Peter (and interested others),
You might want to peruse the article "Antennas, Transmission Lines, and
Tuners" at my website http://w3fpr.qrpradio.com.
The article tries to flatten several myths in terms that should be
understood by most hams - no math, but I believe a good explanation of
principles.
73,
Don W3FPR
Peter Howson wrote:
I have been following this thread and find it very interesting,
particularly as a Mechanical Engineer I have to read it slowly but may
I comment --
1, I don't know the details of Icom's protection system, but the ones
I have seen sense reflected power, well actually it will be the
voltage detected at the sensor against an assumed impedance of 50 ohms
(50+j0). The actual SWR at which the protection will come into play
will be a function of feedline impedance (which will, in practice,
have a reactive element), feedline losses, antenna impedance and
efficiency. -- If it's a problem - just add a few feet of coax!!
2, It shows an underlying belief in SWR as the be all and end all of
antenna systems.
3, Many years ago now I had the opportunity to visit the main Naval
transmitting station in the UK. Utilising frequencies from VLF to VHF
their antenna systems were out of this world. But the majority were
fed with 600 ohm open line across many kilometres of open ground. I
recall one of our guys asking about SWR. The station chief just
shrugged. One of the main HF transmitters was opened for maintenance
and contained a large number of 4CX250 valves connected in parallel -
not the easiest valve to run.
So what should we be doing; as an instructor I see and hear a hell of
a lot of myths about SWR and do try to correct them within my own
understanding of the subject. But I do have to hand it to those who
fully understand the subject and who take the time to explain it at
the level of our examination system, which cannot be easy.
Perhaps we should send this to the mythbusters and see if thay can
blow it away!!
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