How to Reduce the Effects of High SWR for Radio Transmitters
HTML-код
- Опубликовано: 1 авг 2024
- When using a radio transmitter for CB, amateur radio or for professional radio communications use people strive to get the best SWR in the antenna feeder. But it's not always possible to get the perfect 1:1 SWR reading. In this video we look at the effects of high levels of SWR - Standing Wave Ratio (or VSWR - it's almost the same thing as it looks at the voltages associated with SWR).
We also take a look at the methods of reducing SWR and how far to go as often the last 10% of improvement can take 90% of the time and is it worth it.
The video first takes a look at what SWR is and how it arises. In this way it is easier to understand how SWR can affect transmitters and the resulting effects on the transmitted signal.
There are some really useful tips that we provide to get the best performance from any radio communications transmitting system whether it is for two way radio communications, amateur radio communications, CB or any other form of radio communications transmission.
First try to get the best match between any coaxial feeder and the antenna system as this will ensure that the maximum amount of power is transferred and the minimum level is reflected back along the coax feeder with all the resultant losses that may entail.
Next, use an antenna tuner at the output of the transmitter - this though is more applicable for HF radio stations. The antenna tuner will ensure that the transmitter sees the best impedance match and the PA isn't subject to high levels of SWR. By doing this, it can prevent possible damage to the PA output devices or the reduction in power level when the protection circuitry starts to act.
The maximum level of SWR that many transmitters are specified to tolerate is 3:1, so keep well below this by using an ATU or improving the matching at the antenna itself . Keeping the SWR below a maximum of 2:1 or preferably lower is a much better aiming point. But once the transmitter is happy there is little benefit in getting the SWR down from, say 1.3 to 1 to 1.1 to 1, so don't worry too much about those last little improvements.
Don't totally rely on the PA protection as it is meant to be a last ditch circuit. Most of these circuits work very well, but very occasionally small time delays can mean that it doesn't kick in fast enough and damage may still result. It's not worth taking the risk and it is always best to sort any known problems first.
Finally, the use of high quality coaxial feeder is always a good idea as it will reduce the losses in the feeder and it will mean that more of the transmitter power reaches the antenna, although it will mean that the SWR level seen by the transmitter for any mismatch at the antenna will be high, but that will be the subject of another video.
More information about SWR and how to reduce the effects of high SWR levels: www.electronics-notes.com/art...
Check out the Electronics Notes website: www.electronics-notes.com
Please don't forget to subscribe to our RUclips channel: ruclips.net/user/electronicsnotes?... 
Наука
Thank you for this information - very well presented.
But my understanding or SWR ( the amount of power reflected back to the TX) suggests that the higher the SWR, the less of the original TX power is actually being radiated.
So, if you have a high SWR (say 6:1), and you use an ATU to bring it down to say 1.1:1, all you are doing is using the LC circuits in the ATU to absorb the reflected power before it can get to the TX and damage the finals.
The ATU may make the TX see an SWR of 1.1:1, but if you put an SWR meter between the ATU and the antenna, you will still see an SWR reading of 6:1 (in the case above).
So the ATU basically 'protects' the finals in the TX, but there is still significant loss of transmitted signal at the antenna - am I correct?
This is a good point. However if the ATU provides a good match between the transmitter and the overall antenna system including the feeder then in turn it will cause the power to be reflected back towards the radiating element of the antenna where it can be radiated. HF antennas with open wire feeders can operate very efficiently with high levels of SWR. The issue is that each time the power is reflected there will be power loss. Coax cable gives good screening, etc but introduces a relatively high level of loss. Open wire feeders when used correctly have much lower levels of loss snd can operate effectively in this mode.
I hope this helps.
@@ElectronicsNotes I understand your point about 'high swr being reflected will be radiated'., but in the case where you use an ATU to reduce the swr that the TX sees, and thus protect the final amps on the TX from overheating, the 'absorbed' rf energy just goes into heating the LC circuits in the ATU, and does not make it back to the antenna to be radiated.
The scenario you describe is the case where there is no ATU to artificially 'resonate' the antenna-feedline by absorbing the reflected power in the LC circuits of the ATU.
@@ElectronicsNotes LOL No.
This is incorrect. The ATU does not absorb reflected power, but reflects it with a phase shift. The main function of an ATU is to create an impedance match between the transmitter output and feedline. The PA has an output circuit which is actually an impedance transformer, so without a tuner any reflected power is reflected - see Poynting's Vector Rule. If the output circuit "sees" a difficult load, heat can develop in the finals, or high voltage across the finals will punch a hole. Some of these wild ideas need to be measured up against fundamental truths such as Ohm's Law. Most new solid state radios have a power foldback circuit to protect those expensive finals. Placing a tuner at the feedpoint is generally not preferable. Signal loss is not directly proportional to VSWR. At HF frequencies, loss in coax is not really significant. See "Reflections" by Walter Maxwell. This subject needs to include more qualifiers, such as - frequencies of use, length and type of feeder, etc. This is possibly the most misunderstood subject in radio, and abounds with myths and incorrect assumptions, and people simply repeating these incorrect facts without researching the science. Losses happen almost entirely in the feedline, but as mentioned in the video, power wavefronts undergo numerous reflections wherever there is an impedance inconsistency, but large portions of those waves will be radiated by the antenna. VSWR has no bearing whatsoever on the efficiency of the antenna. And the antenna does not need to be resonant to radiate well. See the ARRL Antenna book. The video is correct in stating that balanced feedline is virtually lossless, and very high feedpoint VSWRs do not contribute to losses, so a basic dipole antenna fed with balanced feedline (on HF for example) can work multiple bands quite well, with a few qualifiers.
@@mikeymouse4629 Reflected power does not get absorbed in the ATU, but gets reflected (and now is in phase with forward power producing reflection gain) because of the impedance mismatch at the output of the ATU. The term resonance simply means that voltage and current are in phase, so there is no reactance and it appears to have DC resistance. For example - a half wave dipole at resonance has a feedpoint impedance of about 73 ohms resistive. If fed with 50 ohm coax, there will be a VSWR of about 1.5:1, and therefore some reflected power. An ATU will not change this situation at all. The best way to remedy this is to use 75 ohm coax and put up with a small mismatch at the transmiiter end, which is insignificant, but will reduce the feedline losses quite a bit. This would be useful if working QRP where every milliwatt matters.
So the idea then, is that standing waves are NOT desirable on the feeder - coax or other type - whereas on the other hand, we WANT standing waves on the ANTENNA, as this phenomenon facilitates the radio waves detaching from it and then moving out into space. Ball park?
Standing waves in themselves are not a problem. The issues arise because they can cause high voltages and current levels - this can be bad for a transmitter if the mismatch and high voltages / currents are seen by the transmitter (the levels are unlikely to cause problems with a feeder, but this could be possible if it is running very close to its limits). Also the power that is reflected backwards and forwards when standing waves are present suffer losses. This means that the signal can be attenuated. However soem antennas use an open wire feeder where feeder losses are very low and the standing waves are perfectly acceptable.
I hope this helps.
There are standing waves throughout the entire antenna system. There are two mian types of losses. One is because of I squared R losses due to feedline current, and there will be some dielectric loss too, not to mention losses from the radiated field of the antenna to surrounding structures, ground etc. The ideal is to have every drop of energy delivered to the antenna being radiated out into space.
Ok