What is an Antenna?

IEEE Standard for Definitions of Terms for Antennas is a document that defines almost all antenna radiator terminology.

Interestingly even the IEEE document does not define what an antenna is.

Here are 3 definitions for an antenna from different perspective.

Antenna is a transitional device, transducer that converts:

  1. Guided electromagnetic wave from a transmission line to plane wave in the free space, and vice versa (Electromagnetic wave view)
  2. Electrons motion to photons (Physics view)
  3. Transmission line impedance to intrinsic impedance of the free space (Circuits view)

Although, this question appears basic yet it has significant implications in designing and developing antenna as well as documenting related technical content and patent.

It gets even trickier to define where that air interface is.

That could explain why AIEE, IRE, and IEEE have avoided the antenna definition since 1884.

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Why Antenna Radiates Electromagnetic Waves?

The electrical current is flow of electron [Coulomb per second] in a wire or conductor.

That electrical current represents the velocity of electron movement in the wire.

Velocity is vector quantity and has magnitude and direction, so does electrical current.

When there is constant current in a wire, it creates magnetic field around that wire and right-hand rule applies to determine the magnetic field.

The magnetic field around the wire or conductor is static or fixed, just like the magnetic field around a magnet.

You can experience the magnetic field around the magnet by putting some iron filings around the magnet to see direction of magnetic field of magnet.

If the electrons in the wire accelerate/decelerate (because of the shape of the wire or conductor), the magnetic field responds to that electron acceleration/deceleration.

By disturbing the magnetic field, the electric field is also disturbed, therefore the electromagnetic wave is generated.

James Clark Maxwell was the Scottish physicist and mathematician who discovered the missing term in Ampere’s Law that show the coupling of magnetic and electric field.

While James Clark Maxwell mathematically proved the coupling of electric and magnetic fields and time rate change of one cause the other, which effectively disturbs the electromagnetic fields, John Daniel Kraus intuitive descriptions of electron acceleration requirement for electromagnetic wave helps to design and develop antenna.

John Daniel Kraus explanation of electromagnetic radiation can be utilized to synthesize radiation pattern.

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Multi Band Antenna

BT and WiFi share 2.4 GHz ISM band.  In addition, WiFi technology has 5 and 6 GHz bands.

A router or handheld device which supports both BT and WiFi would need to support multi band between 2 to 7 GHz.  Therefore, designing antenna aperture and radio front end that support multi bands is highly desirable.

In the case of wearable devices, the form factor of the mobile device is of prime concern.

In addition, the material and proximity in close space becomes challenging for typical antenna designer.

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Triple Bands VHF and UHF Antennas

VHF and UHF mobile handheld (Walkie Talkie) radios are utilized by many.

Due to operating wavelength, they have range from few to tens of kilometers depending on the environment and condition.

The mobility requires compact, low weight, low power radio.  Antenna design Triple Bands topology is highly desired.

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Antenna Geometry Scaling

Antenna physical dimensions have always been challenging.

In fact, number one reason that radio frequencies, RF are used for radio communication is the antenna size for handheld devices.

That’s in order for our mobile radio or mobile phone to wirelessly connect, the radio frequencies must be utilized to physically realize an antenna.

Human audio frequency is anywhere from 20 – 20 KHz, which prohibits any handheld device antenna.

This is the history of radio frequency.

Now let’s fast forward to future, where 100 GHz to multi-THz radio are sought.

The outlook for Microwave or Nanowave signals may require another challenge yet in opposite to miniaturization.

At these frequencies it may be required and challenging to physically realize a much larger antenna than the operating wavelength.

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Antenna Radiation Efficiency vs. Resonance Frequency

Antenna optimum radiation occurs at or near its resonance frequency.

Resonance frequency is the frequency where the antenna impedance is real.

In general antenna impedance is in the form of Z(f) = R(f) + j X(f), and at the resonance frequency, f0, the reactance part is Zero, X(f0) = 0.

R(f) is the desired radiation resistance.  The larger R(f) is relative to X(f), the more EM radiation occurs, and vice versa.

When antenna is not near its resonance frequency, X(f) is larger than R(f), therefore most of power delivered to the antenna is not radiated and the antenna is inefficient radiator.

Notice, antenna impedance is frequency dependent, therefor outside of resonance, X(f) is non-zero, could be negative (capacitive) or positive (inductive).

For instance in case of dipole antenna, the dipole resonance frequency is near half wavelength; hence “half wavelength” dipoles are popular linear dipole antenna geometry.

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Why HF Dipole Antennas are Hollowed and Tapered?

As the wavelength increases, i.e. frequency decreases, the length of dipole increases.

High Frequency, HF band stands for 3 – 30 MHz or 100 to 10 m wavelength.

At these wavelengths the weight of a solid material dipole is considerable.

From antenna function perspective, the electrons travel at the skin of dipole, i.e. skin depth.  Therefore, the central part of the dipole is not contributing to the electrical performance of the antenna or negligible.

By designing a hollow dipole, the weight and the material cost of the dipole is reduced without an impact to its electrical performance.

The dipole mounting and installation are also easier.

Tapering dipole along its length increases dipole bandwidth.

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Broadband vs. Multi Band Antenna

Broadband antenna radiates over wide continuous bandwidth with acceptable radiation performance.

In general, antennas are reciprocal device, i.e. their radiation performance is the same regardless whether they are used as receiver or transmitter antenna.

Therefore, any statement about antenna can be taken as either receiver or transmitter antenna performance.

Multi Band antennas are radiator that have acceptable radiation performance over discrete (non-continuous) bands.

It is fallacy to assume that broadband antennas are preferred over multi band antenna.

Think of multi band antennas as frequency selective filter outside of their operating radiating frequency inter-band.  Therefore, technically, multi band antennas have two objectives, first to radiate in the bands of interest.  Second, they filter out of inter band or undesired signals.

Product definition is the most important scoping part of any consumer electronics and comprised of Architecture, Antenna /ASIC /Algorithm of all systems or applications.

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Meander Antenna

Meander antenna topology is utilized to reduce the area or size of monopole antenna.

Meander design enables 2D implementation in printed board and integration with other electronics or 3D high power capabilities.

Wideband Meander antenna can be designed using various techniques.

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Absolute vs. Fractional Bandwidth

Electrical component operating Bandwidth, BW is of great concern when you design any system, SATCOM, Terrestrial, radar, or WiFi.

It is important to realize that operating BW in general is device or even vendor dependent parameter.

Therefore, it is prudent to read datasheets carefully and if not defined, ask appropriate stakeholders about the defined BW for that datasheet.

For instance, in frequency selective filter typically definition for BW is -3 dB for its insertion loss.

That -3 dB BW definition is typically applied for frequency selective filter various topologies, except for Tchebychev filter design.

For Tchebychev filter topology, equal ripple BW is applied which is less than -3 dB.

Here are some various BW definitions, -3 dB, -20 dB, or 99%.

As an electrical component designer, the absolute BW is not of main interest.

For instance, an antenna operating in HF band is supposed to operate between 3 – 30 MHz, or 27 MHz of absolute BW.  That is not an easy design.

To better understand the reason behind this statement, let’s think about half wavelength dipole design as an example.

The wavelength at 3 MHz is 100 m, whereas the wavelength at 30 MHz is 10 m.

The half wavelength dipole antenna at 3 MHz is roughly 50 m long, whereas at 30 MHz is 5 m long, an order of magnitude difference.

And it suffices to say that the half wavelength antenna behavior would drastically changes over this 27 MHz.

Now, if the operating frequency of that dipole antenna is at 300 MHz and requires 27 MHz absolute BW, that is fairly straightforward design.

At 300 MHz, the wavelength is 1 m and 27 MHz absolute BW is less than 10% change or 0.1 wavelength.  Therefore, the antenna design is feasible and straightforward.

Regardless of the antenna topology, dipole or not, that the fraction BW is what should be considered.

In fact, the fractional BW should be considered even for any other electrical components beside antenna.  The same challenges are applicable.

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Why HF Dipole Antennas are Hollowed and Tapered?

As the wavelength increases, i.e. frequency decreases, the length of dipole increases.

High Frequency, HF band stands for 3 – 30 MHz or 100 to 10 m wavelength.

At these wavelengths the weight of a solid material dipole is considerable.

From antenna function perspective, the electrons travel at the skin of dipole, i.e. skin depth.  Therefore, the central part of the dipole is not contributing to the electrical performance of the antenna or negligible.

By designing a hollow dipole, the weight and the material cost of the dipole is reduced without an impact to its electrical performance.

The dipole mounting and installation are also easier.

Tapering dipole along its length increases dipole bandwidth.

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Bode-Fano Impedance Matching Criteria

Ultra Wide Band, UWB antenna or circuit are in high demand.

As the bandwidth, BW, increases, the Q factor, Q, decreases, therefore the return loss decreases.

Bode-Fano criteria puts an upper bound on the quality of impedance matching.

It states the relationship between BW and the return loss.

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Yagi-Uda Antenna

Legacy Terrestrial TV antenna is Yagi-Uda Antenna.

Yagi-Uda antenna is typically comprised of reflector, driver, and handful of director dipole elements, e.g., total of 5 – 7 elements.

The elements are placed within quarter of wavelength to create constructive fields in desired direction and destructive field in the undesired direction.

Yagi-Uda has moderate directive gain, e.g. 7 – 10 dBi, depending on number of directors.

Yagi-Uda antenna has advantage of antenna dipole array due to its additional gain over dipole, yet it is simple feeding network, similar to dipole.

Therefore, Yagi-Uda antenna is an elegant design for its simplicity yet effectiveness.

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Miniaturizing Dipole Antenna

Half wavelength dipole antenna is a typical dipole design for Omni directional transmit and/or receive applications.

The half wavelength dipole antenna resonates slightly below half wavelength, about 95% of half wavelength.

The half wavelength dipole antenna is an efficient radiator at or near its resonance frequency.

Below dipole resonance frequency, the dipole input impedance has capacitive component in addition to radiative component, aka radiation resistance.

In some applications, where the physical size of the half wavelength dipole is larger than the available space, dipole miniaturization is necessary and required.

One of the legacy techniques to electrically enlarge effective aperture of the dipole antenna, hence reducing its resonance frequency, is to add inductive load to the dipole.

The added inductor is selected to cancel the capacitive component of the dipole antenna input impedance below its resonance frequency, therefore the dipole antenna resonates even at lower frequency than the half-wavelength.

The placement of the inductor determines its degree of impact on the effective aperture of the dipole antenna.

Where the current is maximum has the most impact on effective aperture of the dipole antenna and resonance frequency.

The same technique can be applied to printed dipole antenna utilized in mobile devices, such as smart phone.

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Dipole Antenna Miniaturization using Material Coating

Printed dipole antenna miniaturization to fit in tight spaces such as smart phone is desirable and drives the success of many wireless products.

One of the ways to miniaturize printed dipole antenna is to coat it with some special material which enlarges effective aperture of the dipole antenna, therefore reducing its resonance frequency.

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