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Author Topic: colinear vs beam  (Read 2687 times)
KE7VUX
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Posts: 35




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« on: May 21, 2010, 11:42:38 AM »

Planning out my new-to-me tower installation, and I'm in need of a VHF and/or UHF antenna.

I'm in a bit of a bowl geographically, and while I need to fool around with RadioMobile or SPLAT! a bit more, thus far the coverage map doesn't seem to change a bunch when I add more gain or power - my signals slam into a mountainside, and that's all she wrote.

Back to antennas.. I had focused on the Cushcraft A270 10S 2M/440 yagi, with 10dbi/10dbi claimed gain for $150.  There was a chance I had one of these, but that fell through.

Looking at antennas again..

Why would I choose the 5 element 10dbi claimed yagi over 3x5/8 co-linear at 8.7dbi?

The co-linear vertical is omnidirectional, which on one hand means no need to rotate it, but at the same time, no rejection of unwanted signals.

But from a "can I be heard" perspective, they're very similar. 

Even in the case of a mountain-bounce to make it through a pass, wouldn't the vertical/omni/colinear perform just as well as the beam?

Why choose the beam then?

A 5 + 5 element dual-band Yagi is about 0.7sq ft, while the colinears are ~ 1 sq ft of windload.

One advantage of going with a colinear vertical on top of the mast for vertical FM work and keeping it well above the HF beam, is I can start thinking about a modest horizontally polarized VHF beam in between.

Cost of a commercial yagi vs colinear is similar for comparable gain figures, so even that isn't a big consideration.

Am I missing some other good reason to with a vertically polarized VHF beam, or should I opt for the colinear?
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N8EKT
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Posts: 371




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« Reply #1 on: May 21, 2010, 06:18:25 PM »

I guess one bad thing about high gain omni dual banders is the pattern.
The other is very poor construction and longevity
Patterns on these ham grade antennas are VERY irregular.
And the beamwidth of the UHF patterns are so narrow on these antennas that the slightest breeze will cause HUGE fluctuations in signals.
The dual band yagi will have a MUCH wider vertical beamwidth and a much more predictable pattern.
So when given the choice, the yagi is a clear winner all the way around



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KE7VUX
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Posts: 35




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« Reply #2 on: May 21, 2010, 07:27:39 PM »

I guess one bad thing about high gain omni dual banders is the pattern.
The other is very poor construction and longevity
Patterns on these ham grade antennas are VERY irregular.
And the beamwidth of the UHF patterns are so narrow on these antennas that the slightest breeze will cause HUGE fluctuations in signals.
The dual band yagi will have a MUCH wider vertical beamwidth and a much more predictable pattern.
So when given the choice, the yagi is a clear winner all the way around

I was wondering the implications of the narrow beam width, particularly with regard to high mountain repeaters around here and the need to eventually go "up" instead of passing under.

(A breeze here is 25mph, so 17' of antenna sticking out the top of the tower would certainly be affected.)

Now I know - thank you.
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WB6BYU
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Posts: 13021




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« Reply #3 on: May 24, 2010, 10:50:48 AM »

Quote from: KE7VUX
Why would I choose the 5 element 10dbi claimed yagi over 3x5/8 co-linear at 8.7dbi?


Because the former gain claim is consistent with the laws of physics and the latter is not?


A 2 x 5/8 wave colinear will have 3dB gain over a dipole, or about 5dBi.  To add another 3dB requires
that the antenna be more than double the length:  on 2m it would need to be about 20' tall to get
6dB of gain over a dipole (8.1 dBi), while the same gain can be obtained with a 3-element yagi.

Such a colinear with 6dB gain over a dipole will actually be WORSE than a dipole in some directions
if it is sloping off vertical by 10 degrees.  This is generally more of a problem on UHF using dual-band
antennas because the pattern is much sharper on the higher band for the same antenna length.

One advantage of a directional antenna (of whatever type) is with multi-path, where the signal
arrives from different directions (perhaps due to bounding off of two different hills.)  When the
signals arrive out of phase (due to the difference in path lengths) it causes distructive
interference in the antenna pattern.  A beam may allow you to peak performance on one or the other
path and reduce the problem.
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KE7VUX
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Posts: 35




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« Reply #4 on: May 24, 2010, 01:50:08 PM »

Quote from: KE7VUX
Why would I choose the 5 element 10dbi claimed yagi over 3x5/8 co-linear at 8.7dbi?


Because the former gain claim is consistent with the laws of physics and the latter is not?

Well, there's another good reason. Smiley

I've often asked what makes a Mainframe MIPS different from a Mid-tier MIPS - and sometimes the answer isn't as obvious as the math would indicate.

One advantage of a directional antenna (of whatever type) is with multi-path, where the signal
arrives from different directions (perhaps due to bounding off of two different hills.)  When the
signals arrive out of phase (due to the difference in path lengths) it causes distructive
interference in the antenna pattern.  A beam may allow you to peak performance on one or the other
path and reduce the problem.

Multi-path is another angle I hadn't given enough consideration to, particularly knowing that ricocheting off a mountain side is a regular mode of propagation around here.

I guess I should abandon the idea of both a vertical and horizontal VHF antenna at this time and settle for just a vertical beam.

I have about 15sq ft of my 18sq ft of tower capacity with the HF beam and ~6' of mast out the top.

3 sq ft left the VHF and UHF complement.

Putting the upper mast together
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WB6BYU
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Posts: 13021




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« Reply #5 on: May 24, 2010, 04:05:36 PM »

You really can't tell how the various antennas will perform, especially in hilly terrain, until you
have tried them.  It depends to a large extent on what level of performance you expect:  if
your primary goal is to hit a specific repeater, a beam is an excellent choice.  If you want to
chat with several locals on simplex, the omni is likely to be better suited to your needs even
if the gain is lower.

It may also come down to practical issues.  Mounting a vertically polarized yagi with a metal mast
running up though the middle of it can mess up your gain and pattern.  That would be an argument
for using a vertical for simplicity.  Or mount two yagis on either side of a horizontal cross-arm,
a vertical on top, and a relay to switch between them.  That might give you a chance to compare
them and choose one of them before the winter ice season arrives.  Remember that no antenna
is permanent.
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WD8KNI
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Posts: 138




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« Reply #6 on: May 24, 2010, 09:25:41 PM »

Hilly terrain can be very problematic.  Consider some of the repeaters on high mountains.  Operators quickly found out that a high gain antenna on the repeater gave them very long distances, however for 25-50 miles around (below the antenna) was marginal because the antenna gain forced the radiation pattern to horizontal.  Lower gain antenna provided better optimized performance below the repeater.   Operators below the repeater find that a 1/4 wave antenna would outperform (to the repeater) a gain antenna for the same but opposite reason.. It really depends on what you want and experimentation.. gain is not everything, line of site between antenna and the proper radiation pattern for your situation is.... Fred
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KE7VUX
Member

Posts: 35




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« Reply #7 on: May 26, 2010, 10:25:17 AM »

Hilly terrain can be very problematic.  Consider some of the repeaters on high mountains.    Operators below the repeater find that a 1/4 wave antenna would outperform (to the repeater) a gain antenna for the same but opposite reason.. It really depends on what you want and experimentation.. gain is not everything, line of site between antenna and the proper radiation pattern for your situation is.... Fred

I have a few 4x4 friends that are of the opinion that a 1/4 wave is a better choice for precisely the overhead radiation pattern to get UP the mountain to the repeater on top, and there is merit to that.

Of the "near" repeaters, I can hit most or all of them with a 5w HT and a rubber-duck, though some are marginal.  A 1/4 wave ground plane would be enough of an improvement to be worthwhile, even better if it's at a reasonable height.

Given the results with the HT, I'm trying to see to it that I'm at a net gain in signal strength by putting an antenna on the tower and losing signal in the coax (~150' worth) vs the corner of the house and 20' worth of feedline loss (LMR400 is the low end I'm considering, with LDF4 1/2" heliax most likely, though still a bit underrated for UHF)


I'm now looking into a top mounted vertical - either a regular ground plane, or short colinear or coaxial.  Should be useful for random simplex contacts and/or round tables.

I'm also going to plan a modest vertical beam.  I should get a wide vertical pattern with comparatively narrow horizontal pattern (vs the high-gain colinear that would've resulted in a thin vertical area with full 360-deg horizontal coverage), which should let me reach mountain top repeaters and river-bottom fishermen alike, while helping to reject multi-path issues when using reflections to my advantage.


If it's a short beam, I'll end-mount it, if it's longer it'll need a cross boom - unless I get nervous about having the vertical right above the beam and use a cross boom just to get the beam away from the omni.

And if it all works out, I'm tempted to mount a second modest beam horizontally on the other side of the cross boom both to balance it out and to provide a SSB antenna.  I think I'd reverse the direction of the SSB antenna so it would point "S" when the rotor indicates "N" as another attempt to keep the two antennas out of each other's near fields.

There isn't enough excess wind load to put up any big VHF beams and I'm much happier using the capacity for the HF antenna, but I'm trying to cover a few VHF/UHF scenarios while I'm at it.
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WB6BYU
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Posts: 13021




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« Reply #8 on: May 26, 2010, 01:42:37 PM »

Quote from: KE7VUX

Of the "near" repeaters, I can hit most or all of them with a 5w HT and a rubber-duck, though some are marginal.  A 1/4 wave ground plane would be enough of an improvement to be worthwhile, even better if it's at a reasonable height.

In that case it won't take much of an antenna (or much height) to get the marginal ones as well.
Height makes more difference than antenna gain, especially when starting at low heights.   Just a
ground plane on a mast above the edge of the roof may be sufficient to hit most of the repeaters
on low power.

Quote
Given the results with the HT, I'm trying to see to it that I'm at a net gain in signal strength by putting an antenna on the tower and losing signal in the coax (~150' worth) vs the corner of the house and 20' worth of feedline loss (LMR400 is the low end I'm considering, with LDF4 1/2" heliax most likely, though still a bit underrated for UHF)

The relative height of the two antennas will determine how much improvement you get.
You can compare coax losses using VK1OD's handy calculator here:

http://vk1od.net/calc/tl/tllc.php

Quote
... I'm tempted to mount a second modest beam horizontally on the other side of the cross boom both to balance it out and to provide a SSB antenna.  I think I'd reverse the direction of the SSB antenna so it would point "S" when the rotor indicates "N" as another attempt to keep the two antennas out of each other's near fields.

Actually a horizontal yagi should be mounted directly to the mast - a horizontal cross-boom
of conductive material will interfere with the pattern.

I suspect that a reasonably small colinear on top of the mast with a horizontal yagi below it will
do just about everything you want, unless you have multi-path to some of the local repeaters
(which a beam might or might not solve, depending on the relative directions of arrival of the
two paths.)

But you really will have to experiment and find out what works best in your specific situation -
we can't predict it from a distance.  I generally suggest that folks build some cheap and simple
antennas like a ground plane and perhaps a WA5VJB "cheap yagi" to try out different placements
and heights.  Based on the information you gather, you can choose a more permanent solution.
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