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Author Topic: Calculation for inverted Vee  (Read 345 times)
ZL1IW
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Posts: 7




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« on: December 19, 2003, 03:28:53 AM »

Please tell me the calculation required to construct an inverted vee for 75/80 mtrs.

If my memory serves me well, 468/freq in mc's give the answer in feet for a dipole, but seem to recall that the invert v requires a diffeent calculation.

73 Iain
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N1JAO
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Posts: 119




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« Reply #1 on: December 19, 2003, 06:39:22 AM »

if i remember, an inverted v has a feedpoint impedence of 70 ohms and that means you will have to lengthen it. i hope that is correct, i don't want to give you wrong info. here's what i usually do; set it up using the 468/freq formula in the v formation, and use clips with extra wire to lengthen in a test for vswr. you can also fool around with the shape of the v because that too will effect resonance. just remember that the higher the apex, the more directional it will be, which besides the advantage of only having to use 1 support, is the fun of using a good inverted v.

robert
n1jao
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AA4PB
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Posts: 12638




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« Reply #2 on: December 19, 2003, 08:28:40 AM »

A dipole in free space has a feed impedance of 70 ohms. As you bring it closer to earth the impedance lowers. The impedance of an inverted V is generally closer to 50 ohms.

The formula for the inverted V (or dipole) is L(feet) = 468 / F(Mhz). Usually you have to make some minor adjustments because the actual resonant frequency changes from one installation to another. The formula gets you in the ball park. The way I do it in one trial is to cut it to formula, put it up, and then measure the resonance frequency with an analyser or just tuning the transmit freq to find the lowest SWR. Then rework the formula plugging in the measured frequency and solve for a new value for the 468. Using that value and the desired resonant frequency run the formula again solving for the length. Take the antenna down and cut it to the new length. Usually it is right on the money when I put it back in the air.
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AC4FD
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Posts: 26


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« Reply #3 on: December 19, 2003, 09:24:16 AM »

When you Invert or reposition the elements on a dipole it will have a detuning effect on the antenna.  Also, the type of wire that you are using as well as the height above ground can have some effect on the total length of the Element.  At 75/80 Meters where a wavelength is very large, a small change in resonance can mean a change of several feet in the total length of the antenna.  

I have found a couple of sites with some information regarding Inverted V's.  Hopefully they will help you.

http://www.flashwebhost.com/circuit/antennas_for_ham_transmitters.php

http://www.angelfire.com/mb/amandx/dipole.html

Good Luck

AC4FD
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AC5E
Member

Posts: 3585




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« Reply #4 on: December 19, 2003, 12:39:32 PM »

Actually, when you bring the ends of an inverted Vee closer to ground there is an enhanced "end effect." So an inverted Vee tends to be a bit, small bit, shorter than a flat top dipole for resonance.

Since wire is easier to trim than splice, I use the same 468/f formula for either dipoles or inverted Vee's. Of course, I have only put up antennas over a fair selection of ground types but this formula has always resulted in an antenna that's resonant just below where I want it to be.

73  Pete Allen  AC5E
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WB6BYU
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Posts: 12973




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« Reply #5 on: December 19, 2003, 04:51:56 PM »

Antennas are a pragmatic science, because we often do
not know enough details about the antenna environment
to make an accurate calculation.

You have two choices:

Use one of the software packages to model the desired
antenna given the center height, angle of the legs,
height of the tie-off points, ground characteristics,
wire thickness and insulation material, and any other
structures withing a few wavelengths that may have an
effect.

Or just cut it on the long side, install it, and trim
the ends to get the resonance where you want it.  The
formulas are only approximations anyway.

If you like spending your time at the computer, choose
the first option.  If you'd rather spend the time on
the air using the antenna, the second approach is
probably a better choice.
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N8YV
Member

Posts: 103




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« Reply #6 on: December 19, 2003, 07:21:16 PM »

Forget the modelling program, the calculus degree and the hypothetical computations! It's just a DIPOLE, people!  From real world experience, with DOZENS of these inverted vees---you WILL need to shorten the length as compared with a horizontally-oriented dipole.

Exactly how much shorter it will be, depends primarily on three things (yes, there are other factors, but these are the "big three" that will MOST affect it):

1) Its distance above RF ground---a variable height that depends upon soil moisture, etc).  You cannot normally control this, it is a property of the earth area underneath the antenna.

2) Its proximity to nearby objects, especially metal (fences, conduit, siding, underground pipe, etc.)

3) The angle between the dipole halves (the wider this angle is, the less shortening will be needed.  Avoid closing this angle below 90% if possible.

The tried-and-true method of "cut-n-test" with a good SWR bridge is the least expensive way to accomplish this.  There are others, including using an antenna analyzer, computer program, etc.

I have had inverted vees that required as much as 4% of their length to be eliminated, before I got in the ball park.  Watch it, however, once you get pretty close, you should proceed with trimming with care and small "nibbles".  I have seen more than one antenna get close to frequency, then have too much taken off with the next cut.  This results in "overshooting" the mark, requiring adding length to get back to the resonance point.

Better to make a few smaller cuts carefully, than to prune too much and have to splice and solder extra wire.

Good luck, and best wishes for a great Holiday season!
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N6AJR
Member

Posts: 9879




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« Reply #7 on: December 20, 2003, 01:17:50 AM »

fan dipole, I have put your info on here a couple dozen times, look up fan dipole here on elmers search. please, we have done this one before.
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