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VHF Voltage Drop

Discussion in 'General Technical Questions and Answers' started by KC1MUU, Mar 31, 2020.

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  1. KC1MUU

    KC1MUU Ham Member QRZ Page

    Hi Everyone,

    New here, recently passed my tech exam. Looking for some advice on a subject I have researched quite a bit but have come up short. All meetings have been cancelled so I can't ask anyone around here.

    I purchased a Kenwood TM-V71a recently and will primarily use it in the car. However, I want the option to take it inside if needed so I did some research and purchased a 30A switching power supply from Amazon. After setting it to 13.8V I ran through some testing and the supply held dead solid under a 15A load without voltage drop. However, when I connect it to the radio and attempt to transmit on high power the voltage plummets and the radio will restart. This also happens with my HT when it is held near the power supply. It ONLY happens on VHF. On UHF it might drop a few tenths of a volt.

    This leads me to believe there is some form of radio interference to the power supply or to the multimeter.

    Some other findings that I have noticed:
    - AC voltage drops as well. I am using a 14 gauge 6 foot power cable to feed the power supply.
    - The neutral to ground voltage is 60V and the hot to ground is 60V (I know this indicates a problem with house wiring but I am unsure if this could contribute to the current situation). Hot to neutral is 120V.

    Right now I am just using the mag mount antenna I purchased for the car until I get something more permanent. It is mounted to a wire rack.

    Any advice on what I can try to solve this problem would be greatly appreciated! I look forward to learning more about this.

    Thank you in advance.

  2. NE4EB

    NE4EB XML Subscriber QRZ Page

    First things first.

    Where are you measuring, and with what, when you are seeing 60 VAC from neutral to ground?
  3. WA7ARK

    WA7ARK Ham Member QRZ Page

    Horrible CommonMode current from the groundless antenna, along the coax shield, along the back wire into the power supply, thence out the green wire inside the power cord into the wall socket, thence along the bare copper wire in the romex all the way back to the electrical panel and the electric utility ground.

    Get a decent outside antenna, and all your problems will disappear.
  4. AA4PB

    AA4PB Ham Member QRZ Page

    I'm assuming that you were measuring the voltage at the power supply. If so, the first thing I'd do is to unplug the power supply and measure the resistance between ground at the plug and the metal case of the power supply. It should read very close to zero ohms. If it is zero ohms they you have an issue with the house wiring that needs to be checked out before you use that outlet for anything. If it's not zero ohms then you have a broken or disconnected ground wire in the power cord.

    It's also not unusual for a power supply to go "bananas" if you key the radio with the power supply located anywhere near the antenna. A friend of mine did that and his supply put out 28V and fried the PA module in his radio.
  5. W1VT

    W1VT Ham Member QRZ Page

    I had this problem with an old Heathkit variable supply with meters for both the current and voltage. I could see the voltage take a huge drop when I transmitted, even at just a watt or two. Very low power.
    I built a homebrew 13.8 volt supply to run my radios.
  6. KC1MUU

    KC1MUU Ham Member QRZ Page

    Thanks to everyone that has replied already! I measured that at the power supply rail where the AC cable hooks up to it with a Fluke voltmeter. I also measured it in a few outlets around the house as well. All have ~60V between neutral and ground

    AA4PB I did what you suggested and the ohmmeter read less than one ohm. By this measure, you are suggesting a house wiring problem?

    WA7ARK, I understand what you are saying. How would that explain the same effects when I use a HT nearby?
  7. NE4EB

    NE4EB XML Subscriber QRZ Page

    You have a problem either in your distribution panel or you meter base. I would strongly suggest you get that looked at very soon. If you rent let your landlord know. If you own the place you need an electrician.

    By your post I'm guessing you do not have the skills to safely investigate inside your distribution panel. I 'm just a guy on the internet but I don't want you to get hurt.
    AI3V likes this.
  8. KC1MUU

    KC1MUU Ham Member QRZ Page

    It is an old house I am leasing, we're moving out in a few months. I did find a newly wired outlet without the neutral-ground voltage and tested the radio there without any luck. This makes me think it is purely an RF issue.

    So my question is now, is there a way to solve this problem? Should I fork over a ridiculous amount of money for a new power supply or can I work on properly grounding this one to resolve the problem?
  9. WA7ARK

    WA7ARK Ham Member QRZ Page

    The supply is very RF sensistive. It shouldn't be.

    Is it designed for Hams?

    Can you post a model number or data sheet?
    NE4EB likes this.
  10. NE4EB

    NE4EB XML Subscriber QRZ Page

    Is it possible for you to get the antenna outside the house?
    AI3V likes this.

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