What signal might one measure from a simple antenna

[StudentSA]

Good Day,

Firstly I am a complete beginner in RF design but please bear with me. I would like to understand a little more about antennas, lets take a di-pole for eg. I would like to know what electric signal would i measure at the connection point? so if i hooked up an appropriate oscilloscope what would I see? I need to understand this so that I can build on what the RF amp stage is trying to accomplish.

I have this possibly crazy perception form my readings that an antenna could be considered like a band pass filter? meaning that the more you deviate from the quarter(or was it half) wavelength the less signal you will pick up at that frequency.

Thanks,
StudentSA

 

[Nigel Goodwin]

It depends on the incoming signal strength, but probably only uV

You also need a scope that considerably exceeds the frequency you're using, so probably low frequencies and LARGE aerials.

 

[JimB]

StudentSA asked:

so if i hooked up an appropriate oscilloscope what would I see?

Have a look at my pictures and you will see!

Using a ground mounted vertical antenna which is about 10meters long (high?) and connecting to my oscilloscope, I saw several volts of distorted 50hz (mains frequency).
So I terminated the coax from the antenna with a 50 ohm resistor, and the result can be seen in the picture "On the scope".
That band of "noise" gets bigger and smaller ove a period of ten seconds or so but is generally a full 40mV p-p with the odd spike heading for 70mV p-p.

That picture was taken at about 23:00hrs, when I looked in the late afternoon around 16:00 when I first saw your post, the signal levels were only around 20mv p-p.

To give a better idea what is in all that mush we can see on the scope, I connected the spectrum analyser.
The first picture is a scan from 0 to 10MHz.
For this pic I used quite a wide bandwidth, 30kHz, so each peak will contain several radio signals.

The second picture is a scan from 0 to 1MHz.
This time the bandwidth is 3kHz, so each peak is an individual radio signal.

So, does that answer your question?

JimB

 

[StudentSA]

A really big thanks JimB!

This is exactly what I was looking for and more.

A frequency analyzer is pretty nifty if I can say so myself . The 23:00 vs the 16:00 readings really intrigues me, is it that more broadcasts are happening at night or does it have something to do with the atmosphere and how some signals bounce back easier at night? I was really amazed at how distinctly you can see the different bands in figure 2.

In theory I could actually take a small snippet of data from the oscilloscope and feed it into a PC and conduct a Fourier Transform to obtain the spectrum analyzer results?

Kind Regards,
StudentSA

 

[JimB]

A really big thanks JimB!
This is exactly what I was looking for and more.

You are welcome.

A frequency analyzer is pretty nifty if I can say so myself

Yes, a spectrum analyser is one of those things which you wonder how you ever managed without. Especially when it has an add-on called a tracking generator.

The 23:00 vs the 16:00 readings really intrigues me, is it that more broadcasts are happening at night or does it have something to do with the atmosphere and how some signals bounce back easier at night?

Radio propagation changes through the day.
During daytime, the sun ionises the various layers in the ionosphere, very noticably the medium waveband is practically empty during the day with just local (100 miles or so) stations being audible. This is due to absorbtion by the D (from memory) layer.
At night the D layer disappears and the F layers can reflect the signals back to earth, and so then medium wave stations from all over Europe and North Africa can be heard quite easily.
I don't know how the medium waveband is in South Africa, but in UK and Europe is is completely full up with stations every 9kHz, and frequencies re-used so that there can be several stations which are on the same frequencies in different countries.

Similar sorts of things happen with the shortwave frequencies where higher frequencies will propagate during the day than during the night. Again from memory I think this is due to changes in the F layers. (Propagation theory is not one of my strong points).

So, more or less stations, stronger and weaker reflections from the ionosperic layers make for more or less signals on the antenna.

In theory I could actually take a small snippet of data from the oscilloscope and feed it into a PC and conduct a Fourier Transform to obtain the spectrum analyzer results?

In theory, maybe, in practice most unlikely.

But having said that, google for "Software Defined Radio".

With an SDR it is possible to convert the RF signals to "baseband" and feed them into a PC soundcard to create a quite reasonable receiver.
To do this you need to convert the incoming signal to two audio signals with a 90 degree phase shift between them.
Feed the audio into the two channels (left and right) of the sound card and the software can filter the signal and tune up/down the frequency band a few kHz.
How far you can tune is limited by the frequency response and sampling rate of the sound card.

JimB

 

[trash]

SDR has become quite popular with locals around here.
$20 TV dongle and some SDR software and you have a 50-1700MHz receiver.
This is much simpler than using the sound card, but it can be tricky to get the dongle working on some machines.
Just don't install the TV driver that comes with the dongle and the install is pretty straight forward.

Are some of the examples of where to find software and drivers.

https://wiki.spench.net/wiki/USRP_Interfaces
https://rtlsdr.org/softwarewindows

 

[Willen]

Seeing the amazing analyser, I again remembered a question which is always in my mind. There are VERY expensive tools like audio compressor, mixers, some medium speed oscilloscope etc. They have their own parts- transistors, IC etc. Cannot people make a computer software to audio compressing, mixing, processing, and spectrum analysing and like oscilloscope? It would be so cheap for users and free for hackers. Or are there such software exist? Maybe not, because I have seen some FM station, where I saw they are using very expensive audio processor, instead of software version of processor.

 

[Sinedup]

Re Willen's "audio" software query. Try Audacity, WaveEdit and GoldWave. (Free/trial).
Google "audio editor download". Lots of good ones, mixed with "spammers".
I prefer using Audacity as you can edit down to individual sample "bits", analyse, scope, mix, etc.

 

[Sinedup]

Add: (Willen 'audio software'): ... for live analysis/scope, you need fast pc, good soundcard (eg minimum core-2-duo with 100KHZ card) and plenty RAM. (4Gb ok).

 

[Willen]

Add: (Willen 'audio software'): ... for live analysis/scope, you need fast pc, good soundcard (eg minimum core-2-duo with 100KHZ card) and plenty RAM. (4Gb ok).

Thank you for the guidance Sinedup.
Currently I have no more access to a computer described as you. I have a cheap 2.8GHz pentium with 1GB RAM and no sound card. For slower circuit, I am thinking to IN their output signal to the 'Mic IN' and observe the signal wave (by zooming too) from a sound recording software like Cool Edit Pro or Adobe Audition etc. Sounds funny though.

Exactly why the 'Audacity' is designed for? Seems interesting!