Aero65

My Feedback: (2)

Hello Everyone

I fly turbines and was wondering if you could go to a 1/2 (something around 80" total length) wavelength antenna on the RX, so that you could run a whip antenna in the vertical stab. This would partially hide it, but it would still get the antenna out of the ground plane.

If extending the antenna is very detrimental, could you use small coax to extend the antenna without affecting its active length?

I could move the receiver back there but it would make the wire runs much more difficult for the servos.

Thank you in advance for you answers

FLYBOY

My Feedback: (11)

I have been told many times to never change the leangth of the antenna on the reciever. It is tuned that way. I would call the factory for the reciever you have and ask them directly. They will know for sure what you can and can't do. With the price and speed of jets, I don't think I would mess with it unless I talked to them first.

Phil Cole

It depends on the receiver.

Some receivers are tuned to the antenna, so large changes in length, or using coax feedline will detune them. If you are using 72 MHz, the regular 40" antenna shouldn't require too much tuning in the receiver since the 40" is close to 1/4 wavelength.

Others have a low impedance input, so the length of the antenna does not affect their tuning. They do this so shorter antennas can be used, typically for small electrics. The coax extension with a 1/4 wave antenna would work well with this sort of receiver.

An 80 inch end-fed wire at 72 MHz is going to have a highish impedance since it's about half a wavelength. You'd be better going to 3/4 wavelength. The shorter the antenna, the closer to omnidirectional it is. Shorter antennas don't pick up as much signal, though. Quarter-wave (or a bit shorter) antennas are a good compromise, not having too much of a null response off the end, yet giving good signal strength. A straight 1/2 wave (about 80" at 72 MHz) will have deeper, wider nulls off the end, and better signal strength than a 1/4 wave to the side.

Don't forget the grounding. If you use the coax feed, ground the receiver end to the battery negative with a short lead. At the antenna end, put 40" of wire on the coax shield as well as the inner conductor. The two 40" wires should be at least 90 degrees apart. E.g. run the antenna up the fin with a whip to extend it, and the ground plane wire back along the fuse.

Stringing the original antenna wire from the canopy to the tip of the fin actually works pretty well, though it's ugly. Embedding the antenna in the wing can also work well. You can use a connector (make sure it's reliable) or a plastic tube to deal with plug-in wings.

Buy a ham radio antenna book. These will have explanations of various antenna configurations and their radiation patterns. They should also cover what's involved in tuning the antenna to the reciever.

If you do some range-test comparisons in a consistent manner you will soon see what works and what doesn't. You will have to test a few orientations of the receiver antenna to the transmitter.

Aero65

My Feedback: (2)

Wow great info Phil Cole! Thank you for the lengthy response. I guess I should just start reading and experimenting.

I guess I should think about getting a HAM lic. I've been thinking about it for years. I'm sure Ill learn some cool stuff.

Flyboy I have called mfg's and although helpful, they have liability issues and FCC issues they need to keep in mind, so they usually give the don't mess with it answer. If I were a mfg. I would probably say the same thing, and for the most part they are probably right. I just love to tinker with stuff.

Whirley Bird

My Feedback: (1)

I think what you want to do is make a half wave dipole for the plane.
Use a short piece of 50 ohm coax coming out of the RX.
The one wire goes to the braide.
The other goes to the center conductor.
I have one of my trainers setup that way and run each wire under the wing.
More wire = more capture area.
You need to retun the RX but since I can't post that here.
I found a way thatI can pick up RX noise and peak it.
Someone in your club should be able to do it for you

Ladyflyer

My Feedback: (2)

Phils comments as usual are accurate and sound, Thank you Phil

JPMacG

My Feedback: (2)

How can manufacturers tune the receiver to the antenna? Do they make assumptions about the length and dress of the other wiring inside the plane, which makes up the "counterpoise"? The impedance of the antenna is very sensitive to this! For example, the impedance of a 1/4 wave antenna with a 1/4 wave counterpoise (a dipole) is around 72 ohms resistance while the impedance of a 1/4 antenna with a 6 inch counterpoise will be only several ohms reisitance with hundereds or thousands of ohms of capacitive reactance.

Rodney

JPMacG, you have a very large effective counterpoise in your plane; all the ground wires on the servos and wiring harness. No problem there.

JPMacG

My Feedback: (2)

Thanks Rodney - but that is my point. The ground wires in the battery pack, wiring harness, servo leads,etc, do not make an adequate counterpoise. They amount to several pieces of 6 to 12 inch long wire coming out of the receiver. An antenna consisting of a 39 inch wire with several 12 inch wires as a counterpoise will have an input impedance of around ten ohms resistance plus hundreds of ohms of capacitive reactance. If one adds more wire to the counterpoise (e.g. servo extensions in a larger model) the input impedance will change substantially. So how do the manufacturers know what impedance to tune their receivers to?

Thanks-

Ladyflyer

My Feedback: (2)

The input circuit is lo Q and fairly broad band. Tuning the antenna coil for peak is sufficient in most cases. most folks couldn't afford the radios if they had Hi Q tuned inputs that required tuning at operating altitude .
Of course the proof is THE STUFF WORKS ,darn near as far as you can see it . If not FARTHER than you can see it.

The bumble doesn't know it can't fly,and so it does .

JPMacG

My Feedback: (2)

But the issue is not Q, the issue is impedance matching and the efficient transfer of signal power from the antenna to the receiver.

I know that our RC radios work great. But antenna length is a recurring question on this forum and I would like to have the receiver design details so that I can give out correct information.

People don't understand that the "counterpoise" dominates the performance of RC receiver antennas. Phil mentions that a half wave antenna has a highish impedance. This is true of a half-wave ground plane antenna or a center fed full wave dipole, but a half-wave wire fed against an electrically small counterpoise actually has an impedance quite similar to a quarter-wave wire fed against that counterpoise. Maybe in this case it would be more correct to think of the wiring harness as the "antenna" and the "antenna wire" as the counterpoise.

Hope I am not coming off rude or arrogant. That is not my intention.

Jon

Phil Cole

Jon,

Impedance matching at the receiver is not a big deal. If you get enough signal off the antenna to overcome internal noise the job's done.

To make the front end tuning relatively independent of the length of the servo wires, etc. it has to be a relatively low Q circuit. Otherwise, we would all be having the problems that are causing you some concern.

Short antennas (or short counterpoises) will have a relatively high, and very reactive impedance. So you want to design the receiver to have a highish input impedance (more than a few Ohms). You also want the antenna loosely coupled, so any reactance does not detune the front end much.

All this means the signal power transfer won't be anywhere near optimum. Low noise transistors are relatively cheap, so it doesn't matter too much. Even at 72 MHz, internal receiver noise should not be a major limiting factor.

Strong signals and off-channel interfence are more important things to worry about.

As a matter of interest, impedance matching is also the last thing you want in a transmitter as well. We all know that maximum power transfer happens when source impedance equals load impedance. Trouble is, the source has to dissipate as much power as the load, so efficiency is way down.

Impedance matching antenna to feedline is a different matter. Feedlines generally are not loss-free so each reflection (from the antenna and transmitter) results in more dissipation in the feed line. Matching the transmitter here would not be good, since the reflections would be absorbed in the transmitter instead of being reflected back to the antenna.

Ladyflyer

My Feedback: (2)

Thanks ,and a tip of the EllEff hat to you Phil . Very well put .

JPMacG

My Feedback: (2)

Phil, I more or less agree with your comments regarding the receiver and impedance matching. The conclusion we are coming to is that receiver front ends are not tuned to the antennas and there is nothing sacred about the 39 inch length for receiver wires. Aero65 can go ahead with his half-wave wire without degradation in performance. Do you agree?

But on the other hand... your comments regarding transmitters.. I think we're getting into some difficulty regarding the definition of the output impedance of an amplifier. The large-signal output impedance of a non-linear amplifier is a complicated subject and can't be measured using the usual techniques. I don't think we need to open that can of worms.

Best regards,

Jon

Phil Cole

Jon,

I guess from the signal strength point of view there probably won't be a problem with a half-wave wire.

The radiation pattern will depend on the disposition of the wire. Most likely it won't be straight, reducing the width of the nulls off the ends.

I made the amplifier point to show that impedance matching was not the best thing to do in all (or even most) situations. If the [non-linear] output impedance is low it doesn't matter too much what it is.

Ladyflyer

My Feedback: (2)

The 39 inch length is not as critical as many presume. One thing to watch with more efficient antennae is they may become more directional possessing deeper nulls . (?)

Aero65

My Feedback: (2)

I guess the best thing to do is make a cheap coroplast airplane, and test the theory out! We have the walston tracker out here were we fly so if the test aircraft gets lost... no problem just track it down. I think as a precaution I will also put a Parachute on a fail safe release just in case.

Any suggestions on different things to test? Like intentionally putting some electronically noisy things... IE metal to metal contact. Different antenna configs?

Anyway I was thinking of a simple aircraft with a os .25 for power. And longish wing to support weight of experimental stuff.

Thank you for all the responses.

Phil Cole

Ground range testing with reduced transmitter power will get you the information you need. Lowering the antenna is the usual way to achieve reduced transmitter output. While it's not calibrated, a given transmitter will always radiate the same power with a retracted antenna provided you hold it in the same way.

The tests are only comparative, so you establish a baseline with something you know works, i.e. the factory fitted antenna.

To get close to the in-the-air situation, you can sit the rest receiver on top of a 10 to 15 ft non-conducting structure. Measure the range to loss of control while rotating the receive antenna to various orientations with respect to the transmitter. You are looking for the orientation with the least range.

Do the same with your experimental antennas.

Transmitter orientation is important to, so have the same person hold it in the same way each time. While walking out to find the range, retrace the same path each time. Rotate the receive antenna on the top of it's pole, don't walk off in various directions with the transmitter.

This may seem like a lot of work, and it is. However you will get quantitative, repeatable comparsons of the antennas you are testing. This is much better than "I think I lost the plane over those trees this time, and last time it was over the drainage ditch on the other side of the field".