John's homebrew pages
Through-line UHF / microwave wattmeter - the "Sparrow"
Another deviation from the main project, which will be one of the
most useful bits of test equipment I've built. I decided to call it the
"Sparrow", for fairly obvious reasons.
I had already prototyped a directional
coupler and was pleased with the results. So it was not difficult
to lay out a line with a coupler on each side, for forward and reverse.
It's made on double-sided FR4; the 50 ohm stripline dimensions depend on the FR4 thickness.
Use standard references to work out the dimensions! Here's the pcb:
Clearly it's not possible to produce something by hand that has
absolutely identical couplers, but tests of the completed item with the
signal paths reversed show that it's fairly symmetrical, which is fine.
It detects power siphoned off the incoming signal, so needs no
batteries - great. A 100 microamp meter is used as the detector
here since that's what I had available). I set it up so the meter
needle goes in the direction the switch is set to - forward or reverse.
The rotary switch is for the range setting, which has a resistor in
series with the meter for each range apart from the most sensitive;
they're not in this photo which was taken before it had the resistors
added. You can see the signal really does go straight through - in one
N connector, onto a 50 ohm microstrip, and out through another N
connector. Apart from the new pcb and the box, most of the bits came
from rallies - the N connectors cost me 12.5p each!
This is the front view. The range switch has 6 positions, and I have
set it up so that the current read goes in steps of root 10, so that
two steps give a factor of ten. This will be fine for the power levels
I use. The first transverter is a bit too much for range 1, so shows
nicely on range 2; the new 23cm transverter needs range 4. It works at
70cm and 2m as well, and it will be interesting to see how it behaves
at 13cm. At 6m it detects very little, and at HF effectively nothing -
you need much more coupling there since the loops are only a very tiny
fraction of a wavelength even at 6m.
It was good to have the Sparrow available for the first test of my
new 23cm transverter - here it is as a field test instrument! It's very
gratifying to see that RF is actually going up the antenna feeder.
So that's another very useful bit of microwave test equipment to add to the armoury.