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Basic RF design question

Hi everyone,

I am grad student doing some lab research and I need your help concerning a RF question - I'm sorry if the question seems basic to you, I have no experience in the field and couldn't find help anywhere else.

Basically, I am trying to deliver ~10W to a low-impedance load (1-2 Ohms), at 1 MHz. After conducting some research and getting help from this forum, I came up with a power generation + transmission system that seems to work: Signal generator + coax + specific RF power amp + coax + 50:3 Ohms transformer + load.

However, I am having trouble delivering the signal and power to my load, after the transformer. My load is a 3ft x 1ft plate, and I would like to create a voltage drop through it by connecting one side of the plate to the ground and the other side of the plate to the "+" side of the signal. My transformer is designed to have an output impedance that matches with this load, but somehow I can't get rid of reverse power because my transformer-to-plate connection setup is bad.

What I've been using for that connection is a BNC to Banana adapter at the output of the transformer, and then connect one banana wire to one side of the plate and the other to the other side. I am sure this setup is pretty bad in terms of undesired capacitance, impedance mismatch, and that it is why my power doesn't get delivered. However, I do not know what better solution is available out there.

If you have any idea or suggestion, please let me know, I'd really appreciate!

Many thanks,

Ruben

  • Hmmm. I do not follow the purpose of the plate...is this some kind of antenna? How do you know the plate is 1-2 ohms at 1 MHz? How do you determine if power is adequately being delivered to the load?

    The reverse power problem sounds like impedance mismatch. The plate plus adapters and cables may be throwing your calculations off. Perhaps adding a bit of known resistance, say 5 ohms, and adjusting the transformer accordingly will eliminate some of the variability in the set-up.

    --RJH
  • Hi,

    Thanks a lot for your reply. The plate is the final load of our system, so you can visualize is as a termination resistor or antenna. Its purpose is very specific to our research, but basically we want the RF power to be consumed by it.

    The only way we've had to measure its impedance was to measure DC voltage and current going across the (wires+electrodes+plate) setup, and the resistance was constantly around 2 Ohms for any voltage applied. Of course, we're conscious this does not necessarily hold for AC, but we do not know how to make measurements with AC, as we need high current, which is hard to generate and sustain at this frequency. That's why we're using a RF power amplifier now. However, since nothing seems to get out of it (because of reflection), we haven't been able to make AC measurements.

    The main problem for us is with regards to the plate's geometry. While a classic termination load is tailored to be plugged to a coax cable, this is not the case for our plate, which has a rectangular geometry. In this case, we're looking for the best way to send the RF signal with that geometry as a constraint. On our side, we've custom-built specific electrodes and wires that ensure a relatively OK contact with the plate.

    Please let me know if you have any insights as to how to send the signal across the plate. We've been using BNC-banana adapters, and we're splitting a coax cable ourselves without much results, but we hope there's a more elegant, effective and efficient solution that we just don't know of.

    Thanks!