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Author Topic: Switched vertical/horizontal collinears  (Read 1922 times)
AI4KK
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Posts: 107




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« on: January 19, 2006, 07:13:52 AM »

I was wondering if anyone had ever heard of an antenna that would allow switching between various elements to incorporate 3 different polarizations/gain patterns in one antenna?

What I was thinking of was a box with relays and phasing harnesses and NMO connectors on each of the 6 sides. You could switch between the 3 sets of opposing elements for vertical omni, horizontal N/S and horizontal E/W patterns. According to your wishes/needs, you could mount either 1/4 wave, 1/2 wave, or even 5/8 wave elements on the NMO mounts, but the 5/8th wave would be more directional and have more pronounced nulls. Of course, you could always mount 5/8 on the verticals and 1/4 on the horizontal and could even switch the horizontal elements to ground if it was determined that a ground plane was more effective for the verticals.

The whole thing would be mounted on a fiberglass mast to avoid detuning anything and the feedline would have to come in at an odd angle for the same reason, but that would be pretty easy to accomplish.

Thoughts? I would think this would be feasible at least down to 6M and could even be stacked in pairs for more gain.
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WB2WIK
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Posts: 21754




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« Reply #1 on: January 20, 2006, 09:23:45 AM »

Sounds like a project I in which I would not personally invest.  To make the relay/phasing switch box actually work well, with minimal insertion loss, minimal crosstalk, minimal SWR and reasonable power handling all at once is quite a feat -- that, alone, could cost $100 in parts and take many hours of my time tweaking, pruning and testing.

Making the end result really weatherproof might require a NEMA enclosure, or at minimum something strong, shielded and well-caulked, with weather-sealed connectors (NMOs are, but I was thinking in terms of a power connector to relay control).  That could easily add $50 in cost.

Matching networks would be required for some of the configurations you suggest.  You can't directly feed a pair of 5/8-wave whips horizontally opposed and expect the resulting network to look like 50 Ohms: It won't.  But this stuff can be worked out.

What I do, instead, is use commercially available remote coaxial switches and proven antennas mounted on a common mast.  Saves a lot of time and headaches, and possibly some money as well.

For example, for several years I had four 2m (4 element KLM) vertically polarized beams staggered 90 degrees around one of my towers, aimed NE, SE, SW and NW.  With the patterns they had, they just about overlapped in coverage, with only very small nulls in directions I didn't care much about.  I fed each one through a piece of RG213/U to a 4-port remote coaxial switch, and then fed the switch with hardline, and used a separate DC control cable for the switch.  That gave me 8-9 dBd gain in each of 4 directions with the ability to switch directions in milliseconds, instead of waiting for a rotator to go around.  I used in it VHF contesting to increase my FM QSO rate, and it surely did -- in one January VHF Sweepstakes back in the early 1980s I made over 400 QSOs on 2m FM, alone -- before adding in the SSB and CW contacts, and the other bands.

Since I used a commercially manufactured Transco remote antenna switch which was designed for outdoor use, I didn't have to weatherproof anything.  Since I used all 50 Ohm antennas and only fed one at a time, I didn't have to match or phase anything.  Very easy, cheap, elegant in its simplicity, and took up zero mast space since the beams were all tower side mouted.

If I were looking for a solution similar to what you suggest, I'd do something like this again, instead.  Add horizontal antennas to the mix, or whatever.  Transcos are cheap on the surplus market, have 0.00 dB loss at 1 GHz, SWR 1.0 at 1 GHz, and about 60 dB port isolation.  It would take me a lot of experimenting and a lot of time to homebrew something like that.

WB2WIK/6

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AI4KK
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Posts: 107




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« Reply #2 on: January 20, 2006, 01:08:06 PM »

I like your idea of using the commercial antenna relays and had thought about that myself if I could find them for a decent price. Mounting them inside an ammo box should not only increase their weatherproofness, but their physical protection as well.

AS for the co-phasing, if I used commercial 5/8w antennas with the loading coils built in, wouldn't that be a 50-ohm match at the NMO connector? Seems that I would then just have to use 1/4w sections of 75-ohm coax to a t-connector at the switch to have co-phased 5/8w collinears.

One thing that this antenna might be more useful for would be a portable deployent type situation, perhaps hanging high in a tree or on top of a temporary mast. It would be very quick to assemble simply by screwing 6 mobile antennas (1/4 or 5/8w) onto the NMO mounts and plugging the feed and control lines in.
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WB6BYU
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Posts: 17192




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« Reply #3 on: January 20, 2006, 02:00:09 PM »

Might I suggest using term "phased" antennas rather than
"co-phased", which comes from CB jingo and has no definition
in antenna engineering.

One of the problems you run into is decoupling the feedline
from the various combinations of antennas.  From this
perspective, having a single vertical mast with two
horizontal dipoles on it and a vertical whip on top would
probably be more practical.

Also, the impedance of the mobile antennas will depend on
the size and shape of their ground plane.  If you had a
very large box, you could treat each antenna relatively
independently.  With a very small box, it could be ignored
and the antennas would look like dipoles.  If you presume
that the normal 5/8 wave mobile whip looks like 50 ohms
over a groundplane, then two of them as a dipole would
be around 100 ohms, while a pair of quarter wave whips
will be somewhere between 40 and 80 ohms depending on
a number of factors.

There might be some interesting experiments that you
could do with such a box, such as studying the polarization
of incoming signals.  (Especially if you added the option
for circular polarization with any pair of antennas.)

But I can't think of a time when I would choose such a
device for practical communications.  If I were in the
field and wanted to optimize reception of a particular
signal, I'd just pull out my fold-up yagi and spin it
around (both in azimuth and about its axis) for the
strongest signal.  Even with circular polarization, this
would give me a stronger signal than the box.
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WB6BYU
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Posts: 17192




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« Reply #4 on: January 20, 2006, 02:04:12 PM »

Oh, and one other comment.  When matching the 5/8 wave
whips as you proposed, you can't just run them to a T
connector, even ignoring any impedance issues.  Doing
so would put the radiation OUT of phase in the two whips.
It would be correct if they were both pointing in the
same direction, such as two antennas pointing up on the
same car roof.  But in this case you are presuming that
one antenna is upside down, which reverses the direction
of relative current flow.

To make this work, you have to feed them out of phase
at their bases, either by using a 1/2 wave difference in
cable length or some sort of transformer.
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WB2WIK
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Posts: 21754




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« Reply #5 on: January 21, 2006, 04:34:27 PM »

Dale (BYU) had some good suggestions.

I would also avoid the term "co-phasing," as that is absolutely a CB term that doesn't mean anything and is usually frowned upon in professional circles.

But being able to "quick deploy," like pulling something up into a tree for EMCOMMs, has merit, of course.  The problem is, with the kind of antenna system you're suggesting, that's not very "quick-deploy" no matter what you do.  For one thing, the whips would get snagged in tree branches on the way up.  For another, you need not only coax but a control cable (or a method to feed control signals and current through the coax, which is possible).  It's too complicated for a quick-deploy EMOCOMM situation, where you want to be set up and operational in 60 seconds.

Further, EMCOMMs on two meters are 100% -- ALL -- FM.  There isn't any EMCOMM work on 2m SSB, CW or other modes.  If you want to try that, go ahead, but you'd be blazing a new trail.

WB2WIK/6
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AI4KK
Member

Posts: 107




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« Reply #6 on: January 23, 2006, 06:31:46 AM »

Actually, with so many all-in-wonder rigs coming out, both our ARES group and my employers have been studying the use of 2M sideband for long-distance simplex communication since we saw so many repeaters go down in Katrina, but even that doesn't have to be horizontal as long as both stations have the same polarization. While this may not be practical for a truly quick-deployment scenario, having multiple orientations quickly available might be advantageous for a longer-term installation. For the control cable, telephone line would probably be the easiest way to go with less to go wrong than trying to send positive and negative DC voltages on the feedline.

As far as tree branches go, I've found the easiest way to deal with that is to put both ends of the rope up in seperate trees and tie the antenna in the middle in the clear, or if there is only 1 tree available, pull the other end of the rope out to the side until the antenna is as high as it can get before encountering branches. For this purpose (among several others), I have a 1,000' roll of GI paracord. Of course, mobile whips mounted to the sides of a steel box would be pretty rugged on their own.

One thing I forgot to mention earlier that might help understand where I'm coming from with this idea of trying to figure out a multi-directional, multi-orientation antenna. Multiple beams are not an option at my house as I do not have a tower and do not plan to get one even though 4 of us are hams; I won't be able to afford a tower taller than our existing trees until we start kicking kids out (we have 5 total, 2 are hams). What I do have however are several heavy ropes crisscrossing my yard from treetop to treetop 100' from the ground; we can hang all the stuff we want to from that.
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WB6BYU
Member

Posts: 17192




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« Reply #7 on: January 24, 2006, 11:35:13 AM »

From a practical perspective, I'd suggest instead having
at least two different antennas - a vertical of some sort
and a horizontal - perhaps a quad turnstyle as shown here:

http://www.cebik.com/vhf/ex6.html

You could hang them from the same support, or side-by-side
as circumstances permit.  The biggest advantage in my view
is that you can put up one or the other as needed in a
portable installation.  You might end up with a HT connected
to one for local work and a SSB rig on the other for
longer distance links.  For my own use, I'd go with two
(or more) separate antennas as being cheaper and giving
much more flexibility than a single does-everything
contraption.

But I'd encourage you to try the box if that is where
your interests are.  You could start with just two
sets of elements and an open-frame DPDT relay and see
how that works.  You might want to start with a plastic
box and wire elements rather than investing all of the
time and money in mounting NMO fittings until you have
a good feeling about it.  Some of the issues I expect
you will encounter will include decoupling the feedline
and the control cable from the vertical elements and
keeping the box oriented properly while in use. These
are by no means insurmountable.

Using trees for supports needn't be a major limitation
in your antenna farm.  Your technique about hanging
antennas between trees is good when the trees are spaced
somewhat apart, but not too far.  Where trees are more
sparse, you will need to be able to set up using just
one support.  (It is a good idea to be able to support
your antennas from a metal flagpole using the existing
halyard, as this likely will be available at many sites.
This may involve adding a length of PVC pipe to get the
antenna up above the top of the flagpole.)  In other
forests the trees will be too close together to do this,
though you can often manage by taking advantage of the
edges of areas that have been cleared.

But if your yard has some suitable trees, this shouldn't
prevent you from using beams, either.  You can always
attach a rope to both ends of the boom, with some
combination of weights or extra ties to maintain the
desired polarization.  With just 3 trees you can
theoretically maintain fixed beams in 6 directions, or
by tying the back of the boom to one tree and the front
to a rope running between two trees you can steer it
over some range of angles.  In fact, if you had 4 trees
in a square and ran two ropes along the diagonals, with
one attached to the front of the boom and the other to
the rear, I think you could arrange for 360 degree
rotation by manipulating the ends of the ropes.  (But
I would test this theory at a low height where it was
easier to untangle things before trusting it up high.)

Such a yagi installation could be made in the field as
well.  Some useful yagi designs can be found here:

http://www.clarc.org/Articles/uhf.htm

or W4RNL's series of "OWA" yagis would cover the whole
2m band for SSB or FM, as described here:

http://www.cebik.com/vhf/2mowa1.html

And be sure to browse the rest of W4RNL's site - he has
lots of useful antenna designs.
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AI4KK
Member

Posts: 107




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« Reply #8 on: January 25, 2006, 01:59:02 PM »

Very cool. Some of these look interesting, especially for mine (since I'm footing the bills LOL). For the kids, maybe something a bit simpler since if everyone has 3 beams hanging from the ropes, we'll run out of space very quickly. I've also considered a simple loop with a vertical going up through the middle of it; not worried about nulls as the Gulf of Mexico is about all that is South of us.

No matter what I play with, at least half of it is just in fun Smiley
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WB6BYU
Member

Posts: 17192




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« Reply #9 on: January 26, 2006, 10:21:44 AM »

At least for 2m, yagis are quite cheap to make and experiment
with.  I use #8 aluminum wire ("ground wire" from Radio
Shack) for the elements, and #10 solid copper if I need
to solder to it.  If you are going to do a lot of
experimenting, consider investing in a half-mile spool of
the heavy aluminum electric-fence wire.  I use thin-wall
PVC pipe for the booms (though wood will also work) and
just drill holes through it and drop in the elements.  A
bit of tape or glue - or a small bend - will usually keep
them in place.  For portable yagis I can pull the elements
out and put everything in a long skinny box (or larger
diameter PVC pipe) that is easier to fit in the car than
a complete yagi.

The "half-folded" driven element used by WA5VJB is great!
I've scaled designs to other frequencies (from 121.5 to
732 MHz) and they are very easy to build.  Because the
element is continuous through the boom, it is stronger
than splitting the driven element (even with lots of
hot-melt glue) and much simpler than a gamma match.
And you can solder the feedline or a connector to the
element and still be able to remove it from the boom.

If you want to experiment with different yagi designs,
I recommend the Java Applet Yagi Simulator by W9CF:

http://fermi.la.asu.edu/w9cf/yagipub/index.html

This will show you the pattern (horizontal and vertical)
as well as the impedance and gain, and you can tweak
designs for different frequencies, etc.  If you design
the yagi to have a 20 ohm input impedance you can use
the "half-folded" driven element with it.

Quads are also easy and inexpensive to build, again using
stiff wire and PVC pipe and fittings.  If you don't glue
the parts together, you can pull the elements off and
lay them flat, making a 2-dimensional pile from a 3-dimensional
antenna, which is easier to pack in the car.  I use the
W4RNL quad formulas (just enter frequency and wire size,
and it gives dimensions for 2, 3 or 4-element designs)
and they work very well - the 3-element versions have
a 50 ohm feedpoint, and have been "plug and play" with
no adjustments or tuning required.

Good luck playing with antennas!
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KE4SKY
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Posts: 1045


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« Reply #10 on: January 27, 2006, 11:45:35 AM »

Horizontal is much lower noise and works fine for FM too as long as everyone is the same.  During Isabel I used a KB6KQ horizontal loop on FM simplex, packet and SSB from my mobile.  A 15 ft. pushup pole is fast to set up as well.

If you need more directional gain consider a heavy duty, commercial, all welded construction 3-element Yagi such as Antennex.  Fits in your average compact car trunk, can double as a ladder (just kidding, but a small child could!)
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