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Is coaxial cable radiating some radiation?

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dl09

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i bought a 50 ohm female plug, installed it on a breadboard, then connected it to a 50 ohm coaxial cable with a male connector. i also installed a crystal oscillator module and connect the breadboard to a battery, there are spikes on the spectrum analyser. is the load impedance 100 ohms or 50 ohms?
 

Hi,

I assume the coaxial cable is connected to your spectrum analyzer and the 50 \[\Omega\] connector as close as possible to your oscillator.
As the spectrum analyzer has an input impedance matching the characteristic wave impedance of your coaxial cable of 50 \[\Omega\], your oscillator is connected to a load with 50 \[\Omega\] (looking from your oscillator towards the matched spectrum analyzer). The characteristic wave impedance of the coaxial cable can not be represented by a single lumped impedance.

Note, there are also 75 \[\Omega\] system (coaxial cables, connectors, ... ).

Which kind of oscillator are you using, Square or Sinewave?
What is your desired frequency and at which frequency are those spikes located?

greets
 

the 50 ohm coaxial cable serves as an antenna. the coaxial cable radiates radio waves. and the spectrum analyzer picks up the radio waves. the spectrum analyzer is not connected to the 50 ohm coaxial cable.
 


i bought a 50 ohm female plug, installed it on a breadboard
Click to expand...
Installing devices requiring specific impedances on breadboard is usually a bad idea.

Please post a picture of your construction so we can see how you have connected it.

Brian.
 

These are 2 pictures
20200624_143659.jpg


20200624_143726.jpg
 

So why is installing specific devices requiring specific impedance on a breadboard a bad idea
 

The photo clarifies that the SMA ground pins aren't connected at all, only the center pin connects to the crystal oscillator output. That's no correct usage of a coaxial connector.

Even with SMA ground connection, the breadboard doesn't allow appropriate RF circuit wiring. Proper RF wiring involves a continuous ground plane and power supply bypassing near the oscillator

The thread title is still mysterious. Which antenna do you connect?
 

Hi,

It seems he did not install an antenna at all.
He wanted the coax cable itself to act as an antenna. Which makes no sense.

The problem is lack of basic understanding of how HF works.

@OP:
Ignoring informations given in several posts of several of your threads will make you moving in circles.
No learning effect, no progress.
Start with learning basics....as I already recommended..

Klaus
 

may i ask 1 last question? why not use a coaxial cable as an antenna? at 100 megahertz the coaxial cable would be 1/4 of a wavelength long.
 

Hi,

No.
A coax cable is made _not_ to send out RF. Instead it is designed for exactly the opposite.

Compare the coax cable with a water pipe. An intact water pipe.
Is the water pipe designed to spread water like a lawn sprinkler (antenna) or is it made to keep the water inside?

Klaus

Added:
All your non_coax wiring, all your breadboard is like a leaking garden hose.
But all your HF ideas are almost opposite to the real world.
* You use leaking wiring ... where you don't want it leaking
* and you use non leaking, where you want it to be leaking.
 
Last edited:

why not use a coaxial cable as an antenna? at 100 megahertz the coaxial cable would be 1/4 of a wavelength long.
Click to expand...
It's possible if you use the coax outer conductor as antenna wire. In this case you would connect the SMA ground pin to the transmitter output. Of course there's no advantage of having a coax cable, it's used as a thick wire.
 

I have been connecting a coaxial cable roughly 1/4 of a wavelength to a crystal oscillator and I have not removed the insulation and the second layer of copper. I still see spikes appear on the screen. Without an antenna I see 6 spikes and with an antenna I see 11 spikes. Any possibility the second layer of copper is not blocking all the radio waves and some radio waves are leaking out?
--- Updated ---

20200704_075041.jpg

--- Updated ---

Here is a schematic.
 

Incomplete respectively unclear question as many times before. How did you connect the coaxial outer conductor in your test. If it's unconnected, as in the test before, the outer conductor has no shield effect and the cable will radiate similar to a thick single wire due to the strong coupling between inner and outer conductor.

-> Learn about coaxial cable operation principle
-> Design useful test setups
--- Updated ---

Regarding observed "spikes" (spectral lines), it would be interesting to know its frequencies.
 
Last edited:

so if i don't connect the ground terminal, the outer conductor has no shielding effect?
 

Think of your signal as being water and the cable as a pipe. If the pipe is intact, the outside is dry while still carrying water through the middle. If you take the pipe away, the water disperses in all directions.

The shielding effect of a co-axial cable depends on the outside layer (the braided part) having no signal on it because it is grounded. If you remove the ground, as you have done in the photographs, the shield picks up some of the signal from the inner wire and becomes a radiator itself.

Note that the blue '-' strips on each side of that protoboard are not connected together so you have no ground on the socket. Even if they were joined, the inductance of the wiring from side to side would still allow some signal on the socket ground.

Brian.
 

can you post a link to learn how to properly install a coaxial cable?
 

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? i know i ask this question before, but in the previous post nobody told me if the load impedance would be 100 ohms and i have been unable to find the previous post and tried looking under your threads. the coaxial cable serves as an antenna. the ground pins are not connected to ground, so there is no shielding effect. please tell me if the load impedance will be 100 ohms?
 

No. First of all you cannot cosider a cable impedance in the same way you consider a RLC impedance. If you have a cable having a Zo=50 ohm terminated on a 50 ohm load, then the impedance on the other side of the cable will be 50 ohm.
In any case you have a totally different situation. As far as I've correctly understood you want to connect only the inner conductor of the cable to the receiver. In this case the impedance will not be 50 ohm and will depends form the frequency, the cable length and many other factors.
 

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