A Youtuber recently published some enlightenment entitled “The myth of SWR”.
He is obviously a disciple of the late Walt Maxwell and his re-re-reflection explanation:
Here’s a sample of what’s coming which may make your head explode. This is from antenna engineer Walt Maxwell, W2DU.
Now this video has been published just two weeks, it has:
- 13,693 views in two weeks;
- 549 likes;
- 228 comments mostly positive, but a few voices of reason.
So, lets analyse a little of the transcript:
…now here’s the truth… if your transmitter puts out 50 watts and the SWR is about 5.8 that means 25 watts is reflected back to the antenna tuner which every station needs.
That 25 watts is then bounced right back to the antenna which is bounced back to the tuner now this goes on and on like a ping pong game as long as the transmission lasts so if your transmitter is putting out 50 watts and the reflected power is 25 watts.
That means the forward power, the power going from the antenna tuner to the antenna is now 75 Watts, they combine 50 Watts plus the reflected power of 25 watts now this is known as reflection gain but that 25 watts is in reflected back again now the end result is that the full transmitter output power of 50 watts is delivered to the antenna it’s radiated…
Lets add some science, we will consider two different scopes in a lossless system:
- between the transmitter and ATU; and
- between the ATU and antenna.
Assume lossless 50 ohm coax is used, and the ATU is adjusted for VSWR=1, the VSWR meter is calibrated for 50Ω.
Between the transmitter and ATU
Pfwd=50W, VSWR on this section is 1 so Pref=0.
Between the ATU and antenna
VSWR stated as 5.83, so Pref/Pfwd=0.5.
Since the ATU is lossless, the net power towards the antenna MUST be 50W.
In this case where the instrument calibration Z0 is real, then (and only then) \(P=P_{fwd}-P_{ref}\).
By virtue of VSWR=5.83, \(\frac{P_{ref}}{P_{fwd}}=0.5\) we can substitute \(0.5P_{fwd} \text{ for }P_{ref}\).
We get \(P=P_{fwd}*(1-0.5)\) or \(P_{fwd}=2P=2 \cdot 50=100\) and \(P_{ref}=0.5P_{fwd}=50\).
Closing the loop, \(P=P_{fwd}-P_{ref}=100-50\).
Differently to the Youtube, no energy was created or destroyed, there is no “reflection gain”.
What about practical systems?
Now this was a lossless system, it is a lot simpler to analyse than a practical system but some of the concepts apply to a practical system.
The conceptual failure of the quote from the video was the matter of power handling, particularly \(P=P_{fwd}-P_{ref}\). As stated, this applies if and only if Zo is purely real.
Whilst the Zo of practical cables is not purely real, the issue here is the calibration Zo of the directional power meter used, because it is its calibration impedance that is used to render the values Pfwd and Pref, and for a quality instrument, that calibration impedance is usually purely real (eg 50+j0Ω).
If one was to insert good directional wattmeters each side of the ATU (#1 on the tx side, #2 on the antenna side), we could measure and calculate \(P1=P_{fwd1}-P_{ref1}\) and \(P2=P_{fwd2}-P_{ref2}\), and we would expect P2 to be a little less than P1 due to ATU internal loss.
Note that:
- one cannot validly compare Pfwd of both meters, or Pref of both meters;
- one’s ability to accurately read Pfwd and Pref and calculate P is best if Pfwd>>Pref, and this may not be the case for meter #2.
An appeal using anthropomorphism
The author uses anthropomorphism to reach the non technical ham:
…you don’t lose anything of any significance so SWR in a coax line in the amateur HF bands doesn’t matter the only thing that matters is that your transmitter sees a low SWR because otherwise modern transceivers reduce power to prevent damage to the output stage, that’s what an antenna tuner is for, all it does is make the transmitter happy and that’s important…
Happiness of an inanimate object!
Conclusions
It seems there is no shortage of people who swallow specious explanations, possibly assisted by confirmative bias and a bit of anthropomorphism which might suggest they watch these things for self satisfaction rather than learning… learning that might challenge their ‘knowledge’.