My first SDR project: receiving system 80m-40m

[Lambda]

Hello.

First of all, thank you for existing! This forum is a pleasure to read. Thanks a lot to both of you for sharing your SDR experience.

I am a simple SWL listener loving DIY spirit and table corner experiments. So after having drifted in the world of regen, DC and classical super heterodyne receiver, I was offered by a friend a little silver guizmo… the famous RTL.SDR dongle V3…

What a trap! .... I put my interest in that and voilà… I am stucked and become passionated by this new (for me) facet of the radio world!!!

So after having done my first trial with the dongle and the associated dipole in VHF, and tried HF in direct sample mode with some cable “add on” the original dipole, I decided to investigate and invest a bit more in a new project.

Integrating the RTL.SDR with a up converter the Ham it up V1.3, in a standalone unit to be settled on the foot of a tunable magnetic loop antenna (two turns) working at least on the 80m and 40m band.
The unit would be detachable and usable with other aerial system.

I acquired also some elements, DC regulators (LM317), piece of junks, a LNA dedicated to VHF….

The LNA was designed and made by a french radio-ham: F1JKY

https://f1jky.pagesperso-orange.fr/bido ... hubert.htm

It works really well and the radio-ham provided good advices and full of informations. Cool and very instructive contact!

F1JKY was in collaboration with SV1AFN Makis which sells also this LNA.

https://www.sv1afn.com/en/products/pga-103-lna.html

The question may be:
Why a LNA dedicated to VHF for a project more oriented to HF bands?

Well:

In the domain of 80m-40m and generally speaking in the HF domain, noise brought by the antenna and the surrounding environment, especially in urban area should be superior to the intrinsec noise of a decent receiver. And if it is the case, no need to use any HF preamp in front of the mixer stage or up.converter in my case.

But if I wish to optimize the sensitivity of my unit, therefore I would have to use a IF gain stage to be inserted between the output of the converter (Ham it up) and the input of the RTL-SDR dongle.

So this is what I did.

Here you will see the general diagram of my little project.
Sorry, for the naive aspect of my drawing...

Projet SDR full cov (bis).jpg (323.08 KiB) Viewed 1352 times

After having done the loop from scratch with its secondary coupling loop, I made my first tries.

I did first of all shortly a test, to a calm frequency, I tuned the loop antenna and watched the spectrum under SDRsharp while connecting and disconnecting the antenna from the input of the converter…

And indeed, the noise transmitted by the antenna was far above the noise floor intirnsec to the unit itself (20-25 dBfs of difference)… Noise floor delivered by the antenna on my urban crowdy location (Den Haag (NL)) is currently around -60/-55 dBfs

So it confirms the non necessity, in my case, of a front end HF preamplifier (In a rural calm areal, it would be perhaps different…I have to try, definitively).

I put all my effort in the selectivity of my system, and it works quite reasonably.
Here some screen captures made this late afternoon, at different gains adjustment.

I never go above 40%-50% of the LNA gain from the RTL-SDR dongle itself.
The LNA does a good job, as a IF gain stage and allows me to work with not bad SNR when lowering the gain of the dongle.

Picture2.jpg (698.88 KiB) Viewed 1346 times

Picture1.jpg (1.68 MiB) Viewed 1346 times

I have to work on the cable by adding more ferrite and check my USB connectics…. Spurious signals appears time to time when the cable move in the USB plug from the laptop.


Here one video for illustrating the selective effect of the loop antenna…:
Between 1 mn 30 sec and 2 mn, roughly, the drift of the pass-band is a bit chaotic, because I had to tune a secondary capacitor in order to shift the range of tunable frequency higher, for reaching the 40m. It works well, just the ergonomy of using has to be improved, for sure….

https://youtu.be/YMw4wdq3RDc

Next step will be to motorize the tuning capacity of the main loop, the coupling capacity of the secondary loop, and the rotation for the whole assembly, in order to “operate” more confortably behind my screen.

Thank you for reading!

Best regards.

Lambda

[Lambda]

Here is photos. of the "things"...

setup1.jpg (487.85 KiB) Viewed 1344 times

setup2.jpg (345.59 KiB) Viewed 1344 times

Again, thank you for reading.
Hoping it can bring some small useful input....

See you,

Lambda

[rtlsdrblog]

Very nice work! A selective loop certainly does wonders when it comes to managing limited dynamic range.

Have you tried the loop with the V3 running in direct sampling mode (without the upconverter)? The selectivity of the loop combined with the better sensitivity of direct sampling mode might work well.

[Lambda]

Thank you RTLsdrblog!

A selective loop certainly does wonders when it comes to managing limited dynamic range.

Absolutely! What i understood when reading about these cheap but amazing dongles, is that often they do not have any kind of true preselecting front end filter. It seems to be one really weak point of the concept, except more advanced products).
But because of the modular aspect and normalized 5O Ohms input, it is quite easy to fix/fill this lack by adding our own filtering system.... It is in fact extremely evolutive through the using of modular blocks that we can design ourselves... I love it...

Sorry to crush open door, for you... i speak about basic and obvious knowledges for you, guys...

The tunable loop is a great preselector..... Possibly, by the way, that i would update it by making it regenerative, with an adjutable feedback level for pushing a bit further the principe....

Have you tried the loop with the V3 running in direct sampling mode (without the upconverter)? The selectivity of the loop combined with the better sensitivity of direct sampling mode might work well.

Indeed, you right.
I tried this configuration: Direct Sampling + loop. It works and the sensitivity is quite good, (using the up converter bring around 10 dB attenuation, by its nature). But the SNR (at least in my case and location) seems to be a bit, sometimes, lowered, with a floor noise sometimes "dense", and a bit more of spurious/parasitic/intermod signals....
But still usable for sure, and interesting because the setup becomes minimalistic, "plug and play" and less energy consuming for sure.

But i prefer using the current unit because:
- the RTL.SDR dongle works in Quadrature sampling and i have fill access to the tuner's gain controls, which combined with the gain (around 25 dB between 128.5-133-5 MHz) provided by the LNA used as an improvised IG gain stage provides a large latitude of adjustment, and especially allow to lower a lot the gain of the internal tuner of the dongle, which helps to increase the SNR.

- This improvised IF gain stage overcome easily the 10 dB conversion loss provided by the up-converter.

- I think that using the RTL-SDR dongle in the range of 128.5-133-5 MHz is quite confortable for the dongle working in its normal range of receivable frequencies. and in my environment, this portion of spectrum is quite calm and less noisy than the normal decametric HF bands. Therefore, probability of introduction of unwanted strong signals directly through the dongle (by accienditally passing through path like DC line, any kind of unexpectable not wanted coupling...) is quite low.

And of course, thanks to the Up-converter and its high frequency LO at 125 MHz, no worry about image frequency rejection, with the passband filter (-3dB passband roughly a bit below 125 to around 190 MHz ? please correct me if i am mistaking)of the converter.

But again, direct sampling + selective loop or any kind of selective antenna stay a great easy option!

Again thank you for your patience for reading.

Cheers,
Lambda