Rx antennas: can I do this?
 

Can I do this to a Futaba 7-channel 72 MHz FM receiver and still have decent range?

RE: Rx antennas: can I do this?
 

I wouldn't recommend it. The receiver needs to have the antenna extended to its full length to receive signals of a strength to allow reliable operation. Shortening the antenna like that will cause problems. Why not run it in a soda straw (or something similar) inside of the fuselage if you just want to hide it?

RE: Rx antennas: can I do this?
 

I concur, that coil you created will change the tuning of the ant. Stretch it out to it's fullest lenght. however, if you are only flying it indoors, do a range check and if OK, fly it like that. Just set it down and move to the other side of the room the longest distance from the plane and see if you still have control. I would not fly it that way out side though.

RE: Rx antennas: can I do this?
 

This asking for trouble. When you coil up an antenna like that you reduce it's overall length, thus reducing the range.

Ken

RE: Rx antennas: can I do this?
 

The receiver in the picture is a Hitec 555 that I've been flying in that configuration without difficulty for several years. The coil is stacked and at no point does the antenna cross over itself. I was wondering if anybody had done the same with a Futaba Rx and had similar success.

RE: Rx antennas: can I do this?
 

I have done similar with a few different reciever, though I keep at least 1/4" between the wires in the coil, usually wrapped around a piece of cardboard or foam that can be taped to an inside wall. Especially useful for using JR radios in park flyers... (man those jr antennas are long) But done in a couple gas planes with really long range checks and no problems.

RE: Rx antennas: can I do this?
 

We have been doing it for years in the limited space inside a Pylon Racer. Usually wrapped around a dog bone shaped piece of balsa or cardboard.

Important that the antenna coil does not overlap anywhere.

Ed S

RE: Rx antennas: can I do this?
 

A whip antenna http://www3.towerhobbies.com/cgi-bin...?&I=LXKX47&P=7 makes for a neat installation. It keeps everything confined in the fuselage with no significant loss of range.

Dave

RE: Rx antennas: can I do this?
 

Yes, you can do this without significant loss of range. Just make sure the wires don't cross (as you said) and I prefer to have them not touch each other as well.

It works very well.

RE: Rx antennas: can I do this?
 

Thank you all for the replies.

dki

RE: Rx antennas: can I do this?
 

Ditto. Don't cross the wires... I've never had a problem coiling like that. Do a really good range check and call it a day.

As long as you get a good range check you should be fine.

RE: Rx antennas: can I do this?
 

I've been doing it like in the photo for quite a few years in 1/2 A Pylon Racers without any problems. JR 610M RX.

RE: Rx antennas: can I do this?
 

I have to ask why you would think a Futaba RX might behave differently than the Hitec in regards to coiling the antenna ??
As far as decent range goes ..... what do you consider decent range ??

RE: Rx antennas: can I do this?
 

I'm not well versed in radio theory; I just know what you're supposed/not supposed to do. I was shown this technique by my peers. I've only coiled the antenna like this on a GWS receiver and that Hitec shown in the picture and was surprised that there did not seem to be any degredation in range. The GWS receiver was on a Pico-Stick flown indoors and outdoors. The Hitec receiver is in a .25 size combat profile design. I am not too emotionally tied to either airplane however I would like to continue using this technique on more elaborate projects with Futaba receivers as I hate to have "excess" antenna length trailing along behind the aircraft.

Poor choice of words on my part -- "decent range" should have been "no change in range" or as long as the model can be seen.

RE: Rx antennas: can I do this?
 

Anytime you coil wire induction happens! Unless the coils are spaced far enough apart, (how far I don't know) it will change the electrical length of the wire. (resistance) This changes the "tuning" of a reciever. It's the wave-length/electrical-length of the antenna that is critical to the tuning of the reciever. Crossing the coils probably wouldn't make much differance as its still going to be out of tune, though probably not as much. SWR (standing wave reflected) on a transmitter is critical. Thats why some trans. antennas are longer than others. Recievers are "tuned" to a specific frequency and the antennas electrical length is part of it. Change the electrical length with wire cutters or any type of coil, and ya change the frequency. This can't be a good thing even if it works close in! It will cut down on the range. How much depends on how far off frequency it is in a reciever, or how much of the signal is reflected back into the transmitter. Any of ya into amateur radio, or CB will tell ya that 1/4" of antenna length will change everything in recieving or transmitting. A wad (not a coil) of antenna wire inside of the fuse. with a little strung outside is WAY better than any kinda coil. Thats why that "whip" is the best alternative, when the rest of the antenna is waded-up in a hap-hazzard fassion inside the fuse. No coil, no cut! JMO. Q.

Edit; a "wad" of wire ain't a "coil", and does not change the electrical length of the wire. IF the "whip" has a load-coil then the reciever can be fooled to think the overall length is correct because the electrical length is correct due to induction in the coil.
My point is...wad it up inside, pull some out to fit the model and forget it.

RE: Rx antennas: can I do this?
 

That Hitec Rx has performed flawlessly even at 3.7 volts (bad airborne pack cell) far out and at low altitude with the Tx antenna pointed at the model. I guess the other alternative is to make the leap to 2.4 GHz where the Rx antenna is relatively short but I'd like to use what I already have and be able to fly horizon to horizon if I want to. If it means I've got to have that unsightly antenna trailing along behind, then so be it. Its better than losing a model due to range issues.

RE: Rx antennas: can I do this?
 

If you give it a good range check (truly as far as you intend to fly) then you can get by with it. You will suffer a range loss relative to a properly mounted antenna, at least 50% or more. Check out rc-cam's web site for some good info on how antenna changes effect range. Even the best designed shortened whip or best engineered coiling will positively reduce the range over what a normal installation will have.

RE: Rx antennas: can I do this?
 

Yes a whip antenna does slightly shorten the range, but not significantly. Plus it's tuned and provides consistent results, it's a known quantity. Wadding and coiling is not tuned and is an unknown quantity. One installation might work fine for you and another might have problems.

I've been using base loaded whip antennas for years. A range check with the transmitter antenna down will show a few paces less range, but it's still plenty for flying and is safe and consistent from one installation to the next. Whip antennas are inexpensive, easy to install and reliable.

RE: Rx antennas: can I do this?
 

Oh yeah... or use a whip antenna and do a thorough range check.

RE: Rx antennas: can I do this?
 

The whole point of making a coil inside the fuse with NO wire hanging out in the breeze is to reduce drag. These 1/2 A Racers weigh less than 12 oz and the engines turn up more than 30 Grand. Top speed has got to be close to 90 mph, maybe faster. Any additional drag will make a significant difference in speed. ALSO there are two kinds of range, radio signal range and eyesight range. As long as the first exceeds the second all is well. Like I said in my post above I've been wrapping my antenna around a drinking straw to make a COIL for many, many years and have no problems. I'd like to say this is cause I use JR radios but there are a lot of other people out there with Pylon Racers, 1/2 A, Q-500 and Q-40, that user Fubar, Hitec and Joe Blow for all I know.

RE: Rx antennas: can I do this?
 

BTW, IIRC when I was doing 1/2A pylon, my TD with KK needle valve, cyl shims, backplate press, and cut down Cox comp 5X3 was turning somewhere in the 24k rpm range while pulling a ~20 oz airframe and that damn antenna hanging out the rear. A select few had BV Shurikens (gorgeous little engine) that was turning 30k right out of the box!

RE: Rx antennas: can I do this?
 

The engines in use today (if you can find one) are the VA .049's and the Norvel AME .061 that turn the little (4.6 X 2.5) carbon fiber insert props over 30 K with a little work. The 200 sq in wing area models with modern radio gear come in around 11 - 12 oz. The course is a 3 pylon affair with 300" from #1 pylon to #2 and 3 which are 100' apart. We run 10 laps in about 1 min 30 sec. Very addicting indeed. Ya gotta keep yer eye on YOUR model at all times and keep turning left. A well set up racer will fly most of the 10 laps just about on knife edge. AND anything hanging out the back and trailing behind WILL slow ya down!

RE: Rx antennas: can I do this?
 

I'm not telling you guys that what you are doing, and have been doing for years, won't work.
The reason I posted what I did is because alot of guys may think that there is no penalty, when an antenna system is modified.
(I know that I would have if I didn't already know better.)
Maybe on a pylon racer or park flyer that never gets more than a couple hundred yards away, it is fine to shorten a rx. antenna.
My point was/is that for the average "Joe", modifying the antenna is a mistake. Quite a few years ago when I was just learning to fly, I was flying a powered glider. It got so far away that by the time one of the other guys saw what was happening the plane was just a dot in the sky. He had no idea, and neither did I, which way it was going. As he was running towards me he kept yelling "left & up" over and over. We saw the dot move left & up. He grabbed the transmitter, turned the plane around and flew it back. That was at an extreme range and we still had full controal of the plane. When I say a dot,,,I mean like this--->.<. How much range can ya give up? I guess it depends on how far away you plan for your plane to get. Good flying to all.:D

RE: Rx antennas: can I do this?
 

If you want some good info on antenna effects check out http://www.rc-cam.com/ant_exp.htm. You will see that even the best shortened whip will cause an 8 db loss. Remember, the range is decreased by half for every 6 db loss in signal strength.

RE: Rx antennas: can I do this?
 

quepasa,

I assume you're trying to simplify things so they can be understood by the layman, but I think you focus on minor issues. In todays RC equipment (specifically recievers), the front end RF stages allow for much broader variations without major affect on performance. On a transmitter minor changes to the antenna can have significant changes. on a receiver, not so much. On the receiver end, unless your taking 10% of the antenna length and coiling it, I doubt you could measure the difference with the best test equipment. Considering current RC systems can maintain control beyond reasonable limits of vision, even if you cut the range by 10% you'd still be OK. I've been using the coil technique on small scale planes for years. My preferred method is to wrap on a soda straw with wide spacing.
I've worked in electronics for better than 35 years and even though most of my work wasn't in RF, I couldn't have gotten a degree without some knowledge of how things work.