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Author Topic: 2 meter antenna co-phasing  (Read 4518 times)
N4ZYV
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Posts: 90




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« on: October 23, 2000, 06:16:15 PM »

I am planning to build 2 5-element quad 2 meter antennas and mount then on opposite sides of my mast. I am wondering about how to connect them. Should they be in phase with each other? Can I turn one 90 degrees so one is horizontally polarized and one vertically polarized? How far apart should they be? Need input.
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WB6BYU
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Posts: 13113




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« Reply #1 on: October 24, 2000, 04:29:08 PM »

How you phase the antennas depends on what sort of pattern you
want to get as a result.  (The term "co-phase" is used only on CB,
not in any engineering textbook.)

If you are trying to improve the signal in a specific direction, then
the two antennas should be pointed in the same direction and fed
in phase (that is, with the same length feedline to each antenna.)
If each antenna has a 50 ohm feed impedance, connecting the two
antennas in parallel will cause an SWR of 2 : 1 unless you do
something special for matching (commonly by putting a quarter
wave of 75 ohm cable between the junction and the cable to each
antenna, or a commercial splitter working on the same principle.)

If you mount one antenna vertical and one horizontal, and feed
them in phase, the resulting signal will be slant polarized 45
degrees.  If they are fed 90 degrees out of phase in this case (by
adding an electrical quarter wave of cable, or by moving one boom
forward or backward a quarter wave relative to the other one) then
you will get circular polarization.  You may find this an advantage
on difficult paths or satellite work... or maybe not.

If you are really just interested in two main directions, you can
mount one antenna facing each way (towards two different
repeaters, for example).  If the antenna coverage patterns don't
overlap much, then the relative phase of the two antennas won't
matter.

If you point them both the same direction and feed them 180
degrees out of phase (either by making one feedline a half wave
longer than the other, or by turning one antenna upside down
relative to the other) you will get a pattern with a sharp null in the
middle between two strong lobes.  This pattern is sometimes used
for direction finding but otherwise is not very useful on VHF.

I hope this helps!   - Dale  WB6BYU
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N4ZYV
Member

Posts: 90




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« Reply #2 on: October 24, 2000, 08:22:03 PM »

Well, it seems we have found a flaw in all those engineering books (just kidding).
 These antennas will be mounted with the booms parallel to each other, pointed in the same direction.
 I was wondering about having one vertical polarization and the other horizontal because I won't know what the other station will have for polarization and thought I might be able to cover both bases this way. I am going to be getting an all mode 2 meter and wanted to get a beam up. I have no experience with 2 meter ssb. Do you guys think its worth it to have circular polarization? I don't have any plans to work satellites. Would I generally be better off to just have them both one or the other? If so, which polarization would you suggest? I am thinking vertical would be the way to go as most of the 2 meter antennas I see locally are vertical. Also, how far apart? Is the distance apart measured from the booms or the nearest element? Seems to me that the measurement should be from the center of the antenna.
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WB6BYU
Member

Posts: 13113




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« Reply #3 on: October 27, 2000, 01:02:04 PM »

Vertical polarization is standard for FM work, but SSB operation
usually is horizontally polarized.  Circular polarization would have
a 3 dB loss compared to the same antenna of the correct linear
polarization, but would receive either equally well.  (The only
problem would be another circuilarly polarized station if the "sense"
of the polarization - clockwise or counter-clockwise - was
reversed.)

The answer really depends on your operating preferences.  If you
want to DX repeaters, you will want vertical.  If you want local
repeaters and SSB DX, use the yagis horizontal and switch to an
omni vertical for FM.  Maybe you should mount one antenna each
way with separate feedlines initially, while you find out what the
activity levels are:  see which one you like the best, then change
the other beam to match.

The spacing between the antennas is a function of the pattern of
specific antennas you are using.  Think there was an article in
the old Ham Radio magazine on the topic (in the 1980's?) but I
would have to look it up.  As a rough estimate, the optimum spacing
is probably 0.6 to 0.75 times the boom length.  Many commercial
yagis will have a suggested stacking distance in the specifications.

If you are going to use them horizontal, you probably will want to
mount both of them to your mast, one above the other (if there is
space) instead of side-by-side.  This gives the same gain, but
makes aiming less critical because it reduces the vertical beam
width instead of the horizontal beam width.  (Vertical yagis are
usually mounted beside the mast to keep the metal mast out of
the pattern of the yagi elements.)

As is often the case, the best advice is probably, "try it and see."
In spite of all the engineering textbooks, the final decision about
what is the best antenna configuration is highly dependent on the
preferences of the operator.

Good luck!    - Dale  WB6BYU
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N4ZYV
Member

Posts: 90




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« Reply #4 on: November 05, 2000, 07:54:04 PM »

So, I go to a web site and get some dimensions for the quad antenna. They are (of course) nowhere near correct. 3 hours of work tossed in the trash heap. Is there a law that says all antenna dimensions freely shared must be a minimum of 3 mhz off?
 A friend tells me he'll help with the antenna. He's never done something like that and thought it would be interesting. I had a questionable tire on the front of the truck so I spend $140 that I really can't spare so I can travel the 100 miles without worry. After 3 hours of work I realize that these dimensions are so far off, I will have to trim every element and every spreader by so much that the spacing can't possibly any good anymore.
 God, I love this hobby!
 Anyone know where I can find some realistic dimensions for a 2 meter quad?
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WB6BYU
Member

Posts: 13113




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« Reply #5 on: November 06, 2000, 03:57:05 PM »

One thing about gain antennas, particularly at VHF and above:
small details can make a big difference in tuning.  For example,
you could have a 3 MHz difference in resonance on 2m just due
to using insulated wire if the original used bare wire.  One of the
critical skills in antenna building is understanding which dimensions
are critical, and which are not.  And almost all antennas will require
some tuning after assembly.

Let's take a moment to critique what went wrong with the last quad,
so the next antenna has a better chance of working for you.  (We
might even be able to salvage the original one.)  First, did you use
the same materials called for in the original article?  If the only
difference is that your wire had insulation on it, get busy with your
pocket knife and strip it off, and see if it makes a difference.

Generally, quads are relatively broad band antennas (particularly
those with an ODD number of directors.)  Did you check the
performance of the antenna on received signals?  There should
still be a reasonable pattern 3 MHz off frequency.  (Actually, quads
work better 3 MHz below their resonant frequency than they do
3 MHz above it.)  The length of wire in each element will determine
the gain and resonant frequency, but the spacing between the
elements is nowhere near as critical - a 10% variation probably
won't make a significant difference.

The next question is, how did you measure that the antenna was
3 MHz off frequency?  Was this based on the SWR?  If so, it may
just be a matter of adjusting the driven element (or the matching
section, if one is used) and leaving all of the parasitic elements
as they are.  I've had particular problems using the MFJ SWR
analyzers on antennas with a gamma match - the actual frequency
of lowest SWR can be very misleading.  The best approach is to
adjust the antenna for the lowest SWR at the desired frequency.
I don't know what kind of matching system you used:  if it includes
a quarter wave transformer, you might need to adjust the length of
that.  If you are feeding the driven element directly with coax, you
may need some sort of balun at the feedpoint.  One important point
here is that the resonant freqency of the antenna (and the gain and
pattern) do not necessarily correlate with the SWR, especially when
some sort of shunt matching is used.  Even if the SWR is 3 : 1, it
is worth hooking the antenna to a receiver and checking the
performance (gain and pattern).  If the basic antenna seems to be
working, then you can usually find some way to get the SWR down
to a usable number (such as below 1.5 : 1.)

That said, my suggestion for 2m antennas would be the "Quagi"
designs from N6NB (ex-K6YNB.)  I believe they are included in
most of the ARRL antenna books.  The designs have been around
for 25 years or so, and seem to be quite reproducable.  I have
designed 2, 3 and 5 element quads using PVC pipe and copper
wire so they are quite inexpensive to make.  Actually, I would
recommend the 3 element, as the 5 doesn't seem to show any
improvement.  If you still have the parts from the other antenna, I
can see if I can modify the design based on an article in one of the
ARRL Antenna Compendiums that I have found quite helpful.

If you would like further assistance with this I would recommend that
we do it via email.  You can contact me at wb6byu@arrl.net.

Good luck!  - Dale WB6BYU
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K3UOD
Member

Posts: 145




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« Reply #6 on: November 15, 2000, 04:05:44 PM »

You never said what frequency you are building for or what the resonant frequency was.  Could it be that you were measuring VSWR at 147MHz and the antenna is designed for 144MHz?

.
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N4ZYV
Member

Posts: 90




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« Reply #7 on: November 26, 2000, 02:14:52 PM »

The design was for 148 mhz and it actually came out resonant (according to my MFJ 259b) at about 143 mhz. Probably the trouble was my using insulated wire. The original design used a fiberglass boom and I was using a 1' diameter aluminum conduit. The spreaders were 1/4" wood dowel. The aluminum boom was not grounded when I was testing. The feedline was RG8/X, and was connected directly to the driven element. The feedline came off the driven element at a 90* angle and did not run along the boom to the mast. There was no mast at the time, the antenna was hung on 2 wooden boards about 10' off a concrete driveway. I used a wooden ladder to work on it. There were no large metal objects nearby. My friend's wood frame garage with wood clapboards was about 8' from the antenna and his wood frame house with wood clapboards was about 14' the other way.
 I guess I need to get a couple of antenna books and study a bit. I'll try it again but I'm a bit busy with my truck resto right now. Its cold outside here in Minnesota and the truck fills up the garage. I appreciate all the help from you guys.
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WB6BYU
Member

Posts: 13113




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« Reply #8 on: December 08, 2000, 01:59:24 PM »

Sounds like you are off to a better start than it might seem!

The boom material is important for a yagi:  the element lengths
will change depending on the boom material (and whether the
elements are mounted onto, or through, the boom.)  But that
shouldn't make much difference on the quad.

Don't worry too much about the exact resonant frequency as
shown by the SWR analyzer.  Even if it is resonant at 143 MHz,
the quad probably would still work reasonably well at the lower
end of the 2m band - worth a try, anyway.  If the SWR is under
1.5 (or even 2.0) then go ahead and use it.

Restringing it with bare wire (or stripping the insulation off of the
current antenna) may improve matters quite a bit.

Usually I test beams by pointing them straight up, with the
reflector a couple feet off the ground.  A camera tripod or
broomstick lashed to a fence post can make a temporary mount.
Because the ground is behind the reflector, the antenna height
will make less difference, and the feedpoint is reasonably
accessable for adjustments.

If you have specific questions, feel free to contact me via email
at wb6byu@arrl.net.

Good luck! - Dale WB6BYU
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« Reply #9 on: January 18, 2001, 08:07:21 PM »

http://www.softcom.net/users/kd6dks/quad.html

accurate and nice site.

73 joe
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WF0H
Member

Posts: 57




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« Reply #10 on: June 02, 2001, 10:45:49 PM »

You may want to consider your design frequencies again.

Most SSB activity is in the extreme low end of the band - 144.100-300 . Resonance at 144.200 would be a good thing for SSB.

FM Activity is in two bands. The lower band is from 144.5 to 145.5 and the higher band is 146 to 148 Mhz.
Most FM antennas are tuned for a compromise somewhere around 146 Mhz.. If you ever do get into satellites, you will find that they operate in the vicinity of 145.8-146.0 Mhz., so the 146 Mhz. design is going to be good for that too. There is no reason that you can't make one quad - the horizontal one - for the low end of the band and vertical one for the middle of the band, and still feed them as previously described.

Circular polarization can be a great compromise, and actually provides additional gain when working another station with the proper circularity.  Some people believe, but tests have not proven, that circular is better for auroral work, and I believe it is often better for tropo work due to shifts in the polarity of the received signals due to multipath.
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