Instructions for Building a Portable Double Cross Antenna: Great for NOAA/Meteor Weather Satellites
Over on Reddit user merg_flerg has uploaded an imgur post that carefully details a step by step guide for building a double cross antenna. A double cross antenna is great for reception of satellites like NOAA and Meteor since it has a sky oriented radiation pattern with very few nulls. This means that it can receive satellite signals coming from the sky well. Alternative antennas for NOAA/Meteor include turnstiles and QFH antennas, although the double cross antenna seems to have the least nulls, meaning that the signal is less likely to fade in and out as the satellite moves across the sky.
merg_flerg’s design is also modified from the standard design slightly, allowing it to become easily disassembled and carried within a backpack. At the end of his tutorial he writes that he gets much better reception with his double cross antenna than he does with his QFH.
In the post he demonstrates the final constructed antenna decoding a NOAA APT weather satellite image with an RTL-SDR and the WXtoIMG software. See our tutorial for information on decoding NOAA weather satellite images.
Well, I made this antenna using the supplied info. It’s great at long distances i.e 2500 kim, but absolute rubbish at ranges around 500 km, with very deep nulls of 25dB or more.
I’ve spent three weeks playing with it. Some days I get a full pass with only 3 or 4 dB nulls. The next day, it is back with deep nulls, even though I have not changed anything, But there are so many variables that I find it impossible to optimise the performance.
Time to build a QFH antenna methinks, unless anyone has any ideas of how I can fix this double X.
Go with qhf I took mine down not long after building it and replaced it with one made of copper refrigerant tubing. Don’t use coax IMO. Not very rigid and constantly had to dink with it. The copper one I made is still in service and haven’t touched it since install.
Thanks Jonathan for those thoughts, I will make a QFH antenna using “microbore” copper tube (as it is known in the UK) which is 3/8″ OD. I think you can get elbows and in-line couplers. I’ll post back here when it’s up and running
KT
That would be perfect!
Hi Johnathan
I finished my first attempt at a QFH yesterday, and it is so much better than than the double cross at my QTH. The improvement is remarkable.
1. The SNR (when the satellite is overhead or close to overhead ) is 42dB. With the double X I get about 32dB .
2, With the QFH I get about 10 minutes signal for a picture,( counting the contrast bars) where the SNR is 25dB or more. With the double X I get 5 minutes
3. There are no noticable nulls at all, anywhere in the pass.
Also it is smaller and hence easier to move indoors when the weather is bad. It’s a more robust design, so it does not bend if the wind blows it over on to the ground. And finally there nothing to twiddle – it just works.
So for me it as a total no-brainer – QFH rules
Mine looks a bit odd – I rushed the build, so tha elements don’t have a smooth semi-circular appearance. I;m going to swap them out and use elements I have shaped using a template,
Keith G3NNW
I had to roll my tubing between 2 chunks of 2×4 to get them straight. Then bend them around a template I cut out of MDF. You can get the signal off the noise floor with one of those nooelec inline amp/filters for Noaa and by tuning it. Glad you got it built and working better than the Double X. After our last exchange here, I went ahead and built another one and now I’m just waiting for the wife to finish giving me projects to finish tuning it.
Regards.
Johnathan
N5WTA
I submit a suggestion that a properly built DCA will perform better that a QFH. As I read it, the instructions for building a DCA are confusing.
If you use the QST article as a guide, there are several things left VERY unclear.
If you want to “reconfigure” your DCA, I am sure I can assist you to get better images.
I suspect you no longer have interest in building/understanding the DCA. But, if you have interest, contact me
Hi Jerry, I am building a DCA, I was wondering if you have any advice to ensure it is built properly. I am following the procedure described in the imgur post linked above. I have a few questions as of now
Do you think 1/2″ copper pipe would work instead of 3/8″?
How do I connect the coax cable of the antenna to the SDR dongle, is using an adapter to the SDR port all that is required?
Can you provide any info that would be needed to clarify / complete the instructions in the imgur post?
Any assistance would be greatly appreciated
I do have time and interest in clarifying construction of the DCA.
I may need some info on how to post information to you. I suspect the need for extremes security has made Internet communicating complex.
You can send me information here if you would like: [email protected]
Bryan, I am building one too, if you are okay doing.. please share the info you have on building it. My email address is [email protected]
https://github.com/happysat/Double-Cross-Antenna
Given this antenna specifically is for skyward “looking” signal capture. We should take advantage of this property for capturing ADS-B 1090 MHz signals? The only thing needed would be modifying the antenna length for the 1090 MHz signal. Has anyone tried this approach to improve the range of ADS-B signal capture?
This antenna design is to receive circularly polarized signals. I don’t think their would be any gain, or advantage to using this design for ads-b. The size difference would be dramatically smaller as well.
For ADS-B optimization you actually want antennas that look further out towards the horizon, instead of skywards. If a plane is above you, it’s going to be fairly close by with a strong signal which any antenna can receive. When it’s in the distance it’s going to far away and more towards the horizon.
The benefit of a double cross dipole is not only are they circularly polarized, but they are great on low horizon as well. I built this particular DCDP antenna a few years ago, and got great results from it starting at 9 deg elevation. There are plenty of links online for beer can ADS-B antenna’s they are just a piece of wire stuck to a beer can with an SO-239 connector or the connector of your choice. They work VERY well actually. I’ve received signals from more than 600 miles away from airplanes. The higher the antenna, the better.
Regards,
Johnathan
N5WTA
I once started to develop a “L Band” DCA, but found it more intricate than *I* was willing to work with. But, if there was/is a need for building a 1GHZ DCA, I am sure it will provide excellent coverage.
If you ever want to build a 1.09 GHZ DCA, I am available for any assistance I can provide
I read another article with similar design that 75 ohm coax should be used. Anyone try this build with a 75 ohm instead of 50 ohm with good results?
I’ve done several tests with that antenna, here I leave my experience, the first time was a horrible performance I built with 50 ohm cable. there is a fact that little is named and that before building and that should verify is the number of ohms that the receiving device works, be it a LNA or SDR, handy, station base etc. in my case my SDR receiver is 75 ohms and I made the antenna with a 50 ohm cable, as the manual says they will realize that the SDR was practically hunting flies with horrible 90% signal losses, I thought between me, a woki toki toy is more senbile than this. my advice for those who build this antenna is to confirm how many ohms the receiving device works before starting to build, I changed the one of my cable antenna to 75 ohms and a little disillusioned by the first experience, I had already tried to change the connectors, cut a bit the cable move the positioning antenna and nothing was no improvement, I started the second test with that antenna, and a little disappointed by the results of the first experience, grab it and pull it on the roof without mast I already angry because it did not work, and the result of the second test with the cable of 75 omhs was spectacular, the losses were much reduced, I was able to capture the noaa18 without needing to raise the antenna to the mast, now that it works, I will paint it and I will add it to your mastil as it corresponds .. hopefully my experience will serve others, best regards 73 .. from Santiago
From Chile
Where did you find information about SDR regarding if it is 50 ohms or 75 ohms?!
Mine is a cheap chinese DVB-T+FM+DAB receiver
Best regards from Mexico
You will need to look up the information on your particular equipment, Contacting the manufacturer would be my only suggestion. It seems that Daniel Had luck with 75 ohms and I had luck with 50. So there are definitely differences between the manufacturers of our respective pieces of equipment.
I wish you luck!
Johnathan
Could this design be adapted for the 1676 MHz freq range to monitor weather balloons from the National Weather Service?
/agree jim. You used a slightly more scientific method of explaining what I was talking about in my post.
I built this antenna, it works like a champ. I used a little bit of a different method using 3d printed parts, but I used the same specs and I get great signal. I think what a lot of people are having problem with is that they aren’t using 50 ohm cable. Using 75 ohm Coax (TV Cable) Won’t give you a great signal.
jhonattan, thank you very much for the data, people do not notice how many ohms their equipment works .. thanks your information can correct my antenna .. 73rd records of Santiago de Chile
No problem. I am glad you were able to get a readable signal.
It’s interesting to see 3 negative reports so soon! Maybe they have made mistakes in it’s construction or use?
Hi merg_flerg!
What does ‘better than old qfh’ mean? Has it a lower elevation performance? Or do you mean stronger signal power? Let us know :).
Hi.
And uhm.Intetesting.I build one for 70cm.It didn’t work :/
I also built 1 and the performance was horrid. Too bad because I spent a lot of time on it =/
Yeah. Even with LNA I couldn’t get anything worthy out of it :/
Might be issues with phasing cables. The length are for the specific radio shack cable used. Other cable might have different velocity factor and you would need to adjust the length accordingly.
/agree jim. You used a slightly more scientific method of explaining what I was talking about in my post.