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 Volunteer Moderator QRZ Page

    ARK:

    I am very aware of what we were discussing! For several decades, before retiring, I was a telecommunications consultant (still do a little bit of consulting) and one of my specialties was lightning protection and grounding (including for r.f. purposes). I still do occasional presentations, especially at amateur radio club meetings, on such. One of the "slides" in this presentation shows a couple of ways to add a resistor to "bleed off" the charge from Van der Graf build up, etc.

    042a.jpeg

    Precipitation static normally does not reach the level at which damage to the receiver front end would happen. However, antennas at a DC ground potential, such as "plumber's delight" Yagis, are not plagued by such static anywhere near as much as are other antennas.

    Protection from lightning damage is a completely different subject. There are a number of methods that work very well. I suggest certain things and others suggest different methods. Generally, the various methods do work and one can often "pick and choose" things from several of these methods and achieve an excellent system. My methods involve a lot of things that the individual can do themselves which, in turn, keeps the cost of the system as low as possible.

    Glen, K9STH
     
    K3RW and N3AB like this.
  2. WB5YUZ

    WB5YUZ Ham Member QRZ Page

    Thanks for the Mouser link!
     
  3. W5DXP

    W5DXP Ham Member QRZ Page

    The worst P-static I ever encountered was in Queen Creek, AZ, southeast of Phoenix. I awoke one night during a dust storm to what sounded like a machine gun inside my IC-745. I wrapped a towel around my hand and unscrewed the coax connector which continued to arc but was no longer connected to the IC-745. I dropped the cable down on the shag carpet and it started scorching the carpet all the time sounding like a machine gun. I wrapped aluminum foil around the coax connector to stop the noise/arcing and a glass of wine settled my nerves.

    The next morning, fearing the worst, I hooked the antenna back up to the IC-745 and to my surprise, it wasn't damaged. The next weekend, I put up a folded dipole fed with a 4:1 voltage balun and the arcing problem went away because every conductor in the antenna system had a path to ground. One hears the statement, "Loops are quieter than dipoles." and they are - under P-static conditions.

    I have an RTL-SDR dongle that displays RF signals up to about 1.3 GHz. The only time that I have observed P-static here in East Texas is at the leading edge of thunderstorms when it just starts to rain.
     
  4. KC9VFO

    KC9VFO Ham Member QRZ Page

    I think I will try this with my vertical , somedays I get some static. I have the parts in my junk box.
     
  5. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    I've occasionally heard these corona discharge events on my vertical when there are threatening rain clouds overhead. If dark enough it can just be seen with binoculars (the antenna is 100 ft away from the house). It has a variable sound on the radio, anything from rapid clicking to sizzling to a sort of creaking-screeching.

    There isn't anything nearby that is taller, so the discharge affixes to the vertical. I've reduced somewhat it by replacing the sharp wire ends at the top with a round circle of wire to cause less of an electric field gradient.
     
    KB4QAA likes this.
  6. W2WDX

    W2WDX Subscriber QRZ Page

    Yeah well the problem is building small chokes of low inductance like you suggest. A 50uH choke is going to have series resonances close by. Toroids also contribute to series resonance at undesirable points across the bands unless they exhibit very high Q. That's why I build large chokes with values higher than 340uH. I have found that the trick is in building a choke that has a large enough inductance so that at the low end of your frequency range you have a large impedance, but yet at the high end you don't have those resonances right in or near any desired band of operation.

    Spark gaps should not be used to discharge static. All it does is create static noise on receive; exchanging one type of noise for another is not a solution. That's just common sense. BC transmitters use spark gaps for protection, not noise reduction; they are not receiving they are purely transmitting. Using spark gaps for protection is a good idea; using them for noise reduction on receive is counter-productive and counter-intuitive.

    Static voltages should slowly not be allowed to develop in the first place. There is a RC time constant with bleed resistors since all exhibit some stray capacitance (not to mention any capacitance on the line or load). Where less exist with a high inductance choke at DC and near DC frequencies, since their inductive time constants (LR) are more dependent on some HF AC component, not DC or even VLF AC. Chokes are a low resistance short for DC and VLF. A resistor ... it's not. It's a resistor.

    If you're working on the cheap ... OK ... sure. Go for it. It will kinda sorta work ... till it opens.

    BTW, it has been shown that under some circumstances static voltages from simple wind can build-up and exceed 3.5kV at discharge. Spark gaps are very dependent on breakdown voltage of air which is dependent on humidity and air pressure. So very high voltages can form when humidity is high or winds are fast, and the spark gap may be ineffective due to this highly variable air breakdown voltage. Inductors (and resistors) do not exhibit any of these poor characteristics. However you do need to exercise high-voltage awareness for resistors. I have heard of people thinking to keep static bleed resistors out of the weather, they were mounted inside little die-cast boxes. Unfortunately, the insulation breakdown voltage of the resistor was exceeded due to to it close proximity to the case. Corona effect is another consideration over the physical distance of the resistor in these high-voltage pulsed conditions. Power rating of the resistor has nothing to do with its various voltage ratings types. Considering all these variables, this is why I go with inductors every time.
     
    Last edited: Jan 10, 2018
    KB4QAA likes this.
  7. K1ZJH

    K1ZJH Ham Member QRZ Page

    Sorry, I did not suggest a 50uH choke for that application, I stated that only 7 turns yielded that amount of inductance. I did not mean to infer that it was sufficient. That core will easily accommodate more turns.
     
  8. WA7ARK

    WA7ARK Ham Member QRZ Page

    There are two separate causes of weather-caused noise which de-sense a receiver, P-static and Corona. P-static happens as each water-droplet or snow-flake touches an antenna element and the charge difference is equalized (tiny step current). Contrary to what some hams think, this happens if the antenna element has a DC path to ground or not; intrinsically DC-grounded (like a shunt fed tower or Beta-Matched Yagi) do not eliminate P-static. You just have to wait for the rain/snow squall to move on...

    Corona happens when there is an local electrostatic field gradient as when there is a highly-charged cloud overhead preceding a lightening strike in the immediate area. As the cloud moves overhead, there is an "image charge" induced in the ground below. The charge is attracted upwards to an elevated antenna. If the highest tip of the antenna structure (not necessarily the antenna element itself) is a sharp point, then the local voltage gradient can exceed the breakdown voltage of air, and a continuous tiny arc (St. Elmo's Fire) can form. Such an arc off the tip of a vertical antenna can create a tremendous howl or squeal in the receiver, totally blocking reception. This howl is modulated by lightning; it seems to disappear right after lightening strike, and then it builds back up. I deal with this issue every time I fly my airplane through rain or snow...

    There are two methods that help mitigate RFI due to Corona.

    1. Put a sphere on the highest part of an antenna structure. The voltage gradient around a charged sphere is less than what would happen around a sharp point. This is one reason why there is a "ball" on the end of every mobile whip you have ever seen... This helps, but will not totally eliminate the Corona at the antenna tip. (The ball is there to make it less likely that you put your eye out, too)

    2. Put many sharper points high as possible on the antenna structure as far as possible from the active antenna element. This is the "static wick" solution. A static wick is a small aluminum tube with about 1,000 tiny carbon fibers swaged into it. The far ends of the carbon fibers look like a paint-brush. When they get charged (by being part of the antenna structure or airplane), the carbon fibers splay apart, just like the gold leaves of the electrometer you played with in high school.

    Each fiber end is a very sharp point, so can breakdown the air to create a tiny local Corona at its tip. Each fiber is a series carbon resistor, so acts to equalize the currents that flow in each fiber, so we have 1000 equal, but very small Coronas. We have haven't stopped Corona, we can now move it to a different place than the tip of the real antenna.

    To be effective to mitigate RFI in the receiver connected to the antenna, the static wicks must be located higher than the tip of the antenna, and as far away from the antenna as possible. Impossible to do on a vertical antenna. Possible to do on a tower-mounted HF yagi or other horizontal wire antennas like dipoles. The reason that when static wicks are mounted on an airplane, they are typically placed near the wing tips, and near the tip of the rudder; as far away as possible from the VHF comm antennas, which are typically mounted on top of the main cabin. This way the Corona is still there, but is now as far from the antenna element itself as possible, instead of forming right at the tip of the vertical antenna...
     
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  9. N0TZU

    N0TZU Platinum Subscriber Platinum Subscriber QRZ Page

    Excellent summary!

    (A conductive ball of effective size was impractical on my vertical for mechanical reasons, so a loop of wire in the vertical plane was used.)
     
    NH7RO likes this.
  10. AI0K

    AI0K Ham Member QRZ Page

    This is to protect from lightning. It does nothing to help with static noise on reception - except maybe make it worse when it arcs over.
     

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