The mystery of radials
Erecting the antenna above ground (II)
The first effect of placing a 1/4l vertical at ground level is that the mirror-image of the "missing half" has consequences on the feed point impedance that displays about half the impedance of a dipole, or about 35 ohms plus ground loss resistance. At the risk to being repetitious, its lower end displays a low-voltage/high-current, conform to its large lobe firing all the output power at 0° and not higher above the horizon.
But other effects look more amazing... Some manufacturers state for example in their advertisings that they sold high performing antennas that do not request any radial or counterpoise (ground plane). Does it mean that if we raise our vertical a couple of meters above the ground, we can remove all our radials and keep its efficiency ? No, you can't. This is not really so simple, excepting if you are willing to loose all its efficiency... and your money.
Do you remember the 120 radials 1/2l long each lay on the ARRL's estate to make up all ground losses... ? Good news ! According Bencher one more time, "at heights between 2 and 8m about ground [...], four 1/4-waves radials for 40 meters do suffice to provide enough capacitive coupling to earth to work on 40, 75, 80 and even 160 m bands". And indeed, to avoid the detuning of wires running at ground level and reduce the ground currents you must raise the radials as low as 1 meter off the earth. Of course you will raise the antenna by the same occasion, and probably much higher as we will see in one moment.
In fact here also some amateurs confuse the height of radials above ground and the height of the antenna above ground. As both constitute the radiative part of a vertical antenna and that one without the other is ineffective, the melting is understandable. But try to see clearly in this new shallow fog.
For example, if one states like Bencher that for the 40m band radials can be placed between 2 and 8 m high (1/20l to 1/5l high) to remove all ground effects, the antenna itself displays its best radiation pattern when placed at least... 3/4l above ground (see table below), so about 30 m high ! Find the mistake... Both values seem incompatible, but they are not. In fact your radials will raise with your antenna and will be thus placed high enough above ground to be "out of reach" from the soil effects.
Ground plane antennas
We have not told yet, but although our vertical is now raised in height, it needs always its mirror-image to work properly. In this case, the earth is too far and our connection running to the ground will be detuned as we told before. So, instead of being grounded the vertical works against a simulated ground made of some 1/4l radials or stubs attached to the base of the antenna to create an artificial ground plane at a few meter high, hence its name. These radials are either open at 45° like a tripod or attached perpendicular to the antenna axis. For mobile operations the radials are removed at the benefit of loading coils in order to reduce their profile.
If we raise the antenna base at a height of 1/2l or so, as states Bencher usually four or six 1/4l radials only provide about the same efficiency as dozen or even an hundred 1/2l radials running underground. However, whatever state advertisings, when the height above ground decreases say below 1m, the number of radials required to get the same efficiency increases naturally. This is pure technical question, not of marketing...
From 14 MHz and above (because it is impracticable on lower bands) the best radiation pattern for a vertical appears from 3/4l high above ground where an additional lobe appears at 43° of elevation. At 1.5l high the vertical antenna sees its main radiation lobes splitted in 4 parts, the main horizontal one splitting in 3 parts at 0°, 20° and 43° of elevation and a fourth one appearing at 90° (vertical).
The "Bencher" configuration using some 1/4l radials placed in height is called a counterpoise. Used in HF and V/UHF bands, these Ground plane antennas are installed on small masts from 2 to 5m high usually erected above the common obstacles using as few as three radials or stubs. Rather efficient, these vertical antennas are largely used, not only by the ham community but also by other services using shortwaves, hence the presence of a large amount of V/UHF Ground plane antennas here and there at the HQ of many companies.
High, Aie !°
As we saw in the table displayed above, for a vertical cut for the 20m band, say 5m high if we use its mirror-image in the ground, the best efficiency is reached erecting the antenna center up to 30m high (100 ft) ! High, Aie ! If some amateur installations exceed largely this height, for most of us this is neither feasible, nor economical. So for the lower bands of 40 to 160m, amateurs have found some hybrid solutions, like using together a vertical from 9 to 15m high associated to a radial system made of 4 wires as long buried in the ground. The antenna being less than 1/4l high, the input reactance without loading is capacitive. In this case a simple series loading coil placed at the base of the antenna will ensure the matchting to the feed line and the transceiver. QST magazine and the other publications edited by ARRL about antennas have provided several models of such designs (including several models using wire antennas tight in inverted-V or forming a loop).
° Ouch ! in English, but doesn't rhyme...
Radiation and SWR
Another question arises when speaking of efficiency : what becomes the power put in the antenna and how losses affect the SWR ?
The radiation resistance or RF energy radiated by your antenna has to be consider as a "good lost" compared to the lost induced by the ground and conductor resistance that are consider as a total loss. Thus, knowing that some "conductors" (including traps, loading coil, etc) loose more energy than others, we can use the concept of form factor, the famous Q-factor.
But due to ground losses, than can easily exceeded the losses in conductors, traps and coils, a well tuned vertical cut at 1/4l can display a high SWR in the middle of a band (over 2:1) what means that some dozen of ohms vanished in pure ground loss resistance. Here are some tests made in situ. The first shows the efficicency before (first row) and after (second row) installing 6 radials of 2m long at ground level around the base of a 1/4l vertical :
SWR is measured at the antenna feed point in place of the RTX end to avoid transmission losses and increase the accuracy. Total losses are due to radiation lost in coax, traps, loading coils, and ground loss resistances. The efficiency is expressed as the ratio of power radiated to the total power fed to it (or the ratio between the radiation resistance to total losses).
Below is the efficiency before and after installing 6 radials at the base of the previous vertical but resonating at half-frequency (e.g. on 80m in place of 40m), the third case using 120 radials to reach a zero ground resistance :
For comparison purposes, here is the efficicency of a dipole :
From these figures we can conclude that several parameters are very important to get the highest efficiency of a vertical :
1°. The radiation resistance must be kept as high as possible, but it depends on total losses thus,
2°. The radiation resistance depends on the height of the vertical, but as height over 1l are not always practicable, we can reduce the ground loss resistance using as many radials as possible in using high-Q loading inductors of large diameter. Therefore the slim loading coils and traps made of thin wire encasted in metal usually found in ham stores are NOT at all adapted to this usage.
3°. A low value of SWR does not mean that your antenna system is operating efficiently. Even the fence installed in the end of your backyard or an electric heater could be fine-tuned with an ATU to display a SWR 1:1 but it will not radiate the least watt of power as explained on this page dealing with SWR.
By way of conclusion
I think that this time we emerged safe and sound from our fog so much dreaded. We discovered that a ground plane radial system is recommended as it provides low-loss "return" paths to currents that can be "recycled" in the antenna, that might otherwise flow on the lossy earth. If these return of current come back to the shack they can be stopped using a current choke, a variant of balun 1:1.
Even if a "no-radial" system displays a low SWR, by itself it tell us nothing about the antenna efficicency, and mainly how the ground interacts with the system. For short, a simple vertical 8m long using radials and a commercial "no-radial" vertical will display the same performances, but the SWR of the second antenna at the feed point can reach 20:1 or more without losing any energy but without neither emitting the least watt !
When an antenna is placed near ground level, the earth losses are the major factor limiting the antenna performances, and no antenna tuner or matching device can do anything about that physical law, excepting maybe some bad advisers.
Thus if you have to remember only one think, that will be the next one : radials reduce the ground losses and increase the antenna efficiency. A side effect, too few or too short radials affects the SWR.
With all this information, it looks no more as an idle fancy or diligence from a manufacturer if he tells us that radials are useful ! Now you know why.
For more information
Two books are particulary complete about vertical antennas and radial systems :
- the unavoidable "ARRL Antenna Book", ARRL
- and the more recent "Vertical Antenna Classics", ARRL
as well as the tens of other books about antennas edited by ARRL and available via the previous link or to RSGB.
About radials, Reg Edwards, G4FGQ provides on his website several DOS programs among which RADIALS2.EXE that calculates the efficiency of any system of radials.
Technical notes about antennas are also available in PDF format on the website of major manufacturers and portals :