Bleeding off static from a vertical

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K3RW, Jan 6, 2018.

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  1. K9STH

    K9STH Ham Member Staff Member QRZ Page

    ARK:

    There are receivers that do not have a DC connection between the antenna input and ground. Modern SDR receivers most certainly fall into that category and there are a fair number of other receivers that have an untuned input that is definitely above ground.

    Glen, K9STH
     
  2. WA7ARK

    WA7ARK Ham Member QRZ Page

    I just used my Fluke in DC Ohms mode:

    IC-7300 100K
    FT-857 VHF 47K
    FT-857 HF 3K
    R-1000 300Ohm
    Vertex VHF 125Ohm
    Misc VHF radios show a DC short.
    Nothing I have is open.

    As long as an antenna is connected to the radio, the antenna coax center conductor has a path to DC ground, and assuming that the antenna itself doesn't have a intrinsic DC ground (most do), then any "static" charging of a driven element will be drained to ground.

    I have installed 3.3MegOhm 2W carbon resistors inside my antenna tuner and a coax selector switch so that DC-blocked antenna feedlines have a discharge path to ground. I do not like shorting coax switches, because you can get interaction between antennas if the non-selected antenna is "shorted".
     
    K3RW likes this.
  3. K3RW

    K3RW Ham Member QRZ Page

    What determines what value of choke to use? I see a few out there with a wide variety of values.
     
  4. K3RW

    K3RW Ham Member QRZ Page

    But what value of resistor(s)? I have yet to see a tutorial or calculator that says use X amount per MHz, or anything else that gives me a hint what value to shoot for. I'm not sure if more is better, or there is an optimal amount.
     
  5. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Optimal would be zero Ohms.:)

    But as a "static drain" hundreds of KOhms or even a MegOhm or two will usually suffice.

    Key is voltage rating. Static can build up to thousands of volts (miniscule energy, it's in milliJoules or less, but can "break down" components that can't handle the voltage) and of course if you're also going to transmit with the same load attached and run high power with a reasonably high VSWR, that can impose a few kVp across the termination as well.

    The "lightning" transient suppressors (for antennas) commercially sold usually break down in the 1kV -> 3kV range, but they're open-circuit until a transient occurs.

    Why not just try using a resistor across the feedline with clip leads to see if this will actually help anything or not? That's a 10-second experiment.
     
    WB5YUZ, K3RW and N0TZU like this.
  6. KD2ACO

    KD2ACO Platinum Subscriber Life Member Platinum Subscriber QRZ Page

    I had an SX-110 that made noise with static on the antenna that was cured by (3) 180k ohm Caddock resistors in the junque box. It's probably good for voltage around where the surge protector fires. I removed the arc plug from the dipole center insulator and put the resistors there instead... many years ago. It's still fine.

    Friends radios wind up on my bench from time to time and I don't want to find out which ones have a delicate choke inside to get blasted on a windy day.
     
    K3RW likes this.
  7. K9STH

    K9STH Ham Member Staff Member QRZ Page

    ARK:

    Even though your particular radios have some sort of DC path to ground, there is a very good chance that the input circuit might be damaged if the voltage, caused by things like Van der Graf effect of wind blowing across the antenna, induced voltage from nearby lightning strikes, etc., reaches a high enough level. That is why some sort of continuous discharge device should be included within the antenna feed-line.

    During the first Gulf War, when the U.S. military first deployed to Saudi Arabia, there was widespread failure of the Harris HF SSB equipment due to the Van der Graf action of the sand blowing across the antennas. Basically, the receiver "front ends" were "blown out" due to the high voltage induced. As a stopgap, until Harris could devise a modification to prevent this from happening, quite a number of the tube-type Collins KWM-2A transceivers were retrieved from storage, hastily reconditioned, and flown to the middle east to be used. The Collins equipment was much less likely to have problems with the voltages induced upon the antennas.

    The cost of a resistor is extremely low and the protection from such is very great.

    Of course, in most of this country Van der Graf induced voltages from blowing sand are not that common. However, wind blowing across antennas can have the Van der Graf voltage reach a high enough level to damage the front end of quite a number of units. As such, prevention from static electricity, induced voltages, and so forth, should be of concern to any amateur radio operator with outside antennas.

    Glen, K9STH
     
    WB5YUZ and NL7W like this.
  8. N8CMQ

    N8CMQ XML Subscriber QRZ Page

    Inside some of the radios I have worked on, the input circuit has an inductor (RFC) from the center pin to ground to short any static charge to ground.
    This was done to protect the FET front end of the receiver from over voltage transients.

    The antennas I have seen use a cap in series on the vertical element with no DC path to ground, and that does cause noise from static discharge.
    My vertical is this style, but I added a 1 Meg resistor across the cap and element to ground. Originally it was an RFC, but I removed it when I rebuilt the
    base years ago. I have been using a 1/2 watt carbon resistor with no problem.
     
    KD2ACO likes this.
  9. N8CMQ

    N8CMQ XML Subscriber QRZ Page

    One form of static noise I didn't mention is the Saint Elmo's fire, corona discharge that can happen with antennas.
    Even with grounding and bonding, the antenna gets a very large voltage buildup with ionization of the air around it. Needless to say, the noise is very high and can cause receiver desense.

    To help reduce the noise and bleed off the charge, static wics can be used. One source I use is from Dayton- Granger and were made for aircraft use. I only had one location have the problem, and it also had many lightning strikes to the tower and the ground surrounding it.

    The owner finally gave up and moved to another location due to the damage from the strikes and near strikes. He tried many grounding techiques and even disconnected his equipment when not in use. But he still had antenna failures and coax switch failures from the lightning.

    If you have this problem, I wish you the best of luck!
     
  10. WA7ARK

    WA7ARK Ham Member QRZ Page

    We were talking about eliminating arcing (heard as an acoustic "snap") between the coax center conductor and its shield, especially when the coax is disconnected from the radio during thunderstorms. In a darkened room, you may actually see the tiny spark that forms as the feed-line conductor connected to an antenna element that has no other conductive path to earth ground gets charged to thousands of volts, finally breaks-down some air gap, ionizes the air, and suddenly discharges. Just like a relaxation oscillator using a high-voltage source, a several MegOhm resistor, a capacitor and a NE-2 Neon lamp.

    The effective current (number of charged particles per unit time) arriving at the un-grounded antenna element is so low that any reasonable shunting resistance less than a few MegOhms connected between the antenna element and earth ground will prevent the element charging up to a voltage sufficient to ionize an air gap... If the feedline remains connected to a receiver that has an internal dc resistance across its antenna terminals, then you will never hear the "snap". If the feedline passes through a coax switch or tuner that can break the dc path to ground, then install a suitable "bleeder", choke or resistor.

    I am very familiar with Precipitation Static heard as noise in a receiver by flying through rain or snow. It is caused by charged snow grains or water droplets hitting antenna elements (never been to Saudi Arabia). P-Static is not eliminated by using a "grounded" antenna.

    Lightening protection is something else entirely, which gets into spark gaps and gas-gaps..., not just adding a bleeder.

    Maybe I should have offered my services to Harris...

    Harris.png
     

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