# SWR vs Field Strength?

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by W5DXP, Dec 12, 2019.

1. ### KT1FHam MemberQRZ Page

I guess it all ends up as heat or unwanted radiation.

2. ### BILLYBOB560QRZ Member

If a Harmonic Filter is present, the harmonic energy is simply reflected back to it's source (0 dB return loss.) The source sees a pure Reactance at the harmonic frequencies, so there's no real power contained in the harmonics.

The energy STORED in a reactance in the first quarter-cycle is RETURNED in the next quarter cycle. There's thus ZERO net energy contained in the harmonics when a filter is present: no heat, no radiation.

BB

Last edited: Dec 14, 2019
3. ### AI3VHam MemberQRZ Page

Watch out there BILLYBOB560, you are going to ruffle a lot of feathers of the "add a choke"( hmm, choke, a coil of wire, i.e. reactance) crowd.

Rege

5. ### W5DXPHam MemberQRZ Page

Question: When my IC-756PRO says it is putting out 50 watts and the SWR is 1.5:1, is that 50 watts generated or 50 watts forward?

6. ### SM0AOMHam MemberQRZ Page

This was explained to me in the Engineering Electron Physics course back in the 70s, as part of the material about tuned power amplifiers.

The plate current pulse contains a lot of harmonic components, more the shorter the conduction angle becomes.

In the tuned Class-C amplifier, only the fundamental component is transmitted through the tank circuit, and all other are reflected back towards the tube.

As the conduction angle is short, only a small fraction of the harmonic currents can be dissipated in the dynamic impedance of the tube, and the rest will change the operating point of the tube, so an equilibrium forms between DC power converted into plate current pulses, fundamental current transmitted through the tank circuit, and harmonic currents reflected back, all according to the principles of conservation of energy.

This mechanism is partly the reason why very high efficiency amplification is possible with an amplifier operating in hard Class C.

73/
Karl-Arne
SM0AOM

KM1H, KX4O and K0UO like this.
7. ### SM0XHJHam MemberQRZ Page

Not necessarily. When you design a common mode filter (choke), you aim for a high resistive impedance at the operating frequency. That's why you use materials such as Fair-rite 31 or Amidon 43 for chokes, materials that would be terrible in a balun (transfomer). Combinations of material, core size and turns where Rs is larger than Xs is clearly marked in G3TXQ's (SK) excellent charts.
But just throwing a random choke (or coil of coax) at a common mode current problem isn't likely to fix anything, you are right about that. There isn't one combination of material and turns that will give a high resistive impedance over the whole HF spectrum.

N0TZU, KX4O and K0UO like this.
8. ### W9XMTHam MemberQRZ Page

However, fairly recent design improvements of AM broadcast transmitters go beyond the operating class of the r-f amplifier(s) to produce very high efficiency for the entire a-c mains input power to the transmitter to produce its rated, fully-modulated r-f output power.

For example, Harris Corporation designed and patented a method of synthesizing the complete, amplitude-modulated, r-f output waveform of the transmitter by turning on or off various combinations of r-f modules of different power outputs. The incoming program audio (at about +10 dBmW) is converted to a digital form in a "modulation encoder," which provides the logic for controlling the active state of each r-f module — whose outputs then are summed together to produce an a-m waveform with very low harmonic, IM and TIM distortion.

This digital synthesis approach eliminates the need for a separate, large, and inefficient high-level modulator stage.

Below is a block diagram of this approach, which also shows the total a-c input power to r-f output power for a 10 kW version of this transmitter (>86% typical, in this case)

AI3V and KX4O like this.
9. ### SM0AOMHam MemberQRZ Page

The most probable reason for what W5DXP observed is that the tuner
presented a more favourable load-line to the power amplifier at 1.4:1 SWR than at 1:1.

It is indeed possible to tune the antenna + tuner combination for a good SWR but having less antenna current, especially in automatic tuners.

This is usually prevented by both measuring both the phase and magnitude of the reflection coefficient and at the same time the magnitude of the antenna current, trying to converge at a point where the reflection coefficient is as small as possible and the antenna current is as large as possible.

73/
Karl-Arne
SM0AOM

KD0CAC and KX4O like this.
10. ### AI3VHam MemberQRZ Page

Except the coil wound on materal 31 or 43 is almost perfectly inductive.

Likewise the often posted advice to wind several turns of coax around a pvc former.

Rege