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