Antenna too long

The antenna length is important to the tuning of the rx. Since its a "little airplane", I would guess that you fly it close in. You can double up the antenna but leave as much space between the doubled up wire as possible. In other words, dont fold it back on itself. Double it in as big an "S" as you can. If you are flying it close in, the range loss shouldnt affect you but I would still range check!

Neither Tx nor Rx antennas in RC gear are "tuned". They are sized to get maximum signal strength for a nominal length that is kept reasonably short.

I took a 3/8" square x 3/4" piece of balsa, rounded the edges, and wound about 15 turns of the antenna wire on it, about 6" from the Rx, so that I had about 14" left to go to the tail of my Crazy 8 electric. I flew that plane more than far enough away without any problem. BTW, the Rx was a GP electri-fly single convert with 30A esc built-in.

David

Well, that contradicts everything I was ever told. Go figure. Disregard my post.

David,

Interesting!

I never gave it much thought, but you must be right. Given the frequency (and wavelength) of 72 MHz and 50 MHz Txmtrs, the antennae supplied are pretty pathetic. Do you know specifically about the compromises made by the industry to achieve reasonable performance while pushing the limits of good design standards? What do you think the SWR (or how far off resonance) is the Tx antenna? Obviously, the antenna would have to be trimmed within the limits of the final RF stage's ability to withstand the SWR, and at the same time radiate the required output for a reasonable range. I am sure the Rx antenna is less picky, given the expected distance from the Tx is only as far as we can see to fly the plane (not exactly DX).

The coil you wound, is it based on any known design, or just a trial and error experience thing? I would be concerned about creating an RF choke. On a park flyer I could see that the reduced effectiveness of the Rx antenna would be less of a concern than on large model flying further away (unless an interfering signal comes along). Of course, on a large model the length of the antenna is not a problem to accommodate.

It would be great to see a technical article on this subject in one of the R/C mags. Hmmm, I will have to write RC Report.

Tom KA7TAM

electric.

| Neither Tx nor Rx antennas in RC gear are "tuned". They are sized to | get maximum signal strength for a nominal length that is kept | reasonably short.

I'm not sure that's quite true ... sure, they're not *carefully* tuned, but they do seem to at least aim for a 1/4 wavelength antenna, which would be the proper length to aim for.

If you measure your 72 mHz receiver antennas, you'll find that just about all of them, except for the cheap single conversion receivers, are almost exactly one meter long, which corresponds almost exactly to a 1/4 wave antenna for 72 mHz.

Now, I did measure some transmitter antennas (I removed the metal part and measured that, because some is hidden inside the unit.) These seemed to vary a lot more -- a JR radio was 1.16 m, Airtronics was

1.17 m, Futaba was 1.02 meters, and the Megatech transmitter was 0.97 m.

A bit shorter than 1/4 wavelength I can understand -- there's a bit of wire inside the transmitter that will radiate as part of the antenna, but being longer than 1/4 wavelength suggests that they're just not being very careful. But even so, they're sort of close to 1/4 wavelength.

I guess if they wanted much better performance, they'd go for exactly

1/4 wavelength at 72.5 mHz, and they'd include radials. You'd look like a dork with those radials, running the risk of poking somebody in the eye with them, but you'd have a nice strong signal :)

(For receivers, they'd go for a full dipole rather than a half dipole whip antenna.But you'd also pick up more noise, so it may not help that much.)

In any event, since current systems will let you control your plane much further away than you can see already, there's no real need to improve things.

Also, the 50 mHz equipment uses antennas that are exactly the same lengths as that for 72 mHz. So either there's a loading coil in both TX and RX, or they're *really* letting things get sloppy.

And for 27 mHz, again, similar antenna lengths, but there really must be a loading coil or it wouldn't work well at all.

Basically yes, you lose some range, but there should still be more than enough: Indeed there are sound reasons why having an antenna all in one direction leads to nasty nulls in its polar response, and loss of signal with the model pointing right at the transmitter

TX antennae most certainly are quite highly tuned. RX antennae, in general, are not that highly tuned.

And if you measure the 35Mhz ones that we use, they are also the same length.

They TX antennae are fed via matching impedances that tune them broadly to the frequency in use.

RX antennae - subject to random colocation with all sorts of other junk, are not nearly so critically tuned. In general they are loosely coupled

- very loosely coupled - into a tuned front end stage that has a little effect, not much, in rejecting out of band signals and boosting the in-band.

You can mess around with an RX antenna considerably with very little effect on either range or bandwidth. The same is NOT true of the transmitter. That is designed to operate at full extension and some distance away from metallic objects.

Its worth building radials into models. One day I will test this out - wires running all over the structure...

Gets a little worse with cheap park flyer receivers. Here you may at best have little more than 500 yards.

They do, or its functional equivalent. Most TX antennae are fed through a tuned Pi filter, of which the impedance of the fully extended antenna forms a critical part.

Most RX antennae are either taken to a turn on the input tuned circuit, or fed via a very small value capacitor to it. However the Q of this is very low. There is seldom any attempt to get the antenna to resonate - merely decoupling to make sure it doesn't affect the resonance of that coil too much.

Yes. I built one years ago, and it was completely necessary to add a bottom loading col - that design did not use a PI filter.

Any antenna length will work -- until it runs out of range :-) I strongly suggest you fly as the manufacturer designed your radio -- the trailing antenna won't bother anything unless you step on it while taxiing,

I was given a Hots II, 40 size, which is not very long. I drilled a hole in the bottom under the receiver, and dropped the antenna wire out the bottom. I held the end at the tail of the plan and pulled the slack towards the front of the plane. I ran the wire towards the front on one side and taped it to the bottom of the fuse (using yellow electrical tape), then across almost to the opposite side, then down to the back, taping the whole thing to the bottom.. it looks like a big fish hook.... I've had the plane almost out of sight on occasion, and never a problem. I have been flying it this way for 3 years.

On my planes that had external antennas, now have them taped to the bottom to get rid of it sitting ugly on the top. On my new one, I may run an internal tube on my current build, or just tape it to the bottom... it is an easy fix and if for some reason the receiver has to be removed and replaced, easy job on the antenna wire. . Arne, USA (I finally caught up with 'The Joneses') . .

Not really. What he did was to create a base-loaded antenna. Base loads are EXCELLENT for receiving. IMO, he got lucky. The plane could as easily have crashed as flown well. "Don't fool with antenna length" is a good motto. Dr.1 Driver "There's a Hun in the sun!"

David,

I liked your idea. My plane is not a park flyer. It will be powered by an AXI 2212/34 motor. It's very fast and aerobatic. Lickety split, it could be 1,000 feet away. : )

If I made the balsa into a tube about 1" in diameter and 3 inches long, then wound the antenna in a spiral, wouldn't it be better?

Thanks to the folks with their input, Wan

How critical is placement of receiver antennae? I have a glider with carbon tube fuselage. I understand that placing the antennae inside would kill the reception.

What about taping it to but outside the fuselage? I wasn't sure so I ran it outside and at an angle to the tip of horizontal stabilizer.

How can I make it low drag and still have good reception?

thanks, Michal

On 6/22/2004 4:37 PM Ted shuffled out of his cave and grunted these great (and sometimes not so great) words of knowledge:

Hayes makes a "base loaded" antenna that is 18" long. I have used this antenna on several scale models and have had no problems. The only thing I did notice with them is that doing a range test (motor running), I had about 10' - 15' LESS range than with the full length antenna. In regular flying, no problem.

Hey, they make a great 'foot brake.'

Wan, they make aftermarket "loaded" antennas that are only about 6". I've also heard wrapping the antenna around a large drinking straw works. Both methods reduce the range a bit. Since this is a small plane, it won't matter because the model will be out of sight before it is out of radio range.

Morris

Arne, you have an excellent idea. I think I may tape the over long antenna along the inside bottom of the fuse. This way it'll be out of sight and hopefully it will range check out OK. And if not, I can always do it differently.

However, I do have a space problem, so I might loop the antenna a couple times along a foot long piece of light balsa (easier to install and extricate), and lay it on the inside bottom.

This will leave about 17" loose remainng to be dealt with. Hmm.... Any possible problems with this?

Wan

Wan,

There are a number of well engineered small antennas on the market. Check out Azzar's antennas at

They are super lightweight. Dean's makes a fine base loaded antenna that is also compact and lightweight. I use Azzar or Deans in all my planes. It is extra important that they be situated away from the motor, esc, batteries and servos. I have found that they have better range if situated upright - perpendicular to the fuse. Even a well engineered micro antenna will have about a 15% range loss compared to the stock antenna strung out in a straight line. But since you are flying a smaller model, you probably don't need the full 1 mile range anyway.

Mike

I don't have a carbon fuse glider so can't comment from direct experience. I have seen many comments on this on a soaring mail group I subscribe to... mainly Americans. It would appear that there is considerable differences in the layup and weave of carbon fuselages.

Some carbon fuses seem to present no problem with the aerial run inside. Other makes of fuse can cause severe problems and it is virtually essential to run the aerial outside of the fuse... sometimes aerials taped to the outside of the fuse give reduced range.

One method that is sometimes successful is to run the aerial out inside a non-carbon wing and usually use a plug to connect it when the wing is fitted to the fuse.

The comments I have quoted are from guys flying thermal, often at great height and range, and the last thing they want is reduced range on the radio gear.

Unless you are competing, and every little then helps, how much drag do you think the aerial run outside and up to the top of the vertical fin is going to cause ?

You could reduce the drag of an outside aerial by replacing it with smaller diameter enamelled smaller gauge copper wire... but it will not be as flexible.

Try altering your receive aerial, but proceed with caution and make sure you are happy with range tests before you soar up and away in a thermal.

Reg