How High should my Dipole be?

* 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.

(Click on picture for full size image)

This plot shows the elevation patterns at a height of 2 wavelengths, or 280 feet.
White trace is broadside. Yellow trace is axial (off the ends).
Still Lots of high angle radiation.

This plot shows the elevation patterns at a height of 1 wavelength, or 140 feet.
White trace is broadside. Yellow trace is axial (off the ends).
The secondary lobe is down to 47 degrees, but the primary lobe is up to 14 degrees.


This plot shows the elevation patterns at a height of 0.7 wavelength, or 98 feet.
White trace is broadside. Yellow trace is axial (off the ends).
The primary lobe is up to 20 degrees.
Note the large vertical lobe which has appeared!


This plot shows the elevation patterns at a height of 1/2 wavelength, or 70 feet.
White trace is broadside. Yellow trace is axial (off the ends).
Now that's a classical dipole pattern!


This plot shows the elevation patterns at a height of 0.3 wavelength, or 42 feet.
White trace is broadside. Yellow trace is axial (off the ends).
We are entering the "skywarmer" mode here.


This plot shows the elevation patterns at a height of 0.1 wavelength, or 14 feet.
White trace is broadside. Yellow trace is axial (off the ends).


This plot shows comparative elevation patterns, in the broadside direction only,
White trace is for 2 wavelengths, 280 feet.
Yellow trace is for 1 wavelengths, 140 feet.


This plot shows comparative elevation patterns, in the broadside direction only.
White trace is for 0.7 wavelengths, 98 feet.
Yellow trace is for 0.5 wavelengths, 70 feet.

This plot shows comparative elevation patterns, in the broadside direction only.
White trace is for 0.5 wavelengths, 70 feet.
Yellow trace is for 0.3 wavelengths, 42 feet.
Pink trace is for 0.1 wavelengths, 14 feet.

HAMS

Have you ever wondered why we radio amateurs are called "HAMS?" Well, according to the Northern Ohio Radio Society, it goes like this: the word ham was applied in 1908 and was the call letters of one of the first Amateur wireless stations operated by some members of the HARVARD RADIO CLUB. There were Albert S. Hyman, Bob Almy and Peggie Murray. At first, they called their station Hyman-Almy-Murry. Tapping out such a long name in code soon called for a revision and they changed it to HY-AL-MU, using the first two letters of each name. Early in 1909, some confusion resulted between signals from Amateur wireless HYALMU and a Mexican ship named HYALMO, so they decided to use only the first letter of each name and the call became HAM. In the early pioneer unregulated days of radio, Amateur operators picked their own frequency and call letters. Then, as now, some Amateurs had better signals than some commercial stations. The resulting interference finally came to the attention of congressional committees in Washington and they gave much time to proposed legislation designed to critically limit Amateur activity. In 1911, Albert Hyman chose the controversial Wireless Regulation Bill as the topic for his thesis at Harvard. His instructor insisted that a copy be sent to Senator David I. Walsh, a member of one of the committees hearing the bill. The Senator was so impressed, he sent for Hyman to appear before the committee. He was put on the stand and described how the little Amateur station was built. He almost cried when he told the crowded committee room that if the bill went through, they would have to close up the station because they could not afford the license fees and all the other requirements that were set up in the bill. The debate started and the little station HAM became a symbol of all the little Amateur stations in the country crying out to be saved from menace and greed of the big commercial stations who did not want them around. Finally, the bill got to the floor of Congress and every speaker talked about the poor little station "HAM." That's how it all started. You will find the whole story in the Congressional Record. Nationwide publicity associated station HAM with Amateurs. From that day to this, and probably to the end of time, in radio, an Amateur is a HAM.