Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Antenna length

Status
Not open for further replies.

jpanhalt

Well-Known Member
Most Helpful Member
I am faced with a model airplane whose fuselage is too short for the antenna at 72 MHz. Actually, for balance reasons, the receiver is being put in the tailboom. The antenna is about 39 inches (1 m) long.

How much loss would one expect if the antenna were simply loosely coiled on a form, say 3/4 inch dia. X 16 inch length?

Thanks. John
 
It's hard to say. You could try EZNEC 4.0 for an answer. Sounds like it should be a quarter wave length vertical. So winding it loosley on a form is going to change it from a radiating antenna to..well something else. I'd be surprised if it radiated at all in that configuration. You could try a folded dipole mounted on the wings. You could try a partially inverted V from wingtip to vertical tail to wingtip. You could trail the antenna behind the plane like Amilia Earhart.
 
By winding the antenna as described, you are creating a "normal mode helix" antenna.
This type of antenna is what you see on the average VHF/UHF hand held radio as used by police/fire/security etc.

Give it a try, you will probably be quite happy with the result.

JimB
 
Fly a banner behind the plane ("Eat at Joe's" or "Melissa: Will you marry me?") and make it from stranded wire as part of the antenna. All the length you need.

Dean
 
Here is some info on range checks: **broken link removed**

I sometimes run mine out the tail (small gliders or 1/2 A's) but this can be a pain, like when landing in brush. I have had it wrap around stuff and break.

I try to keep it as straight as possible,( usually out the top front of the fusalage and back to the vertical stabilizer, but I have heard of people wrapping it on forms too. Try here too , you will get lots of info.
https://www.rcgroups.com/forums/index.php sam
 
Last edited:
Thank you all for the advice and fast responses. I generally keep the antenna inside the tailboom even with carbon fuselages (sailplanes), and have had no problems, at least to the limit of my eyesight. Like Sam2, I don't like having it dangle much out the back because of tangles, breaking, and general esthetics.

I have seen antennas wrapped around tailbooms, but have not seen actual studies of the effect of that practice on range at 72 MHz (4 M). I was hoping someone on this forum would have a rule of thumb for the expected loss by coiling the antenna versus a straight wire.

I guess it may be worth a try. What do I have to lose? :) I will, of course, range check first. John
 
JimB said:
By winding the antenna as described, you are creating a "normal mode helix" antenna.
This type of antenna is what you see on the average VHF/UHF hand held radio as used by police/fire/security etc.

Give it a try, you will probably be quite happy with the result.

JimB
Are you saying that if you compute the proper length for a 1/4 wave end fed vertical and wrapped it around a form that its characteristics are unchanged? If you had said "add a loading coil" to a shorter length of antenna the response would make a bit more sense. You can do this but you need to provide for the change in geometry.
 
Papabravo,
No I am not saying that.
From what I understand of normal mode helix antennae, they can be made with either 1/4wave or 3/4wave of wire wound into a small former >>0.1wave diameter, and even then the resonant frequency of the antenna can be far from what is expected.

In the case of this model aircraft, we are into the area of Elbonian engineering.
If the antenna were 1/4wave, what is it working against for a ground? I guess not much, just the circuit board and power wiring of the RC receiver.
The performance is not going to be all that brilliant compared with a proper vertical over a ground plane, or a dipole.
I am also guessing that the OP does not have the test equipment and facilities to optimise the antenna when it has been reduced in size .

So, my best guess is to take a 1/4wave of wire, wind it into a helix which fits into the aircraft fuselage and see what happens.
What range are we looking for here, 0.5km max?
I assume the transmitter power is about 200mW, if the receiver cant hear that at 0.5km line of sight, there is no hope for it.

JimB
 
JimB said:
Papabravo,
I am also guessing that the OP does not have the test equipment and facilities to optimise the antenna when it has been reduced in size .

What range are we looking for here, 0.5km max?
I assume the transmitter power is about 200mW, if the receiver cant hear that at 0.5km line of sight, there is no hope for it.

JimB

You are absolutely right. I do not have the facilities or knowledge to optimize antenna design.

As for the other assumptions, you are pretty close. We generally count on about a 1 mile range, but they are pretty small then. Altitude is up to 1000 feet or a bit more. Transmitter power is in the 200 to 250 mW range.

I am still finishing the electric motor installation. When ready to fly, I will do a range check with and without the motor running. If the range with the coiled antenna is significantly shorter than I am accustomed to, I will do the range check with the antenna uncoiled under the same conditions. If anything interesting happens, I will report back.

Again, thanks for the advice. Nothing can prevent a crash, but this particular plane has survived 13 years of my piloting as a pure glider, so I hate to do something that is really dumb. John
 
JimB said:
Papabravo,
No I am not saying that.
From what I understand of normal mode helix antennae, they can be made with either 1/4wave or 3/4wave of wire wound into a small former >>0.1wave diameter, and even then the resonant frequency of the antenna can be far from what is expected.

In the case of this model aircraft, we are into the area of Elbonian engineering.
If the antenna were 1/4wave, what is it working against for a ground? I guess not much, just the circuit board and power wiring of the RC receiver.
The performance is not going to be all that brilliant compared with a proper vertical over a ground plane, or a dipole.
I am also guessing that the OP does not have the test equipment and facilities to optimise the antenna when it has been reduced in size .

So, my best guess is to take a 1/4wave of wire, wind it into a helix which fits into the aircraft fuselage and see what happens.
What range are we looking for here, 0.5km max?
I assume the transmitter power is about 200mW, if the receiver cant hear that at 0.5km line of sight, there is no hope for it.

JimB
I certainly would be interested in seeing a return loss plot of that configuration. I'll have to take your word for it.
 
UPDATE
We finally had a decent day (35 F, winds 5-10 knots) to test fly the design. I made a balsawood tube by wrapping wet 1/32 balsa on a 0.625 rod. After drying, the antenna (39 inches) was wrapped around it with a pitch of about 1". The entire system consisting of motor controller, a SMPS, receiver, LIPOs, and antenna tube was mounted on a piece of coroplast (it's like cardboard made of PVC). Pictures are attached. The system was then stuffed into the tail boom of the model.

Range on the ground was at least as good as was typical for that model when the antenna was left straight and inside the model with the receiver mounted forward. Range in the air was sufficient to the limits of my eyesight.

John
 

Attachments

  • BATTERY SLED.gif
    BATTERY SLED.gif
    107.2 KB · Views: 165
  • Antenna tube.gif
    Antenna tube.gif
    43.4 KB · Views: 154
Nice antenna!

papabravo said:
I certainly would be interested in seeing a return loss plot of that configuration. I'll have to take your word for it.
Inspired by this comment I did some experimentation using a home made Return Loss Bridge (RLB), and the results were both interesting and confusing in some areas.

From memory, (I am away from home at the moment):

1/ Connecting the 1metre wire (straight line) to the Unknown port of the RLB and just the cables to the signal source and detector, I could not find a sensible resonance between 30 and 100Mhz. A very poor result.

2/ Connecting the 1metre wire (straight line), and mounting (a soldered connection at two points) the RLB onto the middle of a strip of copper, about 7cm wide, 1.2m long and 0.2mm thick, I found a good dip in return loss (>20dB) near the expected resonant frequency of the antenna (73Mhz). A good result.

This is what I would expect, the antenna will not give good results without a reasonable earth plane to work against, just like in the model areoplane, where the only earth plane is the reciever chassis and power wiring.

3/ Winding the antenna wire into a helix, about 3cm dai and 30cm long, and connecting it to the RLB as in 2/ above.
I could find a resonance at some frequency, but it was up at about 200Mhz (from memory).

4/ Winding a second antenna from 3 metres of wire (helix 3cm dia, about 80cm long), and connecting as in 2/ above.
I could find a resonance at about 45Mhz (again, from memory).

Some strange results, not quite what I would have expected, from tests done under far from idea conditions.

Another test I did was to measure the relative response of the straing wire and the same wire wound into a helix.
The helix gave 10 to 15dB less signal.
Again I emphasise that the test conditions were far from ideal (verging on a joke).

Relating all this to the practical situation of jpanhalts' model plane, his helix although probably far from ideal, provides a working solution for him.
He is working line of sight to the plane and there will be plenty of signal from his controller with its 200mW transmitter and better mounted antenna.

JimB
 
JimB said:
This is what I would expect, the antenna will not give good results without a reasonable earth plane to work against, just like in the model areoplane, where the only earth plane is the reciever chassis and power wiring.

JimB

That information is very helpful. I had actually made a 1" wide aluminum tray but opted for coroplast for weight reasons. With a few holes, the aluminum tray could probably be made almost as light. I have never seen a model that had an intentional ground plane; although, I am aware of their use in full-size composite and fabric-covered aircraft. Maybe the batteries serve some for that purpose in our models.

It is too cold right now to do much walking around measuring range. If this model survives the winter, I may try the aluminum tray in the Spring.

Thanks for all of the work on this subject. John
 
jpanhalt said:
I have never seen a model that had an intentional ground plane; although, I am aware of their use in full-size composite and fabric-covered aircraft.
Maybe the batteries serve some for that purpose in our models.
Anything electrically attached to the "chassis" of the reciever and running in the opposite direction or at right angles to the antenna will be useful.

However, your solution works correctly in your situation, so no worries.
That is what engineering is all about, finding what works, satisfactorily, for the minimum cost.

JimB
 
Congratulations JimB. A first class piece of experimental work. It is also worth noting that an antenna with circular polarization will work but the geometry needs to be different to get the same resonant frequency. It might also help is the receiving antenna was constructed in the same fashion. At VHF the inductance of and the interwinding capacitance is bound to have an effect.

What does your RLB look like? Could we see a picture?
 
OK here is the Return Loss Bridge, and a couple of the terminations used to calibrate it, an RF short circuit, and an RF 50 Ohm termination.
The short circuit was made from an old BNC plug and a turned brass insert.
The 50 Ohm terminator was made the a BNC plug and two 100 ohm surface mount resistors wired in parallel.

The Bridge was made in a diecast box. It is painted black as it is recycled from some scrap commercial item.
The resistors used in the bridge are 100 ohm surface mount types, two in parallel to give 50 ohm.
The balun is wound on a ferrite core.

Using an HP8640B signal generator as a source and an HP141 series spectrum analyser as the detector, I see a return loss better than 30db for the 50 ohm terminator from 20 to 450Mhz.
Over the same range, a 100 ohm terminator gives a return loss of just under 10db, which is to be expected.
Below 10Mhz, performance drops off quite a lot, I suspect that the balun does not have enough inductance and affects the bridge balance.

It would be nice to have either a tracking generator for the spectrum analyser, or a sweep generator and some other detector and visual display, so as to be able to see a plot of return loss. One day maybe.

The inspiration for this project came from the books:

Solid State Design for the Radio Amateur, chapter 7, page 155
and
Introduction to Radio Frequency Design,chapter 4, page 152
both published by ARRL.

For circuit and pictures of my RLB, see below.
 

Attachments

  • RLB.JPG
    RLB.JPG
    16.3 KB · Views: 196
  • RLB Outside.JPG
    RLB Outside.JPG
    120.8 KB · Views: 164
  • RLB Inside.JPG
    RLB Inside.JPG
    139.6 KB · Views: 174
  • ShortCircuit.JPG
    ShortCircuit.JPG
    50.9 KB · Views: 163
  • 50 Ohm Termination.JPG
    50 Ohm Termination.JPG
    69.2 KB · Views: 180
I'm trying to solve a similar problem for a model helicopter application. I've been using **broken link removed** and helical3.

The numbers I got out of helical3 suggest that winding the rx antenna wire around a toilet paper tube might produce an antenna which resonates at 72MHz, but I haven't tried to determine efficiency/SWR with nec2 yet. It looks like the most economical way to perform any quantitative measurements is via the RSSI pin on the rx.

I am faced with a model airplane whose fuselage is too short for the antenna at 72 MHz. Actually, for balance reasons, the receiver is being put in the tailboom. The antenna is about 39 inches (1 m) long.

How much loss would one expect if the antenna were simply loosely coiled on a form, say 3/4 inch dia. X 16 inch length?

Thanks. John
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top