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Author Topic: VHF 8 dipole phased array  (Read 3148 times)
MAZZ1232002
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Posts: 205




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« on: August 26, 2009, 08:00:00 PM »

I saw an antenna made by a commercial antenna mfg, I think Sinclair Technologies. This thing consists of 8 vertical dipoles fed in phase and mounted on like a 30 foot mast. I modeled the thing on EZNEC and it shows a flat as a pancake elevation pattern. It's for VHF Hi band. It would appear to be killer for long haul as it shoots the radiation pretty much straight out to the horizon in all directions.
  Has anyone ever had any experience with one of these things and compared it to other verticals? You would probably have to be in the two way radio field as I never saw one for amateur use. It's a lot of antennas so it better be worth the bother I would think.

     Pete
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N3OX
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« Reply #1 on: August 26, 2009, 08:09:00 PM »

I think multi-bay dipole arrays get used for ham repeaters a fair amount.  They're simple and rugged.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
N3OX
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« Reply #2 on: August 26, 2009, 08:20:38 PM »

Incidentally one of the other reasons to use these is because you can build downtilt/uptilt into the phasing harness if you want to shoot a little down from the top of a mountain rather than shooting straight out.

If you play with the element phases a bit in EZNEC using "free space" you can see this.  

73
Dan
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WB6BYU
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Posts: 13027




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« Reply #3 on: August 27, 2009, 09:11:24 AM »

4 bay arrays on about a 20' mast are not at all uncommon
in ham use.  Several antenna makers sell them, including
the basic array elements that you can mount on the side
of an existing tower.

But to get the full benefit of an 8 bay array you will
need at least twice the height, and probably close to
50' to get 3dB gain over the 4-pole design (which has
6dB gain over a dipole.)

One problem, as N3OX has already alluded to, is that the
vertical radiation pattern is SO flat that you may need
to add some downtilt for local users.

Another problem is the complexity of the phasing harness:
the more bays you add, the more splitters you need and
lines of a specific length.  There is a point of diminishing
returns, and I think for most repeater owners it is
around 4 elements.

If you have an existing tower that is 100' or more,
you certainly could stack a bunch of dipoles along one
edge and get a lot of gain.  Unless the tower is already
on a hilltop, however, the bottom of the array should
be at least half the height of the top to get the
expected improvement.  (Otherwise a lower gain antenna
with the same top height may be better due to the
average height being greater.)

In the end, even a 50' stack of dipoles will probably
give you somewhat under 9dBd omni, or perhaps 11dBd
using the tower as a reflector.  A single yagi can do
more than this, and a pair stacked one over the other
on a rotator would take up less space and give better
performance for a fixed station.
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K8KAS
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Posts: 569




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« Reply #4 on: August 27, 2009, 11:49:32 AM »

For 10 years I took care of a number of Microwave towers and we used 2 or 3 styles of VHF antennas for our base to car systems. But by far the stacked dipoles with the phaseing lines were the best, at least from a Hams view. I used to DX the bases from long distances with my 100 watt Motorola radio in the car. It was fun driving to work and dialing up the furthest base on VHF, these were always the stations with the stacked dipole antennas. IMHO Denny K8KAS
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WA3SKN
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« Reply #5 on: August 27, 2009, 12:29:15 PM »

4 Bay arrays are the pretty much standard, but 8 bay arrays are certainly do-able, its just getting the phasing harness right!  And yes, they do have a nice, low angle pattern.
73s.

-Mike.
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WA6HDZ
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Posts: 52




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« Reply #6 on: August 27, 2009, 01:53:58 PM »

Check out some of the older references, like Frank Jones' VHF handbook from the early 1960's.  Phased arrays were once very popular for VHF but the phasing harness and balanced feed requirements made the Yagi more popular as a home project.
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K6AER
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« Reply #7 on: August 27, 2009, 02:27:26 PM »

Four dipole collinear arrays are about 6 dBi Omni and in an offset in one direction they are 9dBi in gain. Length of the array is 20 feet. A dual array will add 3 dB but the pattern is very flat and these arrays are used mostly from towers of less than 300 feet in very flat terrine application such as a central valley or for a city wide application.

On mountain tops depending on height most use the four dipole array and in extreme conditions you might have to use some down tilt. Here in the Rockies we generally order the antenna with about a 1-2 degree down tilt depending on repeater height. My repeaters are between 12,000 and 14,000 in elevation but because they are 25 miles from downtown Denver we only use 1 degree down tilt. In a city like Colorado Springs where Cheyenne Mountain is just above the city by 5000 feet they use as much as 3 degree of down tilt.
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W8JI
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« Reply #8 on: August 27, 2009, 05:08:26 PM »

Pete,

Adding 4 more dipoles in that 20 foot or so of space does NOT add significant gain. The performance is virtually identical to a four dipole array of the same length. The best you can hope for is a dB or so of better "roundness" to the pattern.

Four dipoles gives around 5 dBd gain, and eight about 1/2 dB more.

Now if you extended the mast from around 20 ft length to 40 ft length and added dipoles with vertical separation you could get another 2 dB of additional gain, but it is hardly worth it for the expense.

Four bay dipoles, whether each bay consists of one dipole or two on opposite sides of the mast, have been around forever. We used them in the 60's on repeaters, and I have one on a repeater here now. They have been widely used in both amateur and commercial applications for many years.

If you are curious about the gain, look at the stacking gain graphs on my website:

http://www.w8ji.com/stacking_broadside_collinear.htm

A dipole array like being discussed is on the top of a 315-foot tower:

http://www.w8ji.com/Rohn%2065G%20project.htm

Collinear gain takes length, and there is a limit before losses distributing power over the antenna length start eating up any additional gain.

73 Tom
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AC5UP
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Posts: 3825




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« Reply #9 on: September 03, 2009, 02:42:04 PM »

Not to dredge up an old topic... But... I was driving around this afternoon and happened to notice the antenna setup on one of the older fire stations.

Visualize a 25' or 30' tower with a pipe on top at least 10' long and probably closer to 15'. There were eight equidistant radiating elements with a phasing harness taped to the pipe. Each radiating element resembled the letter " H " with the vertical portion on each side a folded dipole sized for approximately 450 MHz. The top four elements were aligned N-S, the bottom four E-W.

Quite a setup, and from a distance I could not tell if both folded dipoles on each element were driven or if one side acted as a reflector.

Newer fire stations use a 25' tower with a ~ 6' white fiberglas mast on top which is likely some flavor of a colinear inside a radome.
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