FM receiver antenna

[BANE]

Hello all,
I have a 6ch FM receiver http://www.lynxmotion.com/p-72-hitec-6ch-rc-set.aspx and i want to know what is the best antenna position or design.  From what ive observed so far, it seems a thin tube with the wire running through it in an upright position is most popular among RC cars.  But i need something a little more robust for my robot.  I have another  expendable antenna that came from an old RC transmitter; could i solder the receiver wire to this antenna? 

any words of wisdom is greatly appreciated

 

[Soeren]

Hi,

[...] i need something a little more robust for my robot.  I have another  expendable antenna that came from an old RC transmitter; could i solder the receiver wire to this antenna? 

A length of piano wire with an eye each end (one for bolting it to a piece of PCB or similar, the other to not loose one of your own) is probably the most robust and well tested that you can find.
The (total) length will depend on the frequency of course.

[BANE]

its 72mhz i believe.  not sure how to do the calculation from that to get the length.  just out of curiosity, should the transmitter antenna length be the same as the receiver?

Also, i was thinking about cutting a car antenna, would this work?

A length of piano wire with an eye each end

like this

[BANE]

i spend an hour or so today with a physics professor trying to determine the length and it turns out to be an nasty calculus problem and our answer was nowhere near reasonable. w=1/(LC)^.5   had to do some guess work on the capacitance and inductance which didnt help.   So it is guess and check time!

[waltr]

The easy, cheating, way is to measure the length of the wire antenna that came with the RC receiver then make the rod the same length.

In practice the antenna length on an RC receiver is not too critical.

[Soeren]

Hi,

i spend an hour or so today with a physics professor trying to determine the length and it turns out to be an nasty calculus problem and our answer was nowhere near reasonable. w=1/(LC)^.5   had to do some guess work on the capacitance and inductance which didnt help.   So it is guess and check time!

Physics professors *tsk tsk*   

Tell him that the formula is:
Wave length, λ [m]= Speed of light [m/s] / frequency [Hz]
So that he won't torture another student for an hour 

The Speed of Light is 299,792,458m/s

A wave length at 72MHz (M means Mega and Hz means Hertz) is thus
299,792,458/72,000,000 = 4.164m

If you round up SoL to 300,000,000 and slice off 6 zeroes each side of the division sign, it gets easier to handle:
300/72 = 4.167m   (an error of less than 0.1%)

For peak reception you'd use (a half or) a quarter of a wave length (not going into other possible fractions here), which makes it (2.08m) 1.04m.
(Now 2.4GHz looks really appealing, right? )

This calculation is only needed for a ball park figure in your app., as there's nothing critical. The transmitter antenna might be more so, but I think most R/C equipment is fairly sturdy these days - just don't transmit completely without an antenna, as you may burn the power stage of the transmitter due to standing waves.

Nothing critical, as mentioned, so you can get by with less than optimum length and a length of 1/8λ should even do for this purpose, so just make it 52 cm if the connecting wire is short (otherwise, subtract its length from the 52cm).


If going for optimum range, it's quite another story, involving trimming the antenna length by nibbling off 1mm at a time until you just passed the point of max (no, you don't glue back 1mm then ). But this is more for comms radios and such - close to electrical motors, it would be a challenge anyway.