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Rx antenna length?

Old 03-19-2004, 01:41 PM
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topwinguk
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Default Rx antenna length?

I recently managed to break a Futaba 35Mhz Rx antenna wire in one of my models and also managed to lose the broken length. I have 8 receivers in total and the Futaba antennas are all around 1 metre in length whereas some of the other makes have antennas of other lengths.Any suggestions as to how I can find out how critical the length is on a Futaba Rx?
Old 03-19-2004, 02:28 PM
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Phil Cole
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Default RE: Rx antenna length?

It's not critical. If you make another one within an inch of the others you will be close enough. It's much more important to use wire that will survive vibration, and to properly strain relieve the wire where it's soldered to the receiver.

The wire should have many strands of fine wire for vibration resistance. If you splice new wire onto the remaining pigtail on your receiver support the wire with heatshrink well past the ends of the solder joint to prevent the wire breaking at the end of the solder that wicked its way into the wire.
Old 03-19-2004, 11:34 PM
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kingair41
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Default RE: Rx antenna length?

As a general rule, most if not all our radios are 1/4 wave. As such take 234 feet times that by 12 to get inches. 2808" divided by the frquency of the radio will net you the length of the receiver wire in inches. You'll see that it's just about where your other receiver antennas lengths are.
Old 03-20-2004, 09:44 AM
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topwinguk
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Default RE: Rx antenna length?

Thanks guys for the replies but am I missing something, as 2808 divided by 35 gives me around 80 inches which is about twice the length of my other Rx antennas? I presume the calcultion goes something like this:-

Speed of light = 186,000 miles/sec
= 186,000 * 5280*12 inches per sec
approx = 10* 10 to 9th power inches

Therefore @35 Mhz, the radio freq wavelength, the above value divided by 35 = approx 300 inches and 1/4 wave would be approx 75 inches which ties up with the 80 inches you give. (no calculator to hand). So where does the 40 inch antenna length come from?
I know, I'm old and confused but something seems screwy!!!
[&o]
Old 03-20-2004, 10:19 AM
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Rodney
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Default RE: Rx antenna length?

A little knowledge can be a dangerous thing. All RC receivers I know of are base loaded (tuned L/C circuit) so length is NOT determined by frequency. Phill Cole is most correct, just make it close to the original length and you will be fine. Your receiver was factory tuned to use a shorter than 1/4 wavelength antenna.
Old 03-20-2004, 11:52 AM
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kingair41
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Default RE: Rx antenna length?

Though the antenna lengths of lower frequency bands would be alittle on the long side as pointed out, a L/C loaded antenna would seem the practical way to shorten the length of the wire while not reducing the range appreciably. If you look at the formula for our frequency bands of 72mhz. and 75mhz. you'll see that it plays a major role in our determination of antenna lengths. To load an antenna is almost the same thing as lengthening it. Although it can reduce the overall performance of the system, we operate the r/c system in line of site, and not at long ranges and any compromise of a loaded base operates fine without a noticeable reduction in performance. When adding or replacing a wire always do a range check.

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