Varactor tunned loop antenna

[Atena]

Hello,

I have made a loop antenna with varactor tuning and capacitive matching.
Attached are the schematic, layout and the images from the antenna.
When I connect one of the power supply cables to the connector marked as "problem" in the images and the other end to any instrument (oscilloscope, multimeter or power supply), even if they are off, the S11 signal gets several unstable and varying resonances around the resonance frequency. Even if I switch the varactor direction and consequently change the polarity of the power supply, the very same trace will have that problem and the other one will be fine.

Just few points about the design:

• Rs footprint is just a footprint to be used in case. It is free right now.
• For both the varactor and the matching capacitor, there are two footprints on the layout in case more capacitance is needed. Both the extra footprints are free now.
• The RF blocking inductances are double wound wire ferrite type.
• The varactor is BB182 from NXP.

Any help will be well appreciated.

 

[CDRIVE]

Make certain that you're using a 10x low cap probe. Also make sure that scope GND is not connected to any node that is not also GND. I.E. you cannot connect scope GND to any node above GND.

Regarding DVM's: In the days of VTVM's the positive probe contained an integral 1Meg resistor. The manufacturers put it in the probe to isolate the the probe lead and the VTVM's capacitance / inductance from the circuit under test. This practice was not carried over to the DMM era. I keep two red probes because of this. The second probe contains a series connected 100K resistor to isolate the + lead and the DMM capacitance and inductance from the DUT. The 100K will produce a small voltage drop but not serious enough to significantly reduce the accuracy of the meter readings.

The resistor should be soldered directly to the opposite end of probe tip with that end of the resistor's lead kept as short as possible.
Chris

 

[CDRIVE]

After looking at your schematic it occurred to me that you have excess components in there. The shell of the SMA should be committed to GND so there's no need for the resistor and choke that you have there. Take a look at how I re-drew it.

Chris

EDIT: Also be aware that Varactors are reverse biased to operate properly. If your steering voltage is not variable you will need to replace R1 with a voltage divider to adjust the bias voltage to the Varactor.

 

[Atena]

Dear Chris,
Removing the extra components, as you suggested, solved the problem
Thank you very much

 

[CDRIVE]

You're quite welcome.

Chris