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Author Topic: Balanced line lightning protection and pass through  (Read 5567 times)
KH6AQ
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Posts: 7718




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« Reply #15 on: September 06, 2017, 11:12:15 AM »

Loop SWR and voltage

What are the loop dimensions, the height, and the bands of operation?

An EZNEC model of a 525' circumference loop up 40' shows maximum 450 ohm SWR at 1.8 MHz where the SWR is 9:1. The SWR decreases over the other amateur bands. At 600 watts the maximum voltage along the line is approximately 1400V RMS or 2kV peak. Placed at the highest voltage point along the line each of the two spark gaps would need to be set to over 1kV to avoid triggering from the RF. Any unbalance will put more voltage across one of the spark gaps. We could analyze other bands but I think the 2700 V GDTs in the Array Solutions unit would work for you.

That offers some protection to the antenna tuner but how much let-thru energy makes it to the receiver? Given the matching network schematic, that can be simulated. It might seem that a spark gap between the tuner and the receiver would help (it wouldn't hurt) but as soon as the ladder line spark gaps fire the one after the tuner may have no voltage to trigger it.

One of the two ladder line spark gaps will trigger first and the other will then see an increased voltage and be triggered in shortly after. The delay of the second spark gap may be shortened by having its gap illuminated by the UV from the triggered spark gap. The UV will cause some ionization in the gas in the other gap and remove the need for a large overvoltage to trigger it. The two spark gaps can be placed so that each gap can "see" the other gap.






« Last Edit: September 06, 2017, 11:16:19 AM by KH6AQ » Logged
KK6RPX
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Posts: 113




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« Reply #16 on: September 06, 2017, 12:23:57 PM »

Thanks again for the time and thought on this subject. A fair bit of what you are saying is above my pay grade but I'm learning a lot.

The length of the antenna is about 650'. That is what I happened to have and so we thought we'd string it up and see how it does. So far I have had no problem tuning up 160, 80, 60,40 and 20. Those are my bands of interest with 80/40/20 being where I spend most of my time. The feed point is about 30' and the other anchor points are at about 40'. I'll gain a little height on the feed point soon, I have set another anchor but have yet to move the antenna over, perhaps this afternoon.

I haven't used spark gaps in RF projects but have some experience with them working with HV projects such as tesla coils, jacob's ladders, kelvin generators and so on. I have always found them finicky and requiring checking, adjusting and cleaning regularly.  I have experienced UV "priming" with gaps (and neon indicators and tubes) so I know what you are talking about there. I'll look and see what is available, both DIY and purchased for ladder line spark gaps as it sounds like a combination may provide the best protection.

Just got home from my 48 hour work shift so have some chores to do. Will do some more reading later. Thanks you again very much for your time and effort.
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KH6AQ
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Posts: 7718




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« Reply #17 on: September 06, 2017, 01:10:17 PM »

How long is the ladder line between the antenna and where the spark gaps will be? With that and the other numbers we'll see what voltage a simulation says is at the spark gaps.
« Last Edit: September 06, 2017, 01:18:59 PM by KH6AQ » Logged
KH6AQ
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Posts: 7718




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« Reply #18 on: September 06, 2017, 03:37:12 PM »

And what antenna tuner are you using?
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KK6RPX
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Posts: 113




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« Reply #19 on: September 06, 2017, 04:19:10 PM »

There is 40' of ladder line to where i plan to put the lightning protection. Then into a 1:1 choke, knife switch and then to the tuner. The tuner is a MFJ-984 Versa Tuner IV.

Thanks!
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KH6AQ
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Posts: 7718




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« Reply #20 on: September 06, 2017, 08:21:15 PM »

Thanks for the details. While an actual antenna may be different than this EZNEC simulation I think it's in the ballpark. The model is a 575' horizontal loop up 40' fed thru 40' of 450 ohm ladder line having a velocity factor of 0.91. The line loss is assigned as zero and the RF power is 600 watts. The differential voltage at the spark gap protection is 2.0kV at 1.9 MHz, 1.0kV at 3.7 MHz, and less than 1kV on the higher bands. The two 2.7kV GDT devices in the Array Solutions open wire line lightning protection device provide a 2X ratio between trigger voltage and operating voltage.

The MFJ-984 is a T-network tuner with a maximum antenna side series capacitance of 250 pF. A quick check with SPICE, using the hypothetical model, shows that the small coupling capacitance greatly limits the energy to the transceiver. I don't see that the 10nF coupling capacitors in the lightning protection device serve any purpose and I'd leave them out.

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KK6RPX
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« Reply #21 on: September 07, 2017, 07:59:16 AM »

So, are you recommending both spark gaps and GDT? If so, what type of spark gap? If I skip the caps in the lightning protection device (which it sounds like as long as I am using a tuner I can do) I could fit it all in the same box as the choke, which would be quite convenient. In terms of protection would it make any difference to have the GDTs before or after the choke?
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N1CZ
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Posts: 23




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« Reply #22 on: September 28, 2017, 03:22:36 PM »

My loop is just under 300'. 600 Ohm ladder line is (I think) 30' to a 4:1 balun. The balun is a DC short between side as well as connection for ground. THIS makes for a quiet antenna because it drains off static build up.

I modeled this antenna as close to how it is installed as I could measure. A tuner gives me a little more band use ... especially at the bottom end of 80M.

No sparks plugs in this system because the balun has the DC ground to both sides of the ladder line. And, it is grounded.
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AH7I
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Posts: 75


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« Reply #23 on: October 08, 2017, 07:04:33 PM »



As to the pass through, the shack is in a trailer so metal walls. I have a metal pass through, I removed a pane of glass in a louver window and have bulkhead fittings. It also has a metal hood shielding it from weather and bringing balanced line through the mess on the other side might be tough. I think it might be easier to just punch a hole through the trailer wall and use twin coax for the feed through the wall.  Thoughts on that?

Thanks!

Jonathan

Here is what I would do. It has nothing to do with the electrical code. Mostly has to do with how lightning behaves.

Antenna - feed - short path to ground outside trailer - loop (impedance) in the line - feed through.
Tuner - line - alligator clips.
Clips to feed through when using the radio. Disconnect and move away when not using the radio.

Resist the urge to connect anything else to that ground outside the trailer. Do not connect the trailer. Do not run a ground wire to your rig. Nothing.

I disconnect both the feed line and the electrical power when I am not using the radio.

73, -Bob ah7i/w4
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W4IA
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Posts: 6




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« Reply #24 on: October 27, 2017, 10:31:02 AM »

I've used open feeders for over 60 years and have yet to suffer lightening damage. Here's the easiest way. Mount your balun outside, run a SHORT piece of RG8 coax through the wall or window from the balun to the tuner. Outside, attach banana plugs to the feed line, banana jacks to the balun. Connect 1 meg 3kv resisters from the balun jacks to a ground rod.

Keep the feed line disconnected from the balun when you are not hamming!

The resistors will bleed off static buildup when hamming from snow, wind, etc. Don't ham when a storm is near!!!

73, W4IA

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N4MQ
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Posts: 139




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« Reply #25 on: November 17, 2017, 05:16:01 AM »

My loop is 860 feet in length and in trees and on poles, for protection I have relays ground the loop when not on the air.  I have 100' of 600 ohm ladder line and the 1:1 balun is on the basement exterior wall in a grounded cabinet for protection of the relays and balun.  When on the air the balun is providing a low resistance ground path to the loop - some few ohms thru the core windings.  This provides protection to the shack and house thru grounding and dc grounding while operating.  I even have a deicing circuit Incorporated to melt ice from storms automatically cause Va gets ice, Enjoy. Woody
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