R/C antenna design query

[Neil Robinson]

I hope this is the appropriate section to post this; it seems to have the most R/C topics.
A mate has an MTH DCS system and the range is rather limited. According to some sources this isn?t uncommon as the system may have been initially intended for indoor O gauge layouts.
A website has details of an improvement involving a longer antenna and quotes a length, in inches, to two decimal places.
I appreciate that optimum antenna length is a function of the frequency but I was wondering where the antenna actually starts.
If the length were that critical, to within one hundredth of an inch, would it involve any PCB tracks from the first active component and/or any part below the pcb joint? This is on the top face of a double sided pcb but the antenna wire extends through and a little below the pcb to ensure a good joint.

 

[bobg]

Neil, try converting the size given to metric, and see if the decimal places came about that way.

 

[treetop]

Hi Neil,
What frequency are you talking about with this system?
If it is 2.4GHz then I expect that the antenna length is quite critical.
If you divide the speed of light by the frequency you obtain the corresponding
wave length. ie 300 million m/sec divided by 2.4GHz = approx 120mm wave length.
This is very small, and a typical 1/4 wave antenna will only be 30mm long.
I have confirmed this on two of my receivers. The antenna is constructed at
the end of a coax cable by removing the outer shield for a length of 30mm.
For lower frequencies, eg 27MHz, the antenna lengths are much greater.
A quarter wave may be over 2.5m long. A much shorter piece of wire must be
used in a modeling application.
I suggest that in your situation, transmitter output power, or lack of, is
a much more likely problem than antenna length.

Dean.

 

[Neil Robinson]

Hi Neil,
What frequency are you talking about with this system?
If it is 2.4GHz then I expect that the antenna length is quite critical. <snip>
I suggest that in your situation, transmitter output power, or lack of, is a much more likely problem than antenna length.
Dean.

Hi Dean,
Yes, the transmitter is relatively weak compared to two others belonging to other mates with the same system, much much weaker than one and a fair bit weaker than the other. Swapping batteries and using different receivers makes no difference. However fixing a longer external antenna to the receiver is quite a simple, reversible, task.
The manufactures quote a frequency of 900MHz which gives a wavelength of 333mm using 300 million m/s for light speed.
The website modification mentioned suggests increasing the existing short antenna to 12.88 inches (approx 327mm).
I'm wondering if the 6mm discrepancy is due to part of the effective antenna length being on the pcb or the quoted 900MHz being nominal or an error on that website.
I assume a 6mm error could make a difference at this frequency.

 

[treetop]

[/quote]
I'm wondering if the 6mm discrepancy is due to part of the effective antenna length being on the pcb or the quoted 900MHz being nominal or an error on that website.
I assume a 6mm error could make a difference at this frequency.
[/quote]

Neil,
I would not get 'hung up' about the exact length to the mm. (The '300' figure is an approximation anyway)
A full wave antenna is probably better than a 1/4 or 1/2 one.
I am interested to know how long the antenna is on the Transmitter?
Dean.

 

[The Devonian]

Neil, I think I am correct in saying that 900 Mhz. is not a legal model control frequency in the UK. Spike or one or two others may confirm or contradict. Most of the band seems to be in use by mobile 'phones and similar devices.