radio transmitter not working?

[dl09]

I bought voltage controlled oscillator from a Chinese seller. I connected a coaxial cable to the voltage controlled oscillator. the coaxial cable is suppose to serve as an
antenna. I shaved off the outer insulation and I shaved off the outer copper layer. the antenna Is 1/4 of a wavelength long. I used a spectrum analyser to detect a signal.
the seller says if the control voltage is 0 volts the out frequency is 70 megahertz. the control voltage is 0 volts. I don't think the seller list the name of the model. I doubt I can find a datasheet. the seller says if the red light turns on, that means the voltage controlled oscillator is on, so I am sure the voltage controlled oscillator is on. the seller says the output is 10 dBm, I think that equals 10 milliwatts. the spectrum analyser does not detect a signal. why not? is the signal too weak? should I use an amplifier?

 

[betwixt]

I could give you an answer as silly as your question but I will restrain myself.
What did you buy? A VCO from a Chinese seller tells us absolutely nothing about it.
Why did you use a co-axial cable as an antenna?

Brian.

 

[dl09]

In a previous post somebody told me to use a coaxial cable as an antenna and connect it to a voltage controlled oscillator to build a radio transmitter and that would work.

--- Updated Jul 18, 2020 ---

https://www.edaboard.com/threads/would-using-coaxial-cable-serve-as-a-transmitter-antenna.389057/

--- Updated Jul 18, 2020 ---

See I have been reviewing previous posts. The voltage controlled oscillator has a sma connector. I didn't use a breadboard.

 

[KlausST]

In a previous post somebody told me to use a coaxial cable as an antenna

No. As far as I remember everybody told you this is nonsense.
But you ignored it...


Klaus

 

[dl09]

I po

No. As far as I remember everybody told you this is nonsense.
But you ignored it...


Klaus

I post a link to this post.https://www.edaboard.com/threads/would-using-coaxial-cable-serve-as-a-transmitter-antenna.389057/

And I shaved off the insulation and the outer copper wire.

--- Updated Jul 19, 2020 ---

No. As far as I remember everybody told you this is nonsense.
But you ignored it...


Klaus

In this posthttps://www.edaboard.com/threads/would-using-coaxial-cable-serve-as-a-transmitter-antenna.389057/
I asked if connecting a coaxial cable to a voltage controlled oscillator would act as a radio transmitter if i shave off the insulation and outer copper layer and somebody said yed. Could you at least check the post?

 

[KlausST]

Hi,

It is possible to use a coax cable, but it makes no sense.
You could also use any other piece of metal. A big knife, a water pipe, solder wire, aluminum profile, steel rope .....
It makes about no difference if you remove the inner wire of the coax cable .... or just use the inner wire (removed outer shield of the coax)

And I shaved off the insulation and the outer copper wire.

This is the other thing that you did not understand: It makes almost no difference if there is isolation around the piece of metal or not.

Klaus

 

[betwixt]

Klaus is absolutely right. Nobody told you to make an antenna out of co-axial cable.
That kind of cable is used as a conduit - a way of carrying a signal some distance away while retaining its integrity and shielding against signal leakage or ingress. Its the pipe that carries the signal, not the antenna.

There is nothing stopping you adding an antenna at the end of the co-axial cable, that is what most of us do.

What we have stated is that if you do not use co-axial cable correctly you will leak signal from it and in doing so it will behave like a leaky pipe, its contents will escape. That is why you could see some of that escaped signal on your analyzer.

If you do not connect the co-axial shield (the braid around the center wire), or you connect the shield to the output of your signal source, it behaves exactly like an ordinary length of wire so there is no advantage over not using a plain wire instead.

Brian.

 

[FvM]

Although beyond the scope of this question, a useful application of a coxial cable can be to make a halfwave dipole:

The ferrite ring is acting as a sheat wave filter.

As for the original question, if you are connecting a single wire or coax cable with stripped outer conductor to the oscillators SMA output(center pin connected!) and don't see any RF signal, the oscillator is either not working or you are monitoring the wrong frequency range.

 

[BigBoss]

Check the VCO works well or not first..
Connect a coaxial cable with an appropriate connector then observe the signal.