I wanted to do long range telemetry so I got myself a dipole antenna for the modem.
I never could check the range because the plane couldn't get a gps lock anymore, with the modem powered on. With the modem powered off, I had a 3D GPS lock, 17 sats in seconds.
What can I do to get both long range telemetry and good GPS?
Replies
Hi Anton,
Anton, I think you are over-complicating the issue. The issue of coax cable lengths is unimportant for this discussion - the fact that your coax is longer than 1/8 Lambda is not a problem. It can be any length desired and has no effect on the signal at all, other than losses in the cable, which will increase with length ( talking meters of length for significant loss). If the antenna impedance is a match to the coax impedance, normally 50ohms for this application, and if the transmitter output impedance is also 50 ohms, then all is matched and the coax length is irrelevant. The impedance of your transmitter should be around 50 ohms - that is a standard in the industry to which mostl transmitters and receivers that couple via unbalanced feeder, ie, coax cable, are designed to.
If you want to short-circuit the process then get an antenna that is known to work, maybe the one Alasdair proposes. If that does not work, then we can go back to the process of elimination.
If on the other hand, you wish to learn more about why it does not work, and more about the antenna and how it works, then maybe have a look at these blogs of mine - Antenna design has been a 30plus year career for me, on commercial and Military Aircraft, and for a small handfull of Amateur Radio Space Satellites..as well as large UAV's 150kg and larger..
The topics may be somewhat advanced, but there are elements that may interest you, and maybe even help!
Joe
http://diydrones.com/profiles/blogs/dipole-style-antenna-for-433mhz
http://diydrones.com/profiles/blogs/some-more-antennae-and-when-is-...
http://diydrones.com/profiles/blogs/antenna-feedpoint-matching-balu...
http://diydrones.com/profiles/blogs/video-and-datalink-antenna-for-...
Hi Alasdair,
This is one of the more insightful answers!
So the feeder of my modem is matched for a lamda/4 monopole antenna and it goes haywire on a lambda/4 dipole? I found no 433 antennas on the site you mentioned. This article is about impedance matching, but I understand only half of it. http://www.radio-electronics.com/info/antennas/yagi/yagi-feed-imped...
This site has a nice selection of 433 antennas. How do I know if any of them are matched?
http://www.getfpv.com/catalogsearch/result/?cat=0&q=433+antenna
Or should go and build my own antenna, get an SWR measurement device and match it?
(Sadly, where I live 1.2GHz is an illegal frequency to use. So I'm stuck with 433 en 2.4. So no 1.2G video for me. :( )
Anton, do not get mislead yet. You have not answered my questions and we need to eliminate the items constructively. Your results were good or better with the SAME dipole, when you had the modem separated and shielded and powered from a separate BEC. It is not proven yet that the dipole is the problem.
Joe
Very interesting topic, following
It's less expensive here :
http://www.fpvpro.com/store
But how to know which one is good between these two : http://www.fpvpro.com/store/receiver-antenna-for-dragon-link-30-cm-... and http://www.fpvpro.com/store/dragon-link-transmiter-antenna-high-per...
I think you might try mounting your 433mhz Transmitter , or at least its dipole antenna on back of your plane.
like in this picture.
move your GPS and RX in fuselage , they are both receivers. they wont bother each other. just move Tx away from Autopilot and RXs. and mount your antenna vertical.
I always use a separate battery for electronics , other than flight battery.
you might try this too.
Hope this will help
An overview of my different tries so far. Will add more experiments as suggested in this thread.
Thx for posting this, very interesting.
It is strange that, when not powering your 433 modem via the autopilot but with a separate power source, you get a better GPS reception. I do not see any logical reason for this result.
The only one I can think of as a result of your experiments is that the autopilot does not have enough amperage capacity to power both your 433modem and the GPS/compass unit simultaneously very well ?
I agree with your thought about not enough amperage... (I've only got about 2 years modern flight controller experience, but about 20 years electrical engineering experience).
I also want to mention... I've been building some small copters lately, with telemetry modems, GPS, UHF control, relatively high wattage everything (600mw video, 500mw telemetry), all shoved into a 250mm sized copter... I definitely have issues with it, in comparison to my bigger vehicles. But, it works... I can fly and do auto missions most of the time....
So, I think there has to be something unique (or wrong, like a bad product) in your situation... Personally, I'd just buy another modem and antenna, before I do anything else....
Hello.
You can get GPS working with following things:
1- Put a "Notch Filter" on 433mhz transmitter. this will kill most of harmonics.
(http://www.getfpv.com/ibcrazy-433-mhz-1-3-ghz-notch-filter.html)
2- Wrap you transmitter with Aluminum foil, if transmitter is emitting RF noise, it will be shielded too.
3- Put an Aluminum foil piece beneath your GPS unit.
4- If still getting noise on gps, then put a ferrite ring on GPS > Autopilot cable.
https://api.ning.com/files/Y3PiDOyKFDJ0R5iLr-z2YQIsV86BVLFNbIzrusdS7...
hope this will help
- Is a notch filter the same as a balun? It looks similar.
- Would a 15nF condensator over the modem power lines help? I still hope to avoid a separate battery for the modem...
- I will try a separate BEC, maybe that helps too.
I will update this thread later with a systematic approach like Nampilot suggested.
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