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fm transmitter kit - new antenna

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brad

New Member
I have a simple fm transmitter I bought from canakit to play mp3's all over the house. When making an antenna it told me it had to be a certain length of wire...they had a formula for it - I can't find it right now though. Why does it have to be that certain size? I keep reading about whip antennas and fancy tv antennas...can I attach anything better to this thing or does it have to be a simple wire? Can I make it as long as I want still adhering to the formula? Thanks a lot
 

brad

New Member
oh yeah...another thing.

how strict is all that FCC stuff? if my mp3's can be picked up on a radio a couple blocks away am I going to be paying a fine or something? I live in Canada..is it any different here than in the states??
 

NightFlight

New Member
There is a direct relationship between the wavelength of the transmitted energy and the physical length of the antenna. Since the electrical signal is coming from the transmitter is an AC signal, the antenna must appear as a reasonant circuit to the frequency that is fed into it in order to present a minimum impedance.
In otherwards, the output signal attempting to aquire access to the antenna (and jump off at the speed of light) will not be able to "see", or rather bounce off the antenna material (impedance mismatch) and not transmit at all.

As for the FCC, I wouldn't worry about that, your transmitter is transmitting on frequencies which are allocated as public, and you can't change that easily , certainly not by messing with the antenna.

nf
 

stevez

Active Member
Regarding the FCC or Canadian equivalent - I do not think either agency is staffed/equipped to go hunting for low power radio stations however there is a risk that you might cause a problem with a neighbor who is trying to listen to a very weak station on your frequency. Your low power may not be much but given the proximity it might overpower the commercial station. That neighbor just might call the agency. In my opinion if you followed the directions in the kit and didn't add amplifiers then even if someone complained, it seems likely that the agency would just ask you to stop.

It would seem that the formula for getting in trouble would be to transmit out of band, and/or at higher power and cause interference with folks who would complain to the appropriate agency. Let me point out that interference with commercial services (or amateur radio for that matter) will get the attention of local techncians who do have the means to track down the source of interference and they'll look to the agency for assistance because it's impacting their ability to make money (or persue their hobby legally). The more complaints the greater the likelihood of a problem. This paragraph is not really aimed at you or anyone in particular but there seem to be numrous posts about jammers or other unauthorized transmissions.
 

brad

New Member
ok thanks, that makes sense. It says it should be 1/4 or 1/2 of the wave length of the frequency and tells me how to figure it out.

Also I keep reading about the vertical/horizontal orientation of the wire and things like that. It says it depends on the radio...well what do most radios need? Does this wire have to be stretched out flat? Coiled up? Would it be better to find an actual telescoping antenna the right size to make it go further or will the copper wire do?
 

glen

New Member
antennas

im no expert but a few things about antennas. like u said the length should be a multiple of 1/4 wavelength. the orientation of the TX antenna should match the orientation of RX antenna so if your recievers have their antennas sticking straight up then so should your transmitter. also get better transmission over conductive surfaces(wet ground or whatever). not very practical in the house. the shape of your antenna will effect the gain of your antenna. gain of a passive antenna just means directivity of the signal. a staight monopole gives equal strenght in all directions so is good if u want to move around. if u want to focus in one direction then u could use a dipole. a bit of metal tubing might make a better antenna than a bit of wirebut im not sure. one thing to check if u want to boost the signal is if there is an attenuator at the output of the transmitter circuit. these consist of a few resistors before or after the output filter that attachs to the antenna. they are sometimes included in kits to make the transmitted power within the legal range. hence its illegal to bypass these resistors.
 
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