The AM Forum

Hi to all,

How do you determine blocking capacitor value and type that is placed inline on both sides of ladder line?

I have read values for blocking capacitors should be .002 pf, 4700pf door knob, mica types and .001mf of unknown type. 

Chuck

Blocking capacitor? What is it blocking?

If you mean series tuning caps, wouldn't you want variable caps?

I think its the final DC on the tank and off the antenner?


klc

Sorry for not being more clear on the blocking capacitor.

The blocking capacitor is a fixed value capacitor that connects the end of the ladder line to the the link coupler.  There will be one blocking capacitor for each side of the ladder line.

Blocking as in DC blocking is what I was looking for.

Chuck

Why would there be any DC on your OWL?

Are you doing a remote relay setup or such?

73DG

D,

"Why would there be any DC on your OWL?"

I think thats what he wants to prevent.


klc

Where would the DC come from?

If you have a link coupler you don't need a blocking cap.  the link will stop DC.  But most rigs, in fact all I have seen have blocking caps if they don't have a link tuned output network.

I suggest you start putting together a vintage reference library.  At a lot of hamfests, there are guys with boxes of books.  Go through them and buy any handbooks especially Bill Orr Radio Handbooks, ARRL, books by Terman, Sterling, NAVSHIPS basic Electronics, old first phone license manuals, just about anything from the 30s, 40s and 50s, and do some reading.  this includes old ham magazines too.   

Chuck,

Yes, a blocking cap isn't needed, but to answer your original question in case you need it for another project...

One thing to consider is how low will the OWL impedance be at a coupling capacitor if the tuner happens to be matching a low impedance like when series tuned?   If we want the capacitive reactance to be at least 1/10th or less of the match impedance, then even .01 uf might not be big enuff.

At 3.8 Mhz, .001 uf = 41 ohms Xc.

.01 uf =  4.1 ohms Xc


So, if the match happens to be very low, like 15 ohms for a shortened dipole, the cap shud be about 1.5 ohms (or less) of Xc =  .03 uf.

If the match is always a high impedance like 400 ohms or more, then  .001 uf will be plenty.


This requirement will get even more demanding on 160M and diminish on freqs above 75M.


T

At that point if you are running some decent power (which will be needed on 160) your problem will not be the cap; it will be the inductor(s) current rating.  you'll need one rated for 10 amps, maybe more on 160 with power and a low Z like 10 ohms. 

Maybe Chuck is afraid of the D.C. potential that a T-368 may put on an antenna when something craps out in its tank circuit.
Most transceivers whether SS or Tubes and even QRO amplifiers will never put D.C. on the antenna terminals.
Fred

How would the DC make it through the link?

Hi Chuck,
Here is what I did, based on a design that AB5WG described in ER Number 284, Jan-2013.

2- 4700pf 5kv doorknob caps
2- MFJ P/N 10-15197 PLATE CHOKE, 1.8-30MHZ, 1.5A, 4KV, AME AMP
2- Gas discharge tubes, Mouser P/N 650-GTCA28-402M-R03
1- Plate of 3/16" copper for the ground. Not shown is the 30' copper strap that connects to the plate, and is buried
at the base of the PT 4x4 that this system mounts to.

(http://i180.photobucket.com/albums/x257/fish1_07/am/ladder_line_protection/complete_assy_zps14ee3ad5.jpg)

(http://i180.photobucket.com/albums/x257/fish1_07/am/ladder_line_protection/gas_discharge_tubes_zpsbe0712ea.jpg)
73
Frank
KJ4OLL

Gotta admit that this thread has been a little confusing.  As has been already stated, There would not be a chance of DC out of the link coupled tuner unless the link accidently touched output coil and there was a failure in the tank of the amplifier.  Even then, the link might short out things unless the whole link got elevated above ground.  In any case I would think that design of the final should be such that that possibility would be more than remote.

Now having an inductive component on the OWL is another matter.  Would that not require some sort of transmitting high power mica cap or a couple of variable caps - either vacuum or breadslicer type?  Isn't the reactive component (albeit inductive or capacitive) on an OWL dependent on the antenna length / height above ground and the actual length of the OWL?

I haven't messed with OWL since I was a jn so have a lot of catching up to do

It's all pretty confusing Al. The photos show series caps in the OWL. What is the purpose of these caps?

Any transmitter properly built will have a choke to ground on the output. In the event that DC from the final somehow makes it to the output, the choke would short the DC to ground and blow a fuse, pop a breaker or otherwise shut down the transmitter - DC gone.

The photos also show a large copper plate in close proximity to the OWL. This creates unwanted capacitance to ground. All other things being equal, the less stray capacitance to ground, the better. It's not clear why you would want to increase this capacitance.

Thanks for the sanity check, Steve.  I’m interested in any OWL / tuna discussion because I see one in my future. 

Al

Maybe there are a good reasons for these additions to the system, but no one has articulated such.

FWIW WAM I once had a 124'center fed with 110 feet of balanced line. It worked FB on 3885 with only a BalUn on the shack end.

Same exact antenna and feedline worked on 7290 with the addition of a pair of 200 pF caps in series with each lead of the feedline at the shack end. It may have been 100 pF. One of those two.

Sure that makes sense. I'm pretty sure you didn't call them blocking caps or put them in there to keep DC off your tuner.  ;)

I didn't call them anything. Just hooked them up and got on 40.

Proper terminology has always been a problem for me so it's nice to see AMFOE coming to the rescue once again for someone else : )

That is good general advice. But in this case, how can the capacitance be more than about 5 pF? Would a few pF of capacitance to ground really matter that much? And I can't see this arrangement unbalancing the line, either.

On the upper H.F. bands and near voltage feed it would make a lot of difference.

It can reduce common mode impedance, thus potentially greater common mode currents and reduced balance.

Famous last words!

There is also a decent chance the amps antenna choke is already open due to a close lightning hit at some point in the past. About 30-35% of the amps I get in for service have a blown choke and if the plate blocking cap lets loose there could be fireworks and expensive repairs.

Same for OWL and link coupling, especially if its a swinger; accidents do happen and a couple of caps is cheap insurance. The cap can be a transmitting mica or a RF rated ceramic.

Carl

If it's a swinger and not a series resonated deal it has a DC short on the 50 ohm side no?

And unless it's an autotransformer design, how would the DC get through the link?

Does anyone on here have access or a copy of this ER article from Jan.2013 that was being referred to earlier in this thread.

Maybe that will help to clue us all in to whats going on with any need? for DC blocking caps inline with each side of the OWL, or AA5WG or KJ4OLL could explain.

Al VE3AJM

AA5WG,

Please take a look at Figures 1 and 2 on Page 45 of this document.

http://www.g4nvh.net/sitebuildercontent/sitebuilderfiles/hfbaltransys.pdf

Is anything here to what you are referring? If not, can you build a schematic using LTSPICE?

All I see here is Hi Impedance to Low impedance tuning capacitors but not what I would denote as DC Blocking capacitors.

Phil - AC0OB


 

I get "ERROR READING the document from Adobe.

I gave a Link to the document. I don't know what happened to the pdf file.

Phil- AC0OB

Since the thread is about a balanced OWL feed why muck things up with an unbalanced 50 Ohm ouput?

Because the thread is also about link coupled couplers. Mentioning open safety chokes in amplifiers also is a fairly safe assumption of 50 ohm to coupler input somewhere.

It is rather rare to see a vintage link coupled amp schematic with an antenna choke and many just copy what they see.

 

Carl, we're talking about a link coupled TUNER not a link coupling amplifier tank.

Nope, the OP never said that and hasnt been back to confirm what he is doing.

OTOH drifting well off track from OWL to 50 Ohm unbalanced adds even more confusion.

Carl

He did confirm in the third post in this thread. It's clear a link is involved whether it's a tuner or a tank. No DC will get through a link.



Emphasis added.

Read it for yourself at the link.

http://amfone.net/Amforum/index.php?topic=34128.msg263589#msg263589

I read that right after it was posted and nothing then or now says it is a tuner. A variable link center COUPLER on the TX/amp can also directly drive OWL and so can a fixed link where the supporting plastic insulators fail.

If I had fired up my HT-9 without taking a close look at the 40M fixed center link tank coil it came with I would very possibly had 1000V on the line to the rear panel feedthrus; it was that close from age deterioration.

My only concern here is safety and not get into another useless nit pickin' discussion with you or the groupie.
I'll leave it up to the OP to clarify what he is doing.

http://is.byu.edu/site/courses/description.cfm?title=READ-045-101

We may never know.  ;)

I can't help it that you choose to run dilapidated equipment. Please don't make it our problem.

If someone has the same point of you, are they your groupie?

AA5WG has been on other forums online in the past discussing and asking questions about link coupled antenna tuners. We won't know for sure? about all of this until he chimes in here again.  ???

I recall Ashtabula Bill always referring to his antenna matching devices as antenna couplers rather than tuners.

http://lists.contesting.com/archives//cgi-bin/namazu.cgi?query=%2Bfrom%3Aaa5wg%40yahoo.com&idxname=Towertalk&sort=date%3Alate

And on another mail list he also interchanges the terms coupler and tuner himself, and he's not referring to a link output circuit of an old transmitter, but of a tuner or antenna matching device.

http://www.eham.net/ehamforum/smf/index.php?action=printpage;topic=65374.0

Al VE3AJM

Thats a typical response from you and your boy wonder sidekick Steve...snotty comments with nothing intelligent to support the thread.

Not everyone is as perfect as you who has never been wrong and constantly likes to remind others.

Just looking at some of the photos on here over the years of potential deathtraps is enough to be concerned, it only takes one slip and BANG

And this is a typical response from you and your name calling...snotty comments with nothing intelligent to support the thread.

Not everyone is as perfect as you who has never been wrong and constantly likes to remind others, as in you capacitor replacement tirades. After all, only dummies would do radio repair differently than the all knowing, almighty radio god KM1H. Then there's your current Heathkit tirades. No kits exist today because Carl says no kits exist. He has spoken. All hail.

Please, continue to bless us with your presence and righteous knowledge. We couldn't live without you.

Steve, you are a serious piece of work and just love to put your own very distorted spin on everything I say. You remind me of the media, simply trash anyone that says something they dont like.

Im thru wasting my time with your replies, just ignoring you is the way to deal with it and many others on here will soon get the same idea and you can have your own little clique to amuse.

Carl

I should remind you in case you've forgotten; this is Steve's website.  You are essentially an invited guest in his home by being here.  It might be wise to behave yourself like a polite guest. 

Im well aware of that fact without your "help", thank you, and have even contributed to the sites financial upkeep fund for the past 2 years. I dont see you on any of the contributor lists tho. Nor do you even bother to list your name in a thread.

There are 3-4 others (not sure of those numbers as the list may have not been updated) in the co admin/moderator capacity on here that are not into the continuous bully mode harassment and like the top people they are they only make their presence known when needed.

As other members have commented to me Steve has been very changed recently in general and having met him a few times and sat with him at an AM Dinner at Nearfest I dont understand the reason in my own case. I dont drink or do drugs either ;D

Carl

Getting back to the original question which was what value and rating of series cap to put in both sides of a balanced line to block DC. This of course depends on the lowest frequency and the highest power to be used. While the original question was (apparently) posed to eliminate the danger of plate potential DC on the antenna the article cited in this thread by KJ4OOL in the January 2013 ER on lightning protection for ladder line is a good reference. The author used two 4700pf 5000 volt series capacitors. He stated that he has used legal limit power, long AM transmissions, and all HF bands without a problem.

If you are worried about plate DC on your ladder line you should also be, maybe even more, worried about lightning or static buildup. The unit shown in January ER deals with all of this using a combination of RF chokes, gas discharge tubes, and series capacitance. See KJ4OOL's message in the first page of this thread.

I might also add that all of this applies to single wire antennas also. You would just use one half of the circuit.

 

WOW!

This all started out with a misnomer concerning OWL and DC.  I think it came about due to inexperience of the poster.  No big deal, we'll figure it out.

As to the rest of the thread, the moderators here are eloquent and guiding folks, however;

Steve is the money guy, so what he says pretty much goes.

There is always the California forum, with a REALLY opinionated owner.

73DG

Gentleman,

I have been out of town and will get back to you all soon.

73,

Chuck

Sorry to "RE-Quote " your reply. But the explanation of the circuitry pictured, and shame on me for not looking in my ER's, takes all of the mystery out of the thread. The picture posted looked quit involved with components that might really affect performance of the ladder line / antenna.
And AMFONE still stands out as a great gathering place of knowledgeable RF people who have years and years of experience with the tech world. Innovators and inventors and folks making things we call goodies or toys for our Man Caves.
Fred

So exactly what do those 2 hockey puck caps in the posted picture do?

That is a good question. I would say that in the scheme of things the Gas discharge and RF chokes are the most important items. The caps would block DC should there be a breakdown event at the transmitter but since the chokes are there if the caps were not it would just short it to ground like the protection RF choke on the output of many tank circuits in transmitters. This is probably what you would want since any exposed wiring on the TX side of the caps would be hot with the DC.

As for lightning any fast rise time spike would probably be coupled by the caps just like they were not there. There may be some benefit to having them there but on the other hand  if they are the right size and rating for the lowest operating frequency there is no real downside to putting them in.

Interestingly the author of the ER article states he always disconnects the antenna from the rig when not in use. If that were always true then I suspect the caps would be of no benefit in the case of a lightning event.   

At the risk of drawing verbal lightning strikes from the more passionate posters here:

If the OWL connects... at the antenna end... to just a pair of wires.... and there is no DC path (like a pair of suitable RF chokes to ground) on the antenna side of the capacitors... then:

Neither of the capacitors would be charged up (i.e. have DC voltage across it) until a load (like someone's body) is placed between either of the OWL wires and ground. Therefore... while they would prevent the continuous flow of direct current... they would not prevent one from receiving a nasty shock in the event that somehow B+ had managed to develop a path from the transmitter to the link at the output of the balanced coupler/tuner. The bigger the value of each of the capacitors, the more charge (current x time) would flow when someone first placed themselves between one of the OWL wires and ground.

As has been pointed out, the RF chokes will need to be placed at a location along the OWL where the wire-to-wire RF voltage is not too high. Doing so will minimize the impedance required to sufficiently block RF, and will minimize saturation effects (if each of the chokes includes a ferrite core). Since the SWR might be fairly high, and the location of voltage maxima will depend upon the frequency/band being used (and the length of the OWL, in wavelengths), the best location of the chokes might be very wavelength dependent. Saying this same thing another way: If the chokes are placed at a fixed location along the OWL (e.g. near the coupler/tuner), the effectiveness of the chokes in blocking the flow of RF may vary greatly with the frequency of operation. 

To be clear, three points.

1. Todd, KA1KAQ is also an "owner" of this site. But it's not our web site. It open for all to use and exchange information. Our part is to keep the thing functioning technically (web server, software, updates, etc) and to maximize the value to the users/members. Only you can be the judge of how well we are doing that job.

2. No one is forced to be here. And no one has a right to be here. If this site is so problematic, stay away. The numbers don't lie, however. We have over 1300 active members. We get new members daily. We average over 200 members logging in and hundreds more visitors (that don't log in) daily. There have been over 200,000 posts on this site. We average almost 80 posts per day. We have 100's who have been active members for over a decade. Yet it's only one or two that have a problem.^** Again, the numbers don't lie.

3. I don't care if someone disagrees with me, as long as they do it without name calling. I'm not here to shout down opinions. You will notice that I don't post in the vast majority of the threads here. If it were about me wishing to control the conversation, I would be much more involved. Vigorously discussing an issue doesn't mean that I am out to get someone or in anyway dislike the person with which I am discussing the issue. The reality is that nearly all the discussions here are inconsequential in the grand scheme of things. In other words, I don't really care that much about any given topic. If you think I do, you are sadly mistaken.



** These troubled few always have the same refrain - they are being singled out, there is a clique, the moderators are heavy-handed, there's a conspiracy. Most have had problems on and/or been kicked off other forums and email lists. They think the rules don't apply to them. When the rules are applied they complain and come up with these silly excuses for their bad behavior. Anyone who thinks the site admins and moderators have the time to single someone out need a reality check. Anyone who thinks there is a clique needs to look up the definition of clique (I'm sure a group of 1300 is not a clique). While they are at it, these troubled few should look up the definition of paranoid.

WHAT??!? Steve gets a boy wonder sidekick?? WHEN did THIS happen??


There's your problem right there. Throw down a couple good, stiff drinks before logging on and you'll see just how 'important' all this online stuff is. ;) Seriously, it's just a hobby Carl. You're one of the most knowledgeable guys I know in this field, but you really seem to place far too much importance or emphasis on it. If someone doesn't know or understand something as well as you it shouldn't get under your skin but be an opportunity to pass on some of that knowledge.

As to the clique Steve has going here, it's somewhere around 1362 members at the moment, and that's after the database was cleaned out of inactive/duplicate accounts last week. He did quite well with the original AM Window site, too. All indications are that this is a pretty active, helpful site despite the occasional dust up.

OWL is a future project for me once we get outta this transitional place and into a place where we can really stretch out. Each time something comes up on the topic, whether this thread or 'KYV/Don's masterpiece line that he constructed, I learn something new. Keep it coming, gang. Everybody benefits from a civil discussion and as Steve said, no one is forced to participate.

I do plan on getting to the bottom of this 'boy wonder sidekick' thing, though....

regarding the AB5WG design mentioned earlier - wondering if the design is counting on the ability to arc to the copper plate from the feeding lines? This in the event of a hit or near hit from lighting. I'd expect the RF chokes to *bamm!* open like fuse wire.

My immediate thought is "I want a spark gap" on both sides of OWL.

Whooo, whoooo?

                         _-_-

If you read the ER article the purpose of the RF chokes are to eliminate static charges and keep the OWL and antenna at a DC ground. You are correct, any kind of direct or probably even a near hit would toast the RF chokes BUT that is the reason for the gas discharge tubes. They conduct and short the spike to ground. He also points out in the article why a spark gap, often used in the past, is not a good idea because of the difficulty in setting it and the variation due to the environment. A spark gap alone would allow very high static levels to build up before arcing if it were set for the maximum operating voltage. Remember you are probably using this antenna for receive also.

The idea here is to, as much as possible, stop the lightning event from going past this point and into the house and equipment. The RF chokes are rated at 4KV and the author pointed out that he has used it at legal limit power on all HF bands through 6 meters. I suspect there could be some combination of antenna design, line length, and frequency that might tax the RF chokes but none were reported.

I would say eliminate the capacitors and the RF chokes would shunt the danger of any DC coming out of the transmitter as well as eliminate static buildup.

I ought to have been clearer, I envisioned spark gaps or gas discharge units across each RFC to ground.

In the case of the spark gap, it would not matter much if the gap was off by a bit because they would only operate when the RFC either saturated, was about to arc over or blew out.

                            _-_-bear

Anyone else have a comment on Stu's point about choosing the placement of the RFCs & box? Seems to me it naturally wants to be at the base of the antenna where the feedline turns to go to the shack?

There is a lot more theory to this than meets the eye. The extremely high rise times of a lightning strike which can be at many hundreds of megahertz or more would not be immediately dissipated by the choke but it would fire the gas discharge tube. The Alpha Delta series of coax switches and lightning protectors use replaceable gas discharge devices.

From a standpoint of RF the position it is placed in the line given the wide frequency range it would be used on would not matter BUT for lightning protection it should go close to the entry point of the house near the ground with as short a ground connection as possible.

Lightning.  The sky's bully.

I've worked in broadcast since '64, also known as the business where now you hook a big computer to a lightning rod, and hope it stays on the air so they make $$ and can pay you.

Old school was protection, nowadays it is avoidance.

Send the bully elsewhere to strike. This technology works, and I have seen it perform many times:

http://www.nottltd.com/lightning.html

Put some of these on your towers and poles, you'll be $$ ahead.

I have no interest in Ron's products, but have used them a lot to complete satisfaction.  There are other firms marketing similar devices.

73DG

People are listening. View from my office window:

But all that low level ionization must raise havoc with ham style reception I would think even when the storm is miles away. It may not bother Beverages or other on the ground antennas far enough away from a tower.

Looks easy enough to home brew however, maybe time to fire up the grinder and TIG welder. There is plenty of SS scrap at the local recycler.

The concept also gives some credence to those who claim their ungrounded towers have never been hit and have a "cone of protection". There are certainly lots of sharp points on the usual Rohn tower....Ive donated enough blood over the years to prove that point ;D

Carl

In a previous thread the suggestion for hams: stainless steel chimney brush.

Tom, Phil, Steve, Carl, Al and all,

Sorry for not getting back sooner.  The flu hit me hard.

I am working with a link antenna tuner.  

My interest is getting rid of static build up on the balanced antenna and ladder line.

When I said DC blocking capacitors I should of said blocking capacitors inline with both sides of the ladder line on the output side of the "traditional" link antenna tuner.  

The author of the January 2013 Electric Radio article states, "The purpose of this capacitor is to block the static currents present on our antenna system from flowing back into your transmitter or receiver.  This will allow the choke and GDT to take care of the static currents and isolate your equipment from the antenna static currents."


Please jump in and correct me.  It seems if I add door knob capacitors inline with the ladder line I would be changing the antenna system electrical length, for a given frequency?


Chuck

Chuck,

First of all if the value of the capacitors is large enough for the lowest frequency of operation they will have no effect on the RF operation of the circuit. They will essentially represent a short circuit at the operating frequency while  blocking DC. Think of this like any coupling capacitor, it is design to pass the desired frequencies while blocking DC.

The reactance of a .01uf cap at 1.8mhz is about 9 ohms, .001uf = 90 ohms. So it depends on the impedance the antenna represents to the load. If it is quite high you  might be able to get away with even 500pf and likewise if you are operating only at higher frequencies you would be able to use a lower value.

I think one could argue that with the RF chokes and Gas discharge tubes the capacitors would be unnecessary. It would be impossible for any DC buildup to exist with the RF chokes to ground. I really think the RF choke or direct DC ground on the antenna is a much better approach then trying to isolate it with only capacitors.

Also if the static buildup was sudden the capacitors would pass it anyhow depending on its rise-time but this would generally not be the case except in a storm situation. 

Im glad we have the actual application answered  ;)

While gas tubes do the job they have a finite life and the series caps add a bit of extra insurance at low cost. Here is antenna specific info that may be of interest and not usually discussed by the generic manufacturers.
http://www.nexteklightning.com/pdf/Gas%20Discharge%20Tube%20Maintenance%20v3.pdf

The choke handles the low level pulses without destructing, the gas tube can be purchased at a wide range of firing voltages and protects the choke in normal use and 20KV 1000pf caps just might save a variable cap or switch from arcing.

The big question now is how to spec the gas tube?

One other thing to consider is a BIG choke such as used in a BC station at the antenna feed point. It will be a very low DC resistance to ground but will have to be swept for the ham bands wanted but the right one will take a lot of near miss abuse.

Carl

Carl

"and 20KV 1000pf caps just might save a variable cap or switch from arcing."

If these capacitors are connected to the two ends of an output link (coil)... how will they protect the variable cap or switch from arcing?

There is no path (except stray/parasitic capacitance) through which DC can flow to charge up these capacitors.

Stray/parasitic capacitance to ground will form a capacitive voltage divider in combination with these series capacitors. But, since the stray/parasitic capacitance will be much less than the capacitances of either of these two series capacitors, and since the leakage resistance to ground associated with the stray/parasitic capacitance is likely to be high compared to the leakage resistances of the added series capacitors, the majority of the static voltage, if any... (i.e. if there are no RF chokes present to provide a DC path to ground, or if for some reason the RF chokes open up)... on the antenna side of these capacitors will still appear between the tuner/coupler side of these capacitors, and ground.

If there is a grounded center tap on the output link of the tuner/coupler, then these capacitors will be able to charge up to drop the static voltage (if any) between the antenna aside and ground... but adding a grounded center tap brings with it a separate set of problems... and obviates the need for the capacitors to block DC.

Stu

Stu, since when are the Johnson Matchboxes NOT considered "Traditional" link coupled tuners?   ;D

Carl

Carl

I give up. Since when?

In a Johnson Matchbox (275W or 1kW model), there is a variable capacitor between each of the balanced output terminals and ground. These variable capacitors, depending upon the settings, will have values that are much less than the 1000pF or 10,000pF capacitors that are contemplated as added series capacitors in this thread. The leakage resistance to ground of each of these variable capacitors can be assumed to be higher than the leakage resistance across each of the contemplated series capacitors. Therefore, in the event of static charge buildup on the antenna and/or the OWL wires... adding the contemplated series capacitors will not block the associated voltage. Most of the voltage will appear between the variable capacitors and ground. 

Stu

Since long before you even had your first license, however short, but you may have seen ads.

With a 20KV 1000pf fixed in series with a 100pf maximum at 3000V Peak DC in the small tuner the fixed cap will protect it until it breaks down and that is with a perfect cap in dry air. It doesnt take much to develop 3000V in a fairly close lightning strike.
The big tuner is 7000V Peak DC

Carl

Carl

I disagree with your analysis. With a 1000pF capacitor in series with the tuner's 100pf... going to ground...charge injected into one of the antenna conductors will result in a voltage across the pair being split such that 10/11 of the voltage will appear across the 100pF capacitor and 1/11 of the voltage will appear across the 1000pF capacitor. The same current must flow through both capacitors. Therefore both capacitors will have the same amount of charge. V= Q/C. Therefore the smaller capacitor will have most of the voltage across it.

Stu

The whole deal of putting anything between your OWL and your tuner other than a grounding switch is 'much a'do about nothing', if I may quote the Bard.

73DG

I did two things. I put high value resistors (6.8 Megohms 10W noninductive) across the line and from each side of the line to ground. My idea is that precipitation/storm-front static can't build up to arcing and snapping levels with the resistive drain, however high it might be. So far it seems to work.

And, to protect against nearby strikes, I added quick-disconnect connectors right at the point where the balanced line enters the house. If I see lightning or hear thunder (or am leaving for a while), I disconnect the feeders and toss them 20 feet or so away from the house. I always do this, even if it's already started raining heavily.

73,

Kevin, WB4AIO.

So far with this thread, Kevin has the correct (IMO) method for lightning and static build-up protection.

High value resistors to continuously drain off static charge build up and disconnect the feeders during rain and lightning storms.

Fred

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

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

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.

OP said:

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.

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.

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.

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

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

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.
 

What's the RC time constant when the feedline is disconnected and grounded?   ;)

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.

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.

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.