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

wrote in message
...
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

Hi Dave

Opinion --
If your options are either 5 radiating elements, all slightly shorter than
1/2 wave, or 4 longer elements, there wil be no decernable difference in
which is best. If you are contemplating the construction of an antenna much
longer than several wavelengths, you will encounter problems much more
siginificant than efficiency,. So, in my opinion, the "best" is whatever
you find convenient.

Jerry KD6JDJ

wrote in news:a706ece1-d519-47d0-8ffe-8e76cf03ff66
@z16g2000prd.googlegroups.com:

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.
Dave,

The popular explanation for these things constructed with reversing coax
sections is that the currents all along the vertical are (exactly) in
phase.

That explanation doesn't seem consistent with the nominal half wave
elements being in fact a halfwave at VF=0.67, ie about two thirds the
length. Nor does it deal with the fact that the element ends are
connected to each other, ie no charge difference permitted.

I have been playing with an NEC model of eight half wave
elements using RG213. The currents are certainly not exactly in phase,
not nearly, and the gain in freespace at 6dBi is less than I see claimed
for this antenna. People seem to justify a claim of dB gain as 3dB for 2
elements, 6dB for 4 elements, 9dB for 8 elements... in which case they
must mean gain to be wrt a dipole, and therefore gain of 8 half waves
would be 11.2dBi... very sus, too simplistic.

You asked specifically about efficiency. The efficiency in my NEC model
(which includes internal loss in the RG213) is very good, 99%.

Confusing part is that some of the designs have a half wave sticking out
the top, and others a quarter wave, similarly different treatments at the
bottom, and they aren't consistent about whether the coax breaks at or
near voltage maxima or minima.

I am still working on this, I suspect the antenna doesn't quite work as
often explained, and not nearly as good as claimed.

Owen

Thanks for the info... My main concern between my two examples was
ease of construction I guess... After stripping the coax and throwing
the braid and jacket away, I starting feeling like it was kind of a
waste. Then I thought... why not just use insulated wire. But that
would bring the element sizes up and not allow as much room for as
many elements. Maybe two less elements, which wouldn't be huge, but it
would be a difference in specs. One version is 10 half wave elements
using a 78 VF center conductor in brass tube, along with a free space
1/4 on the bottom and one on the top. No short at the top as my tests
showed this actually lowered performance. I actually am not sure why.
Independent tests done by a reputable company that does this stuff
full time showed this one to be about a 10 degree angle and about 8 db
gain at peak angle. Which is pretty decent and it does perform quite
well in real world tests. But of course there's always room for
improvement. But anything more than that and there might be some
pretty bad nulls on top.

wrote in message
...
Thanks for the info... My main concern between my two examples was
ease of construction I guess... After stripping the coax and throwing
the braid and jacket away, I starting feeling like it was kind of a
waste. Then I thought... why not just use insulated wire. But that
would bring the element sizes up and not allow as much room for as
many elements. Maybe two less elements, which wouldn't be huge, but it
would be a difference in specs. One version is 10 half wave elements
using a 78 VF center conductor in brass tube, along with a free space
1/4 on the bottom and one on the top. No short at the top as my tests
showed this actually lowered performance. I actually am not sure why.
Independent tests done by a reputable company that does this stuff
full time showed this one to be about a 10 degree angle and about 8 db
gain at peak angle. Which is pretty decent and it does perform quite
well in real world tests. But of course there's always room for
improvement. But anything more than that and there might be some
pretty bad nulls on top.

Hi Dave
Again, opinion --
It is really difficult to gt a 4 or 5 wavelength long colinear antenna to
work as well as the modeling predicts.

Have you modeled the antenna you plan to build?

Jerry KD6JDJ

wrote:
Thanks for any opinions,

The velocity factor of a transmission line applies
to the differential transmission line currents which
don't do much of the radiating.

It's the common-mode currents that do most of the
radiating and the transmission line velocity factor
does not apply to common-mode currents.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

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