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#11
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Two coax as substitute for open line
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#12
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Two coax as substitute for open line
ve2pid wrote:
In the ARRL's Antenna Book 21st ed page 24-21, we see that if we connect the two shields of the coax cables together, we obtain 'Shielded parallel Lines' . In that case, the resultant impedance is simply the sum of the characteristic impedances of each coax. So, there is quite a difference between the two independent coax I mentioned in my first message (we connect the shield to the inner conductor at each of its ends) (A) and the 'Shielded Parallel Lines' case (B). I am trying to understand why and it is the reason I posted my first message... In (A), the Z=276*log(2S/D) applies, so the Zo of each coax does not matter.. but in (B), Z=Zo1+Zo2, so the value of each Zo matters. I am sure that in case B it is as for resistors in parallel, ie 2 50ohm cables in parallel give you 25ohms Zo. Jeff |
#13
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Two coax as substitute for open line
Jeff wrote in news:hm5dj4$4ls$1
@speranza.aioe.org: I am sure that in case B it is as for resistors in parallel, ie 2 50ohm cables in parallel give you 25ohms Zo. Jeff, you you offer more explanation that just that your are "sure". If you can't explain it, it speaks of whether you are sure. Owen |
#14
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Two coax as substitute for open line
Owen Duffy wrote in
: steveeh131047 wrote in : The application is no secret - the first paragraph in my linked page explains it. Steve, Another thought comes to mind, and that is a pair of rectangular conductors, preformed to the parabolic like shape and screwed togeter with insulated fixings. You could achieve the Zo you seek with say 20mm wide flat with manageable spacing... however it would be a water trap to some extent (just like LP feeds using twin booms). Owen |
#15
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Two coax as substitute for open line
Owen Duffy wrote:
Jeff wrote in news:hm5dj4$4ls$1 @speranza.aioe.org: I am sure that in case B it is as for resistors in parallel, ie 2 50ohm cables in parallel give you 25ohms Zo. Jeff, you you offer more explanation that just that your are "sure". If you can't explain it, it speaks of whether you are sure. Owen Well having just tried it for real on a network analyser, and simulated it on Ansoft designer I am now convinced rather than being sure!! Jeff |
#16
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Two coax as substitute for open line
On Feb 25, 4:51*pm, Owen Duffy wrote:
Owen Duffy wrote : steveeh131047 wrote in : The application is no secret - the first paragraph in my linked page explains it. Steve, Another thought comes to mind, and that is a pair of rectangular conductors, preformed to the parabolic like shape and screwed togeter with insulated fixings. You could achieve the Zo you seek with say 20mm wide flat with manageable spacing... however it would be a water trap to some extent (just like LP feeds using twin booms). Owen Owen, Thanks for the suggestion - water shouldn't be a problem because this "band interconnect" TL can be installed within the hexbeam centre post where it would be protected. One chap has already fabricated a "square coaxial" version - second photo, he http://lema.epfl.ch/images/stories/L...HB9MCZ_wb.html but that will be beyond the capability of many constructors! It's not a major issue - most constructors simply use RG213; but some find the physical terminations "fiddly", especially if you try to assemble it inside the centre-post for neatness and weather protection. I'm pretty sure that twin line using the right gauge ecw with teflon tubing on one of the wires would be about the correct Zo and easy to fabricate. It's just another thing on my "to do" list - a list which seems to get longer rather than shorter 73, Steve G3TXQ |
#17
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Two coax as substitute for open line
steveeh131047 wrote in
: .... Thanks for the suggestion - water shouldn't be a problem because this "band interconnect" TL can be installed within the hexbeam centre post where it would be protected. One chap has already fabricated a "square coaxial" version - second photo, he http://lema.epfl.ch/images/stories/L...exbeam/hexbeam _HB9MCZ_wb.html Seeing the construction, I think a pair of rectangular tubes would be easier to construct, and raise less water retention issues. I didn't realise that the line section is straight, I had visualised it taking the shape of one of the spreaders if you understand what I mean. Anyway, my recommendation would be like in the pic, but using two side by side rectangular tubes, say 20mm with 4mm spacing. Owen |
#18
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Two coax as substitute for open line
Jeff wrote in
: Owen Duffy wrote: Jeff wrote in news:hm5dj4$4ls$1 @speranza.aioe.org: I am sure that in case B it is as for resistors in parallel, ie 2 50ohm cables in parallel give you 25ohms Zo. Jeff, you you offer more explanation that just that your are "sure". If you can't explain it, it speaks of whether you are sure. Owen Well having just tried it for real on a network analyser, and simulated it on Ansoft designer I am now convinced rather than being sure!! That is not an explanation at all. Your confirmation might just be confirmation of a wrong interpretation of the B configuration. Owen |
#19
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Two coax as substitute for open line
Owen Duffy wrote in
: .... Anyway, my recommendation would be like in the pic, but using two side by side rectangular tubes, say 20mm with 4mm spacing. I should have added that I would attach the coax at the bottom, and use a ferrite cored broadband balun. Owen |
#20
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Two coax as substitute for open line
Owen Duffy wrote:
Jeff wrote in : Owen Duffy wrote: Jeff wrote in news:hm5dj4$4ls$1 @speranza.aioe.org: I am sure that in case B it is as for resistors in parallel, ie 2 50ohm cables in parallel give you 25ohms Zo. Jeff, you you offer more explanation that just that your are "sure". If you can't explain it, it speaks of whether you are sure. Owen Well having just tried it for real on a network analyser, and simulated it on Ansoft designer I am now convinced rather than being sure!! That is not an explanation at all. Your confirmation might just be confirmation of a wrong interpretation of the B configuration. Owen and you just might be trolling. Jeff |
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