ARLA/CLUSTER: Antenas de radio desaptadas: Explicação sobre ondas estacionárias

[João Costa > CT1FBF]

Radio antenna mismatching: VSWR explained

If you have ever operated any sort of transmitting equipment, you’ve
probably heard about matching an antenna to the transmitter and using
the right co-ax cable.

Having everything match — for example, at 50 or 75 ohms — allows the
most power to get to the antenna and out into the airwaves. Even for
receiving this is important, but you generally don’t hear about it as
much for receivers.

But here’s a question: if a 100-watt transmitter feeds a mismatched
antenna and only delivers 50 watts, where did the other 50 watts go?
[ElectronicsNotes] has a multi-part blog entry that explains what
happens on a mismatched transmission line, including an in-depth look
at voltage standing wave ratio or VSWR.

We liked the very clean graphics showing how different load mismatches
affect the transmission line. We also liked how he tackled return loss
and reflection coefficient.

There was a time when driving a ham radio transmitter into a bad load
could damage the radio. But if the radio can survive it, the effect
isn’t as bad as you might think. The post points out that feedline
loss is often more significant. However, the problem with modern
radios is that when they detect high VSWR, they will often reduce
power drastically to prevent damage. That is often the cause of poor
performance more so than the actual loss of power through the VSWR
mechanism. On the other hand, it is better than burning up final
transistors the way older radios did.

Measuring VSWR without a transmitter is a bit trickier. A network
analyzer can do it. While that used to be a pretty exotic piece of
gear, it has become much more common lately.

https://hackaday.com/2018/08/17/radio-antenna-mismatching-vswr-explained/

• Our thanks to Stephen, G7VFY for spotting this item