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What nose weight material near antenna?

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Kelly

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I'm doing a build that will have an AV bay in the nosecone, but will also probably require some additional nosecone weight for stability. Normally I use lead or steel for this, but this bay will have an eggfinder in it, and I don't want metal near the antenna. What is the best nose weight material for this application?
 

dhbarr

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Carve up a big chunk of fiberglass rod?
 

Igotnothing

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Plaster and cement can be fairly dense while lacking the metals that may interfere with radio signals.
Glass and other non-metallic minerals can be fairly dense. Rocks with low metal content. Sands and silts come to mind. I mean, there is a reason why you pick up a rock when the mean dog threatens - although a stick offers a better return threat and the possibility of making a friend.
 

plugger

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I'm not an RF expert but I believe this is a non issue. It's never been an issue for me. Given you'd have to glue in the ballast it'll be surrounded by epoxy. Given that the whip antenna on your eggfinder will always be below the metal, not surrounded by it. Sure, you might have propagation issues directly above the antenna but given it's a quarter wave whip the radiating pattern of that antenna type is virtually non-existent directly above the antenna, as can be seen below.



If this was an issue I expect more people would have seen it in nosecones with metal tips as well.
 

OverTheTop

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It depends entirely on how much headroom your link budget has. If you have signal to spare you can blow it on proximal materials. If you are really scratching around at the limits of the link every dB matters. So it won't matter, until it does.

Metal nosecone tips will have an effect too. IIRC if it is too close it will add a capacitive element at the tip (effectively shortening the antenna I think).

Here is a timely post I put up today that shows even fiberglass detunes the antenna: https://www.rocketryforum.com/threads/tuning-a-telemetry-antenna-with-a-vna.164715/
 

plugger

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Lol, I opened with "I'm not an RF expert" with you in mind OTT. I hope you're well mate.
 

OverTheTop

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Thanks Plugger. Yeah, good down here. Lockdown 3.0 has just finished for us :).

Considering RF, I am doing some training in ANSYS Maxwell (high-end simulation) in a few weeks and I should be capable of simulating something like this thread is discussing. I might even do it as a practice exercise.
 

plugger

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We're really proud of y'all over there OTT, you've done the hard yards and should be commended for it nationwide. Thank you.

Now when it comes to link budget, once again, I'm no expert. But anecdotally we've seen near order of magnitude increases in transmitter power over the past decade or so in certain transmitters, if not more. I still use my TeleGPS v1.0 which is a 70cm whip antenna connected to a 16 mW radio and I don't miss a packet with that thing in a metal tipped NC even when it's well over 20k'. Obviously I'm not doing the math, but real world experience seems to indicate that works pretty well. The Eggtimer is 100 mW so I would guess it would have budget to spare. That said it is 900Mhz but with line of site I don't expect that would make too much difference.
 

plugger

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I'm also reminded of Ben Brockert's comment regarding link budget on the aRocket mailing list in October last year.

I launched a Byonics APRS transmitter to ~90km. It was picked up by digipeaters in eight states. Be better than me: do a link budget to make sure you’re not transmitting at higher power than needed. I didn’t get any complaints but I could see it happening easily.
I could be wrong but Byonics appears to make transmitters in the 1-10 watt range. I'm unsure but given the all-in-one nature of the 10 watt tracker if I had to guess I would think he was transmitting somewhere between 1-2 watt.
 

Kelly

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I'm not an RF expert but I believe this is a non issue. It's never been an issue for me. Given you'd have to glue in the ballast it'll be surrounded by epoxy. Given that the whip antenna on your eggfinder will always be below the metal, not surrounded by it. Sure, you might have propagation issues directly above the antenna but given it's a quarter wave whip the radiating pattern of that antenna type is virtually non-existent directly above the antenna, as can be seen below.
Thanks. I'm not an RF expert either, but couple comments: First, I don't think the epoxy surrounding a metal ballast helps at all - if there's metal, there's metal. And, yes, you're correct that a quarter wave whip doesn't radiate in the direction the antenna is pointing, I don't know whether this means that metal there can't affect the radiation pattern.

To OTT: I really don't know my link budget. Haven't gotten into that level of detail. But, I expect the flights to be under 10K feet, and hopefully the drift much less than that. I've got the 70cm 100mw Eggfinder, so maybe all this means I have plenty of headroom and shouldn't worry? Still, if there's a very dense material that doesn't mess with the antenna too much, that's what I'd want to use. THe ballast will need to be within a couple centimeters of the antenna tip.
 

OverTheTop

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For under 10k' I don't think you will have any problems. I haven't got the numbers but my intuition is telling me it should work, assuming you don't snub it too severely.
 

JLebow

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Kelly,
I've flown a 900 MHz Eggfinder with the 1/4 wave stick antenna to just over 10,000 feet in the nose of a rocket with a metal tip. This was the same rocket depicted in my avatar (which looks very similar to your avatar!). Maintained radio link for the entire flight (until the last 10 feet of descent) at a range of 2 miles from the pad. Picked the signal up again at about 0.5 mile out during recovery.

On a different rocket, without nose cone metal (no ballast or tip), the eggfinder mini (900 Mhz, rubber ducky antenna) tracked the entire flight 13,700 ft up and 4.5 miles away from the pad (unintentionally deployed main chute at the top).

I think you could gather a bunch of anecdotal data and maybe come up with a reasonable conclusion, but a simple ground test may be better suited to test your exact set-up (ballast mass orientation, radio frequency and power). Try with and without a ballast in the nose, and see if the ballast test has significant reduction in ground range. You may want to run the test with the nose in 3 orientations pointing North-South, East-West, Up-Down. If you do this test, I would be interested in seeing your results.
 

Kelly

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I've flown a 900 MHz Eggfinder with the 1/4 wave stick antenna to just over 10,000 feet in the nose of a rocket with a metal tip.
Thanks, Joey! What was the separation between your antenna tip and the NC tip?
 

JLebow

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Thanks, Joey! What was the separation between your antenna tip and the NC tip?
I measure about 5.5 inches (140 mm) from antenna tip to base of metal nose tip. I also have 5.5" from base of the antenna to the aluminum bulkhead. Antenna length is 3.25" (83 mm), which is 1/4 wavelength of 900 MHz. In the attached picture, the sled is in the correct approx. location (along the length of the rocket) as it gets installed inside the nose cone.
nose.jpg
 

plugger

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Thanks. I'm not an RF expert either, but couple comments: First, I don't think the epoxy surrounding a metal ballast helps at all - if there's metal, there's metal. And, yes, you're correct that a quarter wave whip doesn't radiate in the direction the antenna is pointing, I don't know whether this means that metal there can't affect the radiation pattern.
The epoxy used to pot the metal ballast (if enough is used) will ensure the antenna doesn't physically touch any of the metal in the nose cone used for ballast. That was my point. Combined with the radiation pattern of a quarter wave whip it shouldn't cause issues.
 

kbRocket

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I once glued in gravel to add nose weight to a rocket that was single use. I went with gravel instead of lead because I didn't want to throw lead in the trash or chisel it out of the nose cone after the flight. It was single use because, out of respect for all rockets, I decided to launch the cardboard tube with pressboard fins that I used when learning to do fillets and tip-to-tip. Added paper mache nose cone, recovery, and an ancient H someone sold me cheap. Rocket name was Pinata and it had a good flight with a safe recovery.

Another idea is the opposite end of the cost spectrum: tungsten weight. You can get tungsten fishing weight or pinewood derby car weights off Amazon. Tungsten is about 1.8 times the density of lead. This allows you to use less and get it further away from your antenna, hence blocking less signal.

Which tungsten form factor is better depends on how you want to use them:

The pinewood derby car weights are about 3/8" diameter cylinders of different heights. They glue well into the tips of small nose cones.

The fishing weights tend to be egg shaped with a hole in the middle like a heavy bead. The hole can be nice if you ever need to tie one inside of a rocket for last minute extra weight. Heavy fishing line or thin kevlar may be needed because the hole is narrow diameter. These weights can be larger than the pinewood weights.
 

Budro0

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I once glued in gravel to add nose weight to a rocket that was single use. I went with gravel instead of lead because I didn't want to throw lead in the trash or chisel it out of the nose cone after the flight. It was single use because, out of respect for all rockets, I decided to launch the cardboard tube with pressboard fins that I used when learning to do fillets and tip-to-tip. Added paper mache nose cone, recovery, and an ancient H someone sold me cheap. Rocket name was Pinata and it had a good flight with a safe recovery.

Another idea is the opposite end of the cost spectrum: tungsten weight. You can get tungsten fishing weight or pinewood derby car weights off Amazon. Tungsten is about 1.8 times the density of lead. This allows you to use less and get it further away from your antenna, hence blocking less signal.

Which tungsten form factor is better depends on how you want to use them:

The pinewood derby car weights are about 3/8" diameter cylinders of different heights. They glue well into the tips of small nose cones.

The fishing weights tend to be egg shaped with a hole in the middle like a heavy bead. The hole can be nice if you ever need to tie one inside of a rocket for last minute extra weight. Heavy fishing line or thin kevlar may be needed because the hole is narrow diameter. These weights can be larger than the pinewood weights.
We use "soft tungsten" for fly fishing which is basically a putty. I'm sure you could find larger quantities for cheaper, but this would allow you to get as far away from the antenna as possible.
 

cerving

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Omnidirectional whip antennas have a dead spot at the ends of the antennas, that's why a metal tipped nose cone has no appreciable effect on the transmission. I recommend leaving at least 1/4 wave between the end of the antenna and the tip... about 80mm for the 900 MHz transmitters. Since most metal-tipped nose cones are 3" in diameter or larger, that's usually not much of an issue if it's at least a 4:1 NC.
 

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