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Antenna Length... - again.

R/C Submarine modelers

Postby Bob the Builder » Tue Sep 30, 2003 10:42 am

I have a quick question on a related topic.

I just finished modifying a 16th century galleon model conversion to RC, and (as I was on a strict budget), I utilized the running gear from an old Radio Shack RC car that I had lying around.

The car's antenna said that it's on 27mhz. Somewhere along the way, I misplaced my antenna, and now that I want to run one up one of the masts, I'm not sure of the correct length to use.

A formula that I found on these boards makes my antenna length somewhere around 105 inches.??? Does this formula not apply in this case, or do you use a smaller fraction for the antenna length instead of the 1/4 that is usually used? I'm sure that the original antenna was only about 14" long (approx).

Any help would be appreciated.

P.S. the formula was 11810 / frequency in MHz. time .96 (velocity factor) as posted by Ed Tordahl on June 12th, 2003.
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Postby Ramius-II » Tue Sep 30, 2003 10:58 am

Hi Bob:
The formula you are using is correct, most C.B. (27 MHz) systems use a 109" whip antenna. Electrically the antenna is this lenght. However, if you add a coil somewhere in the antenna system then as far as the transmitter / receiver are concerned, the antenna is 109" long even though physically it is not. You have probably seen this where an antenna has a large "lump" in the center or towards the top. Technical term is the loading coil.
Since this is the receiver end and it is a surface ship, it really does not matter what lenght you use. If you are concerned, you could go to Radio Shack and measure another car's antenna.

Ed :)
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Postby Wayne Frey » Tue Sep 30, 2003 7:11 pm

The antenna length issue was of concern to me on my 3.5 WTC.
Matt Thor gave me an excellent idea for those of you using a D+E WTC.Do not worry about sectioning the antenna from the inside to the outside.Remove the threaded rod in the bulkhead.I then 1/2 drilled the bulkhead to fit a short 1/8" brass rod about 1/2" long.I then sealed the outside of the rod watertight and ran the origional antenna through it.I then used a vacume line for automobile engines that I got from work,running the antenna through it the full length.I then glued a rubber cap to the other end,thus sealing the antenna.
It is very flexable and looks like it should work well.It is untested at the moment though.
The result leaves me without any issues to altering the antenna AT ALL.While this is not entirely related to the topic,it had merit enough to toss out there.
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Postby bo@ti » Tue Sep 30, 2003 7:36 pm

Antenna length
The antenna length of your receiver is important for best range and for better
quality of the reception.
There is a mathematical formula to determine this length:
t=antenna length
c= light speed (for this purpose considered to be 300.000km/s, equivalent to
300.000.000 m/s.
l=transmitter frequency
The most usual frequency in R/C boating is 75 MHz. Within this frequency there are 30 available channels, with frequencies between 75.410 MHz and 75.990 MHz. As you can easily see in the formula, the higher the frequency the shorter the antenna length. Let's check the right lengths for our
receivers, considering the available frequencies.
1. for the shorter frequency of the range (75.410 MHz)
t=300.000.000/75.410=3.97 meters=156 inches
2. for the higher frequency (75.990 MHz)
t=300.000.000/75.990=3,94 m=155 inches
From this formula, two observations arise:
a. the difference in length from the higher to the shorter frequency is negligible - 1 inch in 156";
b. it's impossible use in a boat the antenna length calculated by the formula.
And so, should we forget the ideal length idea?
Not necessarily. Scientists discovered that submultiples from the ideal length can be used with almost no loss. So, 1/2, 1/4 or 1/8 of the calculated length
are almost as efficient as the original. So:
156/8=19 1/2"
155/8=19 3/8"
19 inches and fraction, close to the length we find in our receivers.
And how about the 27 MHz frequency?
Using the same formula, you get 54 inches for1/8 wave or 27 for 1/16.
That's it guys. Science has a solution for (almost) everything.

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Postby Bob the Builder » Wed Oct 01, 2003 10:54 am

I suppose that makes sense now. At 1/16 full length, the antenna should be 27" long. I would assume that the Radio Shack car was using a 1/32 ratio antenna, making it the (approx) 14" antenna that I remember. I'll try a 13.5" antenna and see what kind of performance I get.

I really only need reception to about 50' or so, so this should be adequate. The masts on the galleon are only about 18" tall, so a 27" antenna would need to be strung between two, or some other creative solution.

Thanks to all for the help.
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