Radio Frequency (RF) & Wireless Design

In order for any Radio Frequency, or for that matter, high speed circuitry to work properly all of the major component pieces *must be* balanced (equal to another). This follows one of the most basic rules in electrical engineering called the "Maximum Power Transfer Theorem"  (https://en.wikipedia.org/wiki/Maximum_power_transfer_theorem) in which the SOURCE supplying the DC or AC energy must be equal to the LOAD to make sure that the maximum TRANSFER OF POWER (not maximum efficiency !) is achieved.

In LOW frequency circuits are VOLTAGE driven, whereas HIGH frequency are POWER driven. This is because all of the elements in the circuit, including the interconnects and the inter-relationships of nearby circuits, must be taken into account. Wires are no longer 'dumb' but are inductors and being nearby other wires creates capacitive coupling. These are known as PARASITIC elements (https://en.wikipedia.org/wiki/Parasitic_element_(electrical_networks))

When the subsections (transmission lines, coax, wires, source impedance, output impedance, input impedance, load impedance) are not balanced, the UNBALANCED sections cannot absorb all the energy and the remaining energy is REFLECTED back. This is true regardless which direction the energy is coming from. The RATIO of the source to load is called the REFLECTION COEFFICIENT and the magnitude of the reflection coefficients(https://en.wikipedia.org/wiki/Reflection_coefficient) will yield the STANDING WAVE RATIO or SWR which is what most transceivers use to monitor the power transfer. (https://en.wikipedia.org/wiki/Standing_wave_ratio)

The problem is that each of the elements are frequency dependent and have an exact impedance for an exact frequency. So when the frequency changes the element impedances will change and track the frequency. Moreover, there are MAGNETIC effects from inductances that will radiate and must be contained or will influence other circuits. This is part of Electro-Magnetic Compatibility or EMC and is necessary for all electrical and electronic devices for radiated and immunity (or susceptibility) emission compliance (https://en.wikipedia.org/wiki/Electromagnetic_compatibility).

In order to BALANCE the elements the designer must MATCH the SOURCE and LOAD endpoints. Typically this is done thru the transmission line with other passive or active devices adding or subtracting REACTANCES along the way to equalize the line (https://en.wikipedia.org/wiki/Electrical_reactance).

Sometimes a passive device is used to balance a SINGLE ENDED LINE such as a coax (https://en.wikipedia.org/wiki/Single-ended_signaling) to a DIFFERENTIAL LINE (https://en.wikipedia.org/wiki/Differential_signaling) this device is called a BALUN for Balanced-UnBalanced (https://en.wikipedia.org/wiki/Balun). There are also Unbalanced-Unbalanced or UNUNs used to magnetically couple and isolate circuits.

Differential circuits offer the best SIGNAL-TO-NOISE RATIO (https://en.wikipedia.org/wiki/Signal-to-noise_ratio) however are typically used internally within the RF enclosure, whereas RF interconnections outside of the enclosure are single-ended and typically coaxial cable or coax, or an RF waveguide, which are used in the microwave bands.

So when designing, building, and testing an RF circuit, it is *critical* that all elements must balance out the endpoints or excessive losses will occur (note: there will always be DC losses, but phase losses is where other losses occur).

One final note is that most systems are designed around an optimized CENTER FREQUENCY (https://en.wikipedia.org/wiki/Center_frequency) and that there will be losses as the frequency moves or deviates from this center frequency. However, the good designer will take this into account and design for the best design trade-offs regardless if it is a Receiver, Transmitter, or Transceiver.

NARROW BAND (https://en.wikipedia.org/wiki/Narrowband),

WIDEBAND (https://en.wikipedia.org/wiki/Wideband),

BROADBAND (https://en.wikipedia.org/wiki/Broadband)

or ULTRA WIDEBAND (https://en.wikipedia.org/wiki/Ultra-wideband)