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#1
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Coax Collinear Element Materials and Velocity Factor
Owen Duffy wrote in
: .... Here is a deck for 8 halfwaves at 435MHz with a single stub tuner. .... Ouch, that had some remanents of a matching scheme using RG62. Here is a better deck. CM 8 half waves coaxial collinear on 435MHz CM Assumes lossline TL, VF=0.667, effective choke at bottom of array. CM Matched to 50 ohms with single stub tuner. CM Owen Duffy 2009/04/05 CE GW 2 20 0 0 -0.229885 0 0 0 0.0045 GW 3 20 0 0 -0.45977 0 0 -0.229885 0.0045 GW 4 20 0 0 -0.689655 0 0 -0.45977 0.0045 GW 5 20 0 0 -0.91954 0 0 -0.689655 0.0045 GW 6 20 0 0 -1.149425 0 0 -0.91954 0.0045 GW 7 20 0 0 -1.37931 0 0 -1.149425 0.0045 GW 8 20 0 0 -1.609195 0 0 -1.37931 0.0045 GW 9 10 0 0 -1.83908 0 0 -1.609195 0.0045 GW 200 1 -0.01 0 -2.25308 0.01 0 -2.25308 0.001 GW 201 1 -0.01 0 -2.35308 0.01 0 -2.35308 0.001 GE 0 GN -1 EK EX 0 200 1 1 0 TL 3 1 2 1 -50 0.3448276 0 0 0 0 TL 4 1 3 1 -50 0.3448276 0 0 0 0 TL 5 1 4 1 -50 0.3448276 0 0 0 0 TL 6 1 5 1 -50 0.3448276 0 0 0 0 TL 7 1 6 1 -50 0.3448276 0 0 0 0 TL 8 1 7 1 -50 0.3448276 0 0 0 0 TL 200 1 8 1 50 0.471 0 0 0 0 TL 200 1 201 1 50 0.0855 0 0 1e99 0 FR 0 0 0 0 435 0 RP 0 361 1 1000 -180 0 1 The coax half wave sections are exactly a half wave (electrically). Owen |
#2
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Coax Collinear Element Materials and Velocity Factor
On Apr 6, 11:41*pm, Owen Duffy wrote:
Owen Duffy wrote : ... Here is a deck for 8 halfwaves at 435MHz with a single stub tuner. ... Ouch, that had some remanents of a matching scheme using RG62. Here is a better deck. CM 8 half waves coaxial collinear on 435MHz CM Assumes lossline TL, VF=0.667, effective choke at bottom of array. CM Matched to 50 ohms with single stub tuner. CM Owen Duffy 2009/04/05 CE GW 2 20 0 0 -0.229885 0 0 0 0.0045 GW 3 20 0 0 -0.45977 0 0 -0.229885 0.0045 GW 4 20 0 0 -0.689655 0 0 -0.45977 0.0045 GW 5 20 0 0 -0.91954 0 0 -0.689655 0.0045 GW 6 20 0 0 -1.149425 0 0 -0.91954 0.0045 GW 7 20 0 0 -1.37931 0 0 -1.149425 0.0045 GW 8 20 0 0 -1.609195 0 0 -1.37931 0.0045 GW 9 10 0 0 -1.83908 0 0 -1.609195 0.0045 GW 200 1 -0.01 0 -2.25308 0.01 0 -2.25308 0.001 GW 201 1 -0.01 0 -2.35308 0.01 0 -2.35308 0.001 GE 0 GN -1 EK EX 0 200 1 1 0 TL 3 1 2 1 -50 0.3448276 0 0 0 0 TL 4 1 3 1 -50 0.3448276 0 0 0 0 TL 5 1 4 1 -50 0.3448276 0 0 0 0 TL 6 1 5 1 -50 0.3448276 0 0 0 0 TL 7 1 6 1 -50 0.3448276 0 0 0 0 TL 8 1 7 1 -50 0.3448276 0 0 0 0 TL 200 1 8 1 50 0.471 0 0 0 0 TL 200 1 201 1 50 0.0855 0 0 1e99 0 FR 0 0 0 0 435 0 RP 0 361 1 1000 -180 0 1 The coax half wave sections are exactly a half wave (electrically). Owen Isn't there an inherent problem with this design when using coax sections wich have a velocity factor which differs from that of free space ? The alternating sections rely on radiation from the outer (common mode) and a phase shift occuring along the inner (differential mode subject to the coax VF). Because of this mismatch the cumulative phase error along the length of the antenna will result in it only being close to the required phase shifts over the first few sections. Hence the gain reduction as more sections are added. To work properly the coax sections would need to be air spaced. UKM |
#3
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Coax Collinear Element Materials and Velocity Factor
On Apr 7, 1:55*am, wrote:
On Apr 6, 11:41*pm, Owen Duffy wrote: Owen Duffy wrote : ... Here is a deck for 8 halfwaves at 435MHz with a single stub tuner. ... Ouch, that had some remanents of a matching scheme using RG62. Here is a better deck. CM 8 half waves coaxial collinear on 435MHz CM Assumes lossline TL, VF=0.667, effective choke at bottom of array. CM Matched to 50 ohms with single stub tuner. CM Owen Duffy 2009/04/05 CE GW 2 20 0 0 -0.229885 0 0 0 0.0045 GW 3 20 0 0 -0.45977 0 0 -0.229885 0.0045 GW 4 20 0 0 -0.689655 0 0 -0.45977 0.0045 GW 5 20 0 0 -0.91954 0 0 -0.689655 0.0045 GW 6 20 0 0 -1.149425 0 0 -0.91954 0.0045 GW 7 20 0 0 -1.37931 0 0 -1.149425 0.0045 GW 8 20 0 0 -1.609195 0 0 -1.37931 0.0045 GW 9 10 0 0 -1.83908 0 0 -1.609195 0.0045 GW 200 1 -0.01 0 -2.25308 0.01 0 -2.25308 0.001 GW 201 1 -0.01 0 -2.35308 0.01 0 -2.35308 0.001 GE 0 GN -1 EK EX 0 200 1 1 0 TL 3 1 2 1 -50 0.3448276 0 0 0 0 TL 4 1 3 1 -50 0.3448276 0 0 0 0 TL 5 1 4 1 -50 0.3448276 0 0 0 0 TL 6 1 5 1 -50 0.3448276 0 0 0 0 TL 7 1 6 1 -50 0.3448276 0 0 0 0 TL 8 1 7 1 -50 0.3448276 0 0 0 0 TL 200 1 8 1 50 0.471 0 0 0 0 TL 200 1 201 1 50 0.0855 0 0 1e99 0 FR 0 0 0 0 435 0 RP 0 361 1 1000 -180 0 1 The coax half wave sections are exactly a half wave (electrically). Owen Isn't there an inherent problem with this design when using coax sections wich have a velocity factor which differs from that of free space ? The alternating sections rely on radiation from the outer (common mode) and a phase shift occuring along the inner (differential mode subject to the coax VF). Because of this mismatch the cumulative phase error along the length of the antenna will result in it only being close to the required phase shifts over the first few sections. Hence the gain reduction as more sections are added. To work properly the coax sections would need to be air spaced. UKM So, consider this: with lossless transmission line and each section of transmission line being an electrical half-wave long, the VOLTAGE between adjacent ends of any two sections is identically the same (including being in phase). So every feedpoint -- that is, every gap between sections -- is fed with the same in-phase voltage. Practical feedlines come pretty close to that lossless ideal, short as they are. (You can model this quite accurately to see just what the variation in feedpoint voltages is along the array.) That does not guarantee that the currents on all sections are in phase, nor does it guarantee that they are the same magnitude, but the simulations I've run tell me that they are pretty close to being in phase. It's really not so important that they be all the same magnitude. Were it not for mutual impedances among the elements, having identical feedpoint voltages would yield the same current on each section (except the very end sections, which don't have a feedpoint connection at their outer ends). I note that Owen has reported some different results (larger current phase differences than I recall seeing), and if I can find time, I'd like to explore those with him, but at the moment I'm tied up with other things. Cheers, Tom |
#4
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Coax Collinear Element Materials and Velocity Factor
K7ITM wrote in
: .... I note that Owen has reported some different results (larger current phase differences than I recall seeing), and if I can find time, I'd like to explore those with him, but at the moment I'm tied up with other things. Tom, The models I offered (and they are very similar) do have fairly good cophase operation. My comment earlier was that some designs aren't nearly as good, although it seems to degrade gain by only a small amount (though giving rise to more and narrower lobes). In your own time... 73 Owen |
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