Antenna For Fm Radio

[dasappettan]

Hai friends
I want to make an antenna for my FM radio to receive a channel at 102.6 MHZ. Do you have any idea ?

 

[stevez]

A dipole that is 54.75 inches long would make a good antenna. Google on 'FM dipole antenna' and you'll get plenty of hits and good information.

 

[RadioRon]

An active monopole is sometimes easier to use than a dipole. This would be a wire or small pipe that stands vertical and is 1.17 m in height. At the bottom directly connected to the wire or pipe is a small circuit board with a simple RF amplifier that has a high impedance input and a low impedance output. The amplifier would act mainly as an impedance buffer. This is essential because the feed impedance of that vertical wire length would be fairly high, perhaps about 800 ohms or so.

I recommend this length because it can function well without a groundplane and in fact might degrade if placed on the ground or on a large metal surface like a metal table. It will work fine if placed up on a wood table or shelf.

 

[stevez]

This is a good place/time for a comment/question:

As an amateur radio operator I understand that the polarization of the transmitting antenna and the receiving antenna must be the same for best reception - generally for line-of-sight communication. In US VHF-FM is generally vertical polarization - VHF-SSB/CW is horizontal. The difference is said to be 20 db or so. Anything over the horizon is excluded. Satellite communication is often with circular polarization.

FM broadcast towers look like they are vertical - so I presume the polarization is vertical. Television is similar. FM/VHF/UHF antennas are horizontal polarization.

Now, for FM broadcast stations - what is the polarization of the transmitting antenna?

 

[Nigel Goodwin]

It's fairly country specific, in the UK it works like this:

TV - main stations - horizontal.
TV - relays - vertical (with a small few horizontal exceptions).

FM radio - slant polarity, works on either H or V aerials.

Satellite - both horizontal and vertical used, with dual band as well.

 

[mvs sarma]

Hai friends
I want to make an antenna for my FM radio to receive a channel at 102.6 MHZ. Do you have any idea ?

Hi,

it depends on the distance between the station and your location. if it is far off better use three element ant dimentioned for 102.6MHz.

and it will be horizontal and oriented towards the sation concerned. you may have a single stage amplifier using bft66 or silmilar transistor. output you may couple it to your radio thro a small cap.

 

[Gayan Soyza]

Hai friends
I want to make an antenna for my FM radio to receive a channel at 102.6 MHZ. Do you have any idea ?

There is no use making a FM antenna if there is no enough Signals coming from transmition stations, (due to obstacles,Weather, etc.....it can reduce the power of the wave form.)

I don’t know in which country you are...FM broadcasting Stations are not in a one particular place in most countries.(Not only FM, UHF & VHF as well). One station is in north & the other is in south………. So you need to rotate the antenna to focus each stations. This will not be a problem if you have All the broadcasting stations in a one place, so you need to Focus only to a one direction.

Simple steps to calculate the length of the dipole.
Note:the number after the decimal point represent superscript

V = velocity of the waveform (3x10.8ms-1)
F = frequency (Hz)
λ = length of one cycle of the 360.D waveform (m)

FM range 88 MHz to 108 MHz

Avg F 	= (108+88)/2 MHz
	= 98 MHz

Applying V = F λ formula

(3 x 10.8) ms-1 = (98 x10.6) Hz x λ
300/98m = λ
3.0612m = λ

In λ/2 maximum voltage will present in the middle of the λ/2 conductor.

So the length of the dipole will be λ/2 = 1.5306m

If you need only for 102.6 MHz replace F in the formula & calculate The value of the dipole.

 

[RadioRon]

To answer Stevez question: In North America most FM broadcast uses circular polarization. Ideally, this means that you may use a vertical or horizontal antenna and get equal signal strength.

This link **broken link removed**
shows a typical FM broadcast antenna up close showing two elements mounted to the tower. Each element is a circularly polarized dipole setup.


The simple formula given in the previous post does indeed calculate the half wavelength of a signal in free space, but a practical dipole antenna must be somewhat shorter. I recommend multiplying that figure by 0.8 to account for this. The reason for this is that when the signal induces current to flow in the dipole, that current will be slowed down a little bit (similar in concept to the "velocity" of coax cable) compared to the speed of light, making the distance travelled a bit shorter.

 

[Gaston]

you also need to consider the impedence of the input. for 300 ohm you can use 300 ohm twin pair wire to make the dipole. you can do the same for 75 ohm but you have to use a matching transformer.

 

[RadioRon]

you also need to consider the impedence of the input. for 300 ohm you can use 300 ohm twin pair wire to make the dipole. you can do the same for 75 ohm but you have to use a matching transformer.

If you make a "folded dipole" (eg. https://www.radio-electronics.com/info/antennas/dipole/folded_dipole.php) and put it up in the air at least 6 feet, then it will have a feed impedance of around 300 ohms and you can use the twin pair wire directly.

If you make a dipole (not the folded type) and put it up in the air, then its feed impedance will be closer to 70 ohms. A beginner may solder a 50 ohm coax cable directly to a dipole and expect it to work reasonably well without using a transformer. I have built many wire dipoles without transformers (often called a Balun transformer) and found them to work well. In fact, strictly speaking, the transformer is not actually there to alter the impedance but is in fact there to transform a balanced load (the dipole) to an unbalanced feed (the coax). When you leave out the transformer, you distort the current flow on the dipole in a way that the beginner won't even notice, as the effect is too subtle.

Why do we worry about the feed impedance of an antenna? The reason is that we want to get power from the air, into the antenna and then down to our receiver. In order to get the most power down to our receiver we have to maximise our power transfer by "impedance matching". This link explains this a bit more: https://en.wikipedia.org/wiki/Impedance_matching

 

[dasappettan]

thank you Gayan Soyza

There is no use making a FM antenna if there is no enough Signals coming from transmition stations, (due to obstacles,Weather, etc.....it can reduce the power of the wave form.)

I don’t know in which country you are...FM broadcasting Stations are not in a one particular place in most countries.(Not only FM, UHF & VHF as well). One station is in north & the other is in south………. So you need to rotate the antenna to focus each stations. This will not be a problem if you have All the broadcasting stations in a one place, so you need to Focus only to a one direction.

Simple steps to calculate the length of the dipole.
Note:the number after the decimal point represent superscript

V = velocity of the waveform (3x10.8ms-1)
F = frequency (Hz)
λ = length of one cycle of the 360.D waveform (m)

FM range 88 MHz to 108 MHz

Avg F 	= (108+88)/2 MHz
	= 98 MHz

Applying V = F λ formula

(3 x 10.8) ms-1 = (98 x10.6) Hz x λ
300/98m = λ
3.0612m = λ

In λ/2 maximum voltage will present in the middle of the λ/2 conductor.

So the length of the dipole will be λ/2 = 1.5306m

If you need only for 102.6 MHz replace F in the formula & calculate The value of the dipole.

THANK YOU Gayan Soyza FOR YOUR Detailed explanation.

 

[dasappettan]

THANK YOU for all for your detailed explanation

 

[vkssingh]

I live in India and want to attach a Antenna amplifier for my car FM player. I an planning to construct the circuit given at page https://electronicsdesign-deni.blogspot.in/2011/06/vhf-antenna-amplifier-circuit-using.html.
Will this circuit work for car FM radio to get clear the FM stations in 98 to 102 MHz range?
Any replacement for BFT 66 & BF240 which is available in India.

 

[ronsimpson]

vkssingh,
I think you will not be happy with a amplifier in your car.
A radio does several jobs to get a signal.
(amplify, reject noise, reject other stations)
Only amplifying will help only a small amount.

 

[MikeMl]

Are you driving around while trying to receive a distant station, or are you using an automotive receiver inside your house?

 

[audioguru]

In 1962 I bought a huge Yagi antenna for my FM stereo receiver. It took up the entire ceiling space of my bedroom. It picked up a distant station that I liked.
If you add an RF amplifier to the antenna input of your car radio then the new RF amplifier will become severely overloaded when you drive anywhere near a city. Then you might have the local stations all across the dial.

 

[vkssingh]

Thanks every one for the reply. I live in area near city and I want to improve the reception when driving around. from the replies still appears that installing a Antenna amplifier is not a wise idea, still I want to give it a try. I am also considering circuit at " **broken link removed** ". Any comments! Has some used such circuits or suggest a better circuit to try?

 

[MikeMl]

...I am also considering circuit at "...?

That amplifer only goes to ~70MHz. You need an amplifier that works at 88 to 108MHz.

What kind of FM antenna is on your car? Post a picture of it.

 

[audioguru]

Get a better car radio.
The FM reception on my car radio (the car manufacturer installed it when the car was made) is fantastic. It is very sensitive and it is very selective to receive the almost one hundred FM stations in my area. It is never overloaded by the many strong local stations. Its antenna is on the windshield glass.

 

[Willen]

Hi AG,
Which manufracturer made your car radio? And which thing made the radio so sensitive and selective (if you saw inside the radio) or which chip used there so the radio became so sensitive? I am excited to your car radio from 2 years because it receives signal 2KM far from your simple Mod-4 FM Tx. I made almost 3 or 4 versions of FM Tx same as Mod-4 but got just 500 to 700 meters poorly on my cellphone or cheap Chinese radio. Wishing to get a sensitive radio like an explorer have!