is using a good low pass filter. The 5 pole variant is good and better than no filter at all. But I highly recommend a 7 pole or more filter. 120 watts is a lot of power even for a 5 pole filter. But the 5 pole should be ok unless your still getting localized blanketing in the frequency ranges of the harmonics, which at that point a 7 pole or more filter would be recommended.
Hope that helps!
Peace!
K-ROCKS RadioOne
ZeroPointRadio
AM Stereo 1670
FM Stereo 92.1
In addition to what cmradio and RFburns said I would add that it's possible your mixer is just sensitive to RFI. I have had equipment in the past that would go bonkers around high RF exposure.
If your SWR is low and the antenna is far enough away from the audio equipment then I would suspect a ground loop. It is possible to buy audio isolators for such a situation. Sometimes when mixing XLR balanced audio with RCA style unbalanced connectors you can run into this problem without the proper audio transformers between or many loops of the cable through snap on RF chokes.
If you haven't tried yet, test the mixer without anything connected to it except your transmitters audio cables. Does the hum persist? If you can run the mixer off of batteries then try that also. This will prove if you're getting a ground loop between the wall outlet ground plug and your transmitters grounding and through the audio cable (causing the full loop)
RFBurns i have open air balun 6 turns of 50ohm RG6.
Kage ive tried it just connected to the transmitter it the hum still persists the weird thing is if i turn and unplug my transmitter and just use my mixer normally i can hear the output of the mixer on any fm station it seems like it may be an issue with my mixer as when i use a phone or other audio source the problem isn't there and the signal is as clean as a whistle
RFBurns i have open air balun 6 turns of 50ohm RG6.
Kage ive tried it just connected to the transmitter it the hum still persists the weird thing is if i turn and unplug my transmitter and just use my mixer normally i can hear the output of the mixer on any fm station it seems like it may be an issue with my mixer as when i use a phone or other audio source the problem isn't there and the signal is as clean as a whistle
Yeah most definitely the mixer then. It sounds like it is somehow causing its own oscillation internally. Only thing I can think of that may help with that is using RF choke coil right after the audio connector wired in series going into the mixer, or possibly using a high picofarad value blocking capacitor across the audio terminals to ground. Sounds like the mixer may have been poorly designed in this respect.
What kind of mixer is this? Might help us help you if you were more specific as to it's make, model number and if possible, link to it's documentation.
Never heard of an audio mixer oscillating to a point causing reaching spur or signals into the FM band! I've seen audio mixer input op-amps go nuts and cause oscillations in the audio spectrum and partially into the ultrasonic and low RF spectrum, but not into the VHF spectrum! The typical components used in audio mixers, even digital ones won't have the component structure or values that would throw one into VHF oscillations..but again without knowing exactly what type of mixer you got, it's all a shot in the dark as to what is the problem and what would be the cure.
Peace!
K-ROCKS RadioOne
ZeroPointRadio
AM Stereo 1670
FM Stereo 92.1
RFBurns i have open air balun 6 turns of 50ohm RG6.
Ok then have you checked the VSWR? At 120 watts you would want a VSWR no greater than a 1.5:1!! Anything more and even with the coil balun your going to get problems!
Kage is correct in the placement of the antenna. If you got that thing too close for comfort to your shack gear, no amount of filtering or balun or low VSWR is going to matter! At that much power, your signal could inductively couple onto the overhead power lines and dump high level RF right back into your shack via the power wiring, or telephone line, or tv cable line!
Your antenna should be located well in the clear and above obstructions. And at 120 watts, should be no less than 12 meters away from these conductive objects!!
Also keep in mind, RG-6 coax has a 2 db loss per 100 foot at 100Mhz.
Peace!
K-ROCKS RadioOne
ZeroPointRadio
AM Stereo 1670
FM Stereo 92.1
The mixer is a cheap KAM GMX5 MKII, The antenna is about 22 - 24 meters away i think its just the mixer causing the problem any other audio source and the signal is perfect!
Im getting a 1.3:1 VSWR could probably get that better with a little bit of antenna tuning as its turned for 91.60 and im on 91.70 due to 91.60 being taken over...
Well ive just finished of a 120watt transmitter has a 5 pole LPF along with a tuned 1/4 dipole with matching 50ohm RG6 been using it on special occasions using 2 meter RCA's to separate the audio equipment from the transmitter it works fine when a phone or other audio device is plugged in but when i plug in my mixer to it the vu meter is clipping and there's a hum... Anyoing as hell ive added ferrite chokes on fan wires etc so its pretty well made any tips on reducing humming and why it would make my mixer peak when there's no audio going into it
"Knowledge Is Always Made More Valuable By Sharing It With Others !" Always Remember To Be Kind ! True Greatness Often Has Very Humble Beginnings ! Help A Kid Out Today ! *** High Mountain Radio *** "Broadcasting From Somewhere High In The Remote Appalachian Mountains"
Post by HighMountainRadio on Dec 27, 2017 4:13:01 GMT -6
Greetings !
It sounds as though you either have GROUNDING issues with your setup or possibly RF getting into the mixer, all mixers are not created equal when it comes to being susceptible to RFI. First, make sure all equipment is properly grounded with as short as possible grounding braid (ideal). Also ground loops are a major cause of noise, hum, and interference in audio, video, and computer systems. Wiring practices that protect against ground loops include ensuring that all vulnerable signal circuits are referenced to one point as ground, so that no potential differences can occur. I hope this helps you fix the issue in some small way ! Let me know if I can be of any further assistance !
Best regards, Spooky..
"Knowledge Is Always Made More Valuable By Sharing It With Others !" Always Remember To Be Kind ! True Greatness Often Has Very Humble Beginnings ! Help A Kid Out Today ! *** High Mountain Radio *** "Broadcasting From Somewhere High In The Remote Appalachian Mountains"
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