Greetings friends. With you again Timur Garanin.
Several times I was asked to make a video about 5/8 lambda antennas, and today finally I will cover this topic.
But before moving on to 5/8 lambda antennas, you first need to talk about the evolution of GP antennas.
GP - means ground plane, i.e. horizontal plane.
The task of GP antennas is to spread the signal in all directions horizontally, but not up and down.
Consider the sequence of development of GP antennas.
It all started with a half-wave dipole of Hertz. It can spread the signal in all directions on the horizon and is always aligned with a 75 ohm cable.
But it has a number of problems.
The first problem of the Hertz dipole in its radiation pattern
is that it emits a signal not only in the horizontal plane, but also up and down, where we have no correspondents.
Consequently, part of the radiated energy is wasted.
The second problem is that all the static from the Hertz dipole goes directly into the cable. There is no lightning protection here at all.
And the third problem is that the dipole consists of two halves.
Installing a similar antenna for medium and long waves is a big problem,
because each half can reach a length of several meters or more.
Therefore, they switched from a dipole to a monopoly. The most famous and simplest GP antenna is a quarter-wave pin.
It’s just a quarter-wave rod or wire.
The central core of the 50-ohm cable is connected to it, and the cable sheath in the simplest case is connected to the ground.
If it is not possible to connect the braid to the ground, then counterweights are used -
- rods slightly shorter than a quarter of the wavelength, located at a certain angle to the horizon.
By adjusting this angle, you can make the radiation pattern narrower and raised above the horizon.
The more counterweights, the more uniform the DN in the horizontal plane.
However, the quarter-wave pin still had problems.
There is no protection against static, and the radiation pattern is still too blurry, which is why in the right direction, along the horizon, we get insufficient amplification.
Therefore, the next step in the evolution of GP antennas was half-wave pins.
They really have a flatter radiation pattern, and therefore a larger horizontal gain.
But there are serious nuances. As we know, it is almost impossible to directly coordinate a half-wavelength segment with a cable.
Because the cable is connected to the point of maximum potential, where the reactance of the antenna is much more active.
Therefore, to connect a half-wave dipole to the cable, a resistance transformer is used.
On the one hand, this is an inconvenience, on the other hand, it gives some significant advantages.
Firstly, the cable with the antenna is galvanically isolated, the static does not get into the cable.
Secondly, because at the transformer output minimum current and maximum potential, the lower end of the output winding can be connected to a grounded mast.
This solves the problem of lightning protection completely.
Thirdly, counterweights are not needed here, because all current flows inside the half-wave segment. Even if part of the current flows to the mast, this will not affect the balancing.
The radiation pattern is already sufficiently compressed in the horizontal plane, but it can be made even narrower.
For this, 5/8 lambda antennas are used.
As the name implies, the length of their pin is 5/8 of the wavelength.
A strange number, why not ¾ lambda, which would be quite logical. We will deal with this further.
As we already know, it makes no sense to make the antenna sheet longer than half the wave,
because the direction of the current changes every half period, every half wave in the canvas.
However, in 5/8 lambda antennas, the sheet is indeed more than half the wavelength. What does this lead to?
The radiation pattern becomes narrower, but it is elevated above the horizon, by 15 degrees.
It is precisely because the lower part of the canvas contains an anti-directional current.
But at the maximum of the radiation pattern gain is the largest in comparison with all previous GP antennas.
Because the antenna length is more than half the wave, but less than ¾, then the antenna has capacitive resistance.
To neutralize capacitance, as I already explained in the video about matching elongated and shortened antennas,
we use matching inductance, the inductance of which is equal in magnitude to the capacitance of the pin.
Because the inductance itself contains a wire of a certain length,
then taking into account the total length of the line, it tends to ¾ and the active resistance to the coveted 50 ohms,
which allows the antenna to be matched to the cable without additional devices.
Moreover, by changing the length of the inductance wire, you can change the length of the section with countercurrent in the rod,
thereby controlling the antenna pattern.
If at the inductance input we don’t get the coveted 50 or even 75 Ohm, then we have to use an LC matching circuit.
The cable sheath is grounded directly, if possible.
Otherwise, counterweights of just under ¼ wavelength are used as usual.
In addition to a simple option, you can meet the option of connecting a 5/8 lambda antenna through a transformer connection.
The cable sheath is connected to the lower end of the inductance, and balances are also connected there.
A pin is connected to the upper end of the inductance.
And the central core of the cable is connected to one of the turns inside the inductance.
This option is more complicated, and requires more careful tuning to resonance.
In principle, the 5/8 antenna requires more fine tuning than other GP antennas, but due to the resonance and narrow beam pattern we get a cleaner reception.
When changing the frequency of operation, it is necessary to change the length of the pin
so that it again corresponds to 5/8 lambda.
The antenna is narrow-band, but at least it is tuned by changing the length of the pin, which is impossible for example for collinear antennas that cannot be tuned in frequency.
It is possible to make a cloth of an antenna both from metal tubes, for example, to make a telescopic structure, or from a wire stretched over a dielectric mast.
Copper and aluminum will work equally well.
The inductance material should be selected taking into account the maximum currents flowing through it.
We draw conclusions:
1. The 5/8 lambda antenna is a GP antenna with a pin at about 5/8 wavelength
2. The radiation pattern of antenna 5/8 lambda is the narrowest among the GP antennas and is raised above the horizon by about 15 degrees
3. The 5/8 lambda antenna is narrowband and requires tuning to resonance
4. Due to the narrow band and flat beam, it has the largest gain among GP antennas
That's all for today, dear friends. I hope I answered all the questions that related to this type of antenna.
Other questions and suggestions for the next videos leave in the comments. Share the video if you think it was useful.
Good luck to all!