I agree with John TAG's points on current sharing and keeping the distance of the conductors apart - my spacing was a lot more than that (feet in places) but thats due to the ascent stage of the throwing device being non gyrostabilzed and tended to go off target as it hit a tree (need a bow/spud gun next time!)
Voltages wise - with the typical some 350ft circumference loop I had approaching 50A of current in the loop, but voltages were were still high enough across the matching components for corona, and as John ann Eric and others have found out sharp points and conductors/PVC onto branches at high powers will arc given a choice.
Must still have been over a couple of Kv - I cant afford a forest fire but verticals just wont work for me and give even more issue.
I didnt take a lightweight conductor and insulation solution route - I seem to remember my total copper and insulation weight being close to 350 pounds net - but I adopted a thick insulation, certainly not light or cheap, but I didnt get burn thru at the +24hr >1Kw level
As far as how high you could go - well if you were able to insulate all sections off the tree Im sure You could shove a few Kw round the loop but the current capacity of the components is going to be LARGE Warren -
- Like John I too used heavy duty insulators with tensioning on the horizontal lower leg - My were marine composite material over 12 inches long - the rest just sat on the branches. After a year or more I took the loops down and inspected the insulation and although I had scuffing on the Nylon outer sheath it didnt break thru to the PVC jacket and can/will be used again.
There was a load of discussion from my peers and betters on whether multiple parallel conductors would be stable in the wind as their relative position to each other "up there" does change - but looking at the phase/amp meter during the fierciest Okie storms and my limited experience of single conductors I would say it was manageable and I had very little tuning issues.
I did have heating of my toroids 2T:14T (variable) - more heat from the two turn side but I think I would change my design to see if I can reduce the flux/stress a bit more by adding more ferrite and or changing the ratios a bit more - though I do remember changing to 3T and varying the turns accordingly wasnt so good for my "array".
Current load sharing on the matching capacitor side was very important and many smaller value caps versus a couple of large ones was the way to go. I used a mix of G2 and G3 Sangamos Mica's totallying some 12.5nF and the G3 7.5nF (thats the biggest) was warm at 24 hr operation - the 1.5nF/3nF werent... a typical solution would be to use a number of http://cgi.ebay.com/SANGAMO-Mica-Capacitor-.0012-MFD-10,000-Volts-11-Amps_W0QQitemZ400062564002QQcmdZViewItemQQimsxZ20090719?IMSfp=TL090719159005r24905 or similar in parallel - I do prefer the heftier G3's though.
Differing tree, types, rough/smooth, varying incident branch angles may change your choice on conductor and insulation type - Id love to use a load of u/g RG6 in parallel but I think my Silver Birch in AK will wear it down - I cant shove hose pipe up to protect the insulation as everything has to be invisible (invisible in a forest/!) I used Black 4 or 6 AWG TFFN - pvc insulation with a nylon sheath - that slippery 6 mils of nylon really helps.
As john indicated the proof was in the smell and thermal temperatures of the components - Keeping a weather sealed box, critters out without getting thermal stress was difficult in my case. Going to +2Kw with what Ive got wouldnt work and I would have to redesign everything again -
My Ceramic feed thrus using brass knurled nuts some 1/4inch diameter were getting noticably warm and probably needed uprating - and it was more like high voltage/high current battery telephone exchange wiring techniques than radio.
What was interesting was looking at the efficiency of loops at 137kHz and some of the modelling tools - 350ft was considered by one program (too small <1%) but as you went up to 400-500 ft or more the efficiency really does zoom up - its going to be one of those copper RF losses versus total loop size variable but I can shove up a 500 footer loop but not with the wanted vertical section height - so again it would probably prove not so efficient.
My water table is only 11ft beneath the array so Im not expecting as good as results on the rocky ground of Okie - but Ill proably be wrong again - Empirical measurement rools...
good luck with the combiner Warren -
Off topic but just had a solar eclipse here - just taken on my Blackberry
http://kl1x.com/eclipsejuly2209teda.jpg - didnt have the LF radio on this time.
Laurence
Kl1X China
> Date: Tue, 21 Jul 2009 09:20:43 -0400
> From: [email protected]
> To: [email protected]
> Subject: LF: Loop (was Re: IGBT in 136 KHz TX?)
>
> Laurence,
>
> What do you think is the upper power limit for a loop using
> coaxial cable as a conductor?
> I've run about 1.3 kW into my loop made up of one turn RG-11 and one
> turn RG-214 (both turns parallel as in your case).
> Once one of the coax conductors came into contact with wet leaves near
> the feed point and began to burn through the insulation and the leaves
> - the first indication was erratic aerial currrent.
>
> I'm thinking of running two Deccas into a combiner but not sure
> if the loop insulation is sufficient.
>
> --
> 73 Warren K2ORS
> WD2XGJ
> WD2XSH/23
> WE2XEB/2
> WE2XGR/
>
> On Tue, Jul 21, 2009 at 1:47 AM, Laurence BY3A-KL1X
> China<[email protected]> wrote:
> > Jeff - The "turns" of the mains WD2XDW loop were just distanced parallel
> > loops running in roughly the same plane to reduce the RF copper losses and
> > open up the bandwidth a bit - . I did try a couple of "turns" but never
> > managed to get the ommppff field. Interesting given the fact that some
> > people use 2 turns on their K9AY's successfully -
> >
> > All the loops ends up welded together on lugs and attached to the tuner.
> > Ive just overhauled the tuner ready to put on line for WE2XPQ but it will be
> > running lower power for a while as I dont want to start a forest fire in
> > Alaska!
> >
> > Back home in KL7 on August 14th again...just need time...
> >
> > laurence
> > BY3A-KL1X Tanggu China 34degC
> > We2XPQ/ x Wd2XDW Wasilla AK
> >
> >
> > Subject: Re: LF: Re: IGBT in 136 KHz TX?
> >
> >
> > Hello Markus and Rick!
> >
> > I have only just acquainted myself with your messages.
> > (Just going out from hospital, big blood pressure problem...)
> > Thanks a lot for precious information, of what make fun still!
> > I recovered some ferrites N87 types which should work in 137 KHZ.
> > Another subject interests me, it is the possibility of transmitting with a
> > big spool of several turns (a bit as WD2XDW). But EIRP is much weaker than
> > with a vertical aerial, then it is necessary to "push more" in power!
> >
> > Thanks in advance!
> >
> > 73: Jeff / F6CWN
> >
> >
> >
> > ________________________________
> > Windows Live™ SkyDrive™: Store, access, and share your photos. See how.
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>
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