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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 |
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