Antenna too long

I understand not to cut my antenna, but the Rx I bought is a Hitceh Micro 555 five channel, It came with an antenna 41" long. The location where I want to install the Rx is under the wing of a fuse 36" long.
This leaves the ant. sticking out the tail end about 20". This is way too long for my little airplane. I can just imagine the ant. flopping in the grass.
I wonder if it's all right to have the antenna doubled back into the fuselage? Or maybe I should return it and get one with a shorter antenna?
Wan

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
On Wed, 23 Jun 2004 00:10:42 GMT, "Fubar of The HillPeople"
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Well, that contradicts everything I was ever told. Go figure. Disregard my post.

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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!"

Being a 'ham' for nearly 30 years provides one with lots of experience with antennas.
The BEST antenna for ANY receiver frequencies well up into the VHF region ( 30-300Mhz) is the LONGEST piece of wire that can be strung up so that it does not get grounded. Hams regularly use 'long wire' antennas - 300, 400 , 500 feet long - even more!
For a transmitter, the reason for tuning the antenna is to convert maximum RF power into radiated power. If the antenna is not tuned or matched, then power is reflected back (SWR measures this). If the RF final amp cannot handle the reflected power, it will eventually smoke. The transistors used in ~most~ RC transmitters can handle 100% of the power reflected back - think no antenna screwed in - and do it forever since the transistor can dissapate the RF energy as heat.
And actually, I created a 'center loaded' antenna ! And it was not luck - center loading is commonly used to shorted even TX antennas with minimal signal strength loss.
Check your range with the TX antenna removed - should be about 40'. Now wind your RX antenna up as I did and recheck range - I'll bet you cannot tell the difference.
David On 23 Jun 2004 12:09:41 GMT, snipped-for-privacy@aol.com (Dr1Driver) wrote:
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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
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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

My Crazy 8 is a Cobalt 400 powered screamer with a 31" span. I had it up high enough - about 5 mistakes worth since it was the maiden flight - to get that rascal trimmed. A couple of times I lost orientation with it until I remembered 'dark bottom - light top'.
Never had any hint of range problem.
3/8" to 3/4" is more than enough diameter, and I 'tight wound' mine - which means the turns are touching each other.
It's a BIG plane if you can SEE it well enough to control at 1000' !
David
On 23 Jun 2004 05:13:54 -0700, snipped-for-privacy@toast.net (Wan) wrote:
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David AMA40795 / KC5UH <remove #$%^ from email address> wrote:
| 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.

Doug McLaren wrote:
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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.
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Its worth building radials into models. One day I will test this out - wires running all over the structure...
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Gets a little worse with cheap park flyer receivers. Here you may at best have little more than 500 yards.
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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.
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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.

Another factor to consider when measuring antenna length for a particular frequency is the velocity factor of the wire. According to the egg heads electrons travel at the speed of light in a vacuum but slow down in other conductors. The velocity factor of most copper wire is .65.
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Taking into account the fact that the transmitter aerial is working against ground due to the reactance that your body imposes between tha transmitter and said ground, then adding radials would cock the thing up nicely. Radials are attached to an aerial which is going to be used at a suitable height and they will thus present the correct radiation resistance and the whole aerial system will work as planned. You wafting a transmitter around at a greater or lesser height above ground is not going to give a system which is of any use... it will almost certainly be worse than the currently used systems.
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Now thats an interesting concept.... if it was even remotely correct ! Dipoles are used in preference to end fed aerials because they do actually *reduce* noise. A dipole (suitably fed) is a balanced aerial and as such it greatly reduces impulse and other types of interference. Radio hams living in electrically noisy urban areas usually put up balanced aerial systems in order to be able to work effectively and hear low strength signals.... the dipole is still a very simple and effective balanced aerial.
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Well at least you got this bit right... there really is a loading coil.
I don't just want to single out your comments as being wrong. There are numerous other incorrect statements in this thread but I haven't time to pick them out and reply to each one so I am taking the easy option >:-)
Reg

David AMA40795 / KC5UH wrote:
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TX antennae most certainly are quite highly tuned. RX antennae, in general, are not that highly tuned.

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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

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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
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The style antenna used in RC TXs is a verticle antenna. Verticles MUST have a GOOD ground plane of the proper size to be properly tuned - which RC TXs DO NOT HAVE and therefore CANNOT be 'highly' tuned.
David
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David AMA40795 / KC5UH wrote:
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Depnds on wht you man by 'highly'
They are a lot more 'highly' tuned than RX.
I know. Ive built both.
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Those were YOUR words, not mine........... go back and read your post.
David
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Wan wrote:
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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