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Thread: Receiver aerial wire lengths

  1. #37
    Quote Originally Posted by tak racing View Post
    Another 2 for the list:

    Futaba R114F 40Mhz - from outside of case - 692mm
    Futaba R133F 40Mhz - from outside of case - 532mm


    Intriguing... as I just replaced the wire on my R133F and it was 467mm total length.

    In the past a shop sold me an antenna for the same receiver which was 500mm long (tried the Maplin equipment wire this time - hopefully it will be OK).

    Maybe the antenna length is not as critical as I once thought (within a reasanable margin)?

  2. #38
    Guest Member mad-wolfie has a level 1 reputation
    Join Date
    Nov 2007
    Stoke on Trent (UK)
    I've just come across this thread when googling looking for a replacement transmitter aeriel & it makes some some interesting reading.

    Quote Originally Posted by dhutch View Post
    Your all going hate me for this, but as far as im conserned, the whole thread is flawed.

    Yes, there is an optimum length for a aerial, and yes it is do to with frequency, and harmonics of that.
    - An good aerial is a "quarter wave" aerial, ie, its 1/4 of the length of the wave.

    The relationship between wavelength and frequency is given below.
    - Where F is frequency, c is the speed of light, and lambda is wavelength.

    So if we rearrange the equation:

    - The speed of light (c) is 3 x 10^8 (m/s) (300 million meters per sec)
    - And for a "27meg" radio, the frequency is 27Mhz, or 27 x 10^6 (Hz)

    So for a 27meg radio, wavelength = 3 x 10^8/27 x 10^6 = 11.1 (m)
    - So a quarter wave aerial would be 2.78meters long!
    - Thats quite a lot longer than even your transmitter aerials!
    - And for 40meg its 7.5meters (1/4 = 1.87meters)

    The difference between 26.995Mhz(brown) and 27.245Mhz(blue) is negligible, and therefore its quite acceptable to use the 27.000Mhz for calculations, and you dont need to alter it for different colours as the change in length would only be a few millimetres. (0.025m infact)

    So basically, our aerials are massively shorter than even a 1/4wave aerial.
    - But they still work fine for our purposes, simply because we're never that far away for it to really be an issue.

    So in practical terms, this just means that for best range, you simple have the longest exposed length of aerial that is convenient (up to a length of 2/3meters that is)

    I hope that all make sense, if not, just ask.

    OK time for me to put the spanner in the works..

    those equations & maths Daniel quotes are correct "technicaly" speaking... but realistically it only applies to radio waves that are being transmitted... for recieving radio signals you could use a piece of wet string, a sheet of tin foil & a drawing pin, or simply tie a length of bared wire to an old biscuit tin. But basically the more aerial you have exposed or the more conductive to electricity the wire or aerial the better the signal will be recieved.

    If you ever came through the CB revolution you may of remembered SWRing in which basically meant metering up the aerial wire & chopping bits off the end of the antenna to get the lowest rating possible - which basically meant 1/4 of the wave & you always measure the SWR on the transmit, not recieve. I know you may think that "what's CB got to do with RC" well quite a lot actually as CB runs on 27MHz so a lot of the stuff in the text books regarding transmitting / receiving radio waves is relevent to RC if running on 27MHz & SWR Or (standing wave ratio) is used on all types of transmitters & 2-way radio's. CB aerials are also about 1.5m tall & use a load (basically a coil of wire at the base of the antenna) to simulate the use of a longer aerial because when they legalised CB they also set a maximum height ruling & as was said the optimum height should be 2 metres. the SWR value also depends on the run of the cable & how long it is as that will affect the reading.

    Quote Originally Posted by PDW View Post
    So why do the manufacturers all have different lengths, and why are those lengths too long for the majority of RC cars, leaving us to wrap them up, or find some other method of 'losing' the excess length?

    And why do the manufacturers tell us not to cut the aerial length, in most cases?
    I'm not quoting on this, this is purely from my own experience. Usually if i'm replacing an ariel wire & i don't know the original length, i will try to cut it to about 50mm which seems to be a figure just above what is the average values stated (46cm is about the right value for 1/16th of the main wave on 40mhz if you do the maths in the equation) - this seems to be a good all round number & also gives you a couple of cm to play with the get the wire from the box to the pipe/mast on the car. As has already been said the 1/4 wave ariel can be cut in half & still function, however this now goes back to my previous point.. the more aerial you have the more likely it is to recieve a better signal.

    So to answer Pete's point i'm guessing that the likes of Futaba, Acom, Sanwa etc have worked out that the length they are using on that particular model of their receiever is probably the optimum minimum length for that design, so it probably is best to keep with the length of wire measurements it came from the factory with & probably explains why they say "don't cut the ariel down or shorten it" in their literature & of course they have probably taken into account that a lot of fiddling to make it fit into the car has to be done, so i'm also guessing they use the bare minimum they can & going too far below the magic 1/4 wave will start to have an effect!

    It's also worth noting that some wire materials have different conductive properties & as i said in my previous statement, that can affect how good/bad the signals are being recieved. Personally if i change an aerial wire, i use model Railway wire - the thin stuff for wiring up 00 gauge model railway lines.. it's fairly strong (so it doesn't get cut so easily if going under timing bridges or if the car is sliding on it's roof a lot), the added benefit is it's also often sold in most model shops so easy to get hold of plus it's alluminium or low grade steel based wire so seems a better conductor than the more usual copper & also it helps with Skin effect - Skin effect is basically how good the wire absorbs & distributes AC signals & as we all should know radio waves work in much the same way as AC in an electrical circuit using sine waves. The railway wire is a bit more expensive than using regular copper wire from craplins & no doubt the model shop owner will sell it at a higher cost by the metre than if you bought a roll, but if your club has an overhead timing bridge that your antenna pipe/mast catches as you pass under it rather than using an under-floor loop it's probably worth the extra few pence.

    Hope this helps

  3. #39
    RaceChat Supporter John_Parker has a level 1 reputation
    Join Date
    Apr 2003
    Cambridge, UK
    While this thread might be getting obsolete, I am just replacing my KO 302F aerial, so will note here that the original length is 450mm from the case. Very annoying that it is so long, when the older KOs are more conveniently shorter
    John Parker - Cambridge RC Circuit Car Club

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