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How to compute the required length and turns for a air cored antenna

obones — Sat, 20 Mar 2021 14:13:43 GMT

Hello,

While experimenting with various designs for the 433MHz antenna to be used with my RFM69 receiver, I stumbled across step by step instructions created by Ben Schueler in 2013 and republished a large number of times since then, such as here: https://arduinodiy.wordpress.com/2015/07/25/coil-loaded-433-mhz-antenna/

It works quite well for my application, but now I would like to dabble into the 868MHz range and I was wondering how I can calculate the appropriate dimensions/number of turns to replicate the above design, but tuned for 868MHz reception.

Would you have the link to a reference document so that I can compute it myself? Or should I just go ahead and divide every value by two as 868 is roughly 433 multiplied by two?

Thanks for any pointers.

RE: How to compute the required length and turns for a air cored antenna

obones — Tue, 30 Mar 2021 15:12:42 GMT

Thanks for your answer, yes you are right that for a hobby project, it's not worth the effort and investment into a VNA. But I find it a bit sad that "trial and error" is the only viable option for hobbyist.

RE: How to compute the required length and turns for a air cored antenna

kmikemoo — Sun, 28 Mar 2021 17:12:51 GMT

obones Oh! Antenna building! I'm interested. First things first, shabaz is super smart and really knows his stuff, so I'd encourage listening to his advice.

As a hobby antenna builder... yes, you can try scaling your 868MHz antenna down to see if it works. If you search the Internet, you can find LoRa antennas that look something like what I believe you are trying to make.

https://smile.amazon.com/G-NiceRF-SW915-TH12-Copper-Antenna-helical/dp/B07DPN47VJ/ref=sr_1_35?_encoding=UTF8&dchild=1&ke… is an example. That one is a continuously loaded spiral wound antenna. It has a low takeoff angle perpendicular to the orientation of the antenna and has an omnidirectional pattern.

I second Shabaz's VNA recommendation. The Nano VNA is a great tool for the radio hobbyist. This way, you can tune your antenna to the correct frequency - given the coil spacing you end up with and the length of the radiator you choose to have. Center or end loading is supposed to be more efficient. More efficient on a compromise antenna... anyway. You can see why you're not finding a calculator. SO MANY variables.

Finally, as you search the Internet, you're going to see how inexpensive 868MHz antennas are. Unless your goal is to build it yourself for the sake of building it, it's hard to justify building versus spending. You can buy a few antennas and still spend less than the Nano VNA.

RE: How to compute the required length and turns for a air cored antenna

shabaz — Sun, 28 Mar 2021 14:52:04 GMT

Hi Olivier,

I don't think there is a formula (or at least, not some single universal formula where you can plug in your desired frequency) for such an antenna. There are sets of formulas for the simple antennas (e.g. monopole, dipole, helix) and that's it. Anything else, and you're likely to either need to simulate, or try it out, or both.

The link you have specified is not an antenna calculator, it's just an inductor calculator. By the way, the formula is not exact with real life, although it's close, and in practice you'll have to try it, and tweak it. You can't get away from experimentation at such frequency ranges.

Also, as mentioned, if your board is not designed for that antenna, then there is the matching problem to solve too.

For just a hobby antenna for an off-the-shelf low-cost module, I'm not sure it's worth all that effort/cost.

RE: How to compute the required length and turns for a air cored antenna

obones — Wed, 24 Mar 2021 08:51:45 GMT

Hello,

Thanks, but I already know the value for the basic "whip" 1/4 length monopole antenna.
What I'm looking for here is a way to reproduce the design mentioned above which has a coil at is base, made from turns of the wire, which basically creates an inductor.
Using the following website, I can estimate the inductor value: https://m0ukd.com/calculators/air-cored-inductor-calculator/

But I could not find a way to compute the two lengths of straight wire.

RE: How to compute the required length and turns for a air cored antenna

shabaz — Sun, 21 Mar 2021 22:37:57 GMT

Hi Olivier,

You can just try a length of wire, roughly half, and snip bits off to experiment.

It won't be able to be an optimum value, because each antenna has an impedance. The antenna that you're replacing has an impedance, and the circuit board will have components on it to match the antenna. If you swap out with a different type of antenna then it is now unmatched to the board. Performing the matching requires a VNA and swapping out the matching components on the board with ones determined by using the VNA.

RE: What SDR software do you use?

jomoenginer — Fri, 12 Mar 2021 18:49:40 GMT

The one I see pop up in Job Descriptions and used in certain professional applications is REDHAWK .

https://redhawksdr.org/

Also, you might want to reference the Getting Started Guide to see if they have a suggested SDR software to start with.

https://kb.ettus.com/B200/B210/B200mini/B205mini_Getting_Started_Guides

Looking at the Digilent product page, there is a link to some SDR tutorials from gnuradio,org.

https://reference.digilentinc.com/reference/programmable-logic/b205mini/start

https://wiki.gnuradio.org/index.php/Tutorials

There is one thing to note with the USRP B205mini-i, since they are using the older Xilinx Spartan-6 XC6SLX150 FPGA, it is not supported with the newer design tools from Xilinx like Vivado or Vitis. You have to use Xilinx ISE which technically is not supported on Windows 10 outside of running in a virtual machine. I believe the canned VM that Xilinx provides with the tools already installed has CentOS 6.

RE: What SDR software do you use?

baldengineer — Fri, 12 Mar 2021 18:33:05 GMT

Ahhh okay, maybe I misunderstood what you meant by "assemble an AM/FM receiver." If you want to use an SDR as an AM/FM receiver, then one of the purpose-built software is definitely the way to go. But if you want to build a radio around an SDR, I still think GNUradio is the way to go. Or, more specifically, GNUradio Companion.

Yeah there is still some mix of the GUI widgets and you do need to understand datatypes, but it is a good path to "building an SDR receiver." Like any scientific or engineering software tool, it is complex but not insurmountable.

I've also found for most basic tasks, there are workflows already created like the AM/FM receiver and standard demodulation.

What SDR software do you use?

colporteur — Wed, 10 Mar 2021 01:10:52 GMT

What is your go-to SDR software?

I'm thinking about pitching an application for the Digilent 1x1 USB Software-Defined Radio Platform RoadTest Review. Part of my research is to investigate what software is available for SDR. Here are some SDR application I have used GQRX, CubicSDR and GNURadio. I dabbled in RTL-SDR some two years ago. I recall at that time SDR#(SDRSharp) was popular but I seem to recall it was limited to Windows PC's. Being a Linux user Windows only applications are not something I look at.

I would like to investigate a few SDR applications but avoid the rabbit holes, application you discover after much research they are no longer supported.

What is your software pick for SDR? In your response can you indicate your operating system preference and what frequency band you focus your SDR efforts on? Three responses:

What SDR application?
What operating system?
What frequency band of interest?

RE: What SDR software do you use?

colporteur — Fri, 12 Mar 2021 16:15:34 GMT

My experience with GNURadio is that is not for the faint of heart.

I started exploring it when the software was in the upgrade throws of 3.7 to 3.8. Understanding context was most important. I recall there were components not being supported in the newer version. I seem to recall something about QX & WX widgets I think they were called. One was or had being deprecated.

Two years ago it took some culling on my part to find flowgraphs for working receivers. You needed some RF knowledge to even dabble. I suspect it has changed. Tutorial time it is.

Break...Break...

I did create a list of software from the link you provided. I'm working the data through a spreadsheet to make it more managable.

RE: What SDR software do you use?

colporteur — Fri, 12 Mar 2021 16:07:26 GMT

There website indicates an software upgrade was made in Feb. 202. This demonstrates to me support, which is ideal if you are going to invest in an application. Frustrating to discover there is no one working on issues.

Maybe I need to blow of the dust of blue screen of death and investigate.

RE: What SDR software do you use?

baldengineer — Fri, 12 Mar 2021 03:29:41 GMT

colporteur wrote:

I think being able to assemble a simple AM/FM receiver using a SDR dongle is an ideal starting point for exploring RF.

Frankly, I think that answers your question. You want to use GNURadio. It allows you to "build" a digital radio with the SDR.

All (okay most) of the other software already has the pre-processing blocks designed into their algorithm. You can fiddle with some of the controls, but you're limited to their algorithm.

GNURadio, on the other hand, lets you design the blocks of the radio and experiment directly. (and then perhaps extend in a way that is interesting for experimentation.)

RE: What SDR software do you use?

kmikemoo — Thu, 11 Mar 2021 04:12:33 GMT

colporteur The only one I've used is SDR# - but it was on a Windows machine. The frequencies were Amateur Radio frequencies - primarily 50MHz and 7MHz.

With the Pi4 being the powerhouse that it is, I have plans to try my RTL-SDR in it - but that project hasn't made it high enough on the order of merit list to be worked on. In that application, I plan to use it for radio direction finding.

RE: What SDR software do you use?

colporteur — Wed, 10 Mar 2021 22:28:45 GMT

Really appreciate the replies folks.

I get weekly updates from https://www.rtl-sdr.com/ . I don't know why I never thought to scrape their site. 2014 is a long way back JW baldengineer . Thanks for the scrapings.

A few comments on GNURadio from members. I understand there are some SDR software application have GNURadio as their backend. I think being able to assemble a simple AM/FM receiver using a SDR dongle is an ideal starting point for exploring RF. Commercial FM radio stations still seem to be propagating radio waves. Aircraft to air traffic control communication is AM.

I recognize some of the application with a sprinkling of new ones I'm going to investigate. The posts got no hits on SDR# (SDR-sharp) looks like it is still maintained https://airspy.com/download/ .

Hopefully some more posts will filter in. Again thanks for your responses.

RE: What SDR software do you use?

gdstew — Wed, 10 Mar 2021 17:24:35 GMT

Hi,

I just recently got interested in SDR and investigated programs that were available for Linux. Here are a few that I haven't tried but may interest you:

Skywave Linux - this is a Linux distribution specifically made for SDR

CubicSDR - is a open source cross platform SDR application

Gqrx SDR - open source SDR receiver that uses GNU radio

You might also want to check out passion-radio.com which has lists of Linux, Windows, and Mac SDR software as well as other interesting information.

RE: What SDR software do you use?

rsc — Wed, 10 Mar 2021 14:16:29 GMT

The software I've been using is called CubicSDR for Win32

https://www.nooelec.com/store/downloads/dl/file/id/57/product/265/cubicsdr_v0_2_5_win32.exe

>> What frequency band of interest? - All of it.

Scott

RE: What SDR software do you use?

14rhb — Wed, 10 Mar 2021 08:38:15 GMT

Hi colporteur

It would be GNU Radio for me. Once the driver is setup for your device, which from experience can be a bit hit and miss, you'll be able to see the SDR as a block in the GUI. Everything else is drag/drop and stitch together to create your bespoke RF system. There are even blocks for file read and write - perhaps saving something you wish to Tx like a audio track. GNU Radio only really works on a Linux OS - but you seem to have that covered . There are some awesome output modules available like frequency plot, oscilloscope, waterfall and constellation plots. When you exhaust the pre-built modules and your skills are even higher you can always create your own by diving deeper and scripting one using Python.

Actually you can download and play around with GNU Radio without actually having an SDR dongle attached (again read and display some files, generate some sinewaves etc), so I suggest installing it and seeing what you think and also apply for the roadtest. If you do, good luck

RE: What SDR software do you use?

baldengineer — Wed, 10 Mar 2021 07:25:26 GMT

The larger and first question needs to be: what are you doing with the SDR? That question defines what software options to consider. For example, if you're primarily interested in listening to broadcast radio, you won't be considering GNURadio (despite the name.) If you want a spectrum analyzer then you might choose something else.

With that said, I generally use GNUradio with my SDRs, but that's because I'm usually exploring some RF concept or playing with modulation ideas. Something that GNUradio excels at doing.

For day-to-day playing around, I mostly use SDR Console on Windows. (I believe it is Windows only.)

This list on the RTL blog is a great starting point for looking at the options.

What would be really nice is if someone put together a chart. Maybe software on the Y-axis, Application Use on the X-axis, and OS markers in the middle. (I bet that would look *great* in a road test ...)

Question on Harmonics and Filters

kmikemoo — Mon, 27 Jan 2020 16:31:04 GMT

A discussion going on about filters at Episode 355: Raspberry Pi Pirate Radio has me wondering. I have always thought about harmonics (and the need to filter them out) as things that happen above the base frequency; 3rd, 5th, 9th, etc. Is there such a thing as sub-harmonics?

If my primary issue is harmonics (bit banged Raspberry Pi RF output), why go for a bandpass filter over a lowpass filter? Thanks in advance.

RE: Question on Harmonics and Filters

kmikemoo — Mon, 27 Jan 2020 22:58:44 GMT

fmilburn Thanks for the link. It was extremely helpful. It gives me hope that it will work on slightly higher frequencies - like 146MHz. We'll see... once the new RPi3 comes in. I managed to kill mine over the weekend. Not sure how but all I get is the lightning bolt of death.

RE: Question on Harmonics and Filters

shabaz — Mon, 27 Jan 2020 21:57:43 GMT

Ahh interesting! In that case Mike your're certainly right, a low-pass is all that's needed. Sorry, I should have looked into it before I assumed wrong that it was being generated at some lower freq : (

Hehe that's really interesting Frank that you used to walk past IC. I was there doing EE, and Workshopshed studied there too!

EDIT: I remember now.. seeing the bandpass attempt being mentioned on the video, I'd wrongly assumed that was required (i.e. that the signal was being generated at a lower-freq square-wave).

RE: Question on Harmonics and Filters

fmilburn — Mon, 27 Jan 2020 21:50:04 GMT

Good question Mike. I had the same thought to be honest but did not pursue it. I found this post from the Imperial College Robotics Society (unrelated fact: I used to walk by Imperial College on my way to work when I lived in London) which developed the method: Turning the Raspberry Pi Into an FM Transmitter - Imperial College Robotics Society . They have since made it stereo! From their posting...

it sets the clock generator module to enabled and sets it to output on GPIO4 (no other accessible pins can be used). It also sets the frequency to 100.0Mhz (provided from PLLD@500Mhz, divided by 5), which provides a carrier. At this point, radios will stop making a "fuzz" noise, and become silent. Modulation is done by adjusting the frequency using the fractional divider between 100.025Mhz and 99.975Mhz, which makes the audio signal.

There might be lower frequency noise from elsewhere picked up and present in the signal as well but I have no real idea.

RE: Question on Harmonics and Filters

shabaz — Mon, 27 Jan 2020 18:25:18 GMT

Hi Mike,

I may not have understood the use-case (I didn't follow it entirely). As I understood, the Pi was generating something at some low-ish frequency (e.g. 10 MHz) and a harmonic of that (i.e. an exact multiple) was what was desired to be listened to on the radio.

In that case, all the harmonics below the desired one need to be removed as well as the ones above it.

The OP however was trying to finely bandpass filter down to a few tens of kHz bandwidth, which was impossible with the topology he had, and I couldn't understand why.

Is my understanding incorrect? From what you mention, it sounds like perhaps the Pi instead generates FM directly at the correct frequency, and therefore the harmonics are just exact multiples above this. That may be the case, I don't know.