Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Would using coaxial cable serve as a transmitter antenna?

Status
Not open for further replies.

dl09

Full Member level 4
Joined
Feb 12, 2020
Messages
226
Helped
2
Reputation
4
Reaction score
1
Trophy points
18
Activity points
1,688
i am thinking about buying a voltage controlled oscillator on ebay. the seller is in china and consequently won't be in the office, because of the chinese new year festival. if the voltage controlled oscillator has a sma connector, how do i make a transmitter antenna for it? do i cut a half wavelength length of coaxial cable and connect that to the sma connector? would that work?
 

As a general rule you can attach unaltered coax cable and run a length of it to the roof (or tower, etc). Then the end should be a few feet of bare wire. The reason is because the surrounding conductor (shielding) inhibits emission of radio waves (similar to its ability to absorb them). For testing purposes it's fine to strip away the outer conductor.

- - - Updated - - -

To some extent antenna setups work in both directions. Automobiles once had very simple rod antennas mounted on a fender. Coax cable carried signals to the radio.
 

Reinforcing what Brad said, the co-axial cable is used to add distance between the signal source and the antenna that will radiate it. In many instances, the source (transmitter) and antenna cannot be physically next to each other so the cable carries the signal to a more suitable place for the antenna. A perfect co-ax cable would have no losses, what goes in at one end all comes out at the other end, however in real life nothing is perfect and that includes cables, some signal will be lost in the dielectric (insulating layer) between the center wire and the shield and some will escape through imperfections in the shield and its geometry.
Think of the cable as a bridge between the source and antenna but by design, not good at radiating a signal by itself.

If you think of it in receiving terms where the same rules apply, you wouldn't want to sit on your roof to watch TV because the antenna had to be plugged directly into it. A co-axial cable links the antenna to the TV so it can be watched in comfort.

Brian.
 

As a general rule you can attach unaltered coax cable and run a length of it to the roof (or tower, etc). Then the end should be a few feet of bare wire. The reason is because the surrounding conductor (shielding) inhibits emission of radio waves (similar to its ability to absorb them). For testing purposes it's fine to strip away the outer conductor.

- - - Updated - - -

To some extent antenna setups work in both directions. Automobiles once had very simple rod antennas mounted on a fender. Coax cable carried signals to the radio.

if the end of the bare copper wire is not connected to anything, the transmitter antenna will still radiate radio waves?
 

yes
the voltage in the piece of bare wire still changes, so the electrons are accelerated
and that radiates EM waves
 

Think of the coax as being to electricity what a pipe is to water. You want what goes in at one end to come out at the other without any leaks.
If you block the end of the pipe, nothing comes out, if you don't block it everything comes out. If you attach something at the end that partially blocks it, for example a shower head, the water comes out but may not land where you want it to.

Likewise, co-ax cable ideally will not leak signal in or out, what you put in at one end comes out unscathed at the other end. If you attach any wire or other kind of antenna at the far end, it will radiate the signal according to the efficiency and shape of that antenna.

Brian.
 

so if i connect a coaxial cable to a voltage controlled oscillator, strip off the insulation, the inner conductor is not covered by insulation and the inner conductor is 3 feet long, and the inner conductor is not connected to anything, will the inner conductor still radiate radio waves? just making sure, i understand the responses.
 

so you're connecting one end of a 3 foot piece of bare wire to a VCO
and leaving the other end open

this will act as an antenna at the frequency of the VCO

EXCEPT
it may not be a good antenna
one has to take into account the wavelength of the EM wave to be emitted and set the size of the antenna appropriately
if the wire is the wrong length for the wavelength, it will not be a good antenna

radio waves are a specific range of the EM spectrum
your VCO needs to operate in that range to get "radio waves"

note however that your still a long way from radio
 

so if i connect a coaxial cable to a voltage controlled oscillator, strip off some insulation, the first part of the coaxial cable still has insulation and the outer conductor, the second part consist of the inner conductor with no insulation, and the bare inner conductor is 1/2 wavelength, the end of the inner conductor is not connected to the outer conducter and is not connected to anything, will the coaxial cable still radiate radio waves? just making sure i understand everything.
 

yes
but as i said, its only in the radio region of the EM spectrum if the VCO operates in that frequency range
 

In some situations a small antenna can pick up (or be exposed to) enough photons so you can receive a broadcast. Now picture how a larger antenna is exposed to more photons. Reception becomes better.

My short-wave radio has a 3-foot telescoping rod antenna. It does okay at receiving stations if they're strong. However things improved enormously when I draped a 30-foot wire in a tree and ran it inside the house to my radio.

The same is true when transmitting. Our natural tendency is to go the easier cheaper route, and make the antenna physically small. However a larger antenna (assuming it's driven properly) emits more photons.
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top