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One question about antenna sizes for fm transmitter

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polishdude20

New Member
SO i did the conversion and calculated that the ideal antenna length for my fm transmitter is about 80 cm . But my transmitter only runs on 9 volts! not very powerful! with a 20-30 cm antenna have connected now it only goes like 20 meters or so or even less without walls. so will increasing the size actually do anything even if its 9 volts? wont an antenna that long for 9 volt transmitter cause too much loss and do the opposite?
 

cobra1

New Member
an antenna should be tuned to the transmitter its on, if your antanna is 80cm (halfwave) then i estimate your transmit frequency to be 180mhz.

if you try to transmit with a 20-30cm antenna you will get major losses and also damage to the transmitter can also be caused.

the size of the antenna will not really affect to battery usage.
 

ke5frf

New Member
The size of your antenna has absolutely NOTHING to do with your supply voltage.

If the calculation you have made for your antenna, per your transmitter frequency, is 80 cm, that is exactly how long the antenna should be. The 20-30 cm antenna you have fabricated is presenting an improper impedance to the transmitter power amplifier and will cause it to get hot and burn up. Not only that, but it will simply not work efficiently and much of your transmitter power is wasted as heat in the antenna itself.

If size is a concern, there are loaded antenna designs that allow shorter elements... They are not as efficient as a properly tuned antenna (there will be loss), but they do at least keep your amplifier from getting too hot because the impedance the amplifier sees is a match.

Here is an absolutely thorough discussion of antenna loading:
Mobile antennas, short verticals, loading coil loss,and loading coil current

But keep in mind, your OPTIMAL antenna length depends on frequency. Voltage is not a factor in determining antenna length. If your calculation says you need an antenna X long, either build an antenna that is X long or devise a loading coil to cancel capacitive reactance which allows a physically shorter design.


Oh....one more thing. It is extremely advisable that you either build or purchase an antenna analyzer and perhaps a field strength meter for your experiments. Without test equipment you are running blind and the only practical test of your equipment efficiency would be to measure the temperature of your amplifier while transmitting and perform receiver tests from a distance. But even then you would be working blind.

At a MIMIMUM I might advise an VSWR meter to monitor standing waves on the system, which can be of assistance in tuning your antenna if you use it properly.
 
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audioguru

Well-Known Member
Most Helpful Member
He made an extremely simple very low power FM transmitter that drifts its frequency all over the place and sounds awful. It uses only a single transistor.
It does not have a schematic and is made "dead bug" style without a pcb.

Its range might be more if its antenna is connected to the collector of the transistor instead of to the emitter.
 

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cobra1

New Member
are you trying to send the ipod music to your car stereo???
if so there are adapters that do this on ebay for about £10 and they work properly.

1/2 wave dipoles are most commonly used in radio, a 1/2 wave dipole is made up of 2 1/4 wave antennas thats why you get the 1/4 wave length and the 1/2 wavelength.

if your trying to get this on a stereo, why transmit @ 180mhz??? standard radios i belive work between 88mhz and 108mhz, so you need to transmit somewhere in this range.

what frequency have you built your transmitter to work on
 

Sceadwian

Banned
I've seen VERY few halfwave dipoles used for such a thing, almost all broadcast band FM transmitter and receiver antennas are 1/4 wave whips, using the ground plane as the counter poise.
 

cobra1

New Member
i stand corrected lol, most of my experience with antennas is from amateur radio, so i would either use a yagi or dipole antenna
 

ke5frf

New Member
also whats this with 1/4 wave or half wave? what is optimal?

Horizontally polarized antennas, particularly dipoles, are typically λ/2 long, made of two λ/4 elements on a horizontal plane.

A typical vertical antenna is a λ/4 element working against a conductive surface such as an automobile, a ground plane, radials, or the ground itself, with the vertical element of course vertical and the other elements (4 or more radials) oriented horizontally. Size of the radials need not NECESSARILY be λ/4 themselves, but the more you have and the greater surface area, the more efficiently your radiator will perform.
 

ke5frf

New Member
I've seen VERY few halfwave dipoles used for such a thing, almost all broadcast band FM transmitter and receiver antennas are 1/4 wave whips, using the ground plane as the counter poise.

Technically speaking though, even a vertical λ/4 whip is a λ/2 dipole. But that is another can of worms.
 

audioguru

Well-Known Member
Most Helpful Member
Music on that simple transmitter will be received with no treble frequencies because the simple circuit is missing pre-emphasis that all FM radio stations use and that all FM radios have de-emphasis which is the opposite response.

Its frequency will change if anything moves toward, moves away or looks at the circuit or its antenna :D, and as the battery voltage drops.
 

ke5frf

New Member
I've seen VERY few halfwave dipoles used for such a thing, almost all broadcast band FM transmitter and receiver antennas are 1/4 wave whips, using the ground plane as the counter poise.

Actually, most FM broadcasters would not be using a λ/4 vertical, more likely a 5/8 wave design.
 

audioguru

Well-Known Member
Most Helpful Member
Almost all cars use a half-wave vertical FM antenna about 80cm long.
 

Sceadwian

Banned
5/8ths wave? Could you provide some references links for me Ke5? Antenna's are still mostly black magic to me, I would greatly appreciate any links you could provide on various antenna designs (and why they were designed that way) that aren't excessively heavy on the math. Might as well try to put this thread to good use =)
I know how basic whips and dipoles work they're really simple. I understand the construction of folded dipoles and biquads but not their operating principals. I've always had troubles visualizing the standing waves and how they interact with antennas Dipoles being the exception. Also never quiet understood why the impedance changes on a folded dipole so dramatically and more importantly why aside from compactness you would chose a higher or lower impedance antenna design.
 
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ke5frf

New Member
Yes.

Introduction

You have a small registration procedure now. The amateur who wrote this site recently passed away. L. Cebik, W4RNL and a commercial entity (antenna manufacturer) bought the rights.

He does use a certain amount of math, but is more inclined to go into layman's terms and use pictures and models to help you see the theory in action.

The 5/8 wave antenna has a certain amount of "gain" (a misleading term) over a 1/4 wave vertical because its angle of radiation is lower. This is effective for both long range DX and local broadcasting, because a lower radiation angle from a tower mounted antenna will put a stronger signal over the horizon than a 1/4 wave. The lower radiation angle also is particularly well suited for skipping signals off the ionosphere.

If you learn a few things about antenna design, you will find out that a 1/2 wave dipole, from which all antenna designs are based, is the best all around "average" antenna. But antennas based off multiple wavelengths or off-center fed have properties that can be used to increase gain (again a misnomer) by directing the radiation angle in a desirable direction or angle. Off center fed designs work because of multiple points in multiple wavelength antennas where the impedance can be suitable to feed the antenna, at least with a balun or matching device.
 
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