1.6 GHz ebay QFH antenna "kit". ideas to make it easier to be successful. - Page 1
Forum section: electronics / rf-microwave

author: cdev
author_rank: Super Contributor
timestamp: May 19, 2021, 01:32:53 am
content: So, a while back there was a discussion here about quadrifilar helix GPS antennas and I mentioned that I had seen the Vaisala (aerosonde) QFH antennas and that they were built around a single PCB and would probably be fairly straightforward to copy that design. Well, lo and behold, a few days later a QFH "kit" appeared on ebay which appeared to do just that. The price was a little bit under $9.00 . However I should have realized the word "kit" in this case appears to have meant Absolutely no instructions or pre-forming of the antenna wire. So what it comes with are the PCB which appears to be of decent quality, an edge launch sma connector and a piece of stiff, tinned wire. You have to do the critical winding part, which is likely to be difficult.I had been hoping the wire would arrive pre cut, and formed into the appropriate shape. or come with some kind of instructions. No such luck. What this means is that one must determine the size of the cylinder the wire must be formed around for all the wire, using the calculator which then goves you a file that gives you the ideal cylinder size. (which must be formed into the qfh shape. Note the two loops this createdmust be resonant at exactly the right frequenciesfor it to work, they must be bent in exactly the right manner for this thing to work.I am attaching photos of the Vaisala radiosonde from the web site of jcoppens.com, which are very helpful to getting it right.If one is successful one ends up with a fairly broadband circularly polarized antenna, with of course no LNA. So it should be suitable for multiple GNSS systems, say GPS and Glonass. Also the antenna is a short to DC, while almost all GPS receivers supply a DC voltage to their antennas, expecting to be needed to power a low noise amplifier. One is also going to need a DC block to use it with most GPS receivers. A antenna that's a DC short may even damage some receivers. But you wont get your front end damaged by static since the antenna bleeds off any static by shorting it to ground.I have a commercial "DC block" part, which I intend to use. So, what is the important part? getting the length and shape of the formed wires right. The length is the critical measurement. Here, this should be helpful.http://jcoppens.com/ant/qfh/calc.en.php.Because I want the antenna to work I am going to -before I use the nice, stiff wire provided I am going to do a preliminary build with some stiff copper wire that I have thats approximately the same size. solder it together and test it with the nanoVNA2 that I have. This will be the second L-band QFH that I'm building so I am not expecting any surprises. You should have two loops, one slightly larger than the other. Several ferrite beads can serve as a balun to decouple the feed line. (at the bottom)If you dont have the kit you can use semirigid coax cable instead of the PCB. Make the two loops and since the point where it crosses the center is a null pint for RF, you may want to attach them to the coax to get a more balanced signal. I dont know if that is preferable but the center is at RF ground potential in the kit. Dont forget that this design needs a balun which in this case can just be a split bead (or air coil, coax wound around air common mode choke) A pill bottle, paper towel tube or any other cylinder roughly the right size would make a form suitable for making a nice QFH helix. It sure would be nice if the "kit" came with instruction or at least the link to the calculator. The pattern is quite different depending on the width of the helix.

author: cdev
author_rank: Super Contributor
timestamp: May 19, 2021, 12:03:46 pm
content: I went looking for QFH articles in the scientific literature and indeed there are tons of patents along those lines. And there is no reason why we here cant make them ourselves and get repeatable results. One way that seems amenable to home-makers is to use a paper towel tube, varnish it and wrap it at a precise angle with 5 mm or more copper tape. (this will make it more forgiving of slight errors in length.) Copper tape on cardboard is easy to solder to, plastic will start to melt but if you work fast you should be okay. Semi rigid coax can provide the feedline. and a clip on ferrite can provide the balun.An SMA connector will work to connect the feedline to.A longer QFH will give you one with more gain near the horizon. (if pointed up) a wider one gives you gain straight up. There is also the "eggbeater" antenna which is much like a QFH but it uses two rings and two reflectors like a turnstile, like a turnstile one of the two loops is fed 90 degrees out of phase to give you the CP.(you use a quarter wave stub of a higher impedance coax. A good use for any old Ethernet coax cable. (which has an impedance of around 90 ohms. Ideal for these stubs)Or you can just build a turnstile. Just two dipoles with the quarter wave stub.Or a clover leaf, these are popular with drone/FPV people.I also recently bought some styrofoam cones to see if I can successfully make a conical archimedian or log spiral that works and has gain in the GHz range. Because the small coax has alotof loss as you go up (this is always a problem) I think I am going to try to stick a little broadband LNA at the feedpoint and power it up the cable. That works great with a big archimedian spiral that I have made. The antenna is justmade on a big piece of poster board. Thats the antenna that I have that works the best for 136 MHz APT. I used a poster printing program to print a bunch of letter sized prints, enlargement of an Archimededian spiral made in Inkscape onto a bunch of letter sized sheets with registration marks so the size would be right and I just used black insulated wire for the two arms, taping it to the board with masking tape. It turned out that I had made it just a little bit too small to work well at 136 MHz, but it was easy to add around a meter of additional wire around the outside. Now it picks up 136 MHz well.Thats what I like about frequency independent antennas, they are quite forgiving- if you get the proportions right they are almost guaranteed to work.(reminds me I need to get my laser printer working, its got some problem)

author: cdev
author_rank: Super Contributor
timestamp: May 19, 2021, 12:31:09 pm
content: The polyamide backed copper material might be super useful if its cheap.Quote from: evb149 on May 19, 2021, 11:27:04 am

author: cdev
author_rank: Super Contributor
timestamp: May 19, 2021, 12:41:18 pm
content: Quote from: cdev on May 19, 2021, 12:31:09 pmIf large pieces of it dont cost an arm and a leg this stuff would be great. I wonder how it handles having parts (like baluns, LNAs, etc) soldered to it and then being flexed?

author: hugov
author_rank: Newbie
timestamp: August 19, 2024, 07:05:43 pm
content: @cdev, did you ever figure out working measurements and get the ebay "kit" antenna running?

author: tszaboo
author_rank: Super Contributor
timestamp: August 19, 2024, 09:19:41 pm
content: Quote from: hugov on August 19, 2024, 07:05:43 pmHe's been banned unfortunately from this forum.