Resonant Slot Antenna

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Slot Antenna Resonant Frequency Formula
Resonant Slot Antenna Booster

The holes are not resonant at 10 GHz, so they do not radiate as readily as resonant slots. The result is a high 'Q' sharply tuned antenna. The radiating elements on opposite sides of the waveguide are further apart, so the omnidirectionality is not as good.
Figure 28.3: A cavity-backed slot antenna radiates well because when the small dipole radiates close to the resonant frequency of the cavity, the eld strength is strong inside the cavity, and hence around the slot (courtesy of antenna-theory.com). Slot antenna is a simple antenna to make 132. To improve the radiation e ciency of.
One slot is formed at the bottom side of this cavity as a feeding slot to excite two orthogonal resonant modes. The slot antenna is then constituted at the opposite side of the feeding slot to radiate electromagnetic wave.
The equivalent circuit of a cavity-backed slot antenna is shown in Figure 8-7b; the shunt conductance is the radiation conductance of the slot. The conductance of the cavity-backed resonant half-wave slot is half the open slot, free to radiate on both sides. That is, the shunt resistance is at least 800 rather than 400 Ω.14–16.

Slot Antenna

Figure 1: The length of a slot determines the resonant frequency, the width of the slit determines the broad bandwidth of the slot radiator.

Slot Antenna

Slot radiators orslot antennas are antennas that are used in the frequency range from about 300 MHz to 25 GHz. They are often used in navigation radar usually as an array fed by a waveguide. But also older large phased array antennas used the principle because the slot radiators are a very inexpensive way for frequency scanning arrays. Slot antennas are an about λ/2 elongated slot, cut in a conductive plate (Consider an infinite conducting sheet), and excited in the center. This slot behaves according to Babinet's principle as resonant radiator. Jacques Babinet (1794 - 1872) was a French physicist and mathematician, formulated the theorem that similar diffraction patterns are produced by two complementary screens (Babinet's principle). This principle relates the radiated fields and impedance of an aperture or slot antenna to that of the field of a dipole antenna. The polarization of a slot antenna is linear. The fields of the slot antenna are almost the same as the dipole antenna, but the field’s components are interchanged: a vertical slot has got an horizontal electric field; and the vertical dipole has got a vertical electrical field.

The impedance of the slot antenna (Z_s) is related to the impedance of its complementary dipole antenna (Z_d) by the relation:

Z_d · Z_s = η²/4

where

Z_s = impedance of the slot antenna
Z_d = impedance of its dual antenna
η = intrinsic impedance of free space.

(1)

It follows for Z_s = 485 Ω.

The band width of a narrow rectangular slot is equal to that of the related dipole, and is equal to half the bandwidth of a cylindrical dipole with a diameter equal to the slot width. Figure 2 shows slot antennas different from the rectangular shape that increasing the bandwidth of the slot antenna.

Figure 2: Various broadband slot antenna.

Although the theory requires an infinite spread conductive surface, the deviation from the theoretical value is small when the surface is greater than the square of the wavelength. The feeding of the slot antenna can be done with ordinary two-wire line. The impedance is dependent on the feeding point, as in a dipole. The value of 485 Ω applies only to a feeding point at the center. A shift of the feed point from the center to the edge steadily decreases the impedance.

The application of slot antennas can be versatile. They can replace dipoles e.g. if it is required a polarization perpendicular to the longitudinal extension of the radiator. If a dipole is used for feeding of a parabolic antenna to generate a vertically orientated but horizontally polarized fan beam, then this dipole must be orientated horizontally. This would mean that the edge surfaces of the parabolic reflector will not be sufficiently illuminated, but a lot of energy above and below the reflector would be lost. In addition, the length of the dipole is extended in a plane, in which is demanding a point like source of radiation for the focus of the parabolic reflector. If this dipole is replaced by a slot antenna, in this case don't appear these disadvantages.

Slots in waveguides

Figure 3: Various slot arrangements in a waveguide.

Slot antennas in waveguides provide an economical way of the design of antenna arrays. The position, shape and orientation of the slots will determine how (or if) they radiate. Figure 3 shows a rectangular waveguide with a drawn with red lines snapshot of the schematic current distribution in the waveguide walls. If slots are cut into the walls, so the current flow is affected more or less depending on the location of the slot. If the slots are sufficiently narrow so the slots B and C (Fig. 3) have little influence on the current distribution. These two slots radiate not (or very little). The slots A and D represent barriers to the current flow. Thus, this current flow acts as an excitation system for the slot, this one acts as radiator. Since the wave in the waveguide moves forward, these drawn lines migrate in the direction of propagation. The slot gets one always alternating voltage potential at its slot edges (depending on the frequency in the waveguide). The power that the slot radiates can be altered by moving the slots closer or farther from the edge. The slots A and D (as drawn in Figure 3) have the strongest coupling to the RF energy transported in the waveguide. In order to reduce this coupling, for example the slot A could be moved closer to one of the shorter waveguide walls. Rotating of the slots would have a the same effect (an angle between the orientations of A and B or C and D). The coupling of this rotated slot ist a factor of about sin² of the rotating angle θ.

Slotted Waveguide Antennas

Slot Antenna Resonant Frequency Formula

Figure 4: Basic geometry of a slotted waveguide antenna (The slot radiators are on the wider wall of the rectangular waveguide.)

Several slot radiators in a waveguide form a group antenna. The waveguide is used as the transmission line to feed the elements. In order for radiate in the correct phase, all single slots must be cutted in the distance of the wavelength, that is valid for the interior of the waveguide. This wavelength differs from the wavelength in free space and is a function of the wider side a of a rectangular waveguide. Usually this wavelength is calculated for the TE₁₀ mode by:

Resonant Slot Antenna Booster

a = length of the wider side of the rectangular waveguides
λ_h = “guided” wavelength (within the waveguide)
λ = wavelength in free space(2)

Figure 5: Basic geometry of a slotted waveguide antenna with rotated slot antennas on the narrower wall.

The wavelength within the waveguide is longer than in free space. The distance of the slot radiators in the group is set at this wavelength to a value that is slightly larger than the wavelength λ in the free space. The number and the size of the sidelobes is affected so unfavorably. The slots are often attached to the left and right eccentrically (with reduced coupling). If mounted on the narrow side of the waveguide, it may happen that the length for the resonant slot radiator is shorter than the wall. In this case, the slot can be also guided around the corners, it then lies also slightly on the A-side of the waveguide. In practice, these slots are all covered with a thin insulating material (for the protection of the interior) of the waveguide. This material may not be hygroscopic and must be protected from weather conditions.

A single narrow slot radiator can also work on frequencies ±5 … ±10% besides its resonance frequency. For array antennas, this is not possible so easily. Such a group antenna is fixed strongly to a single frequency, which is determined by the spacing of exactly λ_h, and for which the antenna has been optimized. If the frequency is changed, then these distances not correct, the performance of the antenna decreases. The phase difference arising between the antenna elements are added to the whole length of the antenna to values that can no longer be tolerated. This antenna begins to “squint”, that is, the antenna pattern points in a different direction from the optical center axis. This effect can also be exploited to achieve an electronic pivoting of the antenna beam as a function of change of the transmission frequency.

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Slot Antennas

Attached below is an Excel spreadsheet of the dimensions we used to make the slots that are in the current avionics module.

Found under 'slot antenna' in google:

Very good technical article on the various kinds of slot antennas: http://www.shively.com/choices.html
Designing a slot antenna: http://reality.sgi.com/192.82.208.21/byee_engr/slot_ant.html
An actual measured slot antenna: http://www.qsl.net/kd2bd/slot.html

Found under 'planar slot antenna':

Pictures of a Planar Slot Antenna: http://www.acusd.edu/~ekim/ant_proj/
PDF on designing a planar slot antenna (but no pictures): http://www.imec.be/mcm/pdf/soliman_1999_csp_motl.pdf

Notes: Should we be considering planar slot antennas? Or just slot antennas? It looks like we need some kind of inner conductor, which would be a show stopper It's extremely cool that a single slot gives you omnidirectional coverage Is the outer diameter of the tube related to frequency? If so, another show stopper

You can beam steer! You can beam steer! That's SO cool. But you need multiple slots.

slot.txt - alford slot antennas

~ 200 Ohm impedance, so needs a 4 to 1 balun for matching to 50 Ohm output of transmitter. May be somewhat inductive, so need small cap to tweek for best match. (I think I have suitable caps) (the above relates to a swedish site)

Slot antennas radiate in a manner similar to a dipole, although unlike a conventional dipole, the current circulates (sort of) in a slot antenna. This confirms, as far as I can tell, the thought that a slot antenna is a magnetic radiator rather than an electrostatic radiator, i.e. the B field is used rather than the E field.

Can achieve 50 Ohms without matching circuit if full wave. If less than full wave, then need either off center for half wave, or matching capacitor if less than half wave.

Below are full wavelength figures for the three main bands of interest to us for L.V. #2:

3*10^8 m/s / 900 mhz = ~33 cm = ~13 inches

3*10^8 m/s / 1500 mhz = ~20 cm = ~7.9 inches (futher study shows that the listed frequency is a little low, the actual one is about 1570 MHz. This means the antenna will be shorter than listed.

3*10^8 m/s / 2400 mhz = ~13 cm = ~4.92 inches

Note that these figures are aproximate as to exact length, i.e. if we construct slot antennas we will have to experiment as to length, width, and filling. I have a hunch that filling the slot with fiberglass so as to avoid turbulent drag will not affect the resonant freqency to any substantial degree. (Due to the slot antenna being a magnetic radiator rather than a capacitative one.) If I am wrong in this, then the slot length will be less by a factor of about

sqrt(2.4) = 1.55

which is a significant shortening of the length of the slot. (This presumes that the shortening will behave in a manner much like that of a transmission line, where the square root of the dielectric constant of the insulator is the factor by which the velocity of the traveling wave is reduced.)

In some ways, I think the half-wave slot will work out the best for us, as the off-center location of the RF connection will allow connecting the input such that there will be minimum drag due to minimum horizontal length of the transmission line. Drag is bad, no? Were we to construct full wave antennas, we would need to put the coax connection in the center of the slot which would need a horizontal run of said coax. This might also result in an imbalance and/or increased SWR due to magnetic coupling of the surounding metal into the shield of the Coax.

As far as I can tell, we should be able to construct slot antennas for those frequencies of interest out of material similar in thickness to what the fins are made out of, dial them in, and then transfer them directly to the fins. They **Should** work the same. Note that the two asterisks each side of 'Should' indicates that I hope this is true. The reading I have done implies so, but I have never built a slot antenna and so am hedging strenuously.

?DennisYoung - 23 Aug 2001

picture of full wave slot antenna

half wave slot antenna. Note feed at one end.

less than 1/2 wave, requires series matching cap.

I have some more data on slot antennas, this time of the cylindrical species. The vast majority of this inform- ation is in the form of 2 mounstrous (bigger than huge!) drawings.

The first of these is at: http://www.ece.pdx.edu/~dy/slot2.ps

This drawing, as one may discern from the ending, is a postscript file, and requires a postscript viewer. I suggest GV, which is available for the most commonly used platforms. It works well both on Windows (comments censored) and of course variants of Unix. The file which you will (hopefully) find below is a much less detailed version which will, one hopes, whet your apetite for the real article at the above web address.

?DennisYoung - 02 Sep 2001

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