Why verticals have got radials?
The past week we tried to analyse a vertical monopole 1/2 wl, no radials,
compared with the vertical dipole and the horizontal dipole. The last image illustrated
how the verticals are not so effective while over real ground.
Let reaffirm the fact with a further analysis over very-poor ground. Both the antennas
have the feed point 1/2 wl high, so the monopole is all over the horizontal dipole.
Note how the vertical is somewhat better now than over average ground, because of the
squashing of high lobes.
||But now is the time to add radials.|
Four 1/4 wl radials, horizontally disposed.
What? There is no change in the pattern? If one, no-radial is slight better...
Mmmm, the antenna is high over ground, maybe the radials will be more effective
with a low one. OK. We will lover it, at .02 wl, so around 20 cm. (sic !) at 27 mhz.
Still equals? Where is the white trace? Is the software defective?
Let we came back at the source of knowledge.
- A Modeling Perspective on "Ground" Planes
The first killer idea is *radials can't affect pattern* because they're too near
to the antenna. But there is much more. What unless the ground is needed to
complete the antenna means?
- We are very careless with the ground. We tend to treat it as a single homogenous entity.
In simplest terms, that means that we think about the ground in the same way, no matter
in what radio context it appears.
- We tend to think of the ground relative to an antenna as a single ground.
Hence, we tend to lump together the ground from which signals reflect to contribute to
antenna far field patterns and the ground directly under a monopole antenna. The "only"
difference is their relative distance from the antenna itself.
However, let's see where separating the two ideas leads.
- Far field reflective ground: ... ON4UN did some modeling with some vertical radiators
that suggests the ground kicks in as a reflective medium somewhere around 2 and a half
wavelengths from the antenna--possibly more for highly elevated antennas.
Although the ground immediately under the antenna has some effect on antenna effectiveness
relative to far field patterns, the effect is small (unless the ground is needed to complete
the antenna). Dipoles, for example, exhibit only small gain changes as the quality of soil
is ranged from very poor through very good.
When we run a 1/4 wl monopole, it needs *the other half of the antenna*. We can obtain that
with another straight wire, and ... we've a dipole. We can with some elevated radials,
and that's named a GP. Or we use the surface volume of the ground, but
- Antenna-completion ground: Monopole antennas are generally analyzed as having
their missing pole (relative to a dipole) within the ground. This is often pictorially
presented as an "image" antenna sticking straight into the earth. While this portrayal
allows the solution to certain basic equations, it is actually a very poor picture of
what is going on. It is the surface volume of the ground that provides the completion
of the antenna. Signals penetrate the ground to depths that vary directly with wavelength
and inversely with frequency. However, even with a monopole, the penetration does not act
like a spear into the ground ...
Now where should be that *missed half of antenna* in the case of 1/2 wl monopole? When
it's an off-center feed dipole, as all analysis demonstrate? So such an antenna don't
needs *radials*, also at very low high. However when at 20 cm. by soil, who can negate
any interference... But the 1/2 has also the advantage of very low currents (teorically
zero) at tip, while the 1/4 has a maximum at feed point.
- In fact, the so-called ground plane is never a GROUND plane, ... Hence, the so-called
antenna-completing ground is not a ground at all--even though we sometimes use the ground
to do the work of the plane. This is not the first time we have misnamed something in radio.
It took a long time to weed out the term "condenser." It will likely take even longer to
drop the term "ground" from ground plane.
Now, because I would still to verify, here the same arrangement but with 1/4 wl stick.
Now the four elevated radials add some 2dB gain. But only if the antenna is low a lot.
See below, when the GP is 3m above ground...
So my thought is: in the clear and at decent high, radials don't deal with radiation pattern.
When the antenna is 1/4, of course, they deal with feed-point impedance. But the 1/2 monopole
needs anyway a matching network, so I really can't figure what a radial system can add to
a Antron-like antenna... maybe when not in the clear and maybe somewhat related to rfi, I'm
waiting for hints...
||Now you'd think I hate verticals, and A99 in particular. So here my roof,
with the Moxon rectangle and, in top of it, an Antron-like monopole, the Sigma Simply-27.
The analysis take in count of two distinct levels of ground, the first one (the roof) 6m
under the aerials, the second one (the real ground) 10m under the roof.|
Have fun. 1 AT 1839 Paolo