How High should my Dipole be?--2

Height	Height		Fav Dir	Fav Dir		End Dir	End Dir
				Launch			Launch
Wave			Gain	Angle/		Gain	Angle/		Feedpt	Res.
Lengths	Feet		(dbi)	Bmwidth		(dbi)	Bmwidth		Z	Freq
4.0*	560		7.75	4 / 4		5.57	72 / 13			6.93
3.0	420		7.83	5 / 5		5.25	68 / 14		77+ j11	6.94
2.0*	280		7.80	7 / 7		0	39 /		75 + j12	6.95
1.5	210		7.72	9 / 10		-2.50	33 /		75 + j11	6.96
1.0*	140		7.64	14 / 15		-11.00	20 /		74 + j08	6.96
.9	126		7.03	16 / 17		-8.30	22 /		85 + j13	6.94
.8	112		7.16	18 / 19		-6.40	25 /		84 + j26	6.88
.7*	98		7.95	20 / 22		-4.50	30 /		70 + j30	6.88
.6	84		8.35	23 / 26		-1.95	40 /		60 + j16	6.94
.5*	70		7.45	28 / 33		-0.51	43 / 33		71 - j00	7.00
.4	56		6.06	35 / 47		1.30	59 / 102		93 + j04	6.98
.3*	42		5.59	50 / 137		4.71	90 / 80		100 + j32	6.86
.2	28		6.70	90 / 118		6.70	90 / 67		71 + j56	6.77
.1*	14		8.21	90 / 103		8.21	90 / 66		23 + j39	6.84
.05	7		9.61	90 / 99		9.60	90 / 72		7 + j12	6.95

* Elevation plots shown below



Analysis, Favored Direction:



The first thing to notice is that the gain in the favored (broadside) direction varies very little with height. The important change in the broadside pattern occurs in the launch angle of the primary lobe. As the antenna moves closer to the ground, the launch angle of radiation gets higher and the -3 dB vertical beam width becomes broader. Note that below the benchmark height of ½ wavelength, the launch angle increases rapidly. Once the dipole is lowered to 0.3 wavelengths, most of the radiation goes in a vertical direction. This explains the frequently heard "rule" that a dipole must be at least ½ wavelength high to work. The seeming anomaly with the beam width below 0.4 wavelengths is easier to understand by viewing the plots shown below.




Analysis, End-Fire Direction:



One frequently sees a dipole azimuth pattern depicting a very sharp null off of the ends of a dipole. While technically accurate, this can be very misleading as the table above shows, and is a result of trying to depict a 3 dimensional pattern in 2 dimensions. This often seen null is only evident at the same launch angle as the maximum broadside gain. Of major significance is the large amount of gain off the ends at higher launch angles. Due to multiple lobes forming above ½ wavelength, this is not easily shown in tabular form. I have arbitrarily chosen to list gain and launch angle for the secondary lobe with the lowest launch angle, but recognize that there is frequently a stronger primary lobe at higher angles. Consult the plots below for a better visualization.



Analysis, Feed Point Impedance:



The reference antenna length was chosen to resonate at the ½ wavelength height. As expected, the feed point impedance oscillates significantly as the height changes from our reference point. Thus we verify the old adage that you must trim the dipole to fit your particular QTH (height being very important). The corresponding resonant frequency for each height is shown in the last column for reference, since complex impedance's may be of less practical importance to some.




So, How High should the dipole be to work well?



Now we are back to looking at what we want the dipole to achieve.



For DX work, higher placement is warranted, since more power concentrated between 5 and 15 degrees is reported to be of major benefit. Heights around one wavelength are necessary to get the broadside lobe to launch in this range. However, higher may not always be better. Pay careful attention to the magnitude of secondary lobes in the broadside direction, as well as high angle radiation off the ends. Some heights would appear better than others due to concerns with nulling out local QRM. A complete discussion of of this aspect is beyond the scope of this article, but may be investigated at a later date.



For local work, lower heights appear to be more beneficial. Note especially how omni-directional our dipole becomes at lower heights. Below 0.4 wavelengths, there is less than 1 dB of attenuation in the end fire direction, which suggests a height between 0.4 and 0.3 might be an ideal compromise for local nets and rag chewing.



Feed point impedance and matching does not seem to be of major concern except at very low heights. The effect of height on 2:1 SWR bandwidth was not investigated.