RC Receiver Antenna

I also fly rc planes, and I face the same problem. I would not suggest cutting the antenna, I believe it is tuned to that frequency. You could wrap the length you want to hide aroung a dowel and then trail the rest to the tailfeathers. Or zigzag the wire through the wing core. You may lose some range in both cases. The wings on my models are detachable for transport so wing mounting is not feasible for me.

I don't have any experience designing antennas and based on that, my feeling on the T shaped one is that the length inside the wing core would have to be the original 30" or so in length.

I have heard of reception problems when the antenna has been trimmed, causing the owner some broken parts, and the additional hassle of soldering a new wire to the receiver.

I am curious as to how all of this works out, please post back with any experimentation you do.

Not quite! Wrapping the wire onto a dowel creates a coil, which detunes the antenna. The wire should be as straight as possible.

The RC helo guys wrap the antenna around a soda straw to keep it short, as a trailing wire would eventually find and foul the rotor. This may be a coil, but the antenna still works properly.

According to "Anonmous Hero", my scheme would create a 1/4-wavelength dipole antenna (if doing that to an 82" length makes a 1/2-wavelength dipole). Here's what I tried in several airplanes (both foam and built-up wings): I cut a piece of insulated wire 42" long and rigged it spanwise in the wing. In the center, I soldered a "Tee" connection to a Dean's connector, so I can remove the wings for transportation. Then, I cut the original wire to about 8" long and soldered a mating Dean's to the end of it. So, what I had was a short lead to the Tee, then it branched out 21" in each direction through the left and right wings. Most of my planes have wingspans greater than 42", so the antenna is completely hidden. Every plane I tried it in ranged checked perfectly. I have flown them for over a year with no glitches, even at ranges as far as I could see the airplanes with no problems. I know it works okay, but I don't know if it's reception is slightly diminished (even though it is not noticeable). I know that changing the length will affect reception, because they are tuned to 72 mHz. In hindsight, it might be better to use a shielded cable from the receiver to the Tee, since that short piece also acts as an antenna and probably results in an effective change in length. I am going to try it again with the short piece shielded and I will extensively range check before flight. I'll let you know the results directly if you contact me directly by email. I'm also going to do some homework on antenna theory. Thanks for your interest. I am: marinshaw@stageiii.com

Yes, the length of the antenna is tuned to the frequency that it is designed to operate at. To do what you propose would require an antenna 82" long tapped in the middle to make a 1/2 wavelength dipole antenna.

The center tap would need to be electrically isolated and each leg of the dipole connects to one side of the transceiver's coax output.

If the antenna is not tuned to the right frequency the signal from the transmitter that drives the antenna will be reflected back into the output finals and could burn them out. Look up SWR (Standing Wave Ratio and antennas in general).

The length of your antenna is probably tuned not only by length, but it may also be tuned with a coil to make the antenna electrically 1/4 wavelength.

Since I deal with with general aviation aircraft I can't speak specifically about RC, but the principles will be the same. While the power output may not be enough to damage the RC transmitter with an untuned antenna (I just don't know), it will definitely impact the effectiveness of the transmit and receive operation.

I suggest further reading and there is a ton of stuff on antennat design and theory (i.e., HAM radio) on the net to learn.

"If the antenna is not tuned to the right frequency the signal from the transmitter that drives the antenna will be reflected back into the output finals and could burn them out. Look up SWR (Standing Wave Ratio and antennas in general)."

In RC applications, the tranmitter is handheld and on the ground, the reciever is airborne on the plane. They do not share a common antenna. The transmitter does not receive and the receiver does not transmitt. So how does everything fit now, does it matter it the receiver antenna is coiled around a dowel, or formed into a T, since it is only receiving? Would there be any damage to the transmitter? I don't see how.

The wire emanating from the receiver is a single conductor. How does this affect the formation of a T antenna? The two dipoles can't be isolated.

Your 1/4 wavelength @ 72MHz is 41 inches. (3x10^8 m/s)/(72x10^6 /s)=4.1667m x 1/4 =1.0412m= 1.0412m/[(100cm/m)*(1 in/2.54cm)] =41 inches.

OK, here is the straight scoop from a licensed Amateur Radio Operator (General Class):

Yes, in RC applications, you cannot burn out the transmitter finals if the receiver antenna has poor SWR. You can however diminish the signal received and therefore the range of reception.

Hams use an approximation of the wavelength to frequency conversion formula to figure out antenna length. A half-wave antenna should be 468 ÷ f ( in MHz) = 468 ÷ 72 = 6.5 ft. or 78 inches, which is very close to the 82 inches calculated by the theoreetical formula. The problem with the T-tap scheme is complex. First, at only about 30 inches, what you have is only about a 3/16 wave, or less than a quarter wave, end-fed antenna. Yes, a center-fed dipole must have the two halves isolated and fed with a shielded cable, or a balanced (two parallel conductor) feed line, which itself may radiate and must be trimmed for proper SWR as well. More than likely, as suggested already, your antenna is a shortened version, probably of an end-fed half-wave, because that type of antenna is fairly ground independent. It could also be a quarter-wave monopole, which do not have as much gain, but are easier to match. It probably has a matching device (coil probably, but possibly a coil/capacitor combo) on the receiver itself to allow this. If you change the feed point to the center of your 30 inch length, all bets are off for this fixed matching network to match your new antenna configuration and your receive SWR will probably be huge, resulting in low signal strength. But you never know, it just might be all you need for close range work.

You really don't know what the receiver impedance is that you are trying to match, so about all you really can do is to experimentally try different lengths and see what works best. As a rule of thumb, the longer your antenna is, the higher your signal strength will be. Many hams use "long wire" antennas or full-wave loops, then match the antenna impedance to their 50 ohm transceivers with a tunable matching network, or "trans-match", more often known today as an "antenna tuner". It really doesn't tune the antenna, so much as it tunes the impedance of the radio TO the antenna.

If you really want extended range, here is a thought. Junk your 72 Mhz equipment, which is, by law, for unlicensed operators, study for (really easy, children pass the test!), take and pass the FCC Technician-class Amateur Radio License exam, and buy 50 MHz radio control equipment that is on the market. With a ham license you can legally use higher power on Ham frequencies, as well as make and try out many different types of antennas for both transmitter and receiver.

Check out the ARRL for information on obtaining your ham license. Many local clubs have Volunteer Examiners who have regularly scheduled testing sessions, and sometimes will even schedule one just for you! The license fee is still only around $14 and is good for 10 years, and is renewable without retesting.

Many RC modelers get into Ham Radio for these reasons and find a whole world of other opportunities opens up: emergency radio communications, wide-area local coverage over FM repeaters, satellite and space communications (yes, there are Amateur Radio Satellites and many astronauts are licensed Hams who will talk to you!), long distance (DX) contacts with hams in other countries on the HF (High Frequency, shortwave) bands, contesting (competing with others for the most contact points within an allotted time period), and much more using mobile, portable, or fixed (base station) operation. Many modes are available including FM voice, Single Sideband (SSB) voice, radioteletype (RTTY), computer-based digital (text) modes, the old reliable Morse Code (CW) and Bulletin Board style packet radio. Many bands are available from HF (ten bands between 1.8 MHz to 30 Mhz), VHF/UHF(7 bands from 50 to 2300 Mhz), and up (microwaves)! Many hams also tie their stations to the internet using VoIP protocols. One operating sub-mode of packet is the Automatic Position Reporting System (APRS), which allows a mobile (including air-mobile!) transmitter to send its GPS data either directly or through a network of digital repeaters (digi-peaters) tied to the internet. Some hams even use the moon to reflect VHF/UHF signals back down to other points on the earth using powerful directional high-gain antennas. Cool, huh?

Oh, did I mention the legal operating power for most ham bands? NO, not 100 mw like many unlicensed devices, not 500 mw like the FRS walkie-talkies, not 5 Watts (5,000 mw!) like CB radios, not 25 watts like the VHF Marine radios, or even 50 watts like the (licensed) GMRS two-way mobile radios, but 1,500 watts! That's 1,500,000 mw, or 1.5 KILOWATTS!

Ok, and thanks for the info. I probably asked the wrong question in the first place. Let me try another way. I have a 72 mHz RC radio setup, for which the receiver has an ~40-inch wire antenna. Typically, the airplane fuselages are about 30 inches long, and the transmitter is installed around midpoint, resulting in an usightly long trailing wire which I would like to hide. The wingspans are always greater than 40 inches, so my thought is to install the antenna in the wing. Aside from the options available with switching to 50 mHz, what's the best way to hide the antenna without seriously degrading reception? I don't want to install the receiver in a wingtip; it must remain in the fuselage.

Short answer: it is ok to bend the antenna, as long as you don't fold it back on itself or to an acute angle. 90 degrees would be OK. So however far in back or in front of the wing the receiver is located, just re-route the wire down the fuselage, then bend it 90 degrees to go out onto, into, or below the wing.

As long as the wing is not metal or metallicized in any way (foil, metallic paint, etc.) you should be OK. Metal shields the antenna from the transmitter and the capacitance will through off the impedance of the antenna. Also be sure the antenna is not routed along or touch any other metal in the aircraft.

We're getting closer. With a 48-inch wingspan, I only have 24 inches from fuselage to wingtip. Normally, the wing is removable for transportion and the electronics are located in the fuselage cavity exposed by the removed wing. So, from the receiver to the center of the wing is about 8 inches, max. That leaves 32 inches of antenna to locate in a wing that goes 24 inches in either left or right. Without doubling back some distance, it won't fit. So, I'm probably looking for a compromise solution which will degrade performance a bit. I know from experience that the system is conservative enough that some reduction in signal reception can be tolerated. I could shorten the antenna, coil some of it, double back a portion or modify it some other way. Any suggestions on which would be the least harmful or is there another solution out there that I haven't considered that would be superior (less inferior)?

Least harful in terms of SWR, would be if you can sinuate the remainin wire, that is make it run in multiple "s" curves like a sine wave. That will shorten the run length without changing impedance very much, since the antenna does not fold back on itself. If that cannot easily be done, can the remaining 8 inches hang down of stick up? Another 90 degree bend near the end would be ok. Many hams do this to put full-length HF dipoles in their short attics. Also, if the initial run of the wire can be made near one edge of the wing (leading or trailing) you could curve the wire back so the remainder ran along the other edge. That would put the maximum spacing between the parallel sections, which would still be less harmful than coiling up the wire. Coiling the wire would definitely add inductive impedance and make the SWR far worse.

If you wanted to experiment a little, you could try turning the antenna into a closed loop by adding more wire and grounding the end to the receiver's metal chassis or whichever battery terminal is considered "ground", usually negative. Another alternatiive to improving performance might be to add another wire, known as a counterpoise, connected to your chassis "ground" or the battery "ground" connection. This counterpoise wire should be equal to the length of your antenna, or to its "electrical length" if you could determine it, or to one-half length.

Again, be careful of any metal that might be in your model which might short, shield, or capacitively couple to your antenna or counterpoise.

I've been flying my 59" Piper J-3 CUB with it's antenna wrapped on a drinking straw with closed windings till it's a spot in the sky…!!!...with perfect radio reception.

I can't and shouldn't fly any further…not because the radio wouldn't respond…but because I would loose sight of the model!!!!....

Not only that, I've been flying EVERY model for the last three years with a coiled antenna without a SINGLE range problem.

I think we can read all the theory we want and people can always come up with their own ideas.

This method WORKS and I use it.

Others won't even try it but they go around saying it won't work!! Based on some text book material and that's their prerogative.

I'm sticking to the "coiled on a straw" method….

My 2 cents he,he….

Did you coil it around a straw inside the fuselage? Post a picture, I have heard of this, but haven't seen it in practice, and I don't have a flying club nearby to visit. I would like to try it on mine.

A dipole Google that.