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