[SOLVED] VLF antenna switch to remove ring down

[Robert T]

I have a project which is detecting the amplitude of the return signal of an RF tag. The RX antenna is a separate antenna.
The issue I am having is being able to read the RX signal as close as possible to the TX switching off. The ring down for the TX is too long - 600 us - so I am looking to short the antenna using a switch.
I have tried using a RF transistor - TIP48 - and mostfets but when the TX is on, these seem to short to ground. The voltage on the TX is 200V peak to peak.
Frequency is 58 kHz.

Can anyone suggest a means that I can shunt the ring down signal or short the antenna.

Thank you.

 

[chuckey]

If you use a HV supply (> 200V), feeding a 10K resistor to the collector of a high voltage transistor and connect this point via a capacitor to your aerial, I can't see why it should not work, providing you only switch the transistor on when the RF is off.
In the early days of radar, a neon tube was used, one end connected to the line, the other end was then pulsed to make the neon strike and short the line, but I doubt if you could make it any faster then your 600 micro secs, still you could have a go?
Frank

 

[Robert T]

Thanks Frank. The cap and resistor did the job. I have started with 60V peak to peak using a 473 green cap and a 100 ohm resistor. The ring down went from over 700us to about 80us.
I will start increasing the voltage on the antenna and see what happens.
Interestingly, if I used a 47 ohm resistor with the same cap, the ring down time actually increased. Not sure why. There seems to be a correct combination required for both components.

Thanks again.
Robert