Impact on Signal Strength of Bending an Altus Metrum TeleMetrum Antenna

The Rocketry Forum

Help Support The Rocketry Forum:

kbRocket

Well-Known Member
TRF Supporter
Joined
Nov 14, 2017
Messages
141
Reaction score
164
Location
Portland, OR
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 to fit the TeleMetrum into a short rocket so I thought I would measure the signal strength with the antenna in different bendy configurations.

The TeleMetrum is version 2.0. I attached it to a board with markings in the antenna region 1" apart. I stood about 320 feet away, approximately normal to the surface of the board and I measured the signal strength received by a 3 element Arrow 440-3 hooked up to a TeleBT 4.0. I recorded the RSSI signal strength with the 440-3 antenna pointed at the TeleMetrum with the elements vertical and flat.

Here is a picture of test cases 0 through 6: cases 0 thru 7.jpg
Here is a picture of test cases 7 through 14: cases 7 thru 14.jpg

Here is a graph of the RSSI signal strength measured, both antenna vertically and horizontally:
rssi.png

This shows that from the one measurement location 320 feet away, a straight up antenna has the best signal strength and that covering up the bottom of the antenna with a battery doesn't matter much.

Most of the bent cases had a moderate impact on signal and the inductive center loading wasn't much better. The 'jumbled' antenna did have the lowest signal strength.

I'm not sure how bad these configurations are to the TeleMetrum's radio.

I'm aware that there is a 3 dimensional distribution of signal strength with different powers receive in XYZ orientations at each point in the distribution. Receiving the signals at different distances and angular orientations away from the antenna will have different answers. Maybe there is a good antenna design software that could inform about the 3 dimensional distribution.

This give some insight into what happens when the antenna is bent and folded up and twisted up to try to make it fit into a shorter space.
 

Voyager1

Well-Known Member
TRF Supporter
Joined
Oct 23, 2015
Messages
695
Reaction score
231
Location
ACT, Australia
As you have correctly identified, it is important to consider the 3D far field radiation pattern of any antenna configuration to understand what is happening when you modify the antenna geometry. Ideally, this would be done in a suitable anechoic test facility, but even a simple test performed in the open would yield some useful results by sweeping 360 degrees about the antenna in horizontal and vertical orientations. You do need to be careful about how you do this because you could also be measuring ground reflections, as well as direct radiation from the antenna. This could result in erroneous measurements as it is influenced by polarity, wavelength and height above the ground.

You must also expect that when you deform the antenna into the various configurations as you have in your experiments above, you also influence the loading on the antenna. This loading affects the antenna impedance and can lead to detuning of the antenna’s resonant frequency.

If you want to perform some simulations on this, you could use the EZNEC antenna software.
 

cerving

Owner, Eggtimer Rocketry
TRF Sponsor
TRF Supporter
Joined
Feb 3, 2012
Messages
4,446
Reaction score
1,849
Nice work!

This is why the Eggfinders come with a stiff wire antenna... to keep it from getting bent into a pretzel with the resultant lost of signal strength and radiation pattern. Of course, losing 20 dB of link budget on a 250 mW transmitter would not be as bad as losing 20 dB on a 100 mW transmitter... you get 4 dB of that back in extra power, and another 3 dB or so back because it's a 70 cm transmission vs. 900 MHz. Still, you're down 13 dB... if you're stretching the limit of your link budget that's a big deal.
 

DaveW6DPS

Well-Known Member
Joined
May 28, 2013
Messages
474
Reaction score
227
Location
Barstow, CA
A post on a forum is not really the place to try answer such a question. One thing to consider is if transmitting with a compromised antenna will damage your transmitter. You might want to consult the manufacturer before proceeding, particularly if you expect them to provide warranty coverage if it burns up.

The simple whip antenna that comes with the Altus Metrum, and most other "rocket" transmitters, is pretty compromised to start with. If you don't have room to stretch it out straight, I would suggest looking into external antennas or different designs.

To get a basic idea of what is involved: https://www.arrl.org/shop/Basic-Antennas/

To get a better idea: https://www.arrl.org/shop/Antenna-Physics-An-Introduction/

Then look into using an antenna modelling program, which is the radio equivalent of using Open Rocket: https://www.arrl.org/shop/An-Introduction-to-Antenna-Modeling/
 

Voyager1

Well-Known Member
TRF Supporter
Joined
Oct 23, 2015
Messages
695
Reaction score
231
Location
ACT, Australia
Don't listen to Dave, I think this forum is a great place to post your study given what your goals and constraints were. I like it.
I agree that this forum is a great place to post experimental data such as this, and I wish to see it continue. We can all learn from these investigations. However, I do agree with Dave on the possibility of damaging your transmitter by compromising the load impedance. There is also the issue of de-tuning the antenna's resonant frequency and the reduction of transmit power with a degraded SWR. Personally, I would install your TeleMetrum in the nosecone if it is long enough to house it and the antenna without folding.
 

kbRocket

Well-Known Member
TRF Supporter
Joined
Nov 14, 2017
Messages
141
Reaction score
164
Location
Portland, OR
The reason for this research is that I am planning a small rocket for a TRA record attempt. My sims show that an altitude of 10k' is possible and that I lose about 200' of altitude for every inch of rocket length. I hope to trim off about 2.5 inches of length by trying an antenna configuration that is new to me. Before trimming the body tube I wanted to make sure that enough signal would remain to make GPS tracking very likely. It's only 10k' not 50k'. I think two miles up and two miles over will be fine with the reduced signal.

I know the manufacturer and have discussed my plans with him over email. He commented "...radio range shouldn't be a big deal, so this seems like it should work fine."

I have launched something similar to configuration #5 about 19 times in the past 1.5 years without signal strength or TeleMetrum radio problems. Only one of these flights was under 10k', 7 exceeded 15k', 1 went to nearly 24k'. A number of these flights came down 1.5 to 2.5 miles away. The post three electronics bays shows a few ways that I have built the center loaded inductor antenna into compact ebays.

These results indicate that it is possible to compact the antenna and successfully use it, and I now know something about the impact of different ways of compacting it. The results tell me to not use compact versions when there is little penalty for it, and for when I go really high up or expect a really long walk.
 

OverTheTop

Well-Known Member
TRF Supporter
Joined
Jul 10, 2007
Messages
5,526
Reaction score
2,894
Location
Melbourne Australia
I run a TeleMega and did a flight to 37500' last year. Telemetry was solid the whole way. You are not likely to have a problem.
 

kjs

Well-Known Member
TRF Supporter
Joined
Jan 22, 2013
Messages
291
Reaction score
119
So, I like this experimentation and actual data. My primary job deals in data... I'm sick of data and I still like it... it's a love / hate relationship....

I know Keith and Bdale and have absolutely no 'bad words' about their products or service. I did own a couple Tela* products in the past - and may still have them... I do write software for the Featherweight products so am not biased - but that is what I fly now (obviously). And... I see kbRocket is even from Oregon so he may know KeithP directly... I met them both personally at a Colorado launch a long time ago.

Anyway, I did a 3D av bay for a Featherweight Raven4 and Featherweight Tracker in a 38mm nose cone (pic below). I used 'separate at apogee and cable cutter at main'. It looks like the rocket that kbRocket shows in his "three av bays link" above (except his is prettier). My av bay was this:

- the Raven4 sits on a 24mm av bay base for easy wiring and battery connections
- the Raven4 battey is in the middle of the blue thing (held in place by the blue 'bulkhead)
- the deployment 'pads' went through the base with brass screws for connection below
- the tracker is there (my personal mod to replace the power header with the lower profile battery connector so it fits in the nosecone)
- the tracker battery does also get stuffed in with this - but it fits
- baro hole in the nose cone (yes it works because of mach 'lockout')

if kbRocket wants to try this, we should chat (he may not want to invest at this point and I understand).

I didn't push mine because it was my first time doing this.. (his rocket in the three av bays thread looks much better than my rocket id). i had a aeropack retainer (more drag at the end) and no finish on the rocket (this was the actual nose cone) so someone could do better...

net result is you have no middle av bay - only the (small) rocket and a nose cone. if you want to shorten your rocket, this can help.

I'm trying to figure out how to fit the raven and tracker in a "linear nose bay" for a 29mm rocket... my first 3D print is close other than you can't actually assemble it...! the 38mm does work though...

38mm av bay.png
 

kbRocket

Well-Known Member
TRF Supporter
Joined
Nov 14, 2017
Messages
141
Reaction score
164
Location
Portland, OR
if kbRocket wants to try this, we should chat
Thanks for the offer. I am happy with my plans for this rocket.

I did research to convince myself that the TeleMetrum would work well enough for this application and wanted to pass along my findings to help others who may face similar challenges. Thanks also to others who chimed in with suggestions and links.
 

micro

Well-Known Member
Joined
Apr 29, 2014
Messages
178
Reaction score
135
Interesting. I have a tight space I'd like to place a Telemetrum and was wondering if I could do the jumble antenna. Just kidding, but I was wondering if I could do a Z bend to minimize overall length to 3.5". Basically your #7 but folded down over the board. Do you think I'd get enough signal to locate it in a corn field? Talking 2,000 ft.
 

Voyager1

Well-Known Member
TRF Supporter
Joined
Oct 23, 2015
Messages
695
Reaction score
231
Location
ACT, Australia
In spite of my reservations about doing this to an antenna due to the issues raised above, the only way to find out is to perform a ground test. Go to an open space, e.g., an oval or field; place your rocket with Telemetrum installed on the ground; measure the signal strength from several radial distances (10m, 20m, 50m, etc, to as far as practical) around 360 degrees at no more than 45 degree intervals. This will give you a very basic set of far-field radiation patterns. You might have to re-orient the rocket to from horizontal to vertical to get a better idea of the radiation patterns with a change of polarity. You could also raise it off the ground as it would be if caught in a tree.

I also agree with Bigs - if you have the SMA option!
 

OverTheTop

Well-Known Member
TRF Supporter
Joined
Jul 10, 2007
Messages
5,526
Reaction score
2,894
Location
Melbourne Australia
I found out the other day that there is a NanoVNA available, so I pushed the button and ordered one. One of the uses of a Vector Network Analyser is to analyse the matching of an antenna. It is quite a useful bit of RF kit and modern electronics means we can get one that does 50kHZ-3GHz for the princely sum of around $60USD. Putting that price into perspective is that I just received an Agilent VNA for work that is $42000 new :eek: .
nanovna_v2_tindie.jpg

https://nanorfe.com/nanovna-v2.html
https://www.tindie.com/products/hcxqsgroup/nanovna-v2/
https://www.rtl-sdr.com/tag/nanovna-v2/
https://www.eevblog.com/forum/testgear/nanovna-v2-aka-s-a-a-2/

Beware of some of the clones as quality varies apparently. I ordered from Tindie to support the developers.

I will do a thread writing up this device when I get it and have a chance to use it.

It might be interesting to use it to analyse the impedance of the various scrunching effects on the antenna as you have done.
 
Last edited:

micro

Well-Known Member
Joined
Apr 29, 2014
Messages
178
Reaction score
135
In spite of my reservations about doing this to an antenna due to the issues raised above, the only way to find out is to perform a ground test.
Problem for me is I don't have one yet, trying to figure out my best option to put in there still. But from kbRocket's data I'm guessing it would probably be fine if I could do something clever with the antenna.
 

OverTheTop

Well-Known Member
TRF Supporter
Joined
Jul 10, 2007
Messages
5,526
Reaction score
2,894
Location
Melbourne Australia
Based on the earlier experiments the comments about extending the antenna into the payload bay seem a good idea and will likely give quite a reasonable outcome.
 

kbRocket

Well-Known Member
TRF Supporter
Joined
Nov 14, 2017
Messages
141
Reaction score
164
Location
Portland, OR
There is really no reason to ever bend a Altus Metrum antenna. Just use an SMA through-mounted bulkhead with a whip wire
There actually is one reason: In the rocket I am creating, the distance between the front of the motor and the inside of the tip of the nose cone is less than the length of the TeleMetrum and it's extended antenna. I can either not fly the electronics, or fly it with reduced, yet adequate signal strength.

Interesting. I have a tight space I'd like to place a Telemetrum and was wondering if I could do the jumble antenna. Just kidding, but I was wondering if I could do a Z bend to minimize overall length to 3.5". Basically your #7 but folded down over the board. Do you think I'd get enough signal to locate it in a corn field? Talking 2,000 ft.
Folding the Z bend back over the board will further reduce signal and harsh on the radio. The board has a copper ground plane at one end that helps the antenna function. By folding back over it you are not doing it any favors. You are also covering the GPS chip/antenna with a conductor which I try to avoid. I cannot speak to the RF attenuating properties of corn, but 2000' in is not a great distance. Even if the corn does block signal you should be able to get really close, hence pick up signal again, based on packets received during descent.

My current plan is to have the antenna auto straighten after apogee deployment. Extend the wire TeleMetrum antenna through the bulkhead into the nose cone attached to straight plastic cable tie that acts as a spring and protects the antenna from being ripped off. Bend the cable tie to fit in the nose during the ride to apogee. The cable tie closure rides behind the bulkhead as a hard stop.

1599583654738.png

The antenna will flap around, but more signal should result than anything that remains folded.
 

cerving

Owner, Eggtimer Rocketry
TRF Sponsor
TRF Supporter
Joined
Feb 3, 2012
Messages
4,446
Reaction score
1,849
You could connect a piece of light elastic to the antenna and your Kevlar shock cord so that when it's deployed it straightens the antenna... that should work a lot better than just letting the antenna flap around. I have not tried this, so you may need to take the suggestion with a grain of salt...
 

FredA

Well-Known Member
Joined
Jan 19, 2009
Messages
2,227
Reaction score
400
How about replacing the antenna with a piece of spring-steel?
I'm sure it would be sufficiently conductive and you could have it self-deploy.
 

roytyson

Well-Known Member
Joined
Oct 31, 2017
Messages
309
Reaction score
154
I had mine bent in a U shape, nearly half the straight distance this weekend at Airfest to fit into my av bay of a Mach2. Worked fine at Mach 2.2, and 19,900 feet.
 

OverTheTop

Well-Known Member
TRF Supporter
Joined
Jul 10, 2007
Messages
5,526
Reaction score
2,894
Location
Melbourne Australia
By folding back over it you are not doing it any favors. You are also covering the GPS chip/antenna with a conductor which I try to avoid.
Having your telemetry Tx near theGPS causes receiver desensitisation and can dramatically affect time to lock. I suspect it is also deleterious to the location determination. Any timing jitter induced on the board will be bad for performance and result in similar outcomes. You are better off giving the GPS antenna some breathing room.
 

RocketRev

Lifetime Supporter
TRF Lifetime Supporter
Joined
Jul 25, 2012
Messages
329
Reaction score
190
I'm sort of surprised that nobody has suggested the obvious. If you really can't fit it in, buy a longer nose cone so that the full antenna can fit. If you can't buy one, why not build one? There are lots of ways of making your own custom nose cone.

I would also suggest talking with the manufacturer about antenna options. I would think that the manufacturer of your device has chosen the supplied antenna for good reasons. However the manufacturer should also have info available about signal loss variations due to differing antennas without taking the chance of damaging the unit itself. Maybe they have a shorter antenna. There are many antenna options on the market.

Why do damage to a perfectly good antenna, when there are other options that do not degrade signal strength?

I've played around with all sorts of simulations with various nose cone/air-frame combinations and have a hard time thinking that there are no other options.

Brad
 

kbRocket

Well-Known Member
TRF Supporter
Joined
Nov 14, 2017
Messages
141
Reaction score
164
Location
Portland, OR
Thank you to all who have offered "solutions" to my "problem". I am happy with the design of my rocket, nose cone, payload bay, and recovery system. I make all of my own fiberglass components and they are the dimensions I intended. It should fly high.

My goals for this thread are to:
  • pass along some research results on antenna configurations that may benefit others
  • provide an opportunity for others to chime in if they have relevant expertise or want to discuss ways to fit tracking into small spaces
 

micro

Well-Known Member
Joined
Apr 29, 2014
Messages
178
Reaction score
135
One reason would be if it's not going in a nose cone. Possibly your rocket doesn't have one. My situation is a scale booster with limited space, altering dimensions would defeat the purpose. making everything fit while still being functional is part of the challenge that makes it fun. Extending the antenna through a bulkhead is probably the best option in my case.
 

Alby

Well-Known Member
Joined
Jul 12, 2010
Messages
233
Reaction score
9


KJS:

My whip antenna broke, but I would like to solder a similar antenna like the setup you have in your picture. Could you tell me what antenna parts you used and where to solder the connector? I see 2 pads on both sides of the board. Would a connector be attached to 2 or 4 pads?

Thanks,
 

kjs

Well-Known Member
TRF Supporter
Joined
Jan 22, 2013
Messages
291
Reaction score
119
KJS:

My whip antenna broke, but I would like to solder a similar antenna like the setup you have in your picture. Could you tell me what antenna parts you used and where to solder the connector? I see 2 pads on both sides of the board. Would a connector be attached to 2 or 4 pads?

Thanks,
If you have a Telemetrum, then the frequency and antenna size would be different. For the circuit board connector, you are googling for "sma antenna connectors circuit board" but I can't say which ones are appropriate for the TM board. [The tracker in my picture is the Featherweight tracker.]
 

Alby

Well-Known Member
Joined
Jul 12, 2010
Messages
233
Reaction score
9
If you have a Telemetrum, then the frequency and antenna size would be different. For the circuit board connector, you are googling for "sma antenna connectors circuit board" but I can't say which ones are appropriate for the TM board. [The tracker in my picture is the Featherweight tracker.]

Thanks. I did some digging and found the correct SMA conector. In fact Altus Metrum sells them on their website, but I actually bought one from Adafruit before I knew that.

I also found a similar 70cm monopole antenna to use. Which is the broadcast freq. of my Telemetum. So it looks like I'm set, just waiting for the parts to arrive so I can solder them to the board.
 
  • Like
Reactions: kjs

ksaves2

Lifetime Supporter
TRF Lifetime Supporter
Joined
Nov 25, 2009
Messages
6,234
Reaction score
450
Location
Central Illinois
One thing I did in a small DD 54mm rocket was to have an ebay with an upper bay for the tracker and the lower bay for the deployment electronics. There was a channel for the main chute ematch deployment wires and the wire antenna went through a form fit hole in the bulkhead into the upper main chute bay.

The problem there is the main chute can squish down the antenna wire during boost. I thought about that in the design phase and decided to use clay around the base of the wire to prevent gases from getting to the electronics and use a stent on the wire antenna.

It just so happens that the thick walled cardboard tubes that the Aerotech igniters came in fit the bill perfectly. Just taped the base to the forward bulkhead and the antenna doesn't get squished by the main chute! It runs down the center of the cardboard tube and last I heard, cardboard is radiolucent.

Also when the apogee deploys in my configuration, the nosecone/bay is pointing towards the ground that might help with radio propagation. When the the main blows at lower altitude of course, the antenna is facing vertical and maybe is in a good position for propagation since the rocket is lower to the ground.

In larger rockets, it's better to have a nosecone tracker bay to avoid having to have the tracker next to the deployment stuff.
In smaller diameter rockets that's sometime unavoidable.

Kurt
 

Alby

Well-Known Member
Joined
Jul 12, 2010
Messages
233
Reaction score
9
One thing I did in a small DD 54mm rocket was to have an ebay with an upper bay for the tracker and the lower bay for the deployment electronics. There was a channel for the main chute ematch deployment wires and the wire antenna went through a form fit hole in the bulkhead into the upper main chute bay.

The problem there is the main chute can squish down the antenna wire during boost. I thought about that in the design phase and decided to use clay around the base of the wire to prevent gases from getting to the electronics and use a stent on the wire antenna.

It just so happens that the thick walled cardboard tubes that the Aerotech igniters came in fit the bill perfectly. Just taped the base to the forward bulkhead and the antenna doesn't get squished by the main chute! It runs down the center of the cardboard tube and last I heard, cardboard is radiolucent.

Also when the apogee deploys in my configuration, the nosecone/bay is pointing towards the ground that might help with radio propagation. When the the main blows at lower altitude of course, the antenna is facing vertical and maybe is in a good position for propagation since the rocket is lower to the ground.

In larger rockets, it's better to have a nosecone tracker bay to avoid having to have the tracker next to the deployment stuff.
In smaller diameter rockets that's sometime unavoidable.

Kurt

I've setup my AV Bay to run wires from the flight computer to screws (wingnuts and bolts) to pass the electrical signal to the ignitors on the opposite side of the AV Bay bulkhead. This way I can completely seal off the charge explosion/gases from the electronics. I don't have any lose holes with wires to worry about.

My main design goal was actually to create an AV Bay that never (rarely) needs to be opened. The LiPo battery can be charged from a USB port I've ran to the midpoint band. I also setup an on/off key switch wired to the flight computer. The BP/Ignitors can be replaced w/ease into a charge-well and connected to screw terminals. All this helps to reduce the turn around time to fly it again. 😁
 

LayNLow

New Member
Joined
Apr 13, 2021
Messages
3
Reaction score
0
Maybe think about getting (or making) a flat adhesive antenna to put on the outside of the rocket, and just drill a small hole in the body to feed the small connector through.
 
Top