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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.

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  • \$\begingroup\$ Quarter-wavelength is the normal best. \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 30 '14 at 21:45
  • \$\begingroup\$ @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! \$\endgroup\$ – Ultralisk Sep 30 '14 at 21:52
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    \$\begingroup\$ The original antenna may have been base loaded with an inductor to make it resonate at a shorter length. \$\endgroup\$ – Andy aka Sep 30 '14 at 22:35
  • \$\begingroup\$ @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. \$\endgroup\$ – Ultralisk Oct 1 '14 at 7:29
  • \$\begingroup\$ Be careful to not exceed legal limits for license free transmitting. \$\endgroup\$ – Optionparty Oct 1 '14 at 14:02
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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.

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  • \$\begingroup\$ The length is important if the antenna is loaded with a specific inductor and the new length is very different. \$\endgroup\$ – David Oct 19 '14 at 9:01
  • \$\begingroup\$ 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. \$\endgroup\$ – Ultralisk Oct 19 '14 at 15:03
  • \$\begingroup\$ 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. \$\endgroup\$ – chik man Oct 19 '14 at 15:54

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