900 mhz homemade antenna

[OverTheTop]

Keep in mind such a construction will be electrically short compared a thin wire version at the same frequency/wavelength. The larger diameter tends to add capacitance to it, which you need to cancel out buy adding more inductance (by making the antenna longer).

 

[alvise]

Keep in mind such a construction will be electrically short compared a thin wire version at the same frequency/wavelength. The larger diameter tends to add capacitance to it, which you need to cancel out buy adding more inductance (by making the antenna longer).

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I don't quite understand what you mean exactly here.Can you explain a little more?

 

[OverTheTop]

I don't quite understand what you mean exactly here.Can you explain a little more?

Ok. An antenna needs to be a certain length to resonate and couple the desired frequency to/from the air. Most antennas you see are elements made of skinny wire or metal. So, at a particular frequency the antenna elements will have a particular length. Now imagine making the wires thicker. This add capacitance to the antenna elements. Now, here is the kicker, inductance and capacitance are opposite of each other, so adding one can cancel the other. So, our antenna with thicker elements will add capacitance, which subtracts from the inductance of the antenna, making it appear shorter at the desired wavelength, given the same element lengths. So our antenna has gotten shorter and is not at the resonance for the required frequency. We can compensate for that effect by making the elements a bit longer, bringing the antenna back to resonance.

So the one you drew with the foil elements around the airframe is seriously capacitive, so each of the halves of the antenna (either side of the feed point) will be too short for resonance at the frequency you actually want. The cure is to make those conductive elements longer, bringing the antenna back to the correct "electrical" length. How do you know how much? Really complex software simulations should tell you the resonance point, or you could build a proxy sample and measure it with a VNA for a quicker outcome. I don't have a feel for how much shift your proposal would create.

Personally I would stick with the monopoles inside the airframe. If your link budget is sufficient it becomes a "don't care".

Another alternative is to make dipoles with some coax, and stick them on the outside if you are keen. See the method here:
https://forum.ausrocketry.com/viewtopic.php?f=56&t=6632&start=17If you make them in a slight V shape the impedance matching to the air is better I think.

 

[alvise]

Ok. An antenna needs to be a certain length to resonate and couple the desired frequency to/from the air. Most antennas you see are elements made of skinny wire or metal. So, at a particular frequency the antenna elements will have a particular length. Now imagine making the wires thicker. This add capacitance to the antenna elements. Now, here is the kicker, inductance and capacitance are opposite of each other, so adding one can cancel the other. So, our antenna with thicker elements will add capacitance, which subtracts from the inductance of the antenna, making it appear shorter at the desired wavelength, given the same element lengths. So our antenna has gotten shorter and is not at the resonance for the required frequency. We can compensate for that effect by making the elements a bit longer, bringing the antenna back to resonance.

So the one you drew with the foil elements around the airframe is seriously capacitive, so each of the halves of the antenna (either side of the feed point) will be too short for resonance at the frequency you actually want. The cure is to make those conductive elements longer, bringing the antenna back to the correct "electrical" length. How do you know how much? Really complex software simulations should tell you the resonance point, or you could build a proxy sample and measure it with a VNA for a quicker outcome. I don't have a feel for how much shift your proposal would create.

Personally I would stick with the monopoles inside the airframe. If your link budget is sufficient it becomes a "don't care".

Another alternative is to make dipoles with some coax, and stick them on the outside if you are keen. See the method here:
https://forum.ausrocketry.com/viewtopic.php?f=56&t=6632&start=17If you make them in a slight V shape the impedance matching to the air is better I think.

Thanks for the explanation.
I have 1.5mm thick insulated motor coil wire and I wanted to create an antenna using it.I determined the most suitable one as normal model helix antenna.
I wanted to do this using CST, but I was not able to use it fully.

It's a nice method in the method you shared above,This dipole antenna looks pretty simple to make and I have this cable.As far as I can see, you have turned the shield wire around the cable in one direction(left) and the central wire in the other(right) direction. In my case the length of this dipole is as follows.

If the method you suggest is used with the RFD900, since there are two antenna inputs, antennas can be connected to both sides of the pipe and in this way, a signal transmission close to 360 degrees can be achieved but I don't know exactly how it can be done and its performance.

Another reason why the antenna construction process takes a long time is that I don't have a VNA.

An alternative is to embed the Transmitter (on the rocket) into the side of the tube on either side of the tube using the RFD900's Quarter-wave unipolar 2.1dBi antennas.


And on the receiver side, using 26 dbi grid parabolic+Half wave dipole 3dBi.

 

[OverTheTop]

Check out the thread here:
https://www.rocketryforum.com/threads/kate-in-space.179559/That system uses a standard single whip on the Tx I think. He gets 75k' on 500mW with margin to spare. This is a good yardstick for what you are trying to achieve. Do you really need two Tx antennas, or anything fancy on the rocket? You might want to do it just for the challenge, of course.

I think the Nano VNAs can be got for around $130 or something. Really useful. Or find a friend who has one . Ham radio club to help out maybe?

Remember too, that the lengths quoted in the antenna calculator in your last post are for air dielectric. If you are near the airframe it will alter the permittivity that the antenna sees, affecting tuning length.

 

[VernK]

Check out the thread here:
https://www.rocketryforum.com/threads/kate-in-space.179559/That system uses a standard single whip on the Tx I think. He gets 75k' on 500mW with margin to spare. This is a good yardstick for what you are trying to achieve.

That particular rocket was all metal so the antenna used was a metal one that sticks out the side of the airframe like a very small fin. The radiation pattern from the antenna was very similar to that from a 1/4 wave antenna over a ground plane. Of course, the antenna was also designed to withstand the Mach 5+ airstream! Sorry, I am not at liberty to give out any specific details on the antenna design.

I will also add that a VNA is essential if you are doing anything serious with antenna design. Without a VNA, it is like working with a blindfold on. You can’t see how good or bad the antenna is and you don’t know what to change to make it better.

 

[PhilC]

Rockets made of fibreglass, phenolic or planstic have a very low loss tangent at 900MHz. If you put the antenna inside the losses will be negligible.

Having metal parts close to the antenna could de-tune it, particularly if its a hi Q antenna type. It will affect is polar pattern and efficiency.

Polar pattern will never be omnidirectional. There are always nulls.

Remember that any gain lost by the antenna on the rocket can alwys be made up by increasing the gain of the ground antenna, but at the cost of a narrower beam and hence better tracking on the ground.

I hope these random thoughts help.

 

[AllDigital]

I've flown a dozen different antenna designs inside and outside of our rockets. The "outside the rocket" has never held up well for me, but most of our rockets were high gee going to 15K-20K feet. Even with a lot of epoxy and a layup I wasn't able to get the connection points to be consistently reliable through vibration, pyro charges, etc.

In my experience, a simple 1/4 wave center dipole inside the avBay has worked the best, because I don't like grounding a monopole to my PCB. But, as Chris said, almost every commercial telemetry solution uses some form of a monopole grounded to the board. Below is an example of my simple PCB printed dipole used at 433 Mhz. (900 Mhz would be shorter). These are super easy to make and easy to trim to frequency.

 

[alvise]

Thanks everyone for their replies on this.
I will prefer to stick with the use of a Quarter wave monopole 2.1dBi antenna as it will take a long time to test whether any antenna designed without VNA actually works.

I have a small question Does placing the telemetry antenna near the gps antenna really affect the gps signal reception? (I've never tested this effect before)
Has anyone tested this?

 

[Voyager1]

I have a small question Does placing the telemetry antenna near the gps antenna really affect the gps signal reception? (I've never tested this effect before)
Has anyone tested this?

Yes, it probably will in at least two ways:

1. The RF signal from your telemetry Tx could interfere with the GPS signal reception.
2. The proximity of the telemetry antenna to the GPS patch could interfere with the patch's impedance, and vice versa!

You should separate them as far as possible from each other. If in doubt, test, test and test!

 

[AllDigital]

I have a small question Does placing the telemetry antenna near the gps antenna really affect the gps signal reception? (I've never tested this effect before)
Has anyone tested this?

Absolutely... Think of the GPS receiver as a super sensitive listening device trying to hear a pin drop, while your downlink radio is screaming at the top of their lungs to get a signal as far as possible.

The frequencies are far enough apart to not overlap, but if your antenna is too close and/or your power too high on the downlink the RFI will impact your GPS reception. At 100mw of downlink power it is less of a problem, but as you scale up power it becomes a bigger issue. I've also found that a poorly tuned downlink antenna will also cause more RFI interference.

I often see university teams come out to FAR with two telemetry systems in their avBay, one primary and one "backup". They turn on the first one and get GPS lock and then turn on the second one and the first one loses GPS lock, because their antennas and boards are right next to each other. Commercial altimeters have accounted for this in their design, but if you use more than one commercial solution next to each other and/or you modify antennas this can become an issue.

With our flight computer we run variable downlink power from 250mw to a full one watt. We keep the radio off while the GPS is acquiring fix. In a best case scenario our GPS can acquire about 24 satellites without radio on. Once we power on the downlink radio we lose as much as one third of the satellites. That is important, because when you launch you will lose more sats due to doppler shift and if you lose too many you won't have a valid 3D fix until your GPS regains signal.

 

[cerving]

Another thing to do is to minimize the amount of transmission time... if you're sending out data 100% of the time it's more likely to have a detrimental effect on the GPS reception than if you're only sending with a 10% or 20% duty cycle.

 

[OverTheTop]

Thanks everyone for their replies on this.
I will prefer to stick with the use of a Quarter wave monopole 2.1dBi antenna as it will take a long time to test whether any antenna designed without VNA actually works.

I have a small question Does placing the telemetry antenna near the gps antenna really affect the gps signal reception? (I've never tested this effect before)
Has anyone tested this?

I have seen this. It manifests as longer time to acquire satellites too. I am not sure if it is caused be receiver desensitisation or by introdicing more timing jitter into the digital correlators. Same effect either way I think.

 

[alvise]

I have tested.I placed the RFD 900 Quarter Wave monopole 2.1dBi antennas inside the polyamide plastic and using the Half Wave Dipole 3dBi antenna on the other side (receiving), I was able to transmit data at a distance of 10 kilometers without any problems.