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dl09

Full Member level 4
So if I connect sma plug rated at 50 ohms to a coaxial cable rated at 50 ohms serving as an antenna, the impedance looking into the sma plug will not be 50 ohms or 100 ohms, but will depend on frequency and other factors? Just making sure i understand the response.

betwixt

Super Moderator
Staff member
You are getting very confused and heading off in the wrong direction. It still isn't clear exactly what you are trying to achieve.

Think of connectors and co-ax as being like water pipes and water fittings. If you have 15mm pipes you need 15mm joints between them. You can't fit a smaller or larger one without some blockage or leakage. Similarly, for 50 Ohm cable you use 50 Ohm connectors, you cannot simply add the numbers together, the connector becomes an integral part of the connection and the impedance stays the same.

Brian.

FvM

Super Moderator
Staff member
"coaxial cable rated at 50 ohms serving as an antenna" is a meaningless description. There are possible ways to use coaxial cable to build antennas. The antenna impedance is however defined by the antenna geometry, not the coaxial cable impedance.

volker@muehlhaus

if i use a sma plug rated at 50 ohms impedance and a coaxial cable rated at 50 ohms impedance, will the load impedance be 100 ohms, if the coaxial cable is connected to the sma plus?
No. Not 50. Not 100.

You need to understand, finally, that these 50 Ohm line impedance values are NOT the impedance seen at the transmitter side.

50 Ohm line impedance means a specific ratio of line inductance and line capacitance Zline=sqrt(L/C). If you don't connect the shield, your line behaves line a simple copper wire. Input impedance of that copper wire depends on length, and is totally unrelated to the coax 50 Ohm impedance.

dl09

Full Member level 4
You are getting very confused and heading off in the wrong direction. It still isn't clear exactly what you are trying to achieve.

Think of connectors and co-ax as being like water pipes and water fittings. If you have 15mm pipes you need 15mm joints between them. You can't fit a smaller or larger one without some blockage or leakage. Similarly, for 50 Ohm cable you use 50 Ohm connectors, you cannot simply add the numbers together, the connector becomes an integral part of the connection and the impedance stays the same.

Brian.
I am planning to build a filter that will filter the harmonics produced by the crystal oscillator module, leaving a 100 mhz sinewave. I am trying to scale the normalized filter to the load impedance. I am trying to make sure the load impedance is 50 ohms. The output of the crystal oscillator module will travel through the filter, filtering the output. Then into a sma plug connected a coaxial cable serving as an antenna. Will the filter see a load of 50 ohms? I don't know how to connect the components of the filter together. So please don't ask for a schematic. Just trying to learn how to build a filter.

volker@muehlhaus

If you make that antenna wire 1/4 wavelength long (monopole), and add some ground wires also, the antenna input impedance will be around 50 to 70 Ohm. Look for "ground plane antenna".

dl09

Full Member level 4
So make the coaxial cable 1/4 of a wavelength long, then use the coaxial cable as an antenna?

betwixt

Super Moderator
Staff member
If you want it to be an antenna, why are you thinking of using co-axial cable which is designed to screen signals, not radiate them. Just use any wire you want.

I'm also confused by you wanting to use 50 Ohms impedance at the output of the oscillator. Are you sure the oscillator is designed to drive 50 Ohms, most are designed to drive TTL level logic circuits.

Brian.

dl09

Full Member level 4
I am planning to use coaxial cables, because I want to make sure the load has a known impedance and I don't have any means of measuring the load impedance.

betwixt

Super Moderator
Staff member
Your needs are still confusing us. By 'load' do you mean something connected to the opposite end of the coaxial cable to the oscillator?
If that is what you mean, the characteristic impedance of the cable does not change the load, it just changes how well matched the cable (but not necessarily the oscillator) is to it. A bad match will deliver less power than a good match.

A good system has the same impedances throughout:
Source drives 50 Ohms --> 50 Ohm connector --> 50 Ohm cable --> 50 Ohms connector --> 50 Ohms load.

It could be any other impedance as long as it remains the same at all points.
I strongly suspect your oscillator is not designed with a 50 Ohm output impedance.

Brian.

dl09

Full Member level 4
I am have been studying how to design a low pass filter. The instructions say to scale the components to a higher load impedance. I am just trying to follow the instructions. The instructions use the term load. The instructions don't say what the load is. The output of the filter is fed to 50 ohm sma connector, connected to a coaxial cable., which serves as an antenna, so is the sma connector the load, and the load impedance is independent of coaxial cable impedance? Just trying to learn how to do this.

volker@muehlhaus

I am planning to use coaxial cables, because I want to make sure the load has a known impedance
You can repeat this again and again, but it is wrong. You don't understand coax and antenna basics.

So make the coaxial cable 1/4 of a wavelength long, then use the coaxial cable as an antenna?
Use a simple unshielded cable.

dl09

Full Member level 4
I don't know how to calculate the impedance of a monopole.
In a previous post I think someone said you could determine that using a literature formula. I can't find this literature formula. I have looked a lot on the internet for this literature formula. 1 website said that the input impedance of a monopole is a function of length relative to wavelength. So if I double antenna length of a monopole, I double the input impedance?

volker@muehlhaus

So if I double antenna length of a monopole, I double the input impedance?
I think you are making fun of us.

dl09

Full Member level 4
Is the impedance of a 1/4 wavelength monopole antenna over a groundplane that is large relative to the wavelength 36.5 ohms?

volker@muehlhaus

Yes, approximately. You can tweak it towards 50 Ohm by bending down the ground radials, as shown in the lonk above. But 36 Ohm is already close enough to your 50 Ohm target.

dl09

Full Member level 4
See I have been researching radio technology.