Transmit With 5W FM Transmitter & Dipole Antenna: Range & Radiation Pattern

[dexterdev]

With a 5W FM transmitter and a normal dipole antenna , to what range can I transmit. To be more specific, what will be the radiation pattern like (Can I get signal uniformly as in a circle where the center is antenna). Using which antenna, can I achieve maximum range omnidirectionally.

-Devanand T

 

[sophiecentaur]

Firstly, the antenna must be mounted vertically (just stating the obvious) for an omnidirectional pattern.
By "FM", I am assuming that you mean signals around 100MHz. (FM refers to the modulation system and not the carrier frequency).
The range of such signals is very much determined by the antenna height above ground. Reliable propagation is largely limited to the Horizon. A hand held 5W transmitter (150MHz) has a usable range of a few km from the deck of a yacht. Put an antenna at the top of a 10m mast and the range extends to 20km or more because the horizon is that much further away.

Local features (hills, in particular) can affect things greatly. Also, the location and height of the receiver is just as relevant as for the transmitter. So, I'm afraid there isn't a definite answer to this question. It is quite possible to communicate over much more than 20km (like from Brittany to Portsmouth) on occasions but other times it's hard to hear a boat that is actually visible. Some radio hams spend their lives trying to get as far as possible (DX) with a low power as possible. You'd be amazed at the distances covered at times.

Most transmitting (comms) antenna tend to be vertical monopoles because they can be mounted at the top of a structure. Dipoles are harder to feed because they need a mast to support them from the side, which gets in the way and the feed cable acts as a reflecting element, too. Both of these effects will spoil the omnidirectional radiation pattern.

 

[skeptic2]

If you are referring to a 5W portable, this is perhaps the most difficult situation to predict range and pattern. For instance if you want to talk to another portable that is being worn on someone's belt, that alone will reduce receive sensitivity by 10 - 12 dB. Anytime a portable is held close to the body it will reduce Effective Radiated Power (ERP) and receive sensitivity as well as distort the pattern. The range will also vary greatly depending on whether the transmitter or receiver is indoors or outdoors. I concur with Sophie that in good conditions you should be able to communicate a few km.

If your transmitter and receiver are fixed, the higher you can get your antenna the more range you'll get. Many times, doubling the height of the antenna will increase your range about as much as doubling your power.

 

[dexterdev]

First of all thank you guys for the reply.
My antenna is kept from first floor* of the building. So you tell that when the antenna is at 3rd floor it will cover more distance right.
( [Brit] The first*floor of a building above ground*level :) )

Also do omnidirectional and isotropic means the same?

 

[skeptic2]

Also do omnidirectional and isotropic means the same?

No they are not the same. An omnidirectional antenna has a more or less circular pattern in the horizontal plan. Often the 3D pattern resembles a torus or doughnut.

Isotropic is a spherical pattern which no known antenna can produce. When antenna gain is specified, it is often given as dBi (referenced to an isotropic antenna) or dBd (referenced to a dipole antenna). A gain specified in dBd has 2.15 dB more gain than the same value specified in dBi. DBi is normally used for microwave antennas and dBd is more often used for VHF and UHF. Some VHF and UHF antenna manufacturers specify their antennas in dBi most likely to make it look like their antennas have 2.15 dB more gain than they really do.

Some manufacturers just specify their antenna gain in dB. For those antennas you must contact the manufacturer to find out which it is.

 

[dexterdev]

Thank you.. One more query : If my carrier is 100MHz then should the length of dipole antenna be (corresponding wavelength)/2 meter. I am not clear on the term antenna height. Is it the antenna length or including mast etc.

 

[skeptic2]

The wavelength of 100 MHz is 3 meters, not 2 meters. Dipole antennas are 1/2 wavelength long.

You can make a very effective antenna for 100 MHz by taking 1.5 meters of 300 ohm twin lead, twisting the two leads at both ends together and cut one of the two wires at the center of the length and connect your transmitter or receiver to those two ends. This type of antenna is known as a folded dipole.

 

[berkeman]

First of all thank you guys for the reply.
My antenna is kept from first floor* of the building. So you tell that when the antenna is at 3rd floor it will cover more distance right.
( [Brit] The first*floor of a building above ground*level :) )

Also do omnidirectional and isotropic means the same?

So I have to ask, what band are you planning on transmitting on from your place? Is it a licensed band?

 

[dexterdev]

Thankyou guys...

Any way the location is India and Its actually my friend's experiment (not licensed ). Plz don't inform anyone.

 

[berkeman]

Thankyou guys...

Any way the location is India and Its actually my friend's experiment (not licensed ). Plz don't inform anyone.

Bull manure. Check your PMs. You are temp banned.