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Antennas and DC Shorts

Discussion in 'CB Antennas' started by Retro, Jan 17, 2016.

  1. Retro

    Retro
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    I recorded this video kind of in a hurry in response to an ongoing debate on Facebook. You'll have to excuse the poor video quality, I replaced my old tablet and the camera on this one is less than impressive to say the least. I need to invest on a dedicated camera.

    I'm posting this because I wasted several feet of coax on a Wilson 5000 for not knowing any better, thinking there was a break in the coax, and every now and then I see this question pop up on various Facebook pages, so if you're new and you're curious about your new antenna and why it shows a DC short between hot and ground, then this topic is for you.

    Many antennas come with built-in tuning coils so that they can be adjusted to operate more efficiently on different frequencies. The Solarcon A-99 is a popular entry level CB radio base station, but you can use the tuning rings on it to tune it for use on the 10 and 12 meter ham radio bands. Wilson, Sirio and other companies also make magnet mount mobile antennas for use on cars that have a tuning coil built into their base. With these antennas, if you check it with an ohm-meter on the coax connector or at the feed point, you will find that there's a DC short between the center pin and the shield/ground. This is completely normal, and does not indicate a problem with the antenna.



    Antennas that attach via a stud mount like a 102 inch steel whip, fiberglass trucker antennas (like Firestiks) and such will not show a DC short between the hot and ground because the ground stops at the stud and the antenna uses the body of the vehicle as counterpoise. If you are using one of these antennas that does not have a tuning coil in the base, and there's a DC short between the center pin or radiating element and shield, then you do indeed have a problem. Also, when coax is disconnected from the antenna there should not be any continuity between the center pin and shield. If there is, then your coax needs to be fixed or replaced. This is a common issue with smaller coax cables like RG-58 that have a single center conductor, especially when they're kinked or pinched repeatedly because that single conductor can break and poke through the dielectric insulator and short against the shield. This is one reason I recommend running your coax through drain plugs in the floorboard or through the firewall or some place where they can remain stationary and not get pinched, kinked or cut during normal operation of the vehicle.

    So to summarize, the Solarcon/Antron A-99 and other antennas that have a built in-tuning coil will show a DC short on an ohm-meter and this is totally normal. Put an antenna analyzer on it and check the SWR at your desired operating frequencies, or use a spare radio you don't care about with an in-line meter to measure the SWR. If possible at the time, hit the antenna with as much power as you plan on using, because sometimes the radiating elements can be burned up and will show low SWR readings at low power output levels, but then skyrocket when you actually connect your radio and hit them with higher power levels. This requires you to actually mount the antenna, even if it's only temporary, because checking SWR with the antenna on the ground will not give you accurate readings. Check for obvious physical damage, make sure the drain holes in the bottom of an A-99 and iMax 2000 are clear so that moisture can drain out, and make sure the vinyl cap for the tip of the antenna is still present, etc., but don't be alarmed if your A-99 shows a DC short from hot to ground, :)

     
    #1 Retro, Jan 17, 2016
    Last edited: Jan 17, 2016

  2. Galaxy 959

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    Yea if a antenna says its DC grounded it will show a short. I think the Antron says its a DC grounded antenna. When I started the hobbie with my Antron 99 I was checking my coax after the pl259 installation. Well it was showing a short. Well I started looking around on google and found that the Antron was DC grounded and it was suppose to show a short. I unhooked the coax from the antron and all was good.
     
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  3. Tallman

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    The GAMMA matching on my beam antenna is the same way. RF acts differently than DC power.
     

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  4. M0GVZ

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    Correct tool for the job as always. Multimeter to check you've not got a short in your co-ax when you put on your connectors, antenna analyser to check the antenna and any baluns as baluns can be wound as a direct DC short too.
     
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  5. The DB

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    What your saying about DC grounded antennas is absolutely true, and is a mirror of what some of us have been saying here for years.

    One note...

    If this happens then something is changing somewhere. As far as an antenna is concerned, as long as the antenna is within its power handling capabilities, changing the power within that range will not cause SWR to change. Yes, I know you've seen it, and I have as well. Generally when this happens you have a radio or amplifier that is generating a lot of spurious signals and/or harmonics, and many of these are frequencies that are outside the bandwidth of the antenna, thus to them they see a high SWR and are reflected back. Its not the antennas fault, although many people do try and blame this on the antenna. In most cases this is do to a poor "peak" job by a third rate technician, or one of the many crap boxes that are mistaken for amplifiers on the CB band (that would be a vast majority of amplifiers intended for use on the CB band). In most cases, using said amplifiers with less input power will resolve the issue, and get you near, if not just as much output in the frequency bandwidth as with the full driven amount while keeping your SWR readings about the same. It is also possible that there is a component in the process of failing on the antenna itself, although on the CB band this is less common that the a-fore mentioned reasons.


    The DB
     
  6. Retro

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    That's good to know. I ran a little Power Deluxe 25A power supply/amplifier for about a year (ran short of cash and needed money) and SWR never moved on it, but I turned the radio input down to about 1 watt (swinging to 3.5 or so with modulation) and all was well, but I never considered spurious emissions as a source of SWR changes. I'm always willing to learn new things, :)
     
  7. Captain Kilowatt

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    The Gamma match should show an open circuit from the center pin to ground because the gamma rod forms a capacitor and a capacitor blocks DC.
     
  8. Captain Kilowatt

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    Just a note but I had a mobile HF antenna that would check fine with 5-10 watts into it but go nuts with 100 watts. We use a lot of road salt up here in the winter and there was so much salt built up around the mount that the antenna shaft was arcing to ground because of the salt bridge with 100 watts output.
     
  9. Tallman

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    This one is an adjustable short for a yagi. I'll try to get a larger image. It is a DC short.
     

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  10. Captain Kilowatt

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    I can see it fine.It looks like one half of a Delta match which has no capacitor. What is the material where it says "see text" that the coax attaches too? A normal Gamma match uses a concentric pair of tubes separated with an insulating sleeve which forms a capacitor or a single rod and a variable capacitor. This capacitor cancels out the inductance presented by the Gamma arm itself.
     
  11. Tallman

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    It is sort of like the "J" pole antennas as far as tuning goes. Works good with very low SWR.
     
  12. Captain Kilowatt

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    Yeah it is simply tapping into the point that presents something near 50 ohms however the J-Pole uses a 1/4 wave stub and not a Gamma match. Have you put an analyzer on it to see if there is any reactance? That is the whole idea of the Gamma capacitor, to cancel out any inductive reactance present which is usually cased by the Gamma rod itself.

    My first post above was in concerns too the fact that probably 99% of commercial antennas with Gamma feed are not DC grounded.
     
  13. Tallman

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    I have the very old MFJ analyzer the one with the analog meter, but it does read 50 ohms. Tested before hoisting it up the tower.
     
  14. Captain Kilowatt

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    Any reactance? That is the real question. If not then you got lucky. LOL It can certainly work your way. It's just that I have never seen a gamma match done like that before.
     
  15. Tallman

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    Could not tell you about reactance the analyzer I have reads ohms and SWR. The field strength meter indicates good radiation. So I guess it is working. The see text indicates that the bracket that so239 mounts on the center conductor is insulated at that point but not the outer conductor.
     
: antenna, short, coil, tune

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