# Length of a 40MHz antenna

A few days ago I had the brilliant idea of replacing the ugly antenna of a 40MHz car remote control with an extendable one. Now the range decreased from 50 meters to 3-4m.

I don't know exactly what I broke so I have some questions, maybe I can fix it:

1. How long exactly the antenna has to be? I broke the original antenna so I cannot measure it exactly, but it was about 37.4cm long (+/-2cm). The problem is that if I divide 7.5m to 1/4 or 1/8 I don't get near the original antenna length? So, which is the closest 'round' number to 37cm?
2. The wire from the PCB to the new antenna behaves as an antenna and i has to be taken into account. Right?
3. How exact the length should be? Do I loose lot of power if my antenna is 2%-3% longer than it should be?
4. Maybe the problem is not the antenna but something else. For example: the new antenna is longer and goes very close to the PCB (to the quartz crystal). About 1.5-2cm close. Could it influence the correct functioning of the PCB?

Update:
I put the antenna back. Still doesn't work. I think accidentally touch and deformed some coils in the receiver.

• Quarter-wavelength is the normal best. Sep 30, 2014 at 21:45
• @IgnacioVazquez-Abrams Full wave lenght is 7.5m for 40MHz. Right? Then at 1/4 my antenna would be 1.875m long!! That's too much! Sep 30, 2014 at 21:52
• The original antenna may have been base loaded with an inductor to make it resonate at a shorter length. Sep 30, 2014 at 22:35
• @Andyaka - Actually I see two inductors (SMD) on the PCB. Also the antenna is not connected directly to the PCB but to a 0.3Ohm 'resistor'. But it may well be a coil built as a resistor. Even though the colorful stripes on it makes it look like a resistor. Oct 1, 2014 at 7:29
• Be careful to not exceed legal limits for license free transmitting. Oct 1, 2014 at 14:02

## 1 Answer

The antenna length of those RC remote is not that critical, almost all RC car antenna is underlength, the underlength antenna is very capacitive so a loading coil/inductor is usually added in series to absorption the capacitive component.

I guess your RC radio is just burnt. buy a new one to continue your test.

• The length is important if the antenna is loaded with a specific inductor and the new length is very different. Oct 19, 2014 at 9:01
• How much will the power (distance) drop if I change the antenna length with few centimeters? I think this is what happen in my case. Oct 19, 2014 at 15:03
• If you don't have a spectrum analyzer, and you want to investigate the antenna length vs distance relationship, you can start with a longer copper wire as antenna (say 500mm), test the range, cut 10mm, test the range, cut 10mm more and test the range, iterate until you find the optimum antenna length. Oct 19, 2014 at 15:54