Probe placing and insertion depth.
From: "Gary Peach" <xcg34@dial.pipex.com>
>>From: J-M Noeding LA8AK >> >>Literature seems to show that distance from end wall to N-probe is >>1/8 lambda wg. LA6LCA uses 1/4 lambda waveguide and says it is >>easier to get better vswr. It seems perhaps not critical at all?
>From: "Arie Dogterom" <pa0ez@amsat.org> >A probe in a wg normally should be 1/4 Lg from the closed end. . >Shorter will not work well although woith tuning screws much can > be corrected.
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Generally speaking the shorter that any dimension can be made the broader band it will have.
A short probe having a very high impedance will look more like an open circuit and placing it Lg/4 from a closed end will produce the better result. However if some other structure such as a diode mount is to be placed across the guide then the spacing to the end wall should be as close to the point at which an impedance match occurs. Less than Lg/4. As something like a diode mount will have a complex impedance. In order to find the optimum position for the end wall a movable short can be used whilst measurements of the desired parameters are measured. Tune for maximum smoke. Then check the bandwidth. Having logged the results of the experiment put the permanent end wall into place. If you don't have a nice micrometer adjusted short then use a brass or copper block close fitting to the guide slide it carefully around then scribe a line at the point where best performance is noted make up a short and hard solder it into place. Cut off the surplus guide leaving enough for a stable mechanical joint.
Here is an example table of probe coupling against insertion depth
Here is an example table of probe coupling against insertion depth
Probe insertion 3" x 1" guide at 3 GHz
l/b dB Power
0.00% -70.00dB 0.00001%
2.50% -36.02dB 0.025%
5.00% -26.38dB 0.230%
10.00% -28.54dB 0.140%
15.00% -12.60dB 5.500%
20.00% -17.45dB 1.800%
25.00% -13.67dB 4.300%
30.00% -10.81dB 8.300%
35.00% -9.03dB 12.500%
40.00% -7.70dB 17.000%
45.00% -6.58dB 22.000%
50.00% -5.69dB 27.000%
55.00% -6.09dB 24.601%
60.00% -5.66dB 27.169%
65.00% -5.27dB 29.736%
70.00% -4.91dB 32.304%
75.00% -4.58dB 34.872%
80.00% -4.27dB 37.439%
85.00% -3.98dB 40.007%
90.00% -3.71dB 42.574%
95.00% -3.45dB 45.142%
100.00% -3.21dB 47.710%
The impedance of the probe will fall as the insertion becomes greater and
the spacing to the end wall will need to change slightly to compensate.
Gary G7SLL