RF Absorption in Black Fiberglass Nose Cone

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OverTheTop
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RF Absorption in Black Fiberglass Nose Cone

Postby OverTheTop » Thu Oct 13, 2016 9:28 am

RF Absorption in Black Fiberglass Nose Cone
I recently purchased a Go Devil 54 from Madcow rocketry. It comes with a black filament-wound 5:1 von Karman nosecone :)

The question arose, “What is the RF attenuation of the nosecone?”. The black color is produced by additives which can sometimes include carbon and other products that make the composite more “RF translucent”, rather than transparent like the uncolored mix.

20161012_212454.jpg


I have an RF Beacon (25mW, 433.9MHz) that RocketStar made up a while back, and used that as the signal source:
viewtopic.php?f=32&t=4432

So I took the parts to work and put them in the EMC test chamber. The results on the Spectrum Analyser showed a 1dB attenuation when the NC was added around the TX.

Keep in mind that this result is for the Madcow 54mm black f/w f/g NC only, tested at 433.9MHz, with a sample size of n=1. Other black NC’s or frequencies I can’t comment on with authority!

Some Perspectives on 1dB
    Decibels are on a logarithmic scale. Usually peoples' brains don't instinctively handle logs well. In human terms your ear will barely be able to tell if something is 1dB louder or softer. 3dB is double or half volume.
    The effect of 1dB actually depends on how much signal level you have to spare in your system, which includes things like antenna gain, feedline losses, noise margin on the Rx, humidity, etc...
    The same loss would possibly be achieved by two or three connectors in the antenna feedlines (Tx or Rx).
    25% more Tx power will negate 1dB of loss.

As a system example, my TeleMega system at 21k' currently has a remaining margin of about 5-6dB. 1dB will not affect the link. If I want more range, I can build an antenna with more gain.

Summary:
1dB higher signal is usually not worth worrying about
2dB higher signal is worth having if you can get it
3dB higher signal is definitely worth the effort to get

Outcome:
I will be fitting the TeleMega to my black NC and not losing any sleep over it :)
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Re: RF Absorption in Black Fiberglass Nose Cone

Postby Happy Heyoka » Thu Oct 13, 2016 10:48 am

I have some tables somewhere of attenuation values for various composite materials that I got from an avionics electronics company if you're interested.

I don't have the luxury of having an EMC test chamber or a decent signal analyser (although the latter is on "the list") but have done similar tests with metallic paints and the signal strength register values from a couple of popular radio boards.
(Results similar to yours - yes it makes a difference but probably not enough to worry about)

My stuff is 433... 915 would probably suffer a bit more and 2.4gigs definitely more? (Although at legal power levels I think 2.4gigs is probably pretty useless for rocketry).

I have seen a couple of commercial cylindrical and patch antennas that would probably lend themselves to integration into a scratch built NC.
Have done some eyeballing of a 5/8 433 antenna made from aluminium tube along the axis of a NC and it would fit ok but I'd blow my power budget which would kind of defeat the purpose... guess I'll have to work on getting that amateur licence one day :-)

I wonder if a better strategy is to use a high gain antenna on the ground with a wider beam (I'm pretty sure I've seen you use a small Yagi at Serp?)
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Re: RF Absorption in Black Fiberglass Nose Cone

Postby OverTheTop » Thu Oct 13, 2016 12:25 pm

I'm pretty sure I've seen you use a small Yagi at Serp?

For the 434MHz I have used a patch, 6-element yagi, and mostly a multiply-loaded whip on the car.
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Re: RF Absorption in Black Fiberglass Nose Cone

Postby Happy Heyoka » Thu Oct 13, 2016 2:57 pm

OverTheTop wrote:For the 434MHz I have used a patch, 6-element yagi, and mostly a multiply-loaded whip on the car.


With the std. disclaimer that what I know about radio would fit on the back of matchbox, I had always thought the radiation pattern for a whip would preclude it for tracking stuff immediately overhead?

Returning to the topic, I wonder if, for example, the different amounts of "wall thickness" presented in various orientations through the trajectory make any difference? (also adding in nose weights on descent, retaining hardware on ascent etc).
Did you ever map telemetry dropouts with respect to orientation or relative position to base station?
(yeah I know, a steely eyed missile man like yourself doesn't get telemetry drop-outs :wink: )
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Re: RF Absorption in Black Fiberglass Nose Cone

Postby OverTheTop » Thu Oct 13, 2016 4:03 pm

(yeah I know, a steely eyed missile man like yourself doesn't get telemetry drop-outs :wink: )


Correct :wink:

I do get dropouts if someone keys up a UFH radio near my equipment during the flight though. I got out of that habit really quickly.

Signal drops off quickly on ascent. It is at its worst just before it starts major rolling over. With the antenna horizontal the signal booms in even from apogee. Typically the NC blows at that point and the rocket comes in relativley flat, although flapping about a bit. Signal gets a bit worse than when it rolled over but not greatly, then improves markedly as altitude is reduced. So it looks like the biggest factor is the radiation pattern of the Tx antenna.

The whip on the car works faultlessly for flights up to about 10k' (so far). If I am flying higher I will get out a second ground station and run the patch antenna for a bit more gain. But I have to worry about pointing that one in about the right direction.


guess I'll have to work on getting that amateur licence one day :-)

Your local radio club can usually help out if you need to get that. I went through EMDRC. They did a great job. I was going to get my Standard, but I though "how hard can it be? Why don't I go for Advanced?". As a communications engineer I had a good handle on most stuff and a but of revision/study on Rx, Tx, propagaion, interference and modes (and a little on transmission lines) and I had no trouble.

73's from VK3TRK. Going QRT for now :)
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Re: RF Absorption in Black Fiberglass Nose Cone

Postby SpaceManMat » Thu Oct 13, 2016 5:24 pm

Interesting stuff, do you know if the Madcow NC is the same as the NC that Blake sells?
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Re: RF Absorption in Black Fiberglass Nose Cone

Postby OverTheTop » Thu Oct 13, 2016 5:37 pm

Interesting stuff, do you know if the Madcow NC is the same as the NC that Blake sells?

Referring to my earlier statement, this is all I know:
"Keep in mind that this result is for the Madcow 54mm black f/w f/g NC only, tested at 433.9MHz, with a sample size of n=1. Other black NC’s or frequencies I can’t comment on with authority!"

Remember, this issue is likely only for black f/g NCs I suspect. I don't see anything on the AR site that matches that description.
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Re: RF Absorption in Black Fiberglass Nose Cone

Postby SpaceManMat » Thu Oct 13, 2016 6:28 pm

Ok, I was thinking of the new polycarbonate ones.

http://ausrocketry.com.au/54mm-injectio ... black.html
QRS: 124
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Re: RF Absorption in Black Fiberglass Nose Cone

Postby matthew » Tue Oct 25, 2016 6:26 pm

OTT: did you have a control, such as a non-black FWFG NC to compare against? ie. is the attenuation you experienced directly attributable to the pigment?
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Re: RF Absorption in Black Fiberglass Nose Cone

Postby OverTheTop » Tue Oct 25, 2016 7:48 pm

No control was available. The Tx was kept in a fixed position and the cone just placed and removed. The measurements were confirmed with adding or subtracting the cone to verify the differences.
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Re: RF Absorption in Black Fiberglass Nose Cone

Postby matthew » Wed Oct 26, 2016 8:55 pm

OK, cool. So, even though the attenuation was slight, it wasn't necessarily due to the pigment. My own experience with tracking through a black FWFG NC has been excellent. I tracked the NC of 17:28:59 (a modified Wildman Blackhawk 54) at Thunda to double digit kilometers away while it was still in the air (Eggfinder @921MHz).
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Re: RF Absorption in Black Fiberglass Nose Cone

Postby OverTheTop » Thu Oct 27, 2016 6:24 am

Attenuation in a natural f/g NC: I would expect to see very little, if any, signal loss. Typical RADAR radomes have insertion losses (including scattering and attenuation) approaching 0dB, especially at the lower frequencies. I don't have any data myself on natural f/g but I would expect very low, perhaps in the region of 0.2dB (or less) as a guesstimate.

If I get any data I will let you know :)

Does the Eggfinder system have any indication of signal level in the data available?

It doesn't matter how much attenuation you have in the system. If you are within the link budget it doesn't matter!
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Re: RF Absorption in Black Fiberglass Nose Cone

Postby cerving » Thu Dec 29, 2016 12:13 pm

Early on in the Eggfinder design I tried to use the RSSI data that comes out of the Hope RF module, it's pretty worthless. Since this is a GPS system and not a fox hunter, just about any signal you get from the RF will work... it's the GPS fix that's the variable. Once you get a fix, it tends to stay for quite awhile. The LCD receiver displays the HDOP as a bar graph, which is the quality of the fix, not the RF.

Regarding black nose cones, just about every FG rocket I've every flown has had a black nose cone, that includes kits from Madcow, Wildman, and Rocketry Warehouse. No issues with any of them.

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Re: RF Absorption in Black Fiberglass Nose Cone

Postby OverTheTop » Thu Dec 29, 2016 1:42 pm

Thanks for the info Cris. As my measurements showed it made almost immeasurable difference on the RF side. Carbon is likely another story!
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