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  1. #1
    Join Date
    29th June 2017
    Posts
    8

    Questions about communication from 15000 ft

    I have the eggfinder TRS and LCD kit for my L2 rocket and since the av bay will have a couple threaded rods i want to put the antenna for the TRS outside the av bay. I plan on flying a K660 where I will be reaching 15-16000 ft (according to sims).

    I assume that putting the antenna next to my main charges would not be the best thing to do. I'm looking at putting a 900 mhz rp sma antenna (https://www.amazon.com/Eightwood-900...antena+900+mhz) and I'm wondering what i need to do to protect the antenna when the rocket touches down and when the rocket separates so the shock cord doesn't break the antenna.

    The plan would be to run a short extender to the bulkhead where the antenna would attach to. (https://www.amazon.com/Crazepony-Ant...psma+extension).

    Thanks for the help.


  2. #2
    Join Date
    23rd October 2015
    Location
    Canberra, Australia
    Posts
    102
    You could use threaded nylon rods and a 3D printed sled. Alternatively, you could use an external surface mount antenna such as http://store.rfdesign.com.au/rfdflex...a-300mm-rpsma/

    John
    TRA #14574 L2
    AMRS #199 L2

  3. #3
    Join Date
    29th June 2017
    Posts
    8
    I was planning to use a laser cut sled, is there a place you would recommend getting the nylon threaded rod? or should i get on from amazon and cut it down?

  4. #4
    Join Date
    15th May 2016
    Posts
    2,369
    For what it's worth, I put my antennas next to my main charges. Some day I might melt them... but works so far.
    David McCann
    Dave's Rockets | My Flights
    URRG |URRF 4| Level 2 | TRA# 14210

  5. #5
    Join Date
    23rd October 2015
    Location
    Canberra, Australia
    Posts
    102
    Quote Originally Posted by AndrewJacob View Post
    I was planning to use a laser cut sled, is there a place you would recommend getting the nylon threaded rod? or should i get on from amazon and cut it down?
    I get my Nylon rod from https://www.hiq.co.nz/products/categ...c-threaded-rod
    John
    TRA #14574 L2
    AMRS #199 L2

  6. #6
    Join Date
    6th February 2015
    Posts
    700
    Without any empirical evidence, or extensive research or testing done on the subject, and therefore nothing scientific to bring as an argument, my intuition still tells me that at the level hobby rocketry fliers operate, a couple of threaded rods placed near to and oriented with any of the 900mhz, 433mhz, 2.4Ghz, 5.8Ghz, etc. devices that we use will not attenuate an RF signal to the point one should be concerned and should -in most cases- not be considered an issue.

    My "two cents", as one would say.


  7. #7
    Join Date
    21st July 2015
    Location
    Morgan Hill, CA
    Posts
    66
    I had a similar TRS setup and I placed an sma connector in the forward bulkhead of the AV bay. I connected the antenna cable and antenna on the other side, in the payload section, and used a nomex shock cord protector to cover the cable and antenna. That along with some dog barf kept the antenna from being damaged.

  8. #8
    Join Date
    22nd August 2015
    Location
    Rhode Island
    Posts
    1,327

    Questions about communication from 15000 ft

    I mounted my antenna on the bulkhead, near the drogue charge. I haven't had any problems with the blast of the charge, but twice the harness has gotten wrapped up with my antenna and ripped it off. I may just had
    bad luck, because I haven't heard of anyone else having that problem. Regardless, I now keep my antennae in the AV bay. The two all threads don't seem to bother my reception. (Though I haven't flown to 15k ft yet.)
    Last edited by BDB; 14th February 2018 at 04:56 AM.
    NAR #100940, RIMRA & CMASS
    L1 - 4/17/16, Tyrannosaur (by Binder Design), Loki H144
    L2 - 8/19/17, Terrordactyl (by Binder Design), CTI J250

  9. #9
    Join Date
    29th June 2017
    Posts
    8
    I like the idea of using nylon threaded rods in the AV bay and keeping the antenna in there, seems a lot cleaning and alleviates a lot of the problems.
    Thanks for the help guys!

  10. #10
    Join Date
    3rd February 2012
    Location
    So Cal (ROC, TRASD, SCRA)
    Posts
    2,672
    Lots of people have done what you're doing, it works fine. The antenna that I recommend for transmitters is a Linx Technologies ANT-916-CW-QW, they're about $7 from Mouser or DigiKey, and they're about the same size as the wire antenna (roughly 80mm). The antenna from Amazon that you linked to looks similar to the one that we sell for the LCD receiver, it's pretty long for most builds. It's also not made for the exact radio band (unlike the Pulse Electronics W1063 that we sell) so its performance would be somewhat less. I wouldn't buy an antenna without a data sheet to back up their specifications... good luck finding one for the "Eightwood" antenna. I Googled it and there's a lot of general retailers selling it with exactly the same description, but no data sheet.
    Last edited by cerving; 14th February 2018 at 05:02 PM.

  11. #11
    Join Date
    26th November 2009
    Posts
    5,246
    If one has the room for a longer 1/2 wave antenna in a long carrier or nosecone, this one had potential: https://linxtechnologies.com/wp/wp-c...-916-cw-hw.pdf. Kurt

  12. #12
    Join Date
    23rd October 2015
    Location
    Canberra, Australia
    Posts
    102
    Placing a monopole (or dipole) antenna near the metal rods of an avionics sled will affect the antennas' radiation pattern - period! As to whether you will degrade the signal significantly is dependent on several factors. These include the geometry of the antenna placement relative the rods, the thickness, length and separation of the rods, etc. As an example, the pattern of a monopole antenna can be approximated by a simple dipole antenna in free space. When this is performed the dipole exhibits a maximum gain of about 2.2 dBi with a symmetrical radiation pattern. However, when you introduce the metal rods carrying the avionics sled, the radiation pattern can be deformed such that the gain in particular directions is reduced by 1 - 2 dB. The images below display the radiation pattern of the antenna with its modelling structure beside it in various configurations. The antenna is a half-wave dipole and the avbay metal sled rods are 6mm (1/4") x 200mm (8"), separated by 50mm (2").

    A free space pattern and model of a dipole looks like

    Click image for larger version. 

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    A dipole (Element 1) placed between two spaced rods (Elements 2 and 3) with antenna on the sled could look like

    Click image for larger version. 

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    Note a slight increase in max gain.

    A dipole (Element 1) offset from two spaced rods (Elements 2 and 3) with antenna forward of the sled could look like

    Click image for larger version. 

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    From this you can see that when you position the antenna off the sled just outside the avbay, the metal rods deform the radiation pattern and reduce the gain in the direction towards the rods by about 2-3 dB. This is not a rigorous simulation, but does demonstrate that antenna placement can have an effect. If these results can be believed, then you either have the antenna between the rods, or as far away as you can manage to maintain a symmetric radiation pattern.

    I would invite anyone to do this sim themselves, as I'm a little rusty with RF sims. Others with more recent experience would probably get other results - better or worse! It was an interesting exercise - hopefully not totally pointless!
    John
    TRA #14574 L2
    AMRS #199 L2

  13. #13
    Join Date
    26th November 2009
    Posts
    5,246
    Am with John on this. If one is hoping for the best range from a limited powered tracker (100mW on 900Mhz is limited or 16mW on the 70cm band is limited power)
    best practice is keeping the antenna in "free air". That doesn't mean "free air" literally but inside of a radio-lucent, non-carbon fiber, not metallic painted tube is fine. I've had a Beeline GPS ride forward in an ebay where the wire antenna goes through a tiny hole in the forward bulkhead. I put a cardboard tube stent over the wire so it doesn't get smushed by the main chute. Sure it tilts to the side but I've had no trouble with 13 or 14 totally sight unseen flights. Kurt

  14. #14
    Join Date
    15th October 2016
    Location
    Huntsville AL
    Posts
    2,245
    As I layout the insides of my 5.5" L2 bay, I've been thinking about this topic.

    Let's say I have the T3 transmitter with wire whip and mount it perpendicular to the axis of my threaded rods? What kind of signal alteration am I looking at then?
    "I'm at least 70% confident about whatever I say (90% of the time)"- college me

    NAR 101195
    Level 1: Big SAM, 9/10/16

  15. #15
    Join Date
    31st July 2014
    Location
    Idaho
    Posts
    191
    Quote Originally Posted by Nytrunner View Post
    Let's say I have the T3 transmitter with wire whip and mount it perpendicular to the axis of my threaded rods? What kind of signal alteration am I looking at then?
    Generally speaking, I wouldn't expect that metal rods perpendicular to the antenna would have much effect on the radiated signal from the antenna.

    Terry

  16. #16
    Join Date
    15th October 2016
    Location
    Huntsville AL
    Posts
    2,245
    Quote Originally Posted by W7AMI View Post
    Generally speaking, I wouldn't expect that metal rods perpendicular to the antenna would have much effect on the radiated signal from the antenna.

    Terry
    Perfect, that aligns with my limited radar/antenna knowledge and thinking.

    If I have that giant bay, Might as well use the real estate!

  17. #17
    Join Date
    23rd October 2015
    Location
    Canberra, Australia
    Posts
    102
    Quote Originally Posted by Nytrunner View Post
    As I layout the insides of my 5.5" L2 bay, I've been thinking about this topic.

    Let's say I have the T3 transmitter with wire whip and mount it perpendicular to the axis of my threaded rods? What kind of signal alteration am I looking at then?
    Good question! As you can see in the plot below, the antenna field does not appear to couple into the rods when perpendicular to the antenna. The gain is similar to the unloaded dipole in free space in the previous post.

    Click image for larger version. 

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    This seems to be the case regardless of the axial displacement of the antenna along the rods (currently at the midpoint along the rods). However, as you can see with the typical donut shape of the antennas' radiation pattern, the two minima of the antennas' on-axis pattern will now be directed laterally (z axis), perpendicular to the rockets' main axis (y axis). This is not as ideal as having the minima along the rockets' main axis, but you might just have two small blind spots on either side of the rocket.

    My suggestion is to try a few placement strategies and do some field tests to see what works for you. I use a small hand-held spectrum analyzer that is perfect for this sort of field testing. What you want is for the rocket to be as 'visible' from all orientations if possible. What Kurt said about maximizing the radiation of the limited power is spot on. This can be achieved through a little planning.
    John
    TRA #14574 L2
    AMRS #199 L2

  18. #18
    Join Date
    23rd October 2015
    Location
    Canberra, Australia
    Posts
    102
    Quote Originally Posted by W7AMI View Post
    Generally speaking, I wouldn't expect that metal rods perpendicular to the antenna would have much effect on the radiated signal from the antenna.

    Terry
    That appears to be the case, Terry. The shape is still the same, but the issue is where the minima are now directed. However, this is probably a moot point because the minima are going to be directed somewhere regardless. Although, in this case, this might be advantageous while the rocket is ascending and descending with the main lobe directed towards the ground and not one of the minima.
    John
    TRA #14574 L2
    AMRS #199 L2

  19. #19
    Join Date
    31st July 2014
    Location
    Idaho
    Posts
    191
    John,

    Yes, the null is going to point somewhere we don't want it to at some point during the flight. I kind of like the idea of the null being off to the side instead of pointing down while the rocket is taking off. Each placement method has it's advantages and disadvantages.

    Cheers, Terry


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