The Inverted-Vee Antenna

In many situations an Inverted-Vee dipole antenna is a convenient choice. It requires one central support and the ends droop down towards the ground. However, getting the best performance from this arrangement requires some knowledge. The classic inverted vee is a dipole with the ends sloped down to give a 90 degree apex angle - but is this the best idea?

Letâ€™s start by considering a horizontal 20 m halfwave dipole, 6 metres above a "high accuracy" ground model. I have modeled this with a dipole length of 10 metres. It is resonant at 14.3 MHz with a VSWR of 1.67:1 ( Z = 83.7 Ohms ). The dipole has a gain of 5.68dBi at Az = 90 degrees El = 50 degrees. These are all entirely normal figures [Straw, 1998].

Now letâ€™s slope the ends down a metre at a time, keeping the centre at 6 metres. I will keep the wire lengths the same (5m each side). The results are summarised in the table below.

Feed point above ground (m)	Ends above ground (m)	F (resonant) (MHz)	Z (Ohms)	G (dBi)	Apex angle (degrees)
6	6	14.3	83.7	5.7	180
6	5	14.3	80.5	5.6	156
6	4	14.3	73.4	5.3	132
6	3	14.5	60.6	4.8	106
6	2	14.6	40.7	4.0	72

This little experiment shows that as the apex angle decreases, the impedance of the antenna decreases and so does its radiation efficiency; these are both bad news! The resonant frequency of the antenna also rises as does the angle for maximum gain. The experiment supports a rule-of-thumb for inverted-vees which suggests using an apex angle of 120 degrees or more for best performance. Other commentators prefer to use an angle that gives a 50 Ohm impedance and accept some loss of performance [Meserve,?].The risk with this approach is that performance drops rapidly as the apex angle reduces below 90 degrees and often, people accidentally use an angle of significantly less than 90 degrees. One other point to note is that the smaller the apex angle, the less the bandwidth of the antenna [Devoldere, 2000].

One further general point to note is to keep the ends of your inverted-vee away from the ground. The rule-of-thumb here is not to have any radiating part of the antenna closer than 1/40th of a wavelength from the ground. This rule-of-thumb was developed from the work of McLeod (1994).

Using these two simple rules-of-thumb will go a long way to ensuring that your inverted-vee dipole works effectively!

73 Richard G3CWI

References

Straw R.D. (ed.), The ARRL Antenna Book, The Effects of Ground, 3.1 et seq.
Meserve M.A.,?. Inverted Vee Antenna Design.
Devoldere J., 1990. Low band DXing, ARRL. Ed. 3.
McLeod W.A., 1994. Low Radiators and High Ground Planes. Amateur Radio (Aus.), Nov. p10-14.

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Heavy Duty Antenna Wire

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73 Richard G3CWI

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