On Apr 5, 2:36*pm, wrote:
I always assumed having more 1/2 elements in a collinear was best,
because that obviously raises gain and lowers angle. So from that
point of view, I was thinking that it would actually be better to use
a low VF coax, since that would give you shorter length elements, thus
being able to fit more elements in a shorter space. But I've also read
that having the element lengths closer to actual 1/2 length (longer)
is actually more efficient than having more elements at a shorter (low
VF) length. Just wondering what people's opinion is on this... So lets
say you have a choice between using the insulated center conductor of
a VF 78 coax through brass tube outer elements and having room for a
few extra elements in a given length, verses using just an insulated
wire that has a 99 VF through brass tube outer elements, thereby
allowing slightly less elements because they're longer, but the
elements you do have are closer to actual 1/2 length. Which would be
best? And this is basically various UHF bands we're talking about.

Thanks for any opinions,

Dave

So, why don't you do some modelling and see? I predict (having done
it myself) that you'll discover that the gain is pretty close to
proportional to length, independent of how many sections are used, at
least for practical velocity factors. In fact, you can optimize the
gain a bit by picking the VF, but it's not a big deal. What's
important here is how the mutual impedances among the elements affects
the current distribution on the elements, NOT how many elements you
have. Ideally, you'll end up with something close to the same
current, in-phase, at the center of each element; what you're liable
to see in a long antenna (say 10 elements) with moderate or low VF is
that one element out near each end will have a much lower current than
the rest. The effect of that on the pattern is probably less than
you'd have guessed.

The other thing that the free-space length of the elements will affect
is the feedpoint impedance. I'd suggest, again, that you model it to
make sure the net impedance of the paralleled feedpoints is something
you don't mind matching to.

Cheers,
Tom

K7ITM wrote in news:9b6a34a7-9578-4660-b4f0-5cf02e3ee862
@z14g2000yqa.googlegroups.com:

The other thing that the free-space length of the elements will affect
is the feedpoint impedance. I'd suggest, again, that you model it to
make sure the net impedance of the paralleled feedpoints is something
you don't mind matching to.


An interesting point. I cannot recall any of the online articles
discussing feedpoint Z, and by assumption it is 50+j0... but that doesn't
happen by accident.

Here is a deck for 8 halfwaves at 435MHz with a single stub tuner.

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 -0.23383 0 0 0 0.0045
GW 3 20 0 0 -0.46766 0 0 -0.23383 0.0045
GW 4 20 0 0 -0.70149 0 0 -0.46766 0.0045
GW 5 20 0 0 -0.93532 0 0 -0.70149 0.0045
GW 6 20 0 0 -1.16915 0 0 -0.93532 0.0045
GW 7 20 0 0 -1.40298 0 0 -1.16915 0.0045
GW 8 20 0 0 -1.63681 0 0 -1.40298 0.0045
GW 9 10 0 0 -1.87064 0 0 -1.63681 0.0045
GW 200 1 -0.01 0 -2.25634 0.01 0 -2.25634 0.001
GW 201 1 -0.01 0 -2.35634 0.01 0 -2.35634 0.001
GE 0
GN -1
EK
EX 0 200 1 1 0
TL 3 1 2 1 -50 0.350745 0 0 0 0
TL 4 1 3 1 -50 0.350745 0 0 0 0
TL 5 1 4 1 -50 0.350745 0 0 0 0
TL 6 1 5 1 -50 0.350745 0 0 0 0
TL 7 1 6 1 -50 0.350745 0 0 0 0
TL 8 1 7 1 -50 0.350745 0 0 0 0
TL 200 1 8 1 50 0.42855 0 0 0 0
TL 200 1 201 1 50 0.0837 0 0 1e99 0
FR 0 0 0 0 435 0
XQ
EN

The antenna bandwidth is quite narrow as might be expected from so many
resonant lengths.

It reaches about 7.8dBi gain, well short of the much touted 9dBd or
11.2dBi.

Owen

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

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

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

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