Antennas

[tclocs]

I have a Hobbico Vnetura sailplane and need to replace the antenna. Lost a part of it and need help or ideas of what can I use to replace it and how long to make it. The freq is 27.145Mhz. Thanks

[groundfx]

You can use any small, light insulated wire to replace the lost section. The type of wire is not critical in terms of the electronics but you do want it to be light so your plane doesn't become tail heavy. Just make sure you make the repair such that the final length is the same (or close) as it was when it was new.

Also, make sure you do a range check after the repair.

[rockbus]

I have a Hobbico Vnetura sailplane and need to replace the antenna. Lost a part of it and need help or ideas of what can I use to replace it and how long to make it. The freq is 27.145Mhz. Thanks


Make it 1/8 wavelength long.

l=C/f

where

C = 3*10^8 m/sec
f = 27,000,000 hz

l = C/f = 11.11 meters = 36.5 ft (an easy way to remember is 30 mhz has a 10 meter wavelength)

1/8 * l = 36.5 ft / 8 = 4.6 ft or about 55 inches.


Good luck.


Good luck

[Rodney]

Most RC receivers are base loaded and adjusted to use an antenna 39 to 41 inches long regardless of frequency; i.e. same length for 35 MHZ, 27MHZ, 72MHZ etc. Any deviation from this may cause reduced sensitivity or increased susceptability to noise. If interested, you might Google RC-CAM's website and see what he has found out by making reliable measurements using different antenna lengths.

[inedesca]

So those small base loaded antennas for helicopters are made to use a 8" long wire and work the same
if so why have they not replaced the long ones, why people keeps using 38" long if a based antenna could do the job?
I mistrust these base loaded ones because no one uses them, but I would like to be certain they work the same, then I would only use these

[Rodney]

inedesca, because you lose range when you shorten the antenna, since you have less capture area, even if properly base loaded, range will be less than with the longer antenna.

[vintage1]

all other things being equal, range is proportional to length.

You can get a bit back by peaking the aerial up, but not that much.

[deh6]

all other things being equal, range is proportional to length.
because you lose range when you shorten the antenna, since you have less capture area These are misconceptions. The conclusions are correct, i.e. shorter antennas give less range. If one drives a constant current into a short dipole the signal is proportional length; however, the issue is power. With constant power the signal strength is constant for 1) short dipoles, and 2) no losses. In terms of capture area, a short dipole's capture area is independent of length, and only 9% lower than a full half wave dipole.

The main problem are losses that result in trying to match to the very low radiation resistance of a short antenna. Most of the power is wasted in the matching network. In practical terms, the higher the Q of the "loading coil" the better the efficiency, but the downside is that the antenna is easily detuned by nearby objects. For example, in 1959 a ham was demonstrating his 40 mobile rig for me in a parking lot. A car parked next to him pulled out and he had to completely retune. He had a whip about 8 feet, which would correspond to a 72 MHz receiver antenna of about 10 inches. Such an antenna would be "touchy" with a low loss matching circuit, and even then the efficiency would be low compared to a longer antenna. Lower the Q of the coil (which generally means smaller so it will fit in a model plane) and the antenna would be less prone to detuning, but the range would suffer greatly. Range suffers when the antenna is short (and this applies to the transmitter as well).

[Texas Buzzard]

With reference to the original question:

I have had to solder antennae twice in my experience when it was broken.

The factory istalled the "best" all around length for that radio. Most modelers don't have the equipment for fine tuning. So why not use the factory specs?

Just solder on enough insulated wire ( I used asw 25) to make that antenna the same length as the original. Then RANGE TEST it at over 100 feet withthe xmitter antenna collasped. If that works O.K. then put a flight keeping the plane close by.