I’ve been experimenting with fractal antennas for my Lantern for several months without success until today , so haven’t seen fit to report my failures in the Discussion Group.
Instead of building an L-band fractal, I decided to build an over the air (OTA) HD plane old TV (POT) fractal since they operate into the high UHF band near our beloved L-band.
From a design by William Ruckman (Ruckman.net) I built a working antenna using 18 gauge wire:
Front side (4 inches by 8 inches overall):
My legs are 5/8 inch while Bill’s are 1 inch. My mistake - I’ll have to redo it
Reverse side with glued aluminum foil:
I ran it up in my antenna test bed:
And compared its reception to my Alpha Lantern sitting about 5 feet away with these results seeing 3 dB less SNR (or gain I think compared to the PATCH) than the 4 dB gain Outernet PATH antenna:
Left side IP address 192.168.0.30 is the Alpha Lantern running with a PATCH antenna, and right side IP address 192.168.0.28 is my test bed running with the Ruckman OTA/HD fractal antenna.
Now I hope I have gotten something started here. My antenna design lacks allot right now, for example, it’s a 300 ohm antenna connected to a 50 ohm feed. I didn’t build an antenna matcher yet, but that’s to come. I’m concerned that the matcher will add too much attenuation, so am going to look at redesigning the Ruckman antenna as a 50 ohm feed, and fixing the leg length.
So folks, let’s get started with Fractal Madness. Ken
Thanks for the encouragement. It takes about 2 hours to fashion the thing from poster board, Reynold’s aluminum foil, wire, and a few nuts and bolts.
I’m thinking of reducing the space between the top and bottom feeds from 1 inch to .5 inches, and of course making the legs longer. The formulas for fractal antennas are mind boggling, so I thought a shot in the dark was easier
That’s very cool. I built one for HDTV a few years ago, saw that it worked, so built one for 70cm ham band. It worked too.
As an aside, for experimentation purposes, I wouldn’t worry too much about getting the impedance down to 50 ohms. This is why 75 ohm coax came into common use. Much easier to match to a 300 ohm feedpoint.
Interesting to note that most if not all of these RTL dongle tuners actually are 75 ohms, not 50 ohms. Remember the provenance, These were originally designed for TV reception using 75 ohm coax. These originally featured F or PAL connectors, until the ADS-B community started howling for 50 ohm connectors to hook up to commercial collinear antennas and ground planes. Then the RTL-SDR crowd chimed in with the same. So, the manufacturer swapped connectors and we have a factory built-in mismatch now
Happily, for receive purposes, the less than 2dB loss and bump in the match going across there isn’t really critical.
My objective is to get the antenna down in size and thickness, that I can strap it onto the old Lighthouse Ku-band terminal (many of us have them), and , , , , when Outernet develops an rxOS for the Lighthouse , , , , package them together with a power pack and then go. Ken
For those of you who want a weather-proof backer board of some type I work for a printer and we use 2 products:
Sintra: Solid plastic material in various thicknesses. Larger pieces over 12x12" will need some type of frame or support. We use it for signs.
Corplast (maybe Coreplast): It looks like corrugated cardboard but is all plastic. Often used for yard signs. Probably comes in various thicknesses also.
For indoor use there is foamcore, which is foam with cardboard on either side. Various thicknesses from 1/4" to 2".
You won’t find any of these at your retail or hardware or office store. You’ll have to find it online.
That fractal looks like a Sirpinksky triangle. What if you got someone to print this design with many iterations, on a PCB, of the same size as your picture? How would that work?
What if you made this design on a PCB yourself as a test? Fractal antennas are supposed to be pretty good.
Have you tried the recommended reflector spacing for this type of antenna? I would think it is on the order of maybe at least 30mm or so. Haven’t seen any diagrams or specifications for it in that configuration.
Not yet - - I have 2 Koch Curve antennas right now:
The quick and dirty I tested yesterday with the 1.25 mm space, and
A new one more carefully constructed with sharp points between the sides that I have not tested yet as our weather here is overcast with rain. My patch SNRs are down to about 4 which I think is too low for a viable comparison. It too has the 1.25mm spacing.
So you think 30 mm is a good number? What kind of spacing do you think I should use? Air gap, or more 1.25 mm poster board?
After the next round of tests I’ll try it. Thanks, Ken
There are several type fractal patterns I have looked at:
The easiest for me to build is a Koch Curve with wire as others have fashioned them as high definition TV antennas here in the US. The ultimate solution eludes me because I can’t make them small enough and effective. That requires more equipment than I have in my workshop.
But I’m hoping to start people thinking about fractal antennas as a viable alternative to PATCH and helix antennas.
Outernet’s ultimate Lantern goal must not be driven by antenna size. Ken
As for the spacing on your fractal reflector, I have yet to find data for L-Band open air fractals or even a guideline for the proper fractal antenna reflector spacing at a given frequency. However, the HDTV antenna DIY builds that include a reflector seem to settle on 5.5" for an antenna formed to resonate in the center of the desired bandwidth, at about 525MHz.
So, just unscientifically extrapolating from there, we are at roughly 3 times that frequency, so 1/3 the wavelength. About 1/3 of 5.5" is roughly 1.8", about 45mm. I would start there by testing the fractal array against the reflector at different separations.
Another dumb question @Syed I’m sure you have answered before.
If SMARTPHONEs with GPS capability can received GPS and GLONASS frequencies at 1.575 - 1.605 GHz with their built-in fractal antennas detecting signal levels on the order of about -145 dBm, why can’t they be used to receive Outernet signals of the same approximate frequency and higher signal levels? Ken
Not a dumb question at all. We looked at this extensively in the early days. One of the issues is that those receiver filter everything out except for GPS/GLONASS. But let’s assume that we can get lucky and eek out a bit of signal from the front end. What happens next is that we’re dealing with a highly integrated piece of silicon that only knows how to do one specific thing–despread/demodulate/decode GPS. We haven’t found a modern GPS radio that was capable of being repurposed as an SDR–with the proper Linux support.
This might change next year (sadly, I can’t comment), but as of now, the RTL2832U and R820T2 is the best bang for the buck that we have in the SDR world.
and as you stated it is filtered all to heck and back. Extremely narrow notch filters for L1-L2 and blocks everything else. you can scrape the filter out but you are left with a mediocre antenna that is marginal at best for our uses. It can work in a pinch for sure, but I would not use it as my primary antenna if I had another choice. Been there done that just to see how it works. It “works” but is not worth the effort imho.
, so haven’t seen fit to report my failures in the Discussion Group.
Ken