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Blocking Capacitor for Ladder Line




 
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Author Topic: Blocking Capacitor for Ladder Line  (Read 57691 times)
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KA2DZT
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« Reply #75 on: June 05, 2013, 12:55:26 AM »

Stu,

Resistors of a much lower resistance could be use, probably down to 500K.  500K anywhere on the antenna feeders would have little or no affect on signal currents or voltages.  If the resistors where never at a high voltage location on the feeders, the resistors could be lower than 500K.

What do you think?

Fred
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wa3dsp
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« Reply #76 on: June 05, 2013, 02:56:51 AM »

The ER article referenced many times in this thread talks about why resistors are a bad idea. One persons opinion but probably worth considering.

This thread has gotten so convoluted it is hard to follow!  I think the original poster was using link coupling on the output of a transmitter or tuner and therefore it was above ground???  Why not just ground a center tap on the output link then you would have a DC ground on the antenna and no other static buildup protection would be necessary.
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Steve - K4HX
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« Reply #77 on: June 05, 2013, 08:49:29 AM »

OP said:

Quote
I am working with a link antenna tuner.
http://amfone.net/Amforum/index.php?topic=34128.msg264792#msg264792


Grounding the CT of the link would take care of the static build up. But it would also reduce the common mode impedance of the system.
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WB4AIO
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« Reply #78 on: June 05, 2013, 09:07:17 AM »

The ER article referenced many times in this thread talks about why resistors are a bad idea. One persons opinion but probably worth considering.

[...]

I'm not a subscriber -- could you summarize or quote the author's reasons for his belief that static drain resistors are a bad idea?

Thanks,


Kevin, WB4AIO.
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« Reply #79 on: June 05, 2013, 09:37:20 AM »

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.pdf

According 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.
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AB2EZ
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« Reply #80 on: June 05, 2013, 11:57:46 AM »

Fred
Kevin
et al.

I did a quick calculation regarding the minimum value of resistance placed between one side of the OWL and the other (with the midpoint grounded to bleed off static charge).  In Kevin's configuration the value of the line-to-line added resistance is: 6Mohms (line to line across the OWL conductors) in parallel with 12Mohms (OWL line-to grounded midpoint-to OWL line)... which equals 4Mohms of combined line-to-line resistance.

The fraction of the net power (net power = forward power - reflected power) that is lost in the resistor... assuming the resistor is located at a line-to-line voltage maximum (worst case) is: Z x SWR / R; where Z is the impedance of the OWL (e.g. 600 ohms), SWR is the standing wave ratio on the OWL (e.g. 6:1), and R is the net line-to-line added resistance (e.g. 4Mohms).

Example: if Z= 600 ohms, SWR = 6:1, and R = 4Mohms, then... when the added resistor is in the worst case location along the OWL (i.e., at a voltage maximum), the fraction of the net power (i.e. forward power - reflected power) that is lost in the added resistor would be (600 ohms x 6) / 4Mohms = .0009 = 0.09%.

Using the same equation:

If Z=600 ohms, and the SWR (under any operating conditions) is expected to be no more than than 10:1... and if one is willing to allow 1% (0.01) of the net power to be lost in the added resistor... then the added resistor must have a value that is greater than 600,000 ohms.

Stu
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AB2EZ
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« Reply #81 on: June 05, 2013, 12:21:55 PM »

The equation in my reply above can be derived as follows:

Let VF be the amplitude of the forward wave on the OWL
Let VR be the amplitude of the reflected wave on the OWL
Let Z be the impedance of the OWL

The net power = the forward power - the reflected power

The net power = [VF x VF/ 2Z] - [VR x VR/ 2Z]

From algebra: the net power = [VF+VR] x [VF-VR] / 2Z

Multiplying the numerator and the denominator of the above equation by (VF+VR), one obtains:

The net power = [VF+VR] x (VF+VR) x [(VF-VR)/(VF+VR)]/ 2Z = [VF+VR] x [VF+VR] / (SWR x 2Z)

The power dissipated by an added line-to-line resistor, having value R,... located at a voltage maximum (worst case position) along the OWL is

Dissipated power = [VF+VR]  [VF+VR] / 2R

Therefore the ratio of power dissipated in the added resistor to net power is: Z x SWR / R
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« Reply #82 on: June 07, 2013, 11:50:03 PM »

The ER article referenced many times in this thread talks about why resistors are a bad idea. One persons opinion but probably worth considering.

[...]

I'm not a subscriber -- could you summarize or quote the author's reasons for his belief that static drain resistors are a bad idea?

Thanks,


Kevin, WB4AIO.


I finally found my issue and re-read it.  The author points out two problems with resistors. One is that you need to use a large value (1 Meg or more) because of the potentially high impedance of the line. This in turn would create a very large RC time constant which would allow large static spikes in near storm conditions.

The other was that most resistors do not have a high enough voltage rating. I really don't see that argument because you could use a number of series resistors similar to what you would use for a high voltage meter or bleeder.

So I guess the argument is whether an RF coke or resistor works better for static buildup. I think the choke would be a better overall choice but if you need to buy the parts it is more expensive. The resistor would probably be OK for fair weather static but maybe not if a storm was near. Maybe someone needs to make some real measurements in areas where there are large fair weather static buildups to see what the effectiveness of high value resistors are.
 
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Steve - K4HX
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« Reply #83 on: June 08, 2013, 07:41:15 AM »

What's the RC time constant when the feedline is disconnected and grounded?   Wink
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wa3dsp
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« Reply #84 on: June 09, 2013, 01:18:09 AM »

If the line is disconnected and grounded then this is all a moot point. As far as I can determine the original question was about eliminating static buildup on an operating antenna that was not at DC ground.
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« Reply #85 on: June 09, 2013, 12:40:18 PM »

In which case RC time constants probably aren't relevant. Or as TFO said, "much ado about nothing."

But, if RC time constants are really important, the ER author was incomplete in his analysis if he left out the time constant of the ground rod/system.
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wa3dsp
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« Reply #86 on: June 09, 2013, 02:16:25 PM »

Very correct and as I mentioned before there is a lot more to this than meets the eye. A lightning event has a very high rise time, certainly in the 100's of Mhz range. So even a perfect ground system with a few feet of connecting wire will sustain a significant charge during a strike or even a near strike.

The question is what is the rise time of the static buildup you want to eliminate. My guess is unless there is a storm nearby probably not very high.
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