Glue Reinforcing GPS Antenna Attachment?

[wonderboy]

I've read about the Eggfinder GPS units and the instruction to add glue reinforcement to the GPS antenna in this thread: LINK.

As this thread mentions, it seems that many GPS units use a very similar GPS antenna and mounting method. I have a few Missile Works RTx/GPS units that use a GPS unit / antenna that looks like this:


Is the idea of adding glue reinforcement applicable here too? Seems to me to be a good idea and I think I'm going to do it, but I wanted to get feedback from the community.

There is a subtle difference on the Missile Works unit in that the PCB is between the antenna and the GPS circuitry (inside a shielded metal can), whereas on the Eggfinder, it seemed that the antenna was directly attached to the "metal can". I'm thinking this detail doesn't matter and that the antenna still could benefit from a glue fillet around the perimeter to help is sustain the G-forces from launch and hard landing.

 

[waltr]

I agree, would not hurt and maybe save the GPS unit if you get a very hard landing.

 

[Voyager1]

Another method that might be a little less permanent, but still offer very good security, is to use some clear heatshrink tubing over the GPS module.

 

[wonderboy]

I like that idea for a couple of reasons. The first is specifically the main issue I raised in my initial post. The other though is a secondary concern due to how the GPS module itself is attached to the main RTx/GPS circuit board. If you look at the picture, the attachment is along a single row of pins. In other words, the GPS module is cantilevered on the main circuit board. I can imagine in a severe G load situation that the GPS board would be bent along this line of pins away from the main PCB. I think the shrink tube (around the entire unit) would help support the GPS module and prevent this failure mode. The only other option I considered for this failure mode was a zip tie around the unit.

Thanks for the suggestion!

 

[UhClem]

Another method that might be a little less permanent, but still offer very good security, is to use some clear heatshrink tubing over the GPS module.

Bad idea. This changes the center frequency of the antenna. Since patch antennas are high Q (narrow bandwidth) this will result in a noticeable decrease in antenna gain.

I tested my BigRedBee GPS with and without the heat shrink it came with and it performed better without. The easiest thing to measure is the time from a cold start to first position. This should be about 30 seconds, the time required to receive ephemeris data. Longer than that and the GPS engine is having a much harder time locking onto the signal.

Care must also be taken if you add a fillet around the antenna. A patch antenna radiates from the slot between the edges of the patch and the ground plane. It would prefer an infinite substrate/ground plane but these are normally truncated. But the material properties near the antenna still matter and you can change the center frequency by altering the size of the ground plane or putting stuff (glue, etc.) into that near field area.

 

[Banzai88]

I've had some very hard landings with one of my RTX units. In fact, it's such bad luck that it's 'crashed' in about 50% of it's launches. It has yet to have any issues.

While I appreciate that folks want to take precautionary measures against damage, from my experience of several main deployment fails and a nose cone separation and subsequent drop from over 1K feet, anything that's gonna cause the antenna to fall off is gonna cause the whole unit to be scrap.

 

[Voyager1]

Bad idea. This changes the center frequency of the antenna. Since patch antennas are high Q (narrow bandwidth) this will result in a noticeable decrease in antenna gain.

I tested my BigRedBee GPS with and without the heat shrink it came with and it performed better without. The easiest thing to measure is the time from a cold start to first position. This should be about 30 seconds, the time required to receive ephemeris data. Longer than that and the GPS engine is having a much harder time locking onto the signal.

Care must also be taken if you add a fillet around the antenna. A patch antenna radiates from the slot between the edges of the patch and the ground plane. It would prefer an infinite substrate/ground plane but these are normally truncated. But the material properties near the antenna still matter and you can change the center frequency by altering the size of the ground plane or putting stuff (glue, etc.) into that near field area.

With respect, this hasn't been my experience. I have used heatshrink on several Eggfinder and Featherweight GPS modules without any noticeable difference in performance. I accept that different GPS modules might behave differently. However, the addition of a very thin layer of low value dielectric material (i.e., the heatshrink) over a patch antenna is not likely to significantly affect its resonant frequency. It might effect the bandwidth, though - if anything, by broadening it.

Using an epoxy fillet around the GPS module might affect the bandwidth if it adds to the edges of the ceramic dielectric on the patch antenna, but unlikely to significantly affect the resonant frequency. What is important is the dimensions of the antenna patch and the dielectric thickness between the antenna patch and ground plane. The epoxy fillet is unlikely to affect the ground plane in any significant way as it's not a conductor.

What will significantly affect the impedance and resonant frequency of the GPS patch antenna (or any RF antenna, for that matter) is the presence of adjacent conductive materials such as all-thread rods, altimeter PCBs, batteries, and other metallic mounting structures. Remember that the rocket body is also a dielectric material: cardboard, fiberglass, PVC, paint, etc, and can potentially have an effect on performance. Here, the benefits of securing the GPS module might outweigh any negative effects, certainly when compared with the effects of different mounting configurations on RF performance.

Ultimately, it is up to the individual to test what works for them.

 

[OverTheTop]

Sounds like there could be a good experiment with an old GNSS receiver, a VNA and some heatshrink . I might even have to use one of the good VNA units at work rather than my cheapie.

 

[Voyager1]

Rather than hijack this thread with a technical discussion on the effects of using heatshrink or similar protective materials on GPS modules, I will initiate a new thread for the purpose of allowing interested individuals to contribute their own experiences and/or technical input to this discussion.

 

[wonderboy]

I appreciate the discussion guys. I would love to learn more about the impact on antenna performance, which to me is voodoo/black magic. Antennas mystify me.

 

[OverTheTop]

Antennas mystify me.

Me too.