connect coaxial cable to FM Transmitter's antenna

[Nepaliman]

-In coaxial cable, there are 2 wires- inner and outer. In Fm transmitter, RF probe should be connected to inner wire and outer wire should be connected to groundplane. BUT I AM CONFUSING coz how should i have to connect this coaxial cable to antenna?? Coax has two wire but antenna is single.
- If i connect 5 meter coaxial cable, and 80 cm antenna as RF out, the total length of antenna will be 5.8 meter??? Or will be only 80 CM? Why?
-This cable known as ''50 ohm load'' for transmitter, is it?

 

[ccurtis]

A proper VHF antenna has a two terminal feedpoint to which the coax (or other transmission line) is connected.

Your coaxial cable is probably 50 ohm characteristic impedance, but the cable itself is not a "50 ohm load" unless it is so long that all energy is lost in the cable and no energy appears at its far end. (High power dummy loads have actually been made using a full spool of coaxial cable without the need for a high power 50 ohm resistor, or resistor bank.)

The physical length of your "antenna" element is 80 cm (about 1/4 wavelength) because that portion is completely exposed. The outer surface of the outer conductor (the shield) of the remaining coaxial cable length will substitute as a poor ground plane (reflector). It will not work optimally because the long coaxial shield is not an optimal ground plane. There will be wasted radiation along the entire length of the coax since the shield is part of the antenna, and it will not have a 50 ohm load impedance. What you have is a poor Marconi antenna. Even a proper one has a feedpoint impedance of about 30 ohms which is not equal to 50 ohms.

 

[Nepaliman]

I'm little beginner, tell me directly that-- at the antenna rod, I've connected inner wire of coax but what to do with outer shied wire? Where should I have to connect it?

 

[ccurtis]

In the picture, the vertical element is your 80cm wire. Positioned around the bottom of the vertical element, and equally spaced, are four additional 80cm wires that, connected together at the bottom, form the ground plane for the antenna. Each of those four wires is angled downward from horizontal at 45 deg angle. The shield of your coax connects to all four angled wires. The center conductor of your coax connects to the vertical element. That is a respectable antenna.

 

[Nepaliman]

I got little idea about dipole antenna which is suitable for fm transmitter. But i dont know how long coax cable is suitable for 1 watt transmitter? I think, very long coax in small powered transmitter is harmfull because of signal loss due to too long coax.

-Suggest me for suitable length in 1 watt FM transmitter.

-Give me the idea about balloon transformer used between FM transmitter and dipole antenna. Give me some idea or URL or some pictures to make suitable balloon transformer.

 

[ericgibbs]

I got little idea about dipole antenna which is suitable for fm transmitter. But i dont know how long coax cable is suitable for 1 watt transmitter? I think, very long coax in small powered transmitter is harmfull because of signal loss due to too long coax.

-Suggest me for suitable length in 1 watt FM transmitter.

-Give me the idea about balloon transformer used between FM transmitter and dipole antenna. Give me some idea or URL or some pictures to make suitable balloon transformer.

Hi N,
Its a BALUN transformer.


https://www.google.co.uk/url?sa=t&r...DmTp8ZesxqpVUucow&sig2=PO2qtxcUu6SUnvAeLwSInw
E.

 

[Mikebits]

+1 on Erics answer

 

[ccurtis]

I got little idea about dipole antenna which is suitable for fm transmitter. But i dont know how long coax cable is suitable for 1 watt transmitter? I think, very long coax in small powered transmitter is harmfull because of signal loss due to too long coax.

-Suggest me for suitable length in 1 watt FM transmitter.

The length and type of the coax required depends on the distance between the transmitter and the antenna locations and how much signal loss you are willing to give up to get the signal between the two locations, not on the absolute power of the transmitter. Look at a data chart for the loss in dB per 100 ft for the coax you are using. Every 3 dB loss means that you lose one-half of the effective radiated power.

-Give me the idea about balloon transformer used between FM transmitter and dipole antenna. Give me some idea or URL or some pictures to make suitable balloon transformer.

The monopole antenna shown in the previous post does not require a balun because the monopole is not a balanced antenna. Unbalanced transmission line (coax) to unbalanced antenna connection requires no balun.

 

[audioguru]

Use your BALLOON with helium in it to hold your antenna up high in the sky.

A single whip antenna is unbalanced like coaxial cable so it does not need a BALUN (BALanced to UNbalanced).
A dipole antenna is balanced and uses a BALUN to match it to unbalanced coaxial cable.

Many years ago my TV had a 300 ohm balanced antenna input. Then 300 ohm balanced cable connected to the balanced antenna.
For cable TV to replace the antenna then I used a balun to match the 300 ohm balanced TV terminals to the 75 ohm unbalanced coaxial cable.

 

[Nepaliman]

Use your BALLOON with helium in it to hold your antenna up high in the sky.

OK I will do as you say. And I will write on it- '' Designed and recommended by audioguru.''