Telemetrum in smaller avionics bay

[cornbeef007]

Hey folks,

Quick question regarding the installation of a Telemetrum in a shorter than optimal avionics bay.

Regarding antenna theory in the UHF range…what are the options when attempting to install a Telemetrum into a 6 “ avionics bay? The antenna is about 7” long and the board is 2.75”. The total length needed is roughly 10” for an optimal install.

Not an expert at UHF propagation but it seems to me that the antenna needs to be relatively straight in order to be the proper form for 1/4 wave length transmission/reception.

Has anyone tried adding a 90 or 180 degree change in the length of the antenna. An example would be something like install the device and have the antenna proceed forward 3” then make a 90 degree turn and proceed along the width of the bay. So the antenna would basically form an L shape.

Another option would be to wrap the antenna around the back side of the board, so the antenna would require a bay of 7” or so as opposed to the 10” total of the both. The problem here is that the signal will be disrupted by the board itself.

The other options I can think of is to wire a remote antenna elsewhere in the rocket or drill a hole through the bulkhead, so the antenna can pass through and extend the full 7” in a straight line. But this requires compensating for the larger area, which means the static vent hole will need to be larger.

What do you guys think? Appreciate the help.

 

[bdale]

What I tend to do in this situation is drill a 1/16" hole in the bulkhead and feed the antenna wire through it into the adjoining bay, trying to keep the wire fairly straight. You can actually measure the effectiveness of different approaches by ground testing. Our ground station software will show you the RSSI (received signal strength) of each packet. Setting up your rocket in the back yard or something and walking around with your ground station will teach you a lot about the radiated pattern from the in-airframe antenna, let you compare how different antennas behave, etc...

 

[cornbeef007]

What I tend to do in this situation is drill a 1/16" hole in the bulkhead and feed the antenna wire through it into the adjoining bay, trying to keep the wire fairly straight. You can actually measure the effectiveness of different approaches by ground testing. Our ground station software will show you the RSSI (received signal strength) of each packet. Setting up your rocket in the back yard or something and walking around with your ground station will teach you a lot about the radiated pattern from the in-airframe antenna, let you compare how different antennas behave, etc...

I didn’t think about the RSSI…that will make it pretty straightforward regarding experimenting around.

The hole it the bulkhead is exactly the method I’ve used so far.

Thanks for the reply!

 

[cls]

Has anyone tried adding a 90 or 180 degree change in the length of the antenna.

Hi, 90 at a time is ok, in other words, two 90s to make two Ls or a Z is ok. But more than 90, a 180 is never ok, because it cancels the effective length.

The reason is two parallel wires make a transmission line, have opposite EM fields, those cancel, so no emission. When the two wires diverge, no cancelling, so emission.

Sorry if this explanation doesn't quite work for you. Maybe later I can draw some pictures.

 

[cornbeef007]

Hi, 90 at a time is ok, in other words, two 90s to make two Ls or a Z is ok. But more than 90, a 180 is never ok, because it cancels the effective length.

The reason is two parallel wires make a transmission line, have opposite EM fields, those cancel, so no emission. When the two wires diverge, no cancelling, so emission.

Sorry if this explanation doesn't quite work for you. Maybe later I can draw some pictures.

Your explanation is very good. I figured that 180 wouldn’t work but I thought I’d ask anyway.

Thanks for the information.

 

[Voyager1]

Check out this earlier thread on a related topic.

Impact on Signal Strength of Bending an Altus Metrum TeleMetrum Antenna

The TeleMetrum by Altus Metrum has a 6.5" quarter wave wire antenna. I heard that I could make it into a center loaded antenna by wrapping the central portion three times around a pencil to add inductance while shortening the overall length. I have flown this way many times. I am now trying...

www.rocketryforum.com

 

[cornbeef007]

Check out this earlier thread on a related topic.

Impact on Signal Strength of Bending an Altus Metrum TeleMetrum Antenna

The TeleMetrum by Altus Metrum has a 6.5" quarter wave wire antenna. I heard that I could make it into a center loaded antenna by wrapping the central portion three times around a pencil to add inductance while shortening the overall length. I have flown this way many times. I am now trying...

www.rocketryforum.com

This is exactly what I needed….I don’t like the results but I love the data

 

[Buster]

I have the same problem with my Tele Mini and Tele GPS. I intend to route the antennas through a static port and tape to the outside of the rocket. I got this idea from seeing it done for test equipment harnesses during flight test. If it works at M0.84 it’ll work on my rocket. My priority is telemetry downlink and recovery. It can’t cause any more issues than a camera on the outside. I’ll let you know how it works.

 

[cornbeef007]

I have the same problem with my Tele Mini and Tele GPS. I intend to route the antennas through a static port and tape to the outside of the rocket. I got this idea from seeing it done for test equipment harnesses during flight test. If it works at M0.84 it’ll work on my rocket. My priority is telemetry downlink and recovery. It can’t cause any more issues than a camera on the outside. I’ll let you know how it works.

That’s actually pretty clever…the only issue I see with that is if something gets jolted. If the Telemetrums shifts position but the antenna stays put(or vice cersa), it would stress the joint between the board and the antenna. I figure as long as you have a little slack at the joint or both are secured to avoid any movement, you would be good.

 

[Buster]

That’s actually pretty clever…the only issue I see with that is if something gets jolted. If the Telemetrums shifts position but the antenna stays put(or vice cersa), it would stress the joint between the board and the antenna. I figure as long as you have a little slack at the joint or both are secured to avoid any movement, you would be good.

That is a really good point I’ll add some slack to account for that. Thanks for the input.

 

[GrouchoDuke]

Has anyone tried adding a 90 or 180 degree change in the length of the antenna.

I put a TeleMini in a 24mm nose cone with the antenna folded in half (180 bend). Yep, I know - don't do it. It still worked great for me to DF to it from several miles away.

I've flown quite a few flights to ~18k AGL and 3-5 miles away with a TeleMetrum antenna coiled up just outside my avbay. No reception issues while it was in flight with a decent dipole antenna on my TeleDongle. A Yagi you point toward your rocket during flight can make up for a lot of tx issues on the rocket end if you need more reception gain.

As others have said, ground test, check RSSI, YMMV, etc. Good luck!