Best way to detect voltage from DIY radio antenna for beginner radio projects

i am trying to do radio projects. if i build an antenna to detect radio signals what should i use to detect the voltage at the antenna?

That's a very open ended question. Firstly, what sort of frequencies are you talking about and what sort of input signal.Simply put, you will  probably need gain, and a lot of it,  since it is not uncommon to find signals in the region of micro-volts.Are you are interested in looking at a broad range of frequencies, such as in scanning, or you are more interested in a discrete frequency.That will determine your antenna type and how signals outside the frequencies of interest are dealt with.There are many approaches to wireless design, and these days a very wide range of integrated circuits to aid the designer. Additionally, there is a move to use very little RF analog processing, the tendency is to use  use DSP methods as early as possible.The simplest radio receiver is a tuned circuit feeding a diode and a pair of headphones, these go back a hundred years, and if you are near a stong AM broadcast transmitter they work just fine.There are many interest groups building wireless equipment, a search on the web will likely turn something up.Likewise, there are as many simple wireless projects which include schematics and building instructions, although you have to be wary of some of the designs. If you provide more details on your possible area of interest we may be able to point you in the right direction.Cheers,
Richard

You can probably get a crystal set kit to get you started.  Don't know where you are but in Australia there is this:
https://www.jaycar.com.au/crystal-radio-kit/p/KV3540
You may be able to get this in the US as well, otherwise google "Crystal set kit" and see what you get.

the frequency is 5.8 gigahertz, now what can i use to detect the voltage at the antenna? you have not mentioned any piece of equipment i can use to detect the voltage at the antenna.

Quite right, since we had no idea what frequency you were interested in until now. Additionally, the carrier frequency by itself is usually not enough, since in order to convey information you have to somehow modulate it, so what is the modulation scheme.If you have enough signal, a small signal RF shottky diode on an antenna, followed by an  low frequency amplifier may detect your carrier. Most likely test instrument would be a spectrum analyser, not a cheap piece of gear. Otherwise you need a PCB with a mounted receiver (although you are normally talking about a transceiver chip)capable of demodulating your signal. Most chips have a thing called an RSSI output, which gives an output proportional to the input signal in dB. It is often only available  by interrogating the device from a microprocessor, as is the demodulated signal. A scope  can be used on the data pin, but a logic analyser is more useful.The ARRL hanbook may have a UHF diode detector, otherwise look it up on the internet.Again, please provide more information.Cheers,Richard

What you need is a radio receiver capable of receiving a signal at the frequency of interest. You might be able to detect a voltage using an oscilloscope (at 5.8 GHZ that would be very expensive) but as Richard mentioned above, you will likely need an amplifier between the antenna and whatever you are using to receive the signal since the signals are generally very low.I'd suggest that you start with radio projects at a lower frequency to gain understanding before going to 5.8 GHz.
Lime Microsystems has some interesting parts and there is a board available that would be a good platform for experimentation. Here is one available from SparkFun.
https://www.sparkfun.com/products/15027Oh, and if your radio experiments involve transmitting signals, please check regulations applicable to your country before doing so. Transmitting on an unauthorized frequency can get you in big trouble.

why does an antenna have to have 50 ohms?

Because it's a historical standard for coax cables so transmitters are designed to to maximize power to a 50 ohm load. In reality, you can design antennas (and transmitters) with a wide range of impedance but it's best to stick with the standard when you first start out.
For instance, coax for TV antennas is usually 75 ohms so TV antennas usually match that. Of course they are receive only so no need to match a transmitter.
I did a Google search for"why antenna impedance is 50 ohms" the link below was the first one. There were a number of other links which I'm sure had variations on the same explanation.
https://www.microwaves101.com/encyclopedias/why-fifty-ohms

The voltage on the receiver antenna is, usually, few microvolts.The antenna does not receive only a single TX station but collects many many waves and many frequencies. The strongest signal is the frequency of resonance.You can amplify the signal in antenna by, for example, 1000 times. Then you can measure this amplified signal (for example 12 mV), then divide 12mV by 1000 and you wil get the voltage of input signal (12 microvolt).The best solution is to use an oscilloscope, bat the signals are very very very low. Usually the signal on antenna is very low and connecting the oscilloscope you will lower this signal.To measure the impedance of antenna, you can design it with a good software (for example: MMana-Gal or EZNEC). I use them to project my antennas.The impedance of the dipole, at his resonance frequency, is, usually 75 ohm in optimal conditions.Regards.

what does the antenna has 50 ohms mean? does it mean the resistance of the antenna is 50 ohms? does it mean the antenna is connected to a 50 ohm load?

When we say that an antenna has 50 ohms, it means that at the frequency it is being measured, it has an impedance of 50 ohms.Impedance is not the same as resistance even though it has the same units. Resistance is what you measure at DC (with your multimeter) impedance is what is measured at the frequency you are using.If you have an antenna that is 50 ohms at 5.8 GHz  it could measure open or short (or anything in between) at DC with your multimeter.This line will take you to a tutorial on AC resistance and impedance. The next tutorials in the series cover inductive and capacitive impedance followed by RLC circuit analysis. Understanding this will be useful when designing radio circuits
https://www.electronics-tutorials.ws/accircuits/ac-resistance.html

so the impedance of the antenna is 50 ohms?

Impedance Z = R + jX has real part R and imaginary part X. j is the square root of -1, a 90-degree rotation in phase. A half-wavelength dipole antenna has an impedance of (72+j0) ohms, so is a good match for 75-ohm coax cable. If the frequency is changed, the dipole is no longer a half-wavelength, and the imaginary part of the impedance rises. Positive imaginary part is inductive reactance. Negative imaginary part is capacitive reactance. David Lopez, are you familiar with complex numbers?

yes, i have studied science before, but i have never designed and built an antenna before. so the impedance of the antenna is 50? you still have not answered that question.

No, the impedance of an antenna will never be 50 Ohms (a nice generalization),  there are a number of parameters to consider in designing an antenna, and you always sacrifice a number of them to get somewhere near a specified target, and impedance is definitely one of them. In reality it is rarely of great importance on low power transmitters or receivers, which is all that you are allowed to use in this band, it's all about  acceptable VSWR against bandwidth, directivity etc.For a start, you say you wish to build an antenna for 5.8GHz. What sort of antenna, and how do you plan to measure it's performance. There are dozens of potential antenna types for 5.8GHz, and in reality, if you want a fully characterized antenna, then you either buy one or have appropriate test gear, which doesn't come cheap. (If you drop down to 2.4GHz, then there are some cheap network analysers available )There is finite analysis software which allows antenna designs to be computed, but don't expect this to be easy to learnPerhaps the simplest to build is a  wire whip antenna, 1/4 wave  are often used to keep them as compact as possible. There is a lot of information on the internet on how do design for 2.4GHZ, which will apply at 5.8GHz. You can tweak these antenna  by bending the arm with respect to the ground plane to get close to 50 Ohms, but in doing so you trade off other performance characteristics. Why bother?Cheers,Richard

As Richard pointed out, the impedance of an antenna will rarely be exactly 50 Ohm and will depend on other characteristics of the antenna. Close is usually good enough. If your antenna impedance is too far from 50 Ohm then you may need to add a matching network between the antenna and the receiver or transmitter. In my experience, matching is less critical on receivers than transmitters.

i am thinking about measuring the voltage from the antenna with an s-meter, will that work at 5.8 gigahertz?

You don't say if you are now understanding more about antenna impedance.I touched on the subject of RSSI in a past post.  I have used Schottky diodes  in applications up to tens of GHz. They are not particularly linear, and require a good signal strength, but are very simple to use, see this application note for more details. The LTC5533 expands on this theme.Analog Devices have a range of stand alone analogue RF detectors, some of which operate at your frequency of interest. Again, they require a good received signal strength, but they are more linear than diodes. They are a specialist surface mount product and you would need to get one pre built on a demo board unless you have experience in designing RF circuitry on PCBs at this frequency.Why are you working at 5.8GHz specifically, you are making a rod for your back if you have no experience in working  with RF, as has already been suggested, it is much easier working at lower frequencies, the antennas may be larger, but  test gear is more readily available. Even dropping down to 2.4GHz and you suddenly find hundreds of products available .
Cheers,Richard

As you can see in the image, the impedance of a dipole (tuned to 433Mhz) changes if you change the frequency of its utilization.For example, a dipole projected for 433 Mhz, will have the following impedances:
300 Mhz: 26.2823 Ohm400 Mhz: 55.8678 Ohm433 Mhz: 70.5316 Ohm500 Mhz: 112.604 Ohm600 Mhz: 232.195 Ohm
CheersGiovanni Di Maria

you still have not answered my question would a s-meter work at 5.8 gigahertz?

What s-meter are you considering?The s-meters that I've seen are built in to a receiver in order to measure at a specific frequency. So if you have a receiver capable of 5.8 GHz, with a s-meter built in then that s-meter would work.

so you plug an s-meter into a prebuilt receiver?  i don't know how to us a s-meter.

Actually the s-meters I've used have been built in to the prebuilt receiver.From Richard Gabric's comment earlier in this thread
" Otherwise you need a PCB with a mounted receiver (although you are
normally talking about a transceiver chip)capable of demodulating your
signal. Most chips have a thing called an RSSI output, which gives an
output proportional to the input signal in dB."The s-meter output is likely to be generated from the RSSI output of the receiver.

how can i build a device that will amplify the signal from the microvolt range to the volt range? i am thinking i will amplify the signal first, then detect the signal.

David, there are now 23 comments in this thread, and we are no further forward than when you started, or apparently so, since you have never acknowledged if you have understood any of the replies. At this stage I would reiterate, why are you working at such a difficult frequency, even 2.4GHz is bad enough, but at least at 2.4GHz there is a lot more information, hardware, and cheaper test gear. Not apparently having done any RF work at all, you need to go back a lot further than where you are now to get a better understanding of the basics, which is usually university level electronics engineering. At this stage I am choosing to bow out with as much grace as possible, you need more than I can possibly supply,I wish you well in your endeavors,kind regards,Richard

David,I agree with Richard that you need to get a better handle on the basics and that starting out at a lower frequency would be beneficial although it appears that there is a certain amount of hobby activity at 5.8 GHz. Based on the search I did for 5.8 GHz receivers, there are a variety of low cost receivers available.I'd suggest that if you want to try out antenna design you should purchase a receiver. Most of them appear to have a connector for the antenna that you could use to connect your own antenna. You should be able to find a receiver with an output you can use  to measure the level of the signal. The receiver will have the amplifier built in.