Building a 70 cm slotted line
 

Actually, not slotted. Here's what I have in mind:

Use some 1/16 inch common FR4 two-sided PCB material. The ARRL
UHF/Microwave Experimenter's book shows that a .105 inch wide trace
will have a characteristic impedance of 50 Ohms. For 434 MHz, I will
make it at least 3.6 inches long (velocity factor = .528) but
probably more like 6 or 7 inches long.

Here is where I have a bit of a problem. I need a probe to run along
the .105 inch trace so I can read the (rectified) signal. From the
data I should be able to calculate the load impedance.

I have read that a small current loop placed near the line would
suffice. But would it be too insensitive at low power (10 mW)?
Also, how would I maintain the separation between the probe and line?
Should I use a probe that would contact the line instead?

Mechanical stuff is my weakest point. Any ideas?

Thanks in advance.

John, KD5YI

You may have some trouble getting enough signal with a loop probe
that's short compared with the line length, but you could easily give
it a try. FWIW, I just made some voltage-doubler diode detectors
using HSMS-285C zero-bias Schottky diodes, and get about 20 microvolts
DC output at -50dBm input at 50 ohms. You could get considerably more
output if you have a resonant pickup. You might also be able to use a
non-contact voltage probe. It should work very well with a diode
detector, for low power, actually. Recommend you make the line at
least 1/2 wave long, so that you can be sure to see both a peak and a
valley, no matter what the phase of the load is. Better still, make
it at least 3/4 wave, so you can see two peaks or two valleys.

Cheers,
Tom

"John Smith" wrote in message hlink.net...
Actually, not slotted. Here's what I have in mind:

Use some 1/16 inch common FR4 two-sided PCB material. The ARRL
UHF/Microwave Experimenter's book shows that a .105 inch wide trace
will have a characteristic impedance of 50 Ohms. For 434 MHz, I will
make it at least 3.6 inches long (velocity factor = .528) but
probably more like 6 or 7 inches long.

Here is where I have a bit of a problem. I need a probe to run along
the .105 inch trace so I can read the (rectified) signal. From the
data I should be able to calculate the load impedance.

I have read that a small current loop placed near the line would
suffice. But would it be too insensitive at low power (10 mW)?
Also, how would I maintain the separation between the probe and line?
Should I use a probe that would contact the line instead?

Mechanical stuff is my weakest point. Any ideas?

Thanks in advance.

John, KD5YI

"Tom Bruhns" wrote in message
m...
You may have some trouble getting enough signal with a loop probe
that's short compared with the line length, but you could easily give
it a try. FWIW, I just made some voltage-doubler diode detectors
using HSMS-285C zero-bias Schottky diodes, and get about 20 microvolts
DC output at -50dBm input at 50 ohms. You could get considerably more
output if you have a resonant pickup. You might also be able to use a
non-contact voltage probe. It should work very well with a diode
detector, for low power, actually. Recommend you make the line at
least 1/2 wave long, so that you can be sure to see both a peak and a
valley, no matter what the phase of the load is. Better still, make
it at least 3/4 wave, so you can see two peaks or two valleys.

Cheers,
Tom

Thanks, Tom. Very good advice and I appreciate the Schottky diode lead. I'll
look in to getting some of them.

73,
John