Kevin
I think the use of high value resistors is an interesting alternative to RF chokes for draining static buildup.
Do you have any experience comparing the behavior of the antenna system with respect to static buildup effects (or lack of those effects) with and without the resistors in place?
I'm wondering whether the values of resistance that you mentioned would be able to drain the charge quickly enough to prevent the buildup of high voltages?
I have no idea whether they would or they wouldn't be able to drain the charge quickly enough to be effective.
Best regards
Stu
Time will tell. This is a new installation so I can't give direct A/B comparisons.
But in the past I have experienced DC static problems on HF antenna systems, and I noticed they appear to arise from several causes: precipitation static from rain or snow, wind static (especially when it's very dry), and static that arises when a front comes through the area bringing a severe voltage gradient with it. The result can range from annoying near-continuous high-rise-time snapping noises on receive to actual arcs in tuners, etc. Sometimes receiver front ends can be damaged by this kind of static discharge, even when no lightning is nearby.
I also have noticed that some antenna systems don't suffer from these effects at all. I speculate that this may be due to the system taken as a whole having a DC path to ground somewhere, so DC charges never build up to the "popping" level. I also think (based only on memory, having never kept records or anything) that antennas that are totally "in the clear" are more prone to this problem that antennas "in the woods" that have actual physical contact with leaves and branches which can provide a (very high) resistive path to ground.
So I decided to provide my own 6-megohm or so DC path across the conductors and from either conductor to ground. It's only been a couple of months and no popping, but that's not conclusive, since these kinds of static only show up a few times a year at most. In a couple of years I'll have an opinion.
To Fred: You are probably right that the resistors could be smaller. But I am using the balanced feeders for multiple bands and on some bands the impedance is very high, and I didn't want to bleed off too much of my mighty 18-Watt carrier!
I suppose the ability of the resistors to keep the voltage below the point at which there is corona and popping could be mathematically modeled, but I don't know how to start doing that. The rise time of the impinging voltage gradient would have to be known, as well as its effective source impedance. I'm guessing the latter is very, very high -- hence my belief that several megohms is enough to pretty much short it out.
Aircraft radio systems are very prone to this sort of static buildup when flying in storms and they go to great lengths to abate it. One technique used is to cover the antenna radome with a resistive coating.
http://www.lockheedmartin.com/content/dam/lockheed/data/aero/documents/global-sustainment/product-support/Service-News/V4N3.pdfAccording to that article, the coating will be effective if the resistance is as follows:
"The antistatic paint is checked in production to measure 2
to 200 megohms between probes spaced six inches apart,
before decorative painting. The resistance should be in
this range when measured between needlepoint probes
about six inches apart which have been pushed through to
the antistatic layer. The antistatic paint-to-fuselage
resistance should be less than 100 megohms as measured
with the needlepoints three or four inches from the edge
of the radome."
Maybe my 6.8-Meg resistors will work!
73,
Kevin, WB4AIO.
