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Author Topic: Inverted V Top Down Angle?  (Read 1983 times)
VE6ETP
Member

Posts: 19




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« on: April 21, 2013, 11:26:11 PM »

That subject may take some explaining.

I have built an inverted V dipole antenna for the 20m band. Currently it sits in my backyard and the end of the legs are only about a foot off the ground. Clearly not ideal.

I want to put it up on the roof of my house. But because of where my existing mast is (attached to the chimney) I would have to put the two legs at an angle if you were to look from the top down. So instead of a straight line they would be at a 90 degree angle or perpendicular to each other. This is because my mast at at one corner of my roof. Does that make sense?

If it does make sense, how will this antenna perform? Slight degradation, massive issues, or will it even work at all?

Secondly, should I use a metal post to hold up the center point or a plastic pipe? I'm thinking about how to ground the thing but also have read to keep metal away from the antenna as normally it is just hung from a rope being held up by a tree branch or something else. Or so I've read.

Also how do you ground these things since the center point is not metallically connected to the mast at all?

Sorry, I'm guessing I'm asking 3 questions and very rookie questions at that. I appreciate any insight and comments.

I am proud that I built my first antenna rather than buying one to first get on the air. Even from the backyard it has being doing well to make some JT65 contacts (I just got licensed this month.) Thanks for any guidance. Smiley

Regards,

Peter
VE6ETP
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K2DC
Member

Posts: 1377


WWW

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« Reply #1 on: April 22, 2013, 06:48:35 AM »

Peter,

   The antenna you describe should work just fine.  In fact if you do a search on "inverted vee angle" several of the articals indicate that an included angle of 90 degrees is just about the optimum, and the orientation of the plane that includes that angle shouldn't matter (i.e., a sloping plane in your case, rather than a vertical one as a classical design would have).  The use of a conductive support for the center insulator will likely cause the azimuth pattern to favor the direction of the antenna ends, but shouldn't have a major impact.  As far as grounding goes most would recommend that at the feedline entry point into the house, you should have a lightning arrester that is grounded, and that that ground be bonded to your power line safety ground.

73,

Don, K2DC
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W5DXP
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Posts: 3631


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« Reply #2 on: April 22, 2013, 06:56:55 AM »

Clearly not ideal.

A self-supporting horizontal dipole is easy to build from telescoping aluminum tubing and easy to make rotatable. That's what I have.
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
WB6BYU
Member

Posts: 13355




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« Reply #3 on: April 22, 2013, 08:14:47 AM »

If you have a 90 degree angle between the wires when viewed from the top,
and the wires are also sloping down from the feedpoint, then the total angle
between them (measured in the sloping plane containing the wires) will be less
than 90 degrees.

Generally, the more you bend a dipole in the center (turning it into a V shape)
the lower the feedpoint impedance becomes, and, especially as you get past
120 degrees of angle between them, the efficiency can drop due to higher
currents in the wires.  This is why you see recommendations to maintain at
least a 90 or 120 degree angle between the wires.

That doesn't mean that the antenna stops working, however:  it just doesn't
work quite as well.  So I'd put it in the "slight degradation" column.  Depending
on the height above ground, you may find that the SWR isn't quite as low as
you might like at resonance.  But it will still make contacts, and that is the
important thing.
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VE6ETP
Member

Posts: 19




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« Reply #4 on: April 22, 2013, 03:13:02 PM »

Thank you all for the responses!

I just want to clarify what exactly I mean here to ensure I'm being clear.

If looking at the antenna from the side it will be the traditional inverted look with the center post and the two legs coming off of it to the ground with 90 degrees (atleast) between them.

If you were then to somehow "fly" above the antenna looking down you would see the top of the post and the two wires coming out from it to either side at 180 degrees from each other. This would be in a traditional inverted V. However what I want to do is that the angle from this view would also be at 90 degrees.

So I guess on the vertical and the horizontal the angle would be 90 degrees rather than 90 and 180 which you see in most pictures.

Does this make sense and will this work?

Thanks again!

Regards,

Peter
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WB6BYU
Member

Posts: 13355




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« Reply #5 on: April 22, 2013, 04:44:17 PM »

That certainly is the way I interpreted your description.

Let's assume both angles are 90 degrees, and that each leg is 17' long.  (It might
not be exactly, but close enough for this purpose.  Using simple geometry that puts
the end of each leg 17 * 0.707 = 12' out horizontally from the support mast.

Now looking down from the top, we see two 12' legs 90 degrees apart, and old
Pythagoras puts the spacing between the ends at 12 * 1.414 = 17' apart.

Now if we look at the resulting triangle in the plane of the two wires, they are
17' long and the ends are 17' apart, so we have an equilateral triangle and the
angle between the wires is actually 60 degrees.

60 degrees is closer than ideal, but still usable.
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VE6ETP
Member

Posts: 19




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« Reply #6 on: April 22, 2013, 10:40:30 PM »

Ah ok that makes a lot of sense now. Thank you!

So perhaps I should go for a higher angle than 90 degrees on the vertical rather to compensate for the horizontal angle?

Regards,

Peter
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K7KBN
Member

Posts: 2817




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« Reply #7 on: April 22, 2013, 11:11:28 PM »

Peter -

One word:  experiment.  In the time it takes to ask a question on a forum and get an answer, you could get an antenna up and on the air.  Then you could change the angle of the vee and see how those changes affect the way it loads, directivity, signal strengths and so forth. 

Antennas are antennas, not consensuses (consensuae?).
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73
Pat K7KBN
CWO4 USNR Ret.
WB6BYU
Member

Posts: 13355




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« Reply #8 on: April 23, 2013, 08:35:29 AM »

A wider angle is usually better anyway.  Make the wires as flat as practical
in your situation and just use it.
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N6AJR
Member

Posts: 9912




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« Reply #9 on: April 24, 2013, 12:42:14 PM »

any thing is better than nothing.  start with this and  see how it does, then improve antennas as you can.
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KD2CJJ
Member

Posts: 369




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« Reply #10 on: April 27, 2013, 08:12:26 AM »

If your V (apex at 90 degrees or so) is horizontal (parallel to the earth) then your effectively your creating a V beam by doing that... there will be gain (dBd) towards the direction of the legs of the V.   At the apex there will be negative gain (dBd) which is now the rear of your beam.  Conceptually this is how a Hex Beam works (the same principle but also very different)

If you bring the legs down to earth directly to the sides of the apex at 90 degrees the signal will become omni directional whereby the gain will be the same all around.

if you as an example place the apex in the air and the legs slanting down away from the apex but out at 90 degrees then you have a slanted inverted v... there will be gain in the direction of the slants, no gain to the sides and negative gain towards the rear of the slant - less omni directional but not all that bad either depending on the slant.

As you raise each leg higher into a T then there will be No gain forward and back (dBd) and Nulls (>15 dBd) off the sides.



I have done a Slanted Inverted V because if I tied the V down below/to the side of the apex then the antenna would be against the house and I could not get to 90 degrees.  When I slanted it towards my fence I could get close to 90 degrees (I had more distance to widen the legs).  This approach worked much better than the inverted V with only about 30 degrees...  I also found that it worked more effectively than raising the legs close to a T actually.  The reason being is that as you lower the legs your impedance match will improve thus more power output to the antenna than the feedline.  This is my experience... and specific to my situation..

Dont bother doing a horizontal V beam unless you have a way to rotate it....


I am not doing any of the above anymore.. I ended up abandoning the wire dipole and went to a Mini Beam TGM 26 attached to the side of my house on a mast - it kills my end fed (another proejcet of mine) and my previous wire dipole... Why?  Because I can get it up higher and rotate it now.  Its significantly quieter 3s points over my end fed and 1 - 2S points over my wire dipole and with about 1S  points better than my wire dipole... Not much better gain but the S/N is a MAJOR difference...

Good Luck!



Thank you all for the responses!

I just want to clarify what exactly I mean here to ensure I'm being clear.

If looking at the antenna from the side it will be the traditional inverted look with the center post and the two legs coming off of it to the ground with 90 degrees (atleast) between them.

If you were then to somehow "fly" above the antenna looking down you would see the top of the post and the two wires coming out from it to either side at 180 degrees from each other. This would be in a traditional inverted V. However what I want to do is that the angle from this view would also be at 90 degrees.

So I guess on the vertical and the horizontal the angle would be 90 degrees rather than 90 and 180 which you see in most pictures.

Does this make sense and will this work?

Thanks again!

Regards,

Peter
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73

Mike
KD2CJJ
WB6BYU
Member

Posts: 13355




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« Reply #11 on: April 27, 2013, 07:17:41 PM »

Quote from: KD2CJJ

If your V (apex at 90 degrees or so) is horizontal (parallel to the earth) then your effectively your creating a V beam by doing that... there will be gain (dBd) towards the direction of the legs of the V.   At the apex there will be negative gain (dBd) which is now the rear of your beam.  



This myth has been around for many years, but, just because you can find it repeated
on the internet doesn't mean it is correct.

If we build a 40m doublet and compare the forward gain between a straight version and
one with a 90 degree horizontal angle at the feedpoint, we get something like this:

40m:  gain drops a bit over 1/2 dB when the antenna is bent.  Antenna remains bidirectional.

20m:  gain drops 2.7 dB when the antenna is bent.  Antenna is nearly omnidirectional.

15m:  gain increases 1.5dB when the antenna is bent.  Front to back ratio is about 2dB.

10m:  gain increases almost 2dB when the antenna is bent.  F/B is about 1dB.

A vee beam requires wires at least 3/4 wavelength long, and typically 2 to 6 wavelengths
to get significant gain.  The optimum angle depends on the wire length in wavelengths, and
can be narrower or wider than 90 degrees.  The optimum vee angle for maximum gain on 15m
is about 130 degrees, while 90 is about right on 10m (1 wavelength legs) and narrow angles
for longer leg lengths.


Front to back ratio is rarely very high unless
the wires are terminated.




Quote

Conceptually this is how a Hex Beam works (the same principle but also very different)



Actually they have no relation to each other.  The gain of a vee beam is achieved by
aligning the lobes that appear at angles along a long wire.  A Hex beam or other yagi
uses parallel (though sometimes bent) half wave elements with parasitic coupling
among them to establish a phase shift that enhances the signal in one direction.
A yagi or Hex is capable of much higher F/B than a vee beam.

A proper vee beam can provide gain in the same direction over a reasonably wide
frequency range (even when the angle isn't optmum)  So a single antenna could
be used on 20m through 10m.  By contrast, a Hex beam or other yagi variant
works only when the parasitic elements are near the optimum length, usually
covering only a single ham band (though they can be built with separate sets of
elements for multiple bands).  In fact, if you use a 2-element yagi and lower
the frequency by 10% or so, the antenna will reverse direction.  That doesn't
happen with a vee beam.
« Last Edit: April 27, 2013, 07:26:35 PM by WB6BYU » Logged
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