Saturday, June 8, 2013

Dipole or Vertical?

Dipole or Vertical? 

Note: This posting uses hyper links extensively. Just click on the appropriate link to see a table or figure and use your back button on your browser to return. 

*For this article a vertical refers to a 1/4 wavelength, ground mounted antenna with 1/4 wavelength radials.

Vertical antennas are probably one of the most misunderstood of the simple antennas that amateurs use. This is partially true because of the interaction that the vertically polarized signals have with the ground. Many believe that a good radial system extending out from the base of the vertical will insure a high gain, low angle of radiation. This simply is not true! The radial system mitigates the ground return losses, which affect the efficiency of the antenna, but not its radiation pattern. The radiation pattern is a function of the psuedo-brewster angle, and hence the ground conditions far away from the antenna base (typically a thousand feet for a 40 meter antenna). In fact, if you can mount a vertical on a dock over salt water, you can drop a plate into the water and establish a good, low loss system with no radials. I once operated a DX contest from a friend's dock with an 8 foot ground rod driven into the mud beneath the water as a ground. It was amazing the number of first call replies I got with only 100 watts and a 1/4 wavelength vertical on 15 meters. The secret was a lobe that had a 4.3 dbi gain at 9 degrees, and a 1.9 dbi gain at 1 degree.

On the other hand, when the vertical is mounted over the soil, even good soil, you see a conductivity decrease from 5000 millisiemens/meter for salt water to about 30 mS/m for good soil. A vertical mounted on the ground requires a radial system to overcome the decrease in efficiency of the antenna due to the ground losses. The extent of the radial system can vary with varying results. Jerry Sevick, W2FMI, measured the feed point resistance of a vertical with different numbers of radials. Since the feed point impedance of a vertical mounted on a perfect ground plane is 36 ohms, then the efficiency of a vertical with radials, referenced to a vertical over a perfect ground plane, can be determined by dividing 36 by the measured feed point resistance. This can easily be converted into db loss which shows up in the far field pattern.. Table 1 shows this relationship. Also, for those who pay homage to the SWR god, a lossy radial system actually gives a better match at the price of efficiency. And, if your vertical is shorter than a quarter wave length, the feed point resistance goes down, and you have the additional loss of the inductor that is most likely used to counteract the capacitive reactance resulting from the shortening. A double whammy, if you will, since the efficiency equation gets an increase in the denominator and a decrease in the numerator resulting in an even lower efficiency.

The radial system is surrounded with controversy. Some say no radials are necessary, while others maintain that 120 radials 1/2 wavelength long are needed. Thanks to Row Lewallen, W7EL, and his EZNEC antenna analysis program many of these questions can be answered.Figure 1 shows the azimuth pattern for a vertical with just one radial and a vertical with two radials. You can see that the pattern is skewed with one radial. However, if you add only one more radial in the opposite direction, the pattern is perfectly round. I heard of one vertical manufacturer that added a third radial just because some prospective customers did not believe that uniform coverage could happen with only two radials. As you have seen from Table 1, the truth of radials lies between the extremes and is dependent on your particular situation. You can never have too many radials, but it is a game of diminishing returns. After all, two radials causes a 3 db drop in signal, which is only half an S-unit, but who wants to give up half of their power.

The map in Figure 2 shows the ground conductivity in millisemiens/meter for the continental United States. This map is purposely shown large for detail, so that you can determine the soil conductivity in your area.

After locating the soil conductivity for your area from the map in Figure 2 , use Table 2 and click on the appropriate row for the number of radials that you can install which corresponds to the column of soil conductivity in your area. Just pick a conductivity in the table that is close to your soil conductivity from the map. You will see a comparison plot of a vertical with the selected number of radials over the selected soil. The primary plot is always the vertical. Plots of a flat top dipole 1/4 wavelength above the soil are shown for comparison. Since you might not have a choice of orientation of the dipole, both broadside and end fire plots are shown. These plots should enable you to determine whether to construct a vertical or dipole to achieve your operating goals. Some of you may have only a single high support, so the Inverted Vee is compared with the Flat top Dipole in Figure 3 and Figure 4. You can easily transition from the table to your Inverted Vee. The Inverted Vee is constructed with a 120-degree angle at the apex with the center 1/4 wavelength above the soil.


According to the ARRL Antenna Handbook, much of the single hop DX arrives at angles between 1 and 5 degrees. Table 3 compares these low angle gains of the vertical with a dipole, a 5-element Yagi, and a vertical Moxon. The top of the vertical Moxon is at the same height as the top of the vertical. As you can see, the low angle advantages that are usually associated with a vertical are realized, but continental operation suffers with the vertical. With 32 radials, you can get within 1/2 db of maximum efficiency, but no matter how many 1/2 or 1/4 wavelength radials you lay down, you cannot see the good gain that you have over salt water or a ground screen that extends many wavelengths beyond the vertical base. However, a dipole or inverted Vee mounted at the same height as the top of the vertical will many times only start to out perform the verticals at angles higher than 20 degrees. On the other hand, unless you can mount the vertical out in a field, away from surrounding objects, you will not realize its full potential. Also, the horizontal antenna is not as influenced by different soil conductivities as is the vertical. Back and forth and back and forth. As most things in life, it is a trade off. You decide! 


Amen! Well done article. One of the best I have seen on eHam in a long time. 

Someone asked about comparing a ground mounted vertical and a vertical dipole. Well, I got curious about that last summer and did exactly that. 

The comparison was between a full size 20 M 1/4 wave vertical over 103 Half wave long radials and a fullsize 20 M Folded Dipole mounted vertically, with the 450 ohm feed line coming off perpendicular to the antenna for about 60 ft. The low end of the folded dipole was about 10" off the ground. The vertical folded dipole was supported by a non-conductive tower guyed with phillystran non conductive guys. i.e. a "clean" installation. The two antennas are separated by about 75 ft. (the antennas are out in the middle of a 2 acre lot in an open, semi-rural area, and well away from anything.) 

I was frankly, slightly surprised by the results. In 95% of the cases, the GROUND MOUNTED 1/4 wave vertical had the best signals...not by much, but it consistantly ran 1/2 to 1 1/2 S units better. (I was able to switch antennas instantly, for comparison.) Also, interestingly, the folded diplole was just a little quieter on receive. Both antennas are at DC ground; I attribute the higher noise on the ground mounted antenna to a stronger TRUE ground wave signal. 

After thinking about it, I realized that the slight advantage the ground mounted antenna enjoyed HAD to be related to earth losses. A vertical dipole (with one end near the ground) has the SAME earth losses as a ground mounted vertical! (I have to respectfully disagree with a posting on that.) The NEAR field effects of earth on the vertical folded dipole are EXACTLY the SAME as the ground mounted vertical; actually they are worse, because the high.max current area is 1/4 wave out in pure dirt (no radials) and the losses are very high; whereas the 1/4 wave max current is carried right at/near the base in copper wires. To make the point: if you fed a Half Wave vertical at the base (voltage feed) and used one basic 8 ft. ground rod, for the "ground system", the relationship between this radiator and the vertical dipole are Exactly the SAME...the ONLY difference is the point of feed! i.e. the vertical dipole is fed in the center at the low impedance point...other wise, it is the exact same antenna...and in this case, BOTH antennas totally LACK a good low resistance ground system in the immedidate vicinity of the antenna. There IS an "image" or return current or whatever you want to call it induced in the ground with BOTH types of feed. 

Bottomline: where you connect the transmission line to a vertical (more correctly, a ground mounted vertical) does NOT eliminate the requirement for a low resistance ground system. (If your vertical dipole is 210 ft. in the air, or mounted as a true Ground Plane antenna, it is a totally different game, obviously.) BUT if the base of your vertical dipole is only inches off the ground, you will have VERY high ground losses unless there is a high conductivity ground system under the antenna (in the near field area). 

As a practical matter, I was quite satisfied with the performance of the vertical folded dipole, especially on DX, but it was NOT the better of the two antennas at the margin. If you want a good, cheap DX antenna and really are not/won't/can't (too much work) going to lay a lot of radials, then the vertical dipole has a lot going for it; they work well as a low angle radiator. You can also feed it with balanced line an a tuner and have a very good multi-band low angle radiator up to the 2nd Harmonic... i.e. a 20 M antenna will give good low angle radiation on 20 thru 10 M. 

Didn't mean to make this so long!...sorry... 

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