Radio Shack field strength/SWR meter

[Lord Snooty]

This useful little box measures "signal strength" both from its antenna or
from its input, and also SWR. Inside is a stripline and diodes, so I presume
it's some sort of RF bridge. Can someone explain in electronic engineering
terms precisely what this instrument measures, on both its Signal Strength
scale and its SWR (presumably VSWR?) scale? I am experimenting with loads of
varying complex impedance, and getting non-intuitive results.

Andrew

 

[Tim Wescott]

This useful little box measures "signal strength" both from its antenna or
from its input, and also SWR. Inside is a stripline and diodes, so I presume
it's some sort of RF bridge. Can someone explain in electronic engineering
terms precisely what this instrument measures, on both its Signal Strength
scale and its SWR (presumably VSWR?) scale? I am experimenting with loads of
varying complex impedance, and getting non-intuitive results.

Andrew

I'm looking at my 15-year old example, yours may differ a bit from mine:

The signal strength scale most likely just measures the voltage across
the diode in the bridge circuit.

When you adjust the "CAL" knob to read at the "CAL" position of the
meter with the switch in the "FWD" position then throw the switch to
"REF" the "SWR" scale reads SWR.

I'm not sure what kinds of readings you're seeing, or if your intuition
matches reality, but as your load diverges from a purely resistive 50
ohms you'll see more and more reflected power and higher SWR.

 

[Lord Snooty]

I'm looking at my 15-year old example, yours may differ a bit from mine:

The signal strength scale most likely just measures the voltage across
the diode in the bridge circuit.

When you adjust the "CAL" knob to read at the "CAL" position of the
meter with the switch in the "FWD" position then throw the switch to
"REF" the "SWR" scale reads SWR.

I'm not sure what kinds of readings you're seeing, or if your intuition
matches reality, but as your load diverges from a purely resistive 50
ohms you'll see more and more reflected power and higher SWR.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Yup Tim, that's it. But what does "the voltage across the diode in the bridge
circuit" actually mean?
Another thing is the SWR. It doesn't seem to change a lot on a matched
reactive load, but on a matched resistive load it works fine. Why do you think
that is?

Andrew

 

[Tim Wescott]

Yup Tim, that's it. But what does "the voltage across the diode in the bridge
circuit" actually mean?
Another thing is the SWR. It doesn't seem to change a lot on a matched
reactive load, but on a matched resistive load it works fine. Why do you think
that is?

Andrew

Mean? Mean? You asked what it _measured_!!

If the meter is truly matched to 50 ohms it measures the forward voltage
wave in the "fwd" position and the reverse voltage wave in the "rev"
position, with heavy frequency dependencies because of the nature of the
sampling loop. So if you terminate it with a good 50 ohm resistance
it'll measure your forward voltage as some real value and show a zero
reverse voltage, implying zero SWR.

What do you mean by "matched reactive load"? A purely reactive load
cannot be matched to a transmission line -- the SWR will always be infinite.

 

[Lord Snooty]

Andrew writes:
This useful little box measures "signal strength" both from its antenna or
from its input, and also SWR. Inside is a stripline and diodes, so I presume
it's some sort of RF bridge. Can someone explain in electronic engineering
terms precisely what this instrument measures, on both its Signal Strength
scale and its SWR (presumably VSWR?) scale? I am experimenting with loads
of varying complex impedance, and getting non-intuitive results.

Tim Wescott replies:
I'm looking at my 15-year old example, yours may differ a bit from mine:
The signal strength scale most likely just measures the voltage across
the diode in the bridge circuit.
When you adjust the "CAL" knob to read at the "CAL" position of the
meter with the switch in the "FWD" position then throw the switch to
"REF" the "SWR" scale reads SWR.
I'm not sure what kinds of readings you're seeing, or if your intuition
matches reality, but as your load diverges from a purely resistive 50
ohms you'll see more and more reflected power and higher SWR.

Andrew replies:
Yup Tim, that's it. But what does "the voltage across the diode in the bridge
circuit" actually mean?
Another thing is the SWR. It doesn't seem to change a lot on a matched
reactive load, but on a matched resistive load it works fine. Why do you think
that is?

Tim Wescott replies:
Mean? Mean? You asked what it _measured_!!
If the meter is truly matched to 50 ohms it measures the forward voltage
wave in the "fwd" position and the reverse voltage wave in the "rev"
position, with heavy frequency dependencies because of the nature of the
sampling loop. So if you terminate it with a good 50 ohm resistance
it'll measure your forward voltage as some real value and show a zero
reverse voltage, implying zero SWR.
What do you mean by "matched reactive load"? A purely reactive load
cannot be matched to a transmission line -- the SWR will always be infinite.

Andrew replies:
Thanks for clarifying the forward versus backwards voltages. This is now clear
and makes sense.
By "matched reactive load" I mean a capacitor whose reactance is equal to the
nominal source impedance (say 50 ohms). And indeed, at this particular value,
there is a measurable dip in the indicated SWR which,. as you say, is quite a
high value. Why in fact is the reverse voltage high for a purely reactive
load? Feel free to use an equation with jw's if you like. I already realise
that no real power is absorbed by a reactance, but I cannot visualise why the
same impedance, this time pure imaginary instead of pure real, would change
the reflected voltage so.