# Antenna output voltage vs. received RF power

1. ## Antenna output voltage vs. received RF power

Hi everyone,

I'm new to RF design and aware that this may be a trivial and/or silly question for the RF experts on here.

I'm wondering which parameters, for planar antennas (such as patch antennas), are affecting the output voltage of the antenna (Vpp). Is that the frequency of the signal(s), the power of the received signal(s), a combination thereof, or something completely different? Second part of the question: for a certain antenna design, is it possible to deduce the received power strength and/or frequency from the output voltage, possibly with calibration?

If I wanted to measure the received power level in a very narrow band of frequencies, what would be the best approach to do this?

If anyone could direct me to relevant literature on this topic, I would be very grateful.

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2. ## Re: Antenna output voltage vs. received RF power

the output voltage of the antenna is dependent on the combination of the power of the received signal, the bandwidth of the antenna and the impedance of the load connected to it.

To measure the received power level in a very narrow band of frequencies , best approach is to have the test instrument for that purpose.

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3. ## Re: Antenna output voltage vs. received RF power

Originally Posted by srizbf
the output voltage of the antenna is dependent on the combination of the power of the received signal, the bandwidth of the antenna and the impedance of the load connected to it.
Assuming that the load impedance is constant, it can be deducted that the power of the received signal can then be correlated to the output voltage, if the bandwidth of the antenna is reasonably flat. However, I do not have much experience with antennas yet. Which type(s) would be good choices in a range from 433 MHz to 2.45 GHz?

Originally Posted by srizbf
To measure the received power level in a very narrow band of frequencies , best approach is to have the test instrument for that purpose.
Unfortunately I don't have any high end test instruments available, and they're way out of my budget range. I do, however, have a CCC 2015 Rad1o Badge (SDR dev board) at my disposal. Can I work around it using that board?

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4. ## Re: Antenna output voltage vs. received RF power

It would be probably helpful if you tell what your actual design/measurement problem is.

Is the primary quantity of interest field strength or received power with a specific antenna? What's the field strength range, how accurate want you measure it?

Quantitative level measurements "in a range from 433 MHz to 2.45 GHz" will typically use a wide band antenna like log-per which have a fairly constant gain over a wider frequency band.

Alternatively you can use dipoles with variable length that are tuned to the respective frequency.

For high level measurements, e.g. calibration of susceptibility test systems, active wide band antennas are used as field strength monitors.

I doubt that you'll have much success without acquiring some basic RF knowledge.

5. ## Re: Antenna output voltage vs. received RF power

Originally Posted by FvM
It would be probably helpful if you tell what your actual design/measurement problem is.
Ideally, I'd like to be able to measure the field strength in the frequency range between ca. 430 MHz and 2.5 GHz, in bands of 1 to 2 MHz wide, with a fixed antenna.

Originally Posted by FvM
Quantitative level measurements "in a range from 433 MHz to 2.45 GHz" will typically use a wide band antenna like log-per which have a fairly constant gain over a wider frequency band.
Most of these log periodic antennas seem to be quite large and cumbersome. Is it a viable alternative to use multiple smaller antennas (with narrower bandwidth) and switch between them with an RF switch? Ideally I'd like to use wire antennas or PCB antennas.

Originally Posted by FvM
I doubt that you'll have much success without acquiring some basic RF knowledge.
Indeed, so I'm doing my best to catch up with the basics in the context of this project, but unfortunately it's -- as usual -- a tight deadline.

6. ## Re: Antenna output voltage vs. received RF power

The grid dip meter is mentioned often as an important rf measuring instrument. Its response is based on LC resonance. The jfet version causes the meter to dip when it detects rf energy. (There are versions which use a bjt, in case a jfet can't be obtained.)

Its operating principle may be suitable to adapt to what you wish to do.

One article describes how you can easily measure different rf bands. Wind different size coils, and attach them cardboard forms which are cut to fit over the detection element in your meter. It does not require making and breaking electrical contact. No soldering is needed.

Example articles:

https://g4rvh.files.wordpress.com/20...-dip-meter.pdf

http://qsl.net/yo4rlp/wshp/gdo.html

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7. ## Re: Antenna output voltage vs. received RF power

You didn't yet answer to the level accuracy point.

Consider that the accuracy of field strength measurements made with "high end" equipment (calibrated antennas and state-of-the-art spectrum analyzers or test receivers) isn't assumed better than +/- 1 dB in EMC standards, you should account a larger error margin with non-standard equipment.

SDRs like the CCC radio badge are basically suited for semi-quantitative measurements, but you'll probably need an RF generator or a power meter and an attenuator set for level calibration.

I prefer a 400 MHz to 6 GHz logper antenna in combination with a regular spectrum analyzer for pre-compliance tests of sub-GHZ small radio devices. Using multiple antennas with a mux doesn't sound reasonable, too much mutual coupling and level error by the mux. I would rather use one or two tuned dipoles or individual fixed dipoles and change the frequency manually.

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