Transmission range impact?

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Cameron Anderson

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I admit my ignorance in terms of antennas and GPS/telemetry transmission range. This is an AIM XTRA 2.0. I added the two 90 degree elbows to fit my antenna into my av bay.

Will the addition of the elbows increase transmission range, decrease range, or have a negligible impact on range?
 

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manixFan

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Hmm, it will be very interesting to hear the experts weigh in, but just based on my personal experience you will likely see an attenuation in the direction of the board. I have mounted antennas parallel to all-thread and it definitely reduces signal strength if the all-thread is between the receiver antenna, and I suspect you may experience the same. Worse, it's possible the RF from the antenna will affect the circuity on the board itself and cause other issues. Have you ground tested the assembly to see how it compares to the normal antenna orientation? That should tell you a lot.


Tony
 

Cameron Anderson

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Hmm, it will be very interesting to hear the experts weigh in, but just based on my personal experience you will likely see an attenuation in the direction of the board. I have mounted antennas parallel to all-thread and it definitely reduces signal strength if the all-thread is between the receiver antenna, and I suspect you may experience the same. Worse, it's possible the RF from the antenna will affect the circuity on the board itself and cause other issues. Have you ground tested the assembly to see how it compares to the normal antenna orientation? That should tell you a lot.


Tony
It worked from 5 feet away on a bench test but I haven't tried any further.
This is going up on a two-stager that sims to 28,000' so I need some range. With that antenna in the normal orientation the device is capable of 30km LOS transmission.
 

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Is there a reason you need to keep the antenna adjacent to the board? I use a flexible cable and a thru hole in the av-bay bulkhead, where I then attach a whip antenna, although if you have the space, the bulkier antenna you have could be mounted in a similar fashion on the outside of the bulkhead. Having the antenna outside the av-bay and fully exposed after the apogee event will give you far more range and less attenuation than laying the antenna on axis with active electronics and the ground plane internal to the pc board.
sma_male_sma_female-10cm-close_1.jpg
 
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cerving

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By folding the antenna back you lose the counterpoise effect of the PC board's ground plane, which will defintely reduce your range. I won't go into the allthread issue... it's been beat to death, and you already have stated that you are aware of it. I suggest you do a ride-along with another NC-mounted tracker to see how well you are receving the packets, and go from there.
 

FredA

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The antenna next to the ground plane is effectively shorting out the RF.
I bet the output transistor won't be happy.
Not recommended.
 

Cameron Anderson

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Is there a reason you need to keep the antenna adjacent to the board? I use a flexible cable and a thru hole in the av-bay bulkhead, where I then attach a whip antenna, although if you have the space, the bulkier antenna you have could be mounted in a similar fashion on the outside of the bulkhead. Having the antenna outside the av-bay and fully exposed after the apogee event will give you far more range and less attenuation than laying the antenna on axis with active electronics and the ground plane internal to the pc board.
The antenna doesn't need to be adjacent to the board per se, that is just there it fits best (which, as discussed, is not optimal for function). It is a short and narrow av bay. I have a smaller antenna good for 20 kilometers, which is sufficient range for the airframe even on the biggest possible motors - it might be time to reconfigure the av bay to support that set up.
 

Cameron Anderson

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By folding the antenna back you lose the counterpoise effect of the PC board's ground plane, which will defintely reduce your range. I won't go into the allthread issue... it's been beat to death, and you already have stated that you are aware of it. I suggest you do a ride-along with another NC-mounted tracker to see how well you are receving the packets, and go from there.
Current NC doesn't support electronics onboard, but your suggestion of using dual trackers to verify function of the questionable XTRA set up makes perfect sense.
 

manixFan

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By folding the antenna back you lose the counterpoise effect of the PC board's ground plane, which will defintely reduce your range. I won't go into the allthread issue... it's been beat to death, and you already have stated that you are aware of it. I suggest you do a ride-along with another NC-mounted tracker to see how well you are receving the packets, and go from there.
Well, since you brought it up, (OK, I did, but anyway) can you quickly explain (or link to a thread) that has a discussion about mounting next to all thread? I've done it with both steel and aluminum and it does affect the signal, and obviously not the best solution, but it would be nice to know if I can predict or model the effects. I've done this to retrofit trackers to several existing nosecones since I can just slide the mount onto the all thread.

Sorry for the thread hijack, but it is somewhat related.


Tony
 

cerving

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Well, since you brought it up, (OK, I did, but anyway) can you quickly explain (or link to a thread) that has a discussion about mounting next to all thread? I've done it with both steel and aluminum and it does affect the signal, and obviously not the best solution, but it would be nice to know if I can predict or model the effects. I've done this to retrofit trackers to several existing nosecones since I can just slide the mount onto the all thread.

Sorry for the thread hijack, but it is somewhat related.


Tony
Unfortunately, it's very difficult to model and/or predict the behavior of interfering object on the signal propagation, I'm sure there are some software models out there but it's not something that I have access to. There are a lot of things in play. In general though, the less stuff between the antenna and the outside world, the better. A larger airframe with the allthreads farther away from the antenna will have less effect on the signal than a smaller airframe with the allthreads right next to the antenna. You could try a different type of antenna, maybe two opposing panel antennas between the allthreads, but you can only do that with the Ham version; the FCC regs for license-free "portable" devices call for a single-element antenna like a whip or a helix.
 

Cameron Anderson

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If I go with a 1" antenna and can keep the correct orientation of the antenna and still fit everything in the av bay. I'm assuming a smaller antenna in the correct direction is better than my original plan using a larger antenna but positioned very poorly.
 

DaveW6DPS

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If I go with a 1" antenna and can keep the correct orientation of the antenna and still fit everything in the av bay. I'm assuming a smaller antenna in the correct direction is better than my original plan using a larger antenna but positioned very poorly.
Either of these is a pretty bad option. You need to have an antenna that is approximately 1/4 wavelength, which is a bit over 3 inches. The board forms a counterpoise, which is essentially the other half of the antenna. It would be better to have a properly designed antenna, but you are limited to the antenna type supplied by the manufacturer and used in their FCC certification.

Things work best if there is no conductive material around the antenna. Having the antenna between or even with foot or so of the antenna will, best case, cause some strange pattern lobes. More likely it will simply couple the RF energy into the metal, which means it will not be radiated.

You want the supplied antenna straight on the antenna connection. Any of the changes you have described will degrade performance and may cause damage.
 

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When you add an antenna next to allthread it reduces the range. Many factors come into play, including capacitance detuning the antenna, distorted fields, and absorbing of the transmitted energy. As others have said this is difficult to model without a decent field-solver program. Folding back next to the PCB will have similar repercussions.

Try to keep antennas in free space where they were designed to be, unless you have a way of measuring their impedance in a modified environment. Alternatively, range-testing is a good alternative.

The coax patchlead that speaknoevil showed is a great idea to get the antenna elsewhere.

I would not be putting the antenna back over the PCB for another reason. The GPS receiver will be desensitised by the RF field (it is very "near field") getting into the front-end of the GPS. This will increase time to lock or re-lock and/or possibly increase jitter on the pseudoranges measured by the GPS module. The outcome will be slower time to lock or increased position error.

Answering your original question about attenuation by connectors, I would guess it would be in the region of around 1dB per connector, or so. If the connectors were low-quality or poorly terminated it could get worse.

Remember that the Tx signal is only one part in the link budget. You can easily reel-in some losses at that end by using a higher gain (more directional) antenna at the receive end.

Does anyone have any experience flying the Aim Extra to similar altitudes (28k')? Are there any signal strenghth measurments that can be extracted from those flight logs?
 

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Hmm, it will be very interesting to hear the experts weigh in, but just based on my personal experience you will likely see an attenuation in the direction of the board. I have mounted antennas parallel to all-thread and it definitely reduces signal strength if the all-thread is between the receiver antenna, and I suspect you may experience the same. Worse, it's possible the RF from the antenna will affect the circuity on the board itself and cause other issues. Have you ground tested the assembly to see how it compares to the normal antenna orientation? That should tell you a lot.


Tony
Yes, I agree with all these points. The PCB adjacent the antenna will load the the antenna’s impedance and affect the resonant frequency. Also, the radiation pattern will be distorted by proximity of the PCB. Additionally, as Tony points out, the RF from the antenna might mess with the reliability of the electronics. I would strongly recommend installing the antenna on axis with the board. Regardless, you do need to ground test before launching it in that configuration.

There is an earlier thread discussing some of these issues.
 
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Yes, I agree with all these points. The PCB adjacent the antenna will load the the antenna’s impedance and affect the resonant frequency. Also, the radiation pattern will be distorted by proximity of the PCB. Additionally, as Tony points out, the RF from the antenna might mess with the reliability of the electronics. I would strongly recommend installing the antenna on axis with the board. Regardless, you do need to ground test before launching it in that configuration.

There is an earlier thread discussing some of these issues.
In that "earlier thread" you mention the use of Nylon threaded rod for your av bays. Do you use anything special ? Is the rod dia. & thread just a one-to-one match to whatever you would have used if it was steel threaded rod ?
 

Cameron Anderson

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Thanks for all the feedback.

I'll test the 1" antenna at my next launch before I fly it (thinking about taping the unit to a 10' PVC pipe and driving a mile or two away across the lakebed to see how the transmission works).

I like GPS receiver/transmitters in my nose cones but since I'm also controlling staging and recovery events with this unit, I had to position it in the ISC between the upper air frame and the fin can. I think I'll order a new coupler for the av bay with a thick switch band...that will give me the extra space I need in the av bay for the standard antenna without encroaching into the fin can or upper air frame.
 

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Well, since you brought it up, (OK, I did, but anyway) can you quickly explain (or link to a thread) that has a discussion about mounting next to all thread? I've done it with both steel and aluminum and it does affect the signal, and obviously not the best solution, but it would be nice to know if I can predict or model the effects...
I'll test the 1" antenna at my next launch before I fly it (thinking about taping the unit to a 10' PVC pipe and driving a mile or two away across the lakebed to see how the transmission works).

I like GPS receiver/transmitters in my nose cones but since I'm also controlling staging and recovery events with this unit, ...
There is really very little software available that will model near-space conductors like this. It is not likely that it is worth the time or effort to try to model the effects of a poorly design AvBay.

I am still not a fan of randomly chosen antenna lengths, but the transmitter may not have enough power to actually burn out. Ground testing is a good start, but I would also suggest flight testing. Fly the AvBay on a single stage rocket and see if it works at lower altitude before relying on it for higher altitude. You should also consider a separate, or redundant, tracker.

This question also illustrates why it can be a problem to put all your eggs in one basket. It makes more sense to me to keep tracking and flight controls separate.
 

Cameron Anderson

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There is really very little software available that will model near-space conductors like this. It is not likely that it is worth the time or effort to try to model the effects of a poorly design AvBay.

I am still not a fan of randomly chosen antenna lengths, but the transmitter may not have enough power to actually burn out. Ground testing is a good start, but I would also suggest flight testing. Fly the AvBay on a single stage rocket and see if it works at lower altitude before relying on it for higher altitude. You should also consider a separate, or redundant, tracker.

This question also illustrates why it can be a problem to put all your eggs in one basket. It makes more sense to me to keep tracking and flight controls separate.
There is really very little software available that will model near-space conductors like this. It is not likely that it is worth the time or effort to try to model the effects of a poorly design AvBay.

I am still not a fan of randomly chosen antenna lengths, but the transmitter may not have enough power to actually burn out. Ground testing is a good start, but I would also suggest flight testing. Fly the AvBay on a single stage rocket and see if it works at lower altitude before relying on it for higher altitude. You should also consider a separate, or redundant, tracker.

This question also illustrates why it can be a problem to put all your eggs in one basket. It makes more sense to me to keep tracking and flight controls separate.
The stock antenna length is 1.5 inches, the new one is just over one, and most of the length reduction is from a different screw attachment so the antenna length is only negligibly smaller, which im sure still have some impact.

Unfortunately the sustainer can't be flown single stage unless I fabricate up a launch tower - it doesn't have rail guides or buttons. It was always intended to fly staged.

Size on this project is the limiting factor. I'm flying a Madcow 4" Nike to 54mm Tomahawk. It has a Raven 4 in the booster with a magnetic switch (and it was tough getting that small bastard in there!) and the XTRA in the sustsiner. It would take extensive modification to get get anything more in the sustainer. I initially looked at the XTRA GPS in the nose for tracking only and the Marsa33 with tilt gadget in the av bay but I was never comfortable with how much squishing I was doing to get all the electronics and batteries in there a s still run e-match leads.

I also have the 5.5" Nike to 3" Tomahawk and I fly that configuration with 5 devices...Marsa33 and RRC2+ in the booster and another Marsa33 with a tilt gadget, the Raven 4, and the T3 GPS. In the sustsiner. Plenty of room there for redundancy and a lot more electronics for dinner control of staging, separation, and recovery events. Having independent components for each function allows you to space them out but a lot of Altus Metrum products are dual GPS/flight computers so having to put all my eggs in one basket isn't unique to me.

I guess my biggest safety blanket is I am flying at Black Rock. The GPS for a 28,000' shot isn't necessary, it's just nice to have. I couldn't clear the edge of the playa with my shakedown motors if I wanted to and it would be almost impossible to do it on the baby L and full J I have for the full-impulse flight.
 

Cameron Anderson

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Until I can create a new av bay, I reoriented the XTRA and mounted the largest stock antenna. I drilled a hope for the antenna to go through the aft bulkplate of the av bay and hot glued a piece of PVC pipe over the antenna to protect it from the ejection charge. If this works and survives a charge test I'll re-glue it with epoxy.
 

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FMarvinS

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As stated above by Cerving & Tony, the steel all thread can cause transmitter signal attenuation ; so consider a titanium one (which I currently use and anecdotally seems to work fine). Amazon sells them. Also, as speak-no-evil recommends and OverTheTop implies, attach the antenna via a flexible cable so that the antenna "sits" on the outer bulkhead surface-in this way (as stated above) the antenna is in the open air upon parachute deployment (cables on ebay & Amazon). Finally, Dave provided excellent advice, use a quarter length wave length antenna to maximize propagation , it does make a difference. That extra length of antenna should easily fit into the payload compartment from the surface of the bay bulkhead. In this manner, you increase the probability of finding your rocket upon landing.
Good luck!
Fred., L2
ICBM,Camden,SC
KG4YGP
 

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Could you rotate the board 180°? It seems that the antenna connector would be closer to the bulkhead, As it is you have brass tube, all-thread, and an eyebolt in the worst possible place.

Coax to run the antenna outside the bulkhead will definitely help. I use a coaxial dipole on coax to put the antenna outside the AvBay. This has given me very good results. The dipole at that frequency is just over six inches long.
 

Cameron Anderson

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Could you rotate the board 180°? It seems that the antenna connector would be closer to the bulkhead, As it is you have brass tube, all-thread, and an eyebolt in the worst possible place.

Coax to run the antenna outside the bulkhead will definitely help. I use a coaxial dipole on coax to put the antenna outside the AvBay. This has given me very good results. The dipole at that frequency is just over six inches long.
The board has position-sensing software so I can orient it antenna up or antenna down (the default configuration per the manufacturer is antenna down, I assume to lessen the G load from acceleration on the board with an antenna pointing up). The issue with antenna up is the forward bulkplate has an eyebolt and two charge wells (primary and secondary main chute charges) so there is even less room than the aft bulkplate.

I think I'm going to test the antenna down orientation (as pictured on the shakedown flight) and if transmission performance is bad I can use a cable to extend the antenna externally to the side of the upper air frame clear of the all thread and charge wells.
 

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In that "earlier thread" you mention the use of Nylon threaded rod for your av bays. Do you use anything special ? Is the rod dia. & thread just a one-to-one match to whatever you would have used if it was steel threaded rod ?
Nothing special. I typically use 6mm (1/4”) nylon rods and nuts for my high power models. It’s much lighter than the steel rods and quite strong. Additionally, it doesn’t interfere with RF if you choose to install any transmitters or GPS modules in the avbay. Personally, I usually install any RF modules in the nosecone.
 

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I have used titanium bike spokes in place of allthread in avionics assemblies. Only 2mm diameter and breaking load is around 280kg each. Find a good bike shop and they make them to custom length for you. They roll-form the threads, which results in work hardening and strengthening in that area. If you thread a rod with a die it actually creates stress raisers and greatly weakens the rod. I paid about $USD5 each for them a couple of years back.

The amount of space saved by the smaller diameter can really provide much more space in the avionics area.
 
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manixFan

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I have used titanium bike spokes in place of allthread in avionics assemblies. Only 2mm diameter and breaking load is around 280kg each. Find a good bike shop and they make them to custom length for you. They roll-form the threads, which results in work hardening and strengthening in that area. If you thread a rod with a die it actually creates stress raisers and greatly weakens the rod. I paid about $USD5 each for them a couple of years back.

The amount of space saved by the smaller diameter can really provide much more space in the avionics area.
That's a great suggestion. I have a very small AV bay I'm working on and will have to give that a try.


Tony
 

QFactor

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I have used titanium bike spokes in place of allthread in avionics assemblies. Only 2mm diameter and breaking load is around 280kg each. Find a good bike shop and they make them to custom length for you. They roll-form the threads, which results in work hardening and strengthening in that area. If you thread a rod with a die it actually creates stress raisers and greatly weakens the rod. I paid about $USD5 each for them a couple of years back.

The amount of space saved by the smaller diameter can really provide much more space in the avionics area.
I have a small dia. design on the drawing board. Your bike spoke suggestion will come in handy. Thanks!
 
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