900 mhz homemade antenna

[alvise]

I am trying to develop antenna for telemetry module to communicate long distance(20km) on rockets.
Antenna shape I want to develop:
900mhz 3dbi Helix normal mode antenna.
For the telemetry module, which receives and transmits signals horizontally, I want to make an antenna whose wire is wound in a helix shape on the outer surface of the rocket by calculating the pitch length and the number of rotations in which the wavelength is determined.
The reason I'm trying to do this is;
If I use the "868-900MHz Half Wave Dipole 3dBi" antenna that comes with the module, I will have to place it inside the rocket to protect the antenna and I think this will reduce the reception quality of the antenna.
Even if I put it outside, since the antenna is only on one side of the rocket, if the rocket turns to an angle where it cannot see the antenna on the ground, the signal quality will still decrease.

My first question is, does anyone have any suggestions about the method of placing the Standard Antenna on the rocket?
My second question: I am trying to make a normal mode helix antenna. I am using ''CST studio suite'' as an analysis application for antenna.
I designed an antenna like below and the test results are as follows. Do you think these results are correct?
On the 115mm diameter pipe, 7 turns of 1.5mm thick wire and 24 pitch length were used.





Does anyone have any suggestions or a different opinion on this?

 

[OzHybrid]

The RF Design RFD 900 diversity transciever module has already been used at horizontal distances of over 70Km
http://rfdesign.com.au/products/

 

[Worsaer]

Just curious, why do you think placing it inside the rocket will impair its performance? Is this for a carbon fiber airframe.

 

[alvise]

The RF Design RFD 900 diversity transciever module has already been used at horizontal distances of over 70Km
http://rfdesign.com.au/products/

Yes, but how should I place the antenna If I place the antenna inside the rocket, will the data transmission distance not decrease?
If I put the antenna out and the rocket spins around, will the signal reception still be unaffected?

 

[alvise]

Just curious, why do you think placing it inside the rocket will impair its performance? Is this for a carbon fiber airframe.

The thickness of the polyamide plastic is 6 mm. I guess that the signal quality will decrease when there is an obstacle in front of it. And extra if I place it inside the rocket, the antenna must be empty around it, which takes up extra space.

 

[OzHybrid]

Yes, but how should I place the antenna If I place the antenna inside the rocket, will the data transmission distance not decrease?
If I put the antenna out and the rocket spins around, will the signal reception still be unaffected?

If it's fibreglass for that section of the body, attenuation will be insignificant. It depends on the other items you have close to it, like the AV bay tie rod. Specifically, make sure the length of it is not a function of the frequency you are using. I use a vertical stubby antenna and a horizontal one with a vertical and horizontal yagi at the receiver end. If it's carbon, you'll get significant attenuation. There were some people getting good results with slots cut in the CF to allow the signal out.
You are better off investing your time in a helical GPS receiver, to get "omnidirectional" GPS signal. There used to be a Sarantel helical GPS antenna. That factory had a fire and never fully got back into production. There was a discussion which suggested that this antenna in a vertical position, favoured satellites that were NOT vertically above and therefore reduced the chance of loss of lock on a satellite as the Doppler signal effect is maximum at high speed vertically.
If you haven't got GPS lock, you don't have positional data to be able to transmit.
Good luck with the project. I cannot fit a 4" pipe inside anything I'd need that for.

 

[Worsaer]

You’re probably over engineering for 900 mhz. Other than carbon fiber, most rocket airframe materials are rf transparent.

 

[cerving]

Spend your money and effort on the receiver antenna. At 20 km a panel antenna or a 3-element Yagi with a relatively wide beamwidth can greatly improve your signal reception. On the transmitter side, you're not going to do much better than about 3 dB and maintain an omnidirectional pattern, assuming you have a good ground plane a solid transmitter antenna is the Linx ANT-916-CW-QW.

 

[plugger]

I've made a number of helix antennas recently, it's worth mentioning that you want to match your transmitter and receiver polarisation otherwise you'll take a 2-3 dB hit in SNR. I've used Derek's scaffolds to make my helix feeds, you can find those here.
https://github.com/sgcderek/dsgc-helix-scaffold
The code is adjustable so you could easily modify it for 900MHz. It might help to compare/confirm your numbers out of CST studio.

 

[Voyager1]

Interesting concept. However, as suggested above, you don't gain (pun intended) much here. An appropriately installed mono or dipole antenna, preferably in the nosecone without adjacent conductive components, so that you do have clean volume around it, will provide optimum results. You will also benefit from a more omnidirectional radiation pattern.

Polyamide has a dielectric constant of about 2.5-3.5, a little less than fibreglass. Attenuation is probably similar to FG - low. The attenuation of the polyamide can be assessed if you have a receiver that has an RSSI readout. Do a 360 degree far-field test, with and without polyamide structure. Simulations are good, when done accurately, but nothing like the real thing - field test!

The VSWR and return loss (S11) plots you attached need to be replotted over the 0.8-1 GHz frequency span. As they are, they don't provide much information.

Good luck! Nothing like pushing the boundaries. I was working on a 3 patch array antenna system a while back. The patches were to be placed around the external surface of the body tube. When I get the opportunity I will continue developing this concept. The sims were promising, but, as I said above, nothing like a real test. So, I will be keen to see how your development progresses.

As suggested by Cris above, invest some time into developing a good receiver antenna, too.

 

[OverTheTop]

I too support the improve the Rx end of things. Very easy to gain signal and stay within your signal link budget. Mass and volume are almost "don't care" status there.

 

[cerving]

One more consideration is that to maintain the license-free nature in the 900 mHz band you can only use a single element antenna on the transmitter. If you're a Ham, however, you can go to town, and you can do whatever you want on the receiver regardless since it's not an "intentional radiator".

 

[schworer]

Since you want a system with a 20KM+ range, my suggestion is you develop a link budget and figure out what you need in terms of transmitter power and gain, free space loss over the signal path, and receiver gain in order to have sufficient link margin. As the guys above have said, the easiest way to boost performance is on the receive side since you are limited in the type of antenna TX gain and available TX power. Start with the basic receiver sensitivity specs and payload transmitter power into a 1/4 wave vertical and then see what you need with a RX gain antenna to get sufficient link margin. You may find out that a simple 100mW to 500mW transmitter and vertical whip in a RF transparent nose cone combined with a multi-element receive yagi or turnstile is sufficient for line of sight at that range, and that no fancy helictical antennas are necessary. Here is a YouTube video on link budget basics and there are calculators on the web plus lots of info.

 

[alvise]

Going through all the posts over and over, I realized the main idea was to stick with the RX antenna instead of the Tx antenna.
I am planning to use a directional (yagi, omni, Grid parabolic antenna, panel) antenna for the receiver, but there are 3 problems that confuse me;
1- I need to place the Tx antenna inside the rocket so that the antenna does not break or deteriorate every time. In another case, I think by centering the "Half wave dipole" transmit antenna inside the rocket so that it can emit a 360 degree signal and there should be no batteries or any material around the antenna. .But I don't have much room for nose cone or other places.(3dbi antenna length is about 20cm)
2- I need to use 6 mm thick Polymide to create a more solid structure. I think this affects the transmission quality of the antenna signal.
3- If I use a directional receiver antenna, sometimes I may not be able to find exactly which direction the rocket is going. Therefore, it is always a problem to find the right direction.


-If I use a 1000mw telemetry module with a half-wave dipole 3dBi antenna, there is a battery etc inside the rocket around the antenna. Is it not likely that the signal quality will degrade when there is?
-Also, if the antenna is not in the center of the rocket, wouldn't the signal transmission be strong in only one direction?

It is always an advantage to have a strong 360-degree signal in the transmitter direction of the best method. For a few reasons I mentioned above, the idea of making an antenna that wraps around the rocket as a helix sounds more appealing.
Of course, if I can't make a correct antenna, that's a big problem

[SIZE=4]Voyager1[/SIZE] The antenna shape you shared caught my attention.

By the way, thanks everyone for their ideas.
It can really come in handy to hear a few sensible suggestions before field testing.

 

[Steve Shannon]

1. It’s simply not necessary to center the antenna. I doubt that any directional attenuation will be significant enough to even detect.
2. 6 mm “polymide” is probably not radio opaque. I was only able to find polyamide and it appears to be radio transparent. Just run some tests. Most polymers are pretty radio transparent unless they have metal films or high content of carbon.
3. Track the rocket in flight. If you lose sight of it and contact with it, keep swiveling the antenna until you find the signal again.

No matter what you do, test it. Especially with antennas, small changes can make large differences. If you really want to try the helix design, use copper foil tape like is used for alarms.

 

[OverTheTop]

Be careful if you are going to use 1000mW transmit power. It has a nasty habit of getting into other electronics through their "antennas", aka the wires you use in hooking everything up. I have had a nosecone blow off on the pad when the 1W telemetry Tx keyed up, just as I was walking away from the rocket. Use lower powers if you can get away with it. Your link budget will tell the story of what you need. I would start getting a bit nervous above about 100mW Tx power.

Also remember you telemetry Tx can desensitise your GNSS receiver, if you are running a tracker in the rocket. Try to keep the Tx antenna away from the GNSS antenna, to improve the situation. I am not sure of the mechanism. It could be either receiver desensitisation or introducing timing jitter into the GNSS Rx. I have seen this and it maniftests itself as longer times to achieve the location solution.

 

[Voyager1]

I agree that polyimide is highly transparent to RF. If in doubt, test it! In fact, they use it in radomes. Check out this document https://www.curbellplastics.com/Res...tics-for-High-Performance-Radomes-White-Paper

Design you receive antenna with a broad enough beam pattern so that you can more easily follow you rocket. It will be a compromise between beam width and gain. As suggested above, work out a link budget for different configurations to assist you in optimising your system.

Do you really need 1W TX power?

You could use an array of Flex antennas around the body tube fed from a power splitter. http://rfdesign.com.au/products/flex-antennas/

 

[alvise]

First of all, thanks for your suggestions.

half-wave dipole 3dBi antenna for rocket positioning ;
-The conclusion I deduced from the suggestions is that if I put the antenna inside the 6 mm thick "polyamide" plastic pipe without centering it (the point I still suspect here is that if batteries, minicomputers etc. are placed next to the antenna, they will cause a break in the signal), the signal transmission will not be seriously affected.
-I have 1000 mw nodules. I've tasted it before by putting it next to the GPS. It did not affect the GPS signal. But other than that I don't know what the problem could be.

For the antenna design used on the outer surface of the rocket;
- Using 1.5mm thick motor coil wire, I created a 7 turns 24mm pitch length design (I didn't test it).
-I have no idea how to use copper foil tape. If there is a method you know, I can try.
-I was thinking of using patch antennas, but all patch antennas I could find are not enough to place the lengths 360 degrees. I will need to combine multiple antennas for this.

an alternative method; It is placing each antenna of the telemetry module on one side of the polyamide tube (inside the plastic). I don't know how that will work.

 

[Andrew Brown]

I'm going to back up and address the original question... something is definitely wrong with your simulations! The VSWR plot is meaningless on that scale, but it won't matter as your return loss is basically 100% reflection. My guess is that you set something up in CST incorrectly, possibly shorting out your port. Without a smith chart or phase information, I cannot tell if your port is open or shorted. However, 1 mm length of copper trace will radiate better than that.

The radiation plots really don't mean much if you are not coupling anything to free space. An efficiency of -35 dB means you basically are not radiating.

 

[alvise]

I'm going to back up and address the original question... something is definitely wrong with your simulations! The VSWR plot is meaningless on that scale, but it won't matter as your return loss is basically 100% reflection. My guess is that you set something up in CST incorrectly, possibly shorting out your port. Without a smith chart or phase information, I cannot tell if your port is open or shorted. However, 1 mm length of copper trace will radiate better than that.

The radiation plots really don't mean much if you are not coupling anything to free space. An efficiency of -35 dB means you basically are not radiating.

Thanks for your reply. It's kind of you to shed some light on this.
The antenna I use is as follows: The discrete port is between the beginning and the end of the wire.
I've included the design I used below.





I would appreciate it if you could help me with this. As a matter of fact, I don't have much experience in using CST.

 

[Andrew Brown]

I haven't used CST in probably 18 years. You need to set the ports up in the same manner you plan to excite the antenna. A helical antenna is essentially a monopole that has been coiled. You need to place the port between the antenna feed and what little ground plane you'll have in the airframe. If you are dabbling in ridiculously priced software, my guess is you are a student. Any chance you have access to HFSS? If so, I could help you set it up. Unfortunately, I don't have access to CST.

 

[alvise]

I don't have much experience with antenna. I know it is a way that takes a lot of effort and time.Just trying to find a solution to the current problem, I decided to use CST.I have experience in programming and electronics, but I have very poor knowledge of antennas.I've done some research to see if an antenna can be built around the rocket, just curious.Because if such a thing could be done, space could be saved and strong signals could be generatedI'm currently having trouble using CST, I'm hesitant to start using HFSS.My aim is to make an antenna example that can radiate 360 degrees in all directions around a 115 mm diameter polyamide pipe and gain some experience from it.I'm not a student, but I have a lot to learn about it.

You are talking about the need to place the port in the middle of the helix, which I understood from your answer.
like the this picture

 

[Andrew Brown]

alvise,


Most helical antennas are of the monopole variety. The feed is at the bottom with a finite ground plane below it like in the figure below (borrowed from internet).


What you proposed in the previous post is more like a dipole. It will give you more of a symmetrical toroidal pattern, which I believe is what you are after. However, you will need to feed it differentially to get any sort of consistent performance. Either your transmitter has a differential output or you feed it with a balun. You could connect one side to the signal and the other to your pcb ground, but then one end acts more as a counterpoise rather than a dipole and will take some different tuning.

Regarding simulation, these field solvers are great, but you really need to know how to set them up correctly. They will solve exactly what you give them, but garbage in = garbarge out. The ports, correct boundary conditions, mesh setup, what to approximate vs what to model, material properties, etc, are all critical for getting anything remotely matching reality. This is learned through a lot of experience modeling, measuring, and comparing. I'm also a little concerned about how you are getting access to these tools. These are $100k tools. When I assumed you were a student, I figured you were using an educational license. If you are using a license illegally, I cannot help you.

Luckily, these frequencies have large wavelengths. You do not need to model this for the sake of modeling. You can easily take your cylinder, use wire or copper tape, make your antenna, and make some return loss measurements. This gives instant feedback. You can then either add to or trim to your antenna until your resonant frequency matches your target. If you really want to squeeze out performance, you can add a matching network between your transmitter and antenna as the impedance match will be sub-optimal. Since the antenna is symmetrical and your effective ground is tiny, you will get a dipole pattern no matter what you do to it. For an antenna like this, the experimental method can get you there within an hour or two vs. the headaches of learning a field solver, understanding what to trust from it, and what approximations to make.

 

[alvise]

andrew brown;

Thanks for your suggestion.
What I'm after is creating a 900 mhz antenna wrapped around a rocket so that it can propagate 360 degrees (omnidirectional) signal around itself.
I think the most suitable design for this is the normal mode helix antenna.

It has a helical antenna similar to your suggestion in the CST examples, but the signal can propagate vertically.
total effic in the photo below. it's still a negative number. Is this really how it should be?




I am a little cautious that I can get results faster with experiments.
My main problem now is that I don't know how to actually simulate an antenna.

 

[cerving]

Maybe you've addressed this earlier, but if your rocket is made of a radio-transparent material why don't you just use a 1/4 wave antenna with your transmitter's PCB ground plane as the counterpoise? Virtually all hobby rocketry transmitters do that... and it works fine. A 900 MHz 1/4 wave antenna is about 80cm long, if you put that in the nose cone there should be plenty of room, even in a 54mm MD build.

 

[Andrew Brown]

At this point, I'd really urge you to do a little background learning on exactly what you are looking at. The CST example of a helical monopole is giving meaningful results for an antenna in isolation. The efficiency is on the order of -0.6 dB, which is reasonable. Note that 0 dB is 100% efficiency. The pattern and gain numbers look reasonable.

Yes, experimental design through verification for a 900 MHz wire antenna is absolutely faster than learning a complicated software tool, learning what should be modeled vs what can be ignored, then building and testing. These numerical method tools really excel in doing the task when it cannot be easily made and tested, doing parametric sweeps, or looking for manufacturing tolerances. For a quick design in the sizes you are looking at, I could have mocked something up and tested it in about 20 min.

I suggest you to take a step back and look fundamentally at your problem and look objectively at potential solutions. You appear to be looking for a radiation pattern that is omnidirection in phi, basically a donut (dipole pattern). You are suggesting doing this by wrapping a helix around the outside of a body tube. I've mainly been addressing the numerical method issues, but as other posters have commented, this is completely feasible by staying inside the airframe, unless you are using carbon fiber. This can be done with a simple dipole or monopole (which really acts like a dipole when the ground is very limited). A helical antenna for 900 MHz will basically allow you to get slightly more compact. The attenuation through cardboard or fiberglass is negligible at 900 MHz. If you are dealing with a carbon fiber, carbon loaded fiberglass, metal, or metalic (true metal, not mylar loaded) paint for the airframe, the airframe is conductive. Wrapping the an antenna filament on the outside, on the surface, effectively shunts out the fields anyway.

 

[Andrew Brown]

... A 900 MHz 1/4 wave antenna is about 80cm long, ...

Cris, I think you meant 80 mm.

 

[Grog6]

Wet paper, copper foil, metal objects, carbon fiber thats sufficiently thick; all those attenuate RF. other, nonconductive materials, not so much

 

[cerving]

Cris, I think you meant 80 mm.

cm, mm.... whatever.

 

[alvise]

Thanks everyone for their answers. I think I've given up on designing my own antenna now. I will embed a "Right Angled Quarter Wave monopole 2.1dBi" antenna on the sides of the polyamide tube.The length of this polyamide pipe will be 50 mm and will be between two aluminum pipes.