The statement is often made that:
VSWR will damage a HF ham transmitter, and the mechanism is that the ‘reflected power’ in a standing wave will be absorbed by the Power Amplifier (PA), increasing heat dissipation and damaging the PA.
There are two problems with this statement:
Anyone with even a modicum of experience knows that 1 is false, they have observed transmitters operating at VSWR>1 and damage has not occurred. So at best, it is an overstatement intended to frighten rather than inform.
Lets analyse 2 in some detail.
Narrowband HF transmitters can usually be analysed with sufficient accuracy by using a Frequency Domain model. A Frequency Domain model means analysis that considers a single frequency sinusoidal excitation. Anyone who talks of reactance, impedance, and VSWR is talking in the Frequency Domain, whether they know it or not.
So, what exactly is VSWR?
A wave propagating (traveling along) an RF transmission line (eg coax) has a natural ratio of voltage to current determined by the geometry of the line. This natural ratio of V/I is known as the Characteristic Impedance or Z0. If the line is terminated in an impedance equal to Z0, then the voltage and current in the traveling wave and that in the load reconcile. If the line termination is not equal to Z0, then there must be a wave traveling opposite direction (the reflected wave), a wave such that the sum of the voltages and currents for the forward and reflected waves reconciles with the ratio of V/I in the line termination. The combination of this reflected wave and the forward wave gives rise to a standing wave on the line, the ratio V/I is not equal to Z0, but changes all the way along the line. VSWR is the ratio of the voltage maximum to the voltage minimum (assuming the line is long enough to develop a full cycle of the standing wave).
So, if VSWR>1, and it almost always is, the ratio of V/I is not equal to Z0, and varies all the way along the line, so that the ratio of V/I at the transmitter terminals is not equal to Z0. So, in a practical situation using a transmitter intended for a nominal 50Ω load, a length of nominal 50Ω coax, and a less than perfect antenna, the load impedance (V/I) at the transmitter terminals is not exactly 50+j0Ω (meaning 50Ω of resistance and no reactance). But that is not necessarily a problem.
Practical transmitters are designed to work with a range of load impedances. That range is often specified as a nominal load impedance, and a tolerance in terms of VSWR, eg 50+j0Ω with VSWR<1.5. Such a specification encompasses a range of impedances, including purely resistive loads and combination of resistance and reactance.
A transmitter that does not have a reasonable tolerance of load impedance is not very practical. What does “reasonable tolerance of load impedance” mean? It means that:
Imagine how impractical it would be if for instance a mobile transmitter was damaged because VSWR increased as a result of a truck traveling alongside changing the antenna characteristics a little, or the antenna touching an overhead light fitting or other metal in a garage. There are countless applications where ham transmitters are required to operate into less than perfect loads.
In practice, a transmitter may deliver more or less than its rated power into a perfect load, and it may deliver more or less than that into a less than perfect load. Importantly also, the DC input to the transmitter may vary, so the efficiency may vary, and the amount of heat dissipated in the PA may vary… up or down. This is easily demonstrated by experiment, and the cases where dissipation is lower on some transmitters with some less than perfect loads questions whether the explanation at 2 is valid. If you delve into the design of a PA and explore its output, input and dissipation on different load impedances, you will find that the variation in power experienced is fully explained by conventional circuit theory considering the ration of V/I (or load impedance) at te transmitter terminals, and there is no conversion of reflected waves to heat in the PA. A steady state analysis of the PA fully explains the change in output power and dissipation in terms of the changed load impedance at the transmitter terminals.
Analysis of the explanation at 2 that shows that it is not an informed position.
So, if the explanation is bunk, the question remains, does VSWR damage ham transmitters?
If a transmitter is sensibly designed, it is designed to work with a range of load impedances. Operation outside of that range may present voltages or currents in parts of the circuit that are beyond safe operating limits for those parts, and may cause damage. For that reason, good designs incorporate protection schemes that limit operation on extreme load impedances to safe power levels. The most common scheme that is used in HF solid state transmitters that may be designed for a load of say 50Ω with VSWR<1.5, is that they automatically reduce power when the load impedance is outside that range. Operation outside the recommended load range is not usually guaranteed by the manufacturer, but experience shows that the protection schemes employed are usually very effective and the risk of damage is very low. So reliable and effective are these schemes that many mobile users tune screwdriver antennas just watching output power, relying entirely on the protection scheme to reduce output at high VSWR.
This article is not to say that high VSWR loads cannot result in transmitter damage, but to explain the effects of high VSWR loads in practical transmitters of good design, and to say that high VSWR loads do not necessarily cause damage, that damage is not caused by the mechanism explained at 2, and that damage to practical transmitters of good design due to high VSWR loads is unlikely.
Notwithstanding that, it makes good sense to supply the transmitter with a reliable antenna that is within manufacturer’s specifications, and to depend on the protection schemes for dealing with unusual circumstances (like keying up without an antenna connected, or keying up on an antenna that has developed a fault).
© Copyright: Owen Duffy 1995, 2021. All rights reserved. Disclaimer.