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Author Topic: Concurrent use of Yagi and Vertical antennas  (Read 391 times)
KF6FRL
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Posts: 5




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« on: April 20, 2005, 01:14:55 AM »

When operating 2 m simplex I'd like to concurrently focus part of my signal in a specific direction and the rest omnidirectional.  Could this be done by using at the same time a 2 meter Yagi antenna and a 2 meter 5/8 wave vertical(with radials)?  
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W8JI
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« Reply #1 on: April 20, 2005, 04:33:35 AM »

That is generally a bad idea.

In directions where the Yagi and vertical both radiate, the overall gain of the combination will add or subtract depending on phase differences and antenna spacing.

The resulting pattern almost always has all sorts of very deep holes (dead directions) and narrow high gain peaks in several directions.

You are much better off to have a single antenna with the pattern you want.

By the way, a 5/8th wl antenna mounted high above ground, even with radials, often has LESS gain along the horizon than a 1/4 wl groundplane. You might consider a better omni antenna, like an Isopole.

73 Tom  
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KE4SKY
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« Reply #2 on: April 20, 2005, 04:51:53 AM »

You might consider having two antennas, with adequate vertical isolation, selectable by an A/B switch.

For instance, for 2m SSB I use a pair of stacked KB6KQ.com loops as an omni for listening and making close-range contacts within about 100-150 miles and switch to a higher gain horizontal long-boom yagi once I have a location and know where to aim it, if they complain about my crappy "little pistol" signal.

For FM simplex I do the same thing, using a Diamond D130N discone as the omni and I switch off to a vertically polarized 5-element yagi for directional gain if needed.
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N3JBH
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« Reply #3 on: April 20, 2005, 08:34:01 AM »

folded dipoles that is the answer many repeater  sites use this idea place say 8 folded dipoles up in a phased harness.  place 3 in direction you want place 3 in another direction you favor and last 2 in the direction of less importance. that was just example you can basicly do this how ever you wish. but that be the best way i can see to get some gain in one direction and still be omni aswell.

if you dont want to build these folded dipoles companys like cell wave and other make them but there not cheap. and to be honest a fairly cheap tubing bender from home depot and some allumiunm tubing froma refrigertaion supply house would be a heck of alot cheaper and  fun to make.

all the information youll need can be found on l.b. cebik's web site just do the famous google search.
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K1CJS
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« Reply #4 on: April 20, 2005, 08:54:11 AM »

Simple answer:  You'll cause yourself more problems and get worse results.  Use a selector switch and two different antenna systems.  Your radio will thank you for it.
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WA6BFH
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« Reply #5 on: April 20, 2005, 10:00:53 AM »

N3JBH I think offered the best overall solution. I too would build such an antenna myself! Sinclair gets a pretty penny for theirs -- and it looks pretty home-brew!

Get yourself a copy of John Kraus book, "Antennas". This will explain what some of the other responders were trying to tell you about signal "nulls" and phase cancellation.
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WB6BYU
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« Reply #6 on: April 20, 2005, 01:01:31 PM »

The answer is, "Yes, it could be done."  It has been
done successfully in a number of installations, and often may
be a good solution for someone who needs a generally
omnidirectional pattern with additional gain in one
direction.  But it isn't as simple as it may seem.

Let's say you have a yagi with a clean pattern and a gain
of, say, 6dBd.  Above that we'll put an omnidirectional
vertical such as an IsoPole or Ringo Ranger with a gain
of about 3dB over a dipole.  (This requires an antenna
at least 8 feet tall.)  First, since each of these
antennas has a 50 ohm impedance, we'll need some sort of
splitter/combiner to connect them to a single feedline.
(If we just connected both to a "T" connector, the SWR
would be 2 : 1.)

Now, off the back of the yagi the radiation will be pretty
low, so the predominant signal will be that from the
vertical, with about 3dB of gain.  But only half of the
power goes to that antenna, so we end up with a signal
level equivalent to using a standard dipole.  This is
quite adequate for a lot of work.

In the front of the yagi we would have 3dB gain from
the vertical and 6dB gain from the yagi.  Here is the
first problem, though... they may not be in phase.
This will depend on the way the antenna feed systems are
designed, how the antennas are mounted, and the lengths
of the connecting cables.  If they happen to be in
phase, the combined signal is still less than that from
the yagi alone (because the yagi gets only half of the
total power).  If they are out of phase, then the
signal from the vertical will subtract from that from
the yagi, which will reduce the gain somewhat further.
You'll still be able to hear signals in that direction,
but they may not be as strong as you would expect.  With
some experimenting you can find a cable length that
optimizes the signal in the desired direction.

However, there are other directions other than straight
in line with the yagi boom.  As you go around the
circle from the peak direction of the yagi, its signal
level drops off while that from the vertical stays
about the same.  At some point (perhaps 60 degrees or
so from boom direction) the two signals will be equal.
Again, the results depend on the relative phases of the
two signals, but if they are out of phase at this point
the signals will cancel each other entirely, leaving a
null in the pattern.  Unfortunatly there isn't much
information on the change in phase in a yagi pattern
in different directions, but I suspect that the phasing
that works best for the peak direction is not the same
as that which minimizes the nulls in other directions.
And if the yagi pattern isn't clean, there may be other
directions in which the signals have similar strengths,
which is the situation most likely to lead to nulls.

If you want to try it, I suggest the following guidelines:

1) use a splitter/combiner to match the impedance of the
two antennas in parallel.

2) Use as high of a gain yagi as is practical, with a
very clean pattern.

3) Expect the signal in most directions to be 3dB down
from that using the vertical antenna by itself.  

4) Similarly expect the signal in the yagi direction to
be 3dB down from that using just the yagi (as an
estimate.)

5) Expect to have some nulls in the pattern around
those points where the gain of the yagi equals that of
the vertical.  You may be able to work around these
in a specific direction by juggling the lengths of the
coax cables between the splitter and the antennas.
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KF6FRL
Member

Posts: 5




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« Reply #7 on: April 24, 2005, 04:19:18 PM »

 To: W8JI, KE4SKY, N3JBH, K1CJS, WA6BFH, and WB6BYU,

Thank you all for your technical insights, ideas, and experience.  I must have been thinking soley as an optimist, believing that when the patterns overlapped that the signal strength would always be greater.  I forgot that the superposition had to include the signs or phases of the signals.

I'm now thinking about using two verticals properly spaced and phased to produce the pattern desired, although the phasing does seem to be a challenge.

Thanks again for all of your advice.  It is impressive the extent and degree of knowledge that exists in the amatuer radio community.  More impressive is the willingness of you all to share that knowledge.

KF6FRL

 
 
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