FM transmitter antenna

[juan123]

Well, I posted about modifying a tunecast FM transmitter. I added an antenna and the range has improved dramatically. Well my query is , how could I build an antenna that would not use too much space, but at the same time give me much more range than what I have now? My current antenna is just a 15ft long wire going up to the 3rd story of my house, from the 2nd story. I'd like to build an antenna that would boost my signal much more than that wire. I'm thinking maybe an electronic antenna that is powered?(If those exist.) I'm kind of a newbie with electronics, but I do know basics-intermediate level of electronics. Thanks for any help!
BTW the FM transmitter is working at 88.1-88.7 mhz, if that helps at all to help you experts with the numbers. Thanks!!!!!!!

 

[mister_e]

I don't know in your country but i believe it's illegal to broadcast within this frequency range for long distance. FM modulator and cheap wireless microphone use it... but for few inches, feet.

 

[stevez]

Juan - I'd encourage you to work within the limits of the laws. With that said I'll offer some information in the form of comments or statements.

A. Some antennas radiate in all directions - some in specific directions. Directional antennas can provide a stronger signal in one direction (at the expense of other directions) with the same level of power. A beam or yagi beam can do that.

B. A transmitter connects to an antenna directly or via a cable/wire called a transmission line. The transmitter output will be maximum when the transmitter is matched to the load - which is the antenna/transmission line.

C. If the transmitter connects to the antenna via a cable/wire - a transmission line - one needs to consider that all of the power won't get to the antenna - some will be lost in the transmission line. The lowest losses occur when antenna and transmission lines match and the transmitter and transmission lines match.

D. An amplifier could be used to increase the power output of the transmitter.

E. Transmitters put out signals on the frequency of interest and other frequencies. Simpler transmitters are likely to be worse in this sense. A better antenna will radiate the stuff you want better - and maybe the stuff you don't want to radiate. The same is true for an amplifier - and it can generate it's own extra RF signals.

I am sure this barely scratches the surface for you however it's not a simple subject.

 

[HiTech]

connect it to one leg of the AC wiring in your apartment, that way you'll use a majority of the wiring as an antenna. That'll do ya!

 

[RadioRon]

Well, I posted about modifying a tunecast FM transmitter. I added an antenna and the range has improved dramatically. Well my query is , how could I build an antenna that would not use too much space, but at the same time give me much more range than what I have now? My current antenna is just a 15ft long wire going up to the 3rd story of my house, from the 2nd story. I'd like to build an antenna that would boost my signal much more than that wire. I'm thinking maybe an electronic antenna that is powered?(If those exist.) I'm kind of a newbie with electronics, but I do know basics-intermediate level of electronics. Thanks for any help!
BTW the FM transmitter is working at 88.1-88.7 mhz, if that helps at all to help you experts with the numbers. Thanks!!!!!!!

I am glad to hear that you have succeeded in improving the range. What you are using right now is a random long wire antenna, one that is not specially tuned for your frequency, but rather just made as long and as high as you can. This is a good first step. The next step may be to try and tune what you have or replace it with a slightly different version that is better tuned. The trouble is that I am not confident that you can measure or sense small improvements in your antenna. How will you be able to tell if it improves by, say, only 20% more range?

A resonant antenna, such as a dipole might work a bit better, but you would have to make the antenna and then place it up high and then run a transmission line. Do you have any coax or twinlead transmission line?

It is possible to make a directive antenna for more signal, but you would need to master the basic resonant half wave dipole first.

 

[chiba]

How about this then?

 

[stevez]

Chiba's attachment shows a dipole. If kept clear of other conductive objects it will have a feedpoint impedance that will be an approximate match for 50 or 75 ohm cable. A common output impedance for transmitters is 50 ohms so this ought to be a good match. If all you have is 300 TV twinlead you can make that work with baluns (transformers) or you can just make sure you use integral odd multiples of half wavelength twinlead. Note that the wavelength must be the electrical wavelength for the cable - not in free space -or about 92% of the free space wavelength. This will essentially repeat the feedpoint impedance at the transmitter and provide a good match if it's 50 to 75 ohms.

For line of sight communications polarization matters. If you orient the transmitter antenna horizontally then best reception will occur if the receiving antenna is horizontal and the same holds for vertical - transmitting antenna and receiving antenna should be vertical. A dipole can be mounted either way.

 

[juan123]

Thanks!

Thanks, I will try all of your suggestions.....Recently I've been having some trouble bc a radio station operates at 88.1 , which is the frequency I use and there's other radio stations at 88.5, 88.9. My transmitter only works with : 88.1, 88.3, 88.5, 88.7...which would mean that I will have a lot of interference, but thanks, I will try all your suggestions to see if I can outpower them, at least in my home. Thanks!

 

[atari7202]

i am trying to do that to. with my ipod transmitter i have all of the parts to make this https://hem.passagen.se/communication/pa313.htmlone but havent yet.
here is the main website it has tons of info that is very usefull https://hem.passagen.se/communication/pa313.html ooh and any body is it possiable to build a toy car amplifier {for the antenna go long distance} like frequeces like 45,27,49mhz to name a few

 

[chiba]

1.5W Amp on 88-108Mhz can get you in a big hot steaming bunch of trouble, I'm not sure what the EIRP limit is in your area but i think it's probably in µwatts, may be ok inside a fallout bunker if it's got lead lining.

 

[Hero999]

connect it to one leg of the AC wiring in your apartment, that way you'll use a majority of the wiring as an antenna. That'll do ya!

I know you're joking but it wouldn't work, all the power will go straight to ground.

 

[runrun395]

for the maximum antenna range you want to have an antenna that is a multiple of your wavelength. You can calculate your wavelength by dividing the speed of light (300Mm/s) by your desired frequency, in this example (93.5Mhz = 93Mλ/s). Big M means Mega or Millions for those who don't know. The Greek lowercase letter lamda "λ" is a scientific symbol for wavelength. So if we divide 300Mm/s by 93.5Mλ/s we can simplify that to 300m/93.5λ. When you divide it out, you get 3.20855615 meters per wavelength. For the best signal you want to take a multiple of this wavelength as your antenna length. So I might take a multiple of 5 and make a 16 meter antenna. The higher the multiple, the better the strength BUT you may get to the point where the resistance of your antenna in ohms (Ω) may reduce the current, and by extension, the power of your transmission. I'm not sure what the optimal factor is, but I'm sure unless you are making a 50ft or higher antenna with crappy wire, you shouldn't have a problem. The best way to do this is to figure out, with a multimeter, the resistance of the antenna that is made for your transmitter and try to find the wire where, at your desired length, has a close resistance to what is needed. This may seem like a lot of effort, but I find that my little 20W Transmitter with the best antenna hookup, beats the pulp out of my friend's 300W radio station because he never bothered with this. The right antenna makes MILES of difference.

Gave those of you without thanks some thanks. You're welcome... or am I welcome? Hmm...

 

[mneary]

1) 4 year old thread.
2) An effective antenna does not require multiples of wavelengths.
3) You do not confirm the correct antenna length with an ohm meter.

 

[runrun395]

1) Didn't see that, my mistake
2) The more the merrier. Why do you think radio towers have such long antennas? They also do this math when they build their antennas.
3) Of course not! I wasn't saying that! But you do need an antenna with the same resistance. You need to find a wire that has the same resistance at _ meters as the other antenna has. So if the antenna that came with the product is 75ohms at 1 meter, I need to find a wire that will provide 75 ohms at 16 meters.

 

[PatM]

1) Didn't see that, my mistake
2) The more the merrier. Why do you think radio towers have such long antennas? They also do this math when they build their antennas.
3) Of course not! I wasn't saying that! But you do need an antenna with the same resistance. You need to find a wire that has the same resistance at _ meters as the other antenna has. So if the antenna that came with the product is 75ohms at 1 meter, I need to find a wire that will provide 75 ohms at 16 meters.

2)With AM radio the tower is the antenna and is generally 1/4 wave length high - Tv towers are high to give line-of -sight transmission.
The tower is tall, but the antenna at the top is very small compared to the tower.
3) When you talk about a 75 ohm antenna, you are talking about "Impedance" not "resistance" You have the terms confused.
The impedance of a transmission line is due to it's construction and has nothing to do with it's resistance due to length.
A 10 foot 75 ohm coax is still a 75 ohm coax if it is 100 ft long. Generally the length of the coax is determined by the frequency you are operating.

 

[mneary]

2) FM (broadcast) antenna towers are tall to get above other objects. Length of individual elements are usually about 1/4λ and arranged to achieve specific radiation patterns. "AM" (broadcast) antennas are tall because 1/4λ to 5/8λ is tall.
3) Resistance of the wire within an antenna is pure waste.
3a) The length of the coax is determined by the distance it must go, generally the more direct the better. (A coax can be used as a tuned element but that is a very specialized function.) Resistance in a coax is waste, too. Impedance of coax is related to the ratio of inductance to capacitance per unit length of coax.

If you become tired of guessing how antennas work and want to actually understand them, some clever guy named James Clerk Maxwell wrote some papers in 1861 thru 1873 that explain it in mind numbing detail. If you want information you can use immediately, I suggest the ARRL Antenna Book isbn 0-87259-613-3. 728 pages plus CD-ROM $30.