Author Topic: RF amplifier (RMS) + antenna as PCB path  (Read 579 times)

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Offline bipolunipol

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RF amplifier (RMS) + antenna as PCB path
« on: September 22, 2021, 09:00:59 am »
Hi,

I would like to make a PCB for a simple RF circuit with a logarithmic RF RMS amplifier, LMH2110, to measure the power of RF signals. I'm only interested in coarse power measurement (indication if the antenna I'm checking with my circuit is transmitting). The dominant frequency in the testing environment will be 1 - 3 GHz. The circuit is more for fun, I'm not interested in bandwidth related parameters. It should be a simple indicator.

And now I am faced with the problem of designing a PCB.
I already have the circuit but I have a problem with placing components on PCB.
I would like to get as much sensitivity as possible and I don't know what position of the antenna will guarantee it. Will it be version 1 or version 2 (image in attachment)?

Thanks

 

Offline T3sl4co1l

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Re: RF amplifier (RMS) + antenna as PCB path
« Reply #1 on: September 22, 2021, 05:29:54 pm »
What is the extent of the full PCB, is it larger than what's shown or is this it?

If the latter, it's not much of a counterpoise and the best you can do is a dipole, which will have to be pretty broadly tapered (i.e. bowtie) to get bandwidth that wide.  You probably want to do this anyway?  That is, have a half bowtie sticking out from your ground plane, whereas the wiggle element will have peaks and valleys.

Standard disclaimer, hand-waving of response here, use a simulator or build a real one to be sure etc.

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 
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Offline bipolunipol

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Re: RF amplifier (RMS) + antenna as PCB path
« Reply #2 on: September 24, 2021, 03:10:01 am »
What is the extent of the full PCB, is it larger than what's shown or is this it?

If the latter, it's not much of a counterpoise and the best you can do is a dipole, which will have to be pretty broadly tapered (i.e. bowtie) to get bandwidth that wide.  You probably want to do this anyway?  That is, have a half bowtie sticking out from your ground plane, whereas the wiggle element will have peaks and valleys.

Standard disclaimer, hand-waving of response here, use a simulator or build a real one to be sure etc.

Tim

Thank you for your help.

Yes, the size of the whole PCB is bigger. In the attachment I am sending the dimensions for these 2 versions. There will also be a small display and a microcontroller on the board.

Regarding the bandwidth - yes, I want to be as wide as possible.

Are there any simple and free programs to simulate such things? So that I can test the antennas in these 2 versions of PCB and design a better antenna?
 

Offline T3sl4co1l

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Re: RF amplifier (RMS) + antenna as PCB path
« Reply #3 on: September 24, 2021, 06:17:02 am »
Unfortunately, not really...  There are some free or cheap/simple ones, but maybe not so easy to use for planar geometry specifically.  Or accurate ones, but you're going to pay for it, and it's not going to be easy to use...  Hopefully others more experienced will chime in with suggestions?

Mainly my comment comes from a general familarity of ultra/wideband antennas -- they have a self-similar (scale independent) geometry, so that fields behave the same over a wide range of frequencies.  The bowtie is the most likely candidate here.  There's a lot of press about general fractal types but it doesn't look like e.g. Koch curves have all that much going for them, honestly; it's the scale-independent ones (typically having some combination of zooming/radial symmetry -- like a cone, hence the conical dipole for example, a bowtie being the planar equivalent).  To cover an octave, you don't need to be quite so in-depth (you can get a little space savings still), but it will have to be something more than a plain old wire dipole, and can't be electrically short (LC tuned, or curled/folded up to make a shorter overall shape for a given target frequency -- such an antenna will have a higher Q and thus narrower bandwidth as it only couples with the resonant band, plus whatever harmonics show up, corresponding to the segments it's built from -- the zigzag will likely have some peaks and/or notches at frequencies corresponding to the zig length or zag spacing, though these will be at some octaves above the intended passband I think so not really relevant in this case).

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 
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