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Discussion about Horizontal, Center-fed, V-dipoles.

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Horizontal VWhat Happens if...
You Bend a Dipole Into a "V"?

The inverted-V form is common where there is only one high place, often a pole, to hang a long wire antenna.   Another often seen upright V-form is in the Buddy Pole or the similar MFJ Big Ear portable dipole.  Upright or inverted, either form reduces the horizontal length to save space.

To answer to the obvious question about "What happens if..",  is where antenna modeling comes in. For this study a model was developed to allow the setting of the arms of the "V" to any angle up or down. The optimizer function of the program iteratively tries various lengths of wire to find the lowest SWR and  reactance (X-in) when the wire is bent into the specified "V" form. Using the model, this study looks at the changes in antenna characteristics every 15 degrees angle of the arms.*

4NEC2 Antenna Model: Here

The characteristics of a Horizontal V-Dipole are most effected by whether the V-form arms move closer or farther from the ground. In Figure 1 below, note the general pattern of low-to-the left, high-to-the right as the arms move upward away from ground.  The blue antenna graphic under each angle number shows the actual angle.

* The degrees of the angle included between each arm is twice the angle number. For example, at the 45 number the included angle is 2 * 45 = 90... a right angle.  Same at the 135 number.  2 * 135 = 270, where 360 - 270 = 90. Altenatively, use negative angles. 135 - 180 = -45.  Then, 2 * -45 = 90.

Horizontal V-Dipole Characteristics Study
Figure 1
A graphic indication of the V-bend is pictured by the blue angles..

Following Red Line at the top, you see that the gains drops away more rapidly as the dipole is bent toward ground.  From the 7.32 dBi gain of the standard horizontal dipole at the 90 mark, the V-dipole gain drops to around 6 dBi  at the 45 mark and 5 dBi at the 15 mark.
On the other hand, when the dipole is bent upward the gain actually rises slightly in the 105 through 130 area.  Even at the extreme of 165 the gain is only slightly affected at 6.5 dBi.

Following the Blue Line, you can see that with bends at 45 and 120, the SWR changes towards a more desirable 1.1 from the 1.38 SWR of the standard horizontal dipole.  After the 45/120 bend points you can see how rapidly the SWR rises.  Not good but workable in a pinch.

When you bend a dipole, the electrical length changes. Because of this, retuning goes with bending. Following the Green Line on the graph, the message is: the more you bend, the more wire you need... up to 4 or 5% longer.

There is a little known configuration that eliminates retuning at any angle, the flat-top V-dipole. 

We now turn to:

"What happens to Radiation Patterns when you bend a dipole into a V-form?"

From antenna modeling in Figure 1 we see that a V-antenna is a shortened dipole  For tuning you need to lengthen it according to bend . It does not become omni-directional. The pattern merely becomes fatter.

An up-bend means more skyward radiation, narrower pattern, good gain.  A down-bend means higher takeoff angle, wider pattern, lower gain.

Lets look at two far field radiation patterns of a V-dipole.  On the left the included angle between arms is 90 degrees pointing Up and, on the right, 90 degrees pointing Down.  Below, every 3D picture is from an identical point in the air (Theta=49, Phi=315) so you can see the relative in change in shape and radiation angles. 

LEFT COLUMN: Arms up at 135                        RIGHT COLUMN: Arms down at 45
TOP GRAPHS:  Total Gain Vertical-Horizontal Patterns
  BOTTOM PICTURES: Total Gain by 3D overhead view
Total radiation up-down pattern-3D composite
V-Up    Gain: 7.1 dBi         Side Null: -2.9 dBi         V-Down     Gain: 6.14        Side Null: -1.8 dBi
Lobe angle: 65 degrees       Width: 30 degrees          Lobe angle: 60 degrees       Width: 40 degees
Note: vertical radiation hump when arms up.         Note: higher angle of radiation and wider form.

Next, the modeling program also allows us to separate the horizontal and vertical components to visualize gain generated towards the ends of the V-dipoles as well as broadside.

LEFT COLUMN: Arms up at 135                        RIGHT COLUMN: Arms down at 45
TOP PICTURES:  3D Vertical Gain towards ends
  BOTTOM PICTURES: 3D Horizontal Gain broadside
Vertical and Horizontal Gain in V-dipoles

Compare the red portions in all four pictures.  Note that with the V pointing upward (left side), the red tends to point more downward . Conversely, with the V pointing downward, the red is more upward. This shows you the relative change in direction of the radiation intensity.

From all this modeling it is clear that the V-Dipole radiation characteristics change little even when the included angle is as narrow as 70 to 80.   It is widely used because it is forgiving of missuse and easy to deploy.

Dick Reid, KK4OBI