Eggfinder GPS Antenna Question

[user 30299]

I'd love to start a dedicated thread on rocket antennas, but not tonight. I've done a lot of testing with different antenna configurations in the rocket and on the ground. Most of my testing has been with higher powered radios, but the principles apply to most commercial products as well. Out at FAR on most weekends about half the student groups lose signal after the rocket takes off and many of them do not recover. It breaks my heart. It is a different class of challenge with large or high altitude rockets, but most of the time it comes down to the antennas. In the rocket I've tried five or six configurations. I've tried 1/4 wave (too bulky), inverted V (works well, but sticks out), whip (ok, but poor ground plane), helical (exotic, hard to get right), patch (too directional), PCB antennas (hard to get right), and my favorite the straight dipole. In my testing the dipole has been the most reliable. I've made them from stiff 14awg wire, but lately I've been printing them on PCBs for about $1 each and trimming them to frequency with a VNA. The only downside is they are long -- at 430mhz they are 12", but at 900 MHz they are only six inches. The dipole has a great radiation pattern for most orientations and is always balanced, although not necessarily the case with commercial products that already incorporate a ground plane.

On the ground, using a high quality yagi or an elevated antenna on a mast makes a huge difference. As others stated above, that is your likely your best investment to improve commercial trackers.

My other suggestion is to aggressively test your radios at home. I've got a lot of hills around me, so I have marked out line of sight locations that are 1km, 2km, 4km, and 10km away. For smaller rockets you could divide by 10. I put the transmitter inside the airframe or avBay and then drive to the different markers (usually 400-500 feet lower in elevation) to test reception. Real field tests will tell you the most about the performance of a receiver or a transmitter. If you can receive 1km away at 500 feet of elevation at home then there is a good chance you will have a solid "last report" up to 2km away.

Getting back to the OP... I recently tested an EggFinder Mini out of the box and found I couldn't even get 1km away with my reception test. I connected the LCD receiver to a ten foot mast and a 10dbi 900mhz antenna and got consistent reception at 1km. Link to antenna here: https://a.co/d/cJjMdAx -- also in my testing it was clear that bending the stock LCD antenna on the 90 degree hinge degrades reception. It doesn't always show up on the VNA, but antennas don't like to have their coax bent at 90 degrees, especially transmitters.

-Mike


Example dipole printed antennas ("long" 433mhz)
View attachment 544545 View attachment 544544

In my part of Ohio I can go a 1/2 mile one way from my home and get rolling rises with corn & soybean fields. I go 1/2 a mile the
other way and I get hills and hollers. I do the same distance testing like you whenever I build a new rocket that uses a tracker.
Some of our launch sites in southern Ohio have very different topography from site to site.

 

[cerving]

Remember that GPS works differently than a RDF tracker... ground range doesn't really matter all that much, it's all about getting the in-the-air packets on the way down. Generally, you'll get them all the way down to about 100'-200', which should get you close enough to your rocket to get a ground packet, if you don't actually see it first.

 

[SolarYellow]

Figured this would be a good place to ask a couple questions.

Is metallic ballast (lead shot or copper BBs) encased in epoxy in the tip of the nose cone going to adversely affect the transmission efficiency of the EF Mini if it's also located in the nose cone with the antenna pointing toward the tip?

Does anyone have experience with "aluminum look" paints on the outside of the part of the rocket (e.g., nose cone) where the EF is located attenuating transmission?

 

[Andrew Brown]

Adding metal near the tip of the antenna does have the possibility to detune the antenna, but probably not significantly and is a strong function of how far the metal is from the antenna. There is a lot of margin for most flights and you can pretty easily buy back more margin the receiving end with a better antenna, if needed.

Paint... Hard to tell. Most metallic paints consist of diced mylar flecks and are non-conductive. However, there are a few exceptions.

 

[Voyager1]

Figured this would be a good place to ask a couple questions.

Is metallic ballast (lead shot or copper BBs) encased in epoxy in the tip of the nose cone going to adversely affect the transmission efficiency of the EF Mini if it's also located in the nose cone with the antenna pointing toward the tip?

Does anyone have experience with "aluminum look" paints on the outside of the part of the rocket (e.g., nose cone) where the EF is located attenuating transmission?

It depends on the volume of metal used and proximity to the antenna. It’s in the near field of the antenna, so if it’s close, it could affect the antenna’s impedance leading to a slight detuning of its resonant frequency. If the volume of metal isn’t too much, its affect on the far field radiation pattern might not be too significant, as there is typically a null in the on-axis pattern anyway.

As for the metallic paint, it depends on whether it uses synthetic or metallic flakes. Best idea would be to paint a cardboard cylinder or box with the paint and enclose the EF inside it to test whether it affects the signal strength.

 

[ksaves2]

Figured this would be a good place to ask a couple questions.

Is metallic ballast (lead shot or copper BBs) encased in epoxy in the tip of the nose cone going to adversely affect the transmission efficiency of the EF Mini if it's also located in the nose cone with the antenna pointing toward the tip?

Does anyone have experience with "aluminum look" paints on the outside of the part of the rocket (e.g., nose cone) where the EF is located attenuating transmission?

I'd avoid metal in a nosecone GPS tracker period. As one mentioned, can get away with it with weights but DO NOT, I REPEAT DO NOT use metallic based paint on a TRACKER NOSECONE PERIOD! It will, I repeat WILL attenuate the range from the tracker. Don't ask me how I found out. It's a given to not use metallic paints in a tracker nosecone that is RF or GPS. With weights, put'em on a reinforced deeply epoxied screw eye behind the tracker bay.
Oh, one can use a metallic paint on the rest of the rocket but anything that is going to contain a tracker, use regular non metallic paint or you can likely lose the rocket anyways.
I don't know what "aluminum look" paints are but if they have anything metallic in them, they need to be nixed from painting a radio tracker bay. Kurt

 

[bad_idea]

So as to interfere with the tracker as little as possible in the nose cone of a rocket I'm building now, I'll be epoxying glass marbles into the nose tip for weight. Through-the-cone-wall anchor will be wooden dowels instead of the metal rods I've used in the past. Any additional metallic weight needed will be kept to the aft side of the nose cone bulkhead.

May be overkill, may not be, but it seems feasible and seems to have little downside on a rocket where I'm not trying to eke out every ounce of performance.

 

[SolarYellow]

On that point, I picked up some Evergreen 1/16 polystyrene round rods from Hobby Lobby and have started using those instead of brass rods as crossbars for nose weight retention. They glue into the Estes nose cones with Tamiya liquid cement really nicely and offer a consistent material surface for any putty, primer, or other surfacing strategy to stick to on the outside of the cone. I still mix the ballast with JB Weld Plastic Weld epoxy before putting it on the inside. The cross rods are just a double-shear mechanical link to positively engage the walls of the nose cone. Belt and suspenders...

 

[Voyager1]

So as to interfere with the tracker as little as possible in the nose cone of a rocket I'm building now, I'll be epoxying glass marbles into the nose tip for weight. Through-the-cone-wall anchor will be wooden dowels instead of the metal rods I've used in the past. Any additional metallic weight needed will be kept to the aft side of the nose cone bulkhead.

May be overkill, may not be, but it seems feasible and seems to have little downside on a rocket where I'm not trying to eke out every ounce of performance.

I believe the practice of limiting any potential interference to a tracker’s RF signal is a sensible approach. I’ve been rather evangelical about this topic over the years and, of course, not everyone agrees with this approach.

Many people prefer to rely on a generous link budget and install their trackers in amongst altimeters, metal all-threads, etc. That practice is probably perfectly fine while the rocket is in the air, or when you’re launching and recovering on flat vegetation-less land. However, in some cases when the recovery area is less than ideal, all bets are off, particularly when operating at the shorter wavelengths where line-of-sight communication is severely challenged.

 

[ksaves2]

On that point, I picked up some Evergreen 1/16 polystyrene round rods from Hobby Lobby and have started using those instead of brass rods as crossbars for nose weight retention. They glue into the Estes nose cones with Tamiya liquid cement really nicely and offer a consistent material surface for any putty, primer, or other surfacing strategy to stick to.

 

[ksaves2]

Don't know how this dual post happened but in smaller rockets, I used a radio-lucent polystyrene tube/stent that went through a hole into the upper main chute bay around the brass wire tracker antenna. I found the main chute would "squish" the brass RF antenna of the Eggfinder down under boost and the ground footprint of the Eggfinder (or any kind kind of tracker) s#cked" with the antenna smashed in the bottom of the bay. Not a fault of the tracker mind you. Need to have a competent tracker and a "good" antenna. The polystyrene stent kept the brass antenna straight with the force of the accelerating main parachute. Luckily on my test flights I used lower motor impulses to keep the "tracker" rocket in visual range so if the tracker had an issue, I could troubleshoot it and at least get the rocket back! My early antennas got squished down projecting into the main chute bay but when I put a polystyrene "tube/stent" to keep the antenna from being squished. I was good to go.
Do not expect Eggfinders to give "up to the second" positions from launch. The deal is they work well when the main chute blows and the descent rate slows. Sometimes the descents from apogee on drogue get position packets through intermittently but I recommend blowing the main chute from as high as one can from the launch site they are flying from and positions will start coming in to give a "final" position to start a search with 900Mhz trackers. I never lost a rocket with a GPS tracker in it be it a 900 Mhz Eggfinder or on the Ham bands and I have a General license in Ham Radio and so I can use the APRS trackers on 70cm and 2 meter bands.
Only thing is don't get enamored with "more" power as Rf from a tracker can interfere with one's deployment electronics. Never had a problem with Beeline or Eggfinder products (all of them) but if possible, put the tracker in a nosecone mount and the deployment stuff down below. Get the Rf emitter and the deployment altimeters as far as you can from each of the respective devices.
PLUS! Do a ground test on one's rocket with the GPS or Rf tracker blasting positions out in of course a ground test with one's chosen altimeter deployment devices with contained ematches onboard like a full up launch sans engine and 4F powder. If the matches pop, you have a problem here. Don't ask me how I found that out! (O.k. core samples) (A couple of my early tracking rockets "core sampled in" when the RF trackers interfered with the deployment electronics.) I've been told that more modern deployment altimeters are more immune to this situation but it doesn't hurt to test it out to "make sure" before one flies a rocket for the first time with new electronics. One other thing, don't use, NEVER USE a metallic paint on a bay that encloses an RF emitting antenna. It will cut down the air and ground footprint "IMMENSELY". Again, don't ask me how I found out. Paint the ass end of the rocket metallic but if one has an RF tracker bay, use NON-METALLIC paints on it. Trust me, I had to paint strip a rocket on this issue and use non-metallic on it in order to fly it safely. Dang, it's still flyable too! Kurt

 

[SolarYellow]

So as to interfere with the tracker as little as possible in the nose cone of a rocket I'm building now, I'll be epoxying glass marbles into the nose tip for weight.

It occurs to me that some glass beads from Hobby Lobby (or likely better yet, Walmart) would work well for smaller rockets, too.
--------------------

In my last order from Cris, I added a spare Eggfinder Mini antenna because I wanted to see what was under the cover and didn't know whether I'd destroy it in the process. I finally stripped it down today. The hard rubber cover snaps into two circumferential grooves in the knurled antenna base. If you cut around the base of the large diameter section and then slit the part below that, you can peel it off easily. I did it in steps because I didn't know what I was getting into. The remaining section then has only one groove it's holding onto and can be pulled off with pliers. Or you could probably cut at the yellow ring and just pull the cap off with your fingers, then slit the rest and peel it.



This makes the antenna considerably lighter and more compact, so it can be fit in smaller nose cones and not impact apogee as much on smaller, lighter rockets that are over optimum weight.

Masses:

cover	1.883 gm
screw	0.697 gm
bare antenna	3.637 gm
total	6.217 gm
bare antenna plus screw	4.334 gm

So that's ~1.9 gm mass reduction.

There is clearly additional mass in the knurled base that could be ground or milled off it if one really wanted to gram-Nazi the thing. I don't know enough about antennas to know whether it would affect the tuning, but I suspect not.

Dimensions:

Length from bottom of knurled section to tip of antenna
w/cover	42.00 mm
bare	37.0 mm
Diameter of tip
w/cover	8.0 mm
bare	5.24 mm

The antenna gets considerably smaller, to fit farther up in the tip of smaller, shorter nose cones, as in my Alpha project concept discussed above. Because it is a coiled spring, it avoids any potential safety concerns that could arise with a straight wire antenna as I also asked about earlier.

If the unit is protected inside a nose cone, it seems more than robust enough mechanically to survive what it's likely to experience. Reducing the mass and freeing the cantilevered part to act as a flexible spring should reduce the loads it puts into the Mini's circuit board in an impact.

I think that for those of us looking to put a Mini in the smallest rockets we can, this is probably the way to go.

 

[bad_idea]

It occurs to me that some glass beads from Hobby Lobby (or likely better yet, Walmart) would work well for smaller rockets, too.

Yes, I used clear glass marbles from Hobby Lobby so as to avoid any potential metal content of colored marbles. (I'd originally bought these marbles for fish tanks years ago.)

This makes the antenna considerably lighter and more compact, so it can be fit in smaller nose cones and not impact apogee as much on smaller, lighter rockets that are over optimum weight.

Very cool. Thanks for the idea, and for the measurements. I'll likely go this way on a future H13 project. (For the rocket I'm working on right now for the H13, too little time to pivot.)