antenne length

[jfinch]

Not sure if this is the right forum to ask this or not, but it seems like the most appropriate. Its been suggested that I solder an extra length of antenne wire to the Rx in my large 3.2 m sailplane. There's room in the fuse and its supposed to help with range(?). My question is: is there a specific length of antenne wire that optimum for 72 MHz radios? And why is it optimum? Thanks!

[Ron W3FJW]

I recall reading some where that around 60" total length seems to be optimum for 72 mhz Rx. I think it was on Mr RC-Cams website.

[alexcmag]

Remember there is a difference between max gain and optimal value.

In Mr RC-Cam testings 60" has more gain, so if you're in middle of nowhere maybe it can be useful.

But it not always the best. GWS for example has cut the antenna for R4 and R6 receivers to half (52cm) a few years ago, probably after complainings about range.

In my city a lot of people also have problems with single conversion (and sometimes with dual conversion) receivers with original 40" antenna. In most cases, cutting it down to 20" solves most interferences.

Currently I'm using 10" antennas with RF choke ( http://www.uoguelph.ca/~antoon/gadgets/flywire.htm ) in my 72Mhz Berg receivers and I can reach over 3000ft without trouble.

[rick.benjamin]

The optimum length for 72 mhz is 39 inches.

[Brandano]

Hmm, what I get for a quarter wavelength aerial is
((speed of light/72,000,000)/4) * 0.95(approximate velocity factor for copper)
0.988 meters, 98.8 cm. This is a VERY rough calculation, purely theoretical and I won't offer any assurance on it working. Seems to work fine for microwave frequencies, though
[edit] hmm, i think the velocity factor only has a real effect at microwave frequencies too, so it probably ought to be left out. Revised length in that case is 104 cm...

[Rodney]

Most receivers (some of the park flyers are exceptions) use a 39 to 41 inch long antenna regardless of frequency;i.e. the same for 27 MHz, 35 MHz, 50 MHz, 72MHz etc. as they are all base loaded. Base loading means that the values of the L & C networks in the receiver are set to optimize the signal for this length of antenna. Check out the experiments that Mr. RC Cam had made on antenna lengths, a very well done test with the proper instrumentation to insure accurate results. You will find that in most cases, changing the length of the antenna will degrade performance in one manner or another. Usually shorting it will greatly reduce range and lengthening it may increase range but also pick up a lot of interferance and/or noise which will degrade performance.

[jfinch]

Thanks guys for all this great info. I ended up adding an extra length of antanne so I have a bit hanging out the tail. I probably shouldn't have done it but I haven't had any glitches so far.

[alexcmag]

Increasing the lenght is not necessarely the best choice...

Currently I'm using Berg receivers (4L), originally with 1m antenna (39 in), and an old GWS R4P, originally with a 50cm antenna (20 in)

A few years ago I tested this receiver with a bigger antenna (1m) flying on a slope. When I was alone it was OK, but with some other TXs on near, the interference was so high that was impossible to fly farther than 10m.

After that I decided to use this RX indoor on a small model (15 in lenght), so 20in antenna was too much, cut the original antenna to 4in, added a 1.5uH choke and more 6in wire (10in total length).

After many perfect flights indoor, I simply forgot it was a indoor receiver with a even more indoor antenna and flown it outdoor, up to 250m (750ft) far, without any interference.

The Bergs are currently using the same short antenna, and I usually fly them over 3000ft high with a big electric model.

[Adam Neat]

For improving your reception longer length is always better. Not just any random length though, you need to be at some fraction of a wavelength for the frequency used.

39" and 60" were mentioned. They happen to be very close to 1/4 and 3/8 wavelength.

Ive also known of guys rigging their antenna vertical to really improve their reception but really just about any good setup will keep you in control well within visual range.

If you want to calculate the exact length required for your frenquency plug it in here.

http://www.csgnetwork.com/antennaevcalc.html

[Rodney]

For improving your reception longer length is always better. Not just any random length though, you need to be at some fraction of a wavelength for the frequency used.

39" and 60" were mentioned. They happen to be very close to 1/4 and 3/8 wavelength.

Ive also known of guys rigging their antenna vertical to really improve their reception but really just about any good setup will keep you in control well within visual range.

If you want to calculate the exact length required for your frenquency plug it in here.

http://www.csgnetwork.com/antennaevcalc.html
This is not often good advice. As an electrical engineer and a ham of many years, length of the antenna is only a small portion of the critical elements that need to be considered. In most cases, with our receivers as they come from the factory, any change of length of the antenna will decrease performance. Unless you consider the LC circuitry withing the receiver itself and the ground plane in use, changing the length is an iffy proposition that is more apt to decrease performance than improve it. The lengths you get from any table or equation will apply only if you have the perfect ground plane, perfect material and are suspended at the proper elevation above earth ground plus many other factors. Bottom line, unless you have some educated experience and the equipment to make the proper measurements, you are most probably best off to use the equipment as provided by the manufacturer.

[Adam Neat]

In most cases, with our receivers as they come from the factory, any change of length of the antenna will decrease performance.


Im sorry but there have been many cases where people have altered the length of their antennas to improve reception.

Im a long time ham as well but only an Asoc in EE, but I do know that the basic RC TX/reciever is far from what I would consider an ideal. Considering the enviroment they function in they do a fantastic job.

I agree that in 99% of the cases that the stock antenna will be more than adequate......but sometimes guys have reception problems for one reason or another.. and at those times youre better off using a tuned length.

Hopefully the RX will be at an adequate distance above the ground ;)


One tip that everyone should always practice, if you go dorking around with your antenna, longer/shorter or whatever, do a range check...on the ground :D

[electrotor]

Some more light on the subject :

http://www.rc-cam.com/ant_exp.htm