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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I recommended the "magic 50k" years ago. I worked it out from Terry Fritz's streamer loading models, and the assumption of a loaded Q of 10. It also agreed with the better performing coils at that time.
Registered Member #30656
Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208
Eric - sounds like testing Arc Attack's inductive breakout point idea might be worthwhile if you're burning up secondaries too.
I may be able to do some arc measurements to try and increase the understanding of what happens with ground strikes etc. I've got a PC based scope which can talk over wifi, so should be able to battery power it and stick it on the topload to get realtime data on arc impedance. It also has a few megasamples of segmentable memory, which is handy to capture a good burst.
No guarantees if/when I can do any measurement, and how applicable it would be to larger coils than my 160x700mm, but it's certainly an interesting topic.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Hydron wrote ...
Eric - sounds like testing Arc Attack's inductive breakout point idea might be worthwhile if you're burning up secondaries too.
I may be able to do some arc measurements to try and increase the understanding of what happens with ground strikes etc. I've got a PC based scope which can talk over wifi, so should be able to battery power it and stick it on the topload to get realtime data on arc impedance. It also has a few megasamples of segmentable memory, which is handy to capture a good burst.
No guarantees if/when I can do any measurement, and how applicable it would be to larger coils than my 160x700mm, but it's certainly an interesting topic.
This is a great topic, my own secondary for my large DRSSTC is around 56K.
Is there any online documention to arc attacks inductive break out point, searching on google did not turn anything up for me since its so generic words.
I recommended the "magic 50k" years ago. I worked it out from Terry Fritz's streamer loading models, and the assumption of a loaded Q of 10. It also agreed with the better performing coils at that time.
Can you give some details on your thoughts at that time? I wonder if this rule can be stretched by other design parameters such as primary inductance, frequency, coupling, choice of poles, high power versus low power etc.
Hydron wrote:
I may be able to do some arc measurements to try and increase the understanding of what happens with ground strikes etc. I've got a PC based scope which can talk over wifi, so should be able to battery power it and stick it on the topload to get realtime data on arc impedance. It also has a few megasamples of segmentable memory, which is handy to capture a good burst.
I'd be very interested in hearing about these kinds of measurements. I've done some of these with a mini DSO on top of the coil but have never looked at ground strikes.
Registered Member #599
Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624
Goodchild wrote ...
To my point, ever sense we have started using a counterpoise ground I have noticed that the coil is more prone to flashing over and burring up secondareis. I thought this was just a coincidence but your guy’s comments have made me think otherwise.
The counterpoise ground helps to reduce radiated interference (lower impedance path to ground) however could it be a problem when dealing with ground strikes?
Now this is even more interesting, this run was first with both counterpoise ground (1x2m metal mesh on ground) and ground rods. It may or may not have something to do with this..
Registered Member #30656
Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208
Kizmo wrote ...
Goodchild wrote ...
To my point, ever sense we have started using a counterpoise ground I have noticed that the coil is more prone to flashing over and burring up secondareis. I thought this was just a coincidence but your guy’s comments have made me think otherwise.
The counterpoise ground helps to reduce radiated interference (lower impedance path to ground) however could it be a problem when dealing with ground strikes?
Now this is even more interesting, this run was first with both counterpoise ground (1x2m metal mesh on ground) and ground rods. It may or may not have something to do with this..
These question are why I'm gonna have a crack at some measurements - topload measurements been done before, but apparently on more of a single-shot basis, rather than looking at a burst, or ground strikes vs normal streamer hits etc. Might have to rig up a counterpoise and include that too. My advantage in this case is that I can grab the data while running the coil, rather than set up a mini DSO for a single capture.
I'm about to do some testing on getting 5.8GHz wifi through a faraday shield (~5cm holes should do the trick), if that works then I'll set up the coil tomorrow and have a go at getting real data. I think I'll try sticking the router only on the topload first though, really don't want to pop the scope if there is an issue!
Edit:
Steve Conner wrote ...
It is interesting indeed! Maybe a poor, high impedance ground absorbs the "whiplash" instead of reflecting it.
Maybe it's time for someone to measure ground impedance too! (at TC frequencies obviously). If it turns out to be anywhere near the secondary impedance then the increased-secondary-destruction-with-counterpoise phenomenon would make a lot of sense, as a terminated transmission line wouldn't have a big reflected wave causing high voltage at the bottom of the secondary.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Hydron wrote ...
Maybe it's time for someone to measure ground impedance too! (at TC frequencies obviously). If it turns out to be anywhere near the secondary impedance then the increased-secondary-destruction-with-counterpoise phenomenon would make a lot of sense, as a terminated transmission line wouldn't have a big reflected wave causing high voltage at the bottom of the secondary.
Here are some interesting papers on earth resistance, impedance, skin depth etc: Theory but some missing pictures:
Lots of practical measurements and data in tables for TC frequencies - seems like a newer version of the same:
I've used Wards values from They amount to about 0.1pF/cm and 2.6mH/cm for his secondary. From the transmission line equations that results in
v=1/sqrt(L*C) i.e. about 600km/s,
which is about what he gets. For a perfect termination of pulses running down the secondary you need a resistance of
R=sqrt(L/C) i.e. about 160k
This is seriously too large, considering the several amps that are flowing out of the base. The only way to achieve proper termination is to do it only for high frequencies, i.e. an induction at the base essentially shortcutting TC frequencies paralled by a resistor. I wouldn't expect a whiplash anyway, since a good grounding of the base would just reflect the pulse without increasing the amplitude.
Registered Member #30656
Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208
Yeah, 160k is way too big. Just did a quick sweep of the ground in my (fairly damp) back yard, between a shallow spike (~5cm) and either a deeper spike a couple of meters away (still not very deep, only about 30cm) or mains earth, or both. Technique was essentially the same as the HAM operator did in Mads Barnkob's last link (at a quick glance, results look very similar too, despite very different distance). Quick answer: about 1k ohm. Results are below, the file names indicate which test is which:
As for the transmission line amplitude stuff, you can indeed get a voltage increase of 2x upon reflection, but only for an _open_ circuit transmission line, a short circuit should reflect the inverse of the incident wave (adds up to 0 at the short). I haven't though too hard (let alone done any modelling) about the applicability of TL theory to the secondary though - might be looking at this completely incorrectly.
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Large DRSSTC: choosing secondary impedance
Maybe a poor, high impedance ground absorbs the "whiplash" instead of reflecting it.
