If you go the other way and increase the capacitance by twisting the wires tighter, the peaks will spread apart and give a large dip between them in the middle of the passband. If you keep decreasing the capacitance, the peak will decrease in magnitude quickly, so your passband attenuation increases significantly. ![]() If the capacitance is less than that, the peaks move closer and closer together until you have one peak. This capacitor also moves the frequencies of the resonances to be closer together or farther apart. The twisted wire capacitor in the middle also has a huge effect on the circuit. This is how I tuned it to be right at 137.5MHz. If I press the coils together, the inductance increases and the passband frequency lowers. If I space the coils, the inductance lowers and the passband frequency increases. Theyre in parallel with capacitors and theres a resonance between each inductor/capacitor pair. The inductors determine the frequency of the passband. Through trial and error while building those, I was able to get an intuitive sense on how those parts affected the whole circuit. In this particular design, theres 3 diy components: the two inductors and the middle capacitor. I was getting around +70dB of attenuation in the FM frequencies, but above the passband the attenuation began to level out at 20dB, which isn't acceptable. I have had people suggest putting my FM bandstop filter on the cable as well for even more attenuation. It doesn't seem like it would be strictly necessary. I'm thinking about trying to shield the enclosure, but I want it to still look nice so I'm not sure how I'd do that. This was even better than 9A4QV's insertion loss, so I'm happy with it. I was able to get the pass-band attenuation down to 0.9dB originally, but when I put it in the case it went to 1dB. When I connect the filter to my NanoVNA, the frequency of the S11 spike tells me the inductance of the coil.Īs you can see from the NanoVNA sweep of the 137MHz band-pass filter, I have fantastic attenuation at 108MHz (FM stations) and 162MHz (NOAA tower broadcasts). I ended up making a mini-bandpass filter to measure the inductance of the coils I was making since my first few freehand attempts were not working at all and the online coil inductance calculators were not even close to accurate. This was very easy to assemble since the copper tape soaks up solder like a sponge. I was on RFtools trying to design a filter with the fewest inductors and ended up getting this same schematic back, so I assume that's what 9A4QV was going for as well. It's a 2nd order Butterworth direct-coupled filter with series capacitor and a direct-coupled inductance value of 68nH. I decided to try the schematic from 9A4QV's blog again. This has the benefits of connecting all the vias (eliminating some stray capacitance, maybe it has some via stitching effects) and provides some shielding on the sides sort of like a coplanar waveguide. Last night I got the idea to use some copper tape to connect the shield and signal lines. It had good characteristics, I was able to get it down to 3.5dB attenuation in the passband, but I brought it with me one day to get a Meteor pass and I couldn't see the signal at all through the filter, so I scrapped it. I posted my last attempt at a 137MHz bandpass filter a few days ago. In contrast, the Planar Disk is almost free and it performs quite well in an indoor setting. Just because an antenna "is sold" does not mean its good or even marginal. Please verify your dongle is working with a good antenna before complaining that you cannot receive something. Here is a great, broadband Planar Disk Antenna that outperforms most commercial antennas of similar size for under $65-75 and can be built in a half hour or less by beginners with less than $10 in parts. See our wiki with tutorials and other resources including a Quick Start page to verify your dongle works. The rtl-sdr project page Source of the rtl-sdr driver software and hub for its continued development. Resources - Please read this first - before posting! Max sample rate: 3.2 MS/s (2.6 MS/s in practice).Frequency range : 24 - >1760 MHz (100 KHz - 24 MHz in direct sampling mode).Typical specs (some depend on specific tuner): All rtl-sdr compatible devices employ the RTL2832U as an ADC and USB controller, but different RF tuners may be used. "rtl-sdr" is a generic term for cheap USB digital TV (DVB-T) receivers that use the Realtek RTL2832U chipset, which can function as general purpose software defined radios (receive only). While originally dedicated just to the rtl-sdr project, relevant legal content related to general SDR, RF and similar projects is also welcomed.įeel free to ask questions but please check the wiki first. Use self / text posts for your immediate inquiries and general posts.Ī subreddit for the low-cost, DIY software defined radio (SDR) community. Link / Image posts are now sent to the Mod Queue before appearing.
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