Receive Antenna & Impedance "Matching"

Useful information regarding antennas for SDR products.
glovisol
Posts: 640
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: Receive Antenna & Impedance "Matching"

Postby glovisol » Mon Jun 17, 2019 12:31 pm

Hi Phil,

The bandwith of the receiver to which most measurements are referred to is the detection bandwith at a certain FFT and at a certain RBW. Receiver noise figure, receiver noise floor etc. are referred to it. This is covered in detail by a post by Roger in my thread here:

viewtopic.php?f=5&t=3685&hilit=assessing+the+performance

At the time, when I started that thread, I also did not have a clear idea on this subject, did and uploaded wrong measurements on noise floor and then Roger corrected me, to the point I had to re-write part of my thread. It is the operator who determines the detection bandwith and the FFT & RBW parameters by selecting them on the SDRplay console and/or changing the filter parameters, as suitable.

So, just to be on the same wavelength, all the measurements I have been doing in this thread are in LSB, 1800 Hz bandwith. I have now modified the pre-selector links and researched higher QU coils and shall soon post the relevant screens taken on the Spectrum Analyser as well as practical reception tests that I will start as soon as the new pre-selector is ready.

However I have also set up an Excel spreadsheet that, starting from data like Qu and Ql, calculates pre-selector half-bandwith in 50 KHz and 200 KHz ranges, as well as insertion loss. I am uploading the results for you, where you can see what selectivity, with Qu=350, the narrow band pre-selector can provide, with a coupling factor of p=0.1, which is identified by the double circuit insertion loss. Keep in mind, however, that p has only moderate effect on close by selectivity. As you can see, both the single tuned (RED) and the double tuned (BLUE) are too wide to affect bandwith down to 1800 Hz or less.

So, apart from my practical tests, all the cards are on the table now and, as you rightly write, the usefulness of the single tuned, or double tuned pre-selector really only depends on the service you need. For instance adjacent signals at 25 KHz are attenuated 6 dB and these 6 dB on interference could be crucial for receiving that minimal signal for WSPR work. This of course is only up to you to judge, as I do not have any experience in this work. As you can see, I now have all facilities to instantaneously calculate & predict all physical and performance quantities of single & double tuned pre-selectors and those are all at your disposal or of any other Forum dweller.

Kind regards,

Gianfranco
Attachments
Narrowband pre-selector 1.jpg
Narrowband pre-selector 1.jpg (126.31 KiB) Viewed 1009 times
Narrowband pre-selector 2.jpg
Narrowband pre-selector 2.jpg (194.74 KiB) Viewed 1009 times

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vk7jj
Posts: 207
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Location: Tasmania

Re: Receive Antenna & Impedance "Matching"

Postby vk7jj » Tue Jun 18, 2019 7:52 am

Hi Gianfranco,

Yes, there are so many varying needs and varying antennas, looking through these threads, which shows an active and healthy community, which is great!

One of the things you have done that plays (pun intended) so well into the above situation was to demonstrate the sliding coupling. By moving the coils further apart or closer together and retouching the trimmers as desired to narrow or broaden the filter is easy to do.

The tools to make those adjustments are within everyone's grasp, Steve's excellent and free Spectrum Analysis software, or SDRuno - being handily calibrated - works well with an inexpensive white noise generator.

Users are therefore not faced with the binary (haha, analogue) choice of the Phil or Glovisol recipe, they can home brew their own and it sounds like your work in progress may make it easier for them to better understand the possibilities and evaluate their own needs.

And the dreaded SNR discussion! Thanks for the link, very interesting, clearly Roger has an appreciation and depth of knowledge of the subject that I lack, his input would be appreciated.

I've learned so much from all my previous mistakes I think I should make a few more, so here I go:

If we look at your picture (Maximum coupling.png) on the previous page, the power meter reading (ie. the dBm number on the SDRuno waterfall just below the signal strength meter) shows the noise power for your tests conducted with an 1800Hz bandwidth.

My concern is that the figure on the left with the filter does not represent the true noise power calculation because the formula used to derive it assumes incorrectly that the spectrum is flat, as it is in the right hand picture. I wonder what the difference is but I can't read the numbers ... is it the case that is the real difference in value between the two SNRs, ie. is that value a true measure of the effectiveness of the preselector?

Regards,

Phil

PS. Off to RockNRoll again tonight, no doubt you've been there and done that, I'm simply catching up. :-)

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glovisol
Posts: 640
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: Receive Antenna & Impedance "Matching"

Postby glovisol » Tue Jun 18, 2019 1:53 pm

Phil, here below answers to your questions.

SDRplay accurately measures input power in dBm independent of detection bandwith. Any zero bandwith carrier within the receiver passband, by definition, will cause a reading equal to the received level in dBm referred to 50 Ohm.

SNR reading depends most on detection bandwith and very, very little on shape (you can check for yourself). Herewith enclosed pics of the same signal with nearly identical level in dBm, but with a different S/N readng because detection bandwith has been set differently. In SAMPLE 83 you can see the window shape selection menu. SAMPLE 83 and SAMPLE 84 show the different SNRs due to different detection windows.

With our external pre-selector, even with an impossible Qu=1000 and a Ql=400 we would be unable to influence the SNR.
Attachments
SAMPLE 1.png
SAMPLE 1.png (455.71 KiB) Viewed 930 times
SAMPLE 83.png
SAMPLE 83.png (456.06 KiB) Viewed 930 times
SAMPLE 84.png
SAMPLE 84.png (446.52 KiB) Viewed 930 times

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glovisol
Posts: 640
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Location: Piedmont, Italy

Re: Receive Antenna & Impedance "Matching"

Postby glovisol » Tue Jun 18, 2019 1:55 pm

Another example here. Have now made 25 mm dia. coils with Qu=500. More tomorrow.

Have a nice dancing evening,

Gianfranco
Attachments
SAMPLE 93.png
SAMPLE 93.png (480.64 KiB) Viewed 930 times
SAMPLE 94.png
SAMPLE 94.png (484.54 KiB) Viewed 930 times

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vk7jj
Posts: 207
Joined: Thu Mar 02, 2017 9:56 am
Location: Tasmania

Re: Receive Antenna & Impedance "Matching"

Postby vk7jj » Wed Jun 19, 2019 7:51 am

Thank you Gianfranco for everything, I'm frustrated I can't test things for myself with the lack of computer so I've rather pushed you.

I went to buy a new Windows laptop yesterday at the June stocktake sales and ogled a vast smorgasbord of the latest shiny offerings and walked away empty handed. Couldn't bring myself to buy one as every machine had an integrated non-user-replaceable-brand-and-model-dependent battery. Not so bad I suppose if you don't live on a smallish remote island and don't have to face the near certainty of paying to post it off somewhere unreliably distant in three or so years time in hope of having a new one fitted or maybe find an eBay replacement. Looking at the large variation in physical size, shape and brand even that might be a long shot. So I'm rethinking.

One test I've been wanting to do is feed the output of a low power WSPR transmitter into an RSP, decode the transmissions and compare the WSPR SNR reports against the SDRuno SNR readings. Just for fun. WSPR's algorithm is necessarily based on what it can see in the audio passband as the receiving/decoding app is fed only audio from the receiver. It would be good to compare with and without filtering and with different preselectors.

Back to the rest of your post!

Fine that you went for the larger diameter coils, improving the length to diameter ratio. You'll end up revisiting your youth by building something that looks very like a couple of quality crystal sets.

Have you looked at the prices and availability of piston / screw adjustable air trimmer caps on eBay?

Regards,

Phil

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glovisol
Posts: 640
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: Receive Antenna & Impedance "Matching"

Postby glovisol » Wed Jun 19, 2019 3:15 pm

Hi Phil,

Many different subjects.

If I can venture advice regarding PC's, please consider that silicon technology has now come to the point of rapidly diminishing returns and the last PC's with major advances for speed and operability came out in 2015. In my opinion it is nonsense to spend over $ 1,000 for a new unit having the identical performance (I5 5th gen. @ 3 GHz ) of a 2015 used unit at $ 300 or less. I just purchased a Lenovo T540p with 8 GB RAM for that price on ebay and the German supplier sent me a unit in a fantastic packing (looking like just out of the factory) from which I extracted a seemingly brand new laptop with a seemingly brand new battery which started working with SDRplay and RSPduo in five minutes flat. So be advised.

I bought piston air trimmers from a (I think) Russian guy on ebay for 1.5 EUR each approx. They are 0.5 - 12 pF and Qu is in the thousands.
https://www.ebay.co.uk/itm/1-14-pF-LOT- ... SwBnVW974M

I tested various tricks for building high Q coils and can now consistently build inductors with Qu=500 @ 7 MHz. It is my intention to start a separate thread on bandpass pre-selection filters (where these practices will be described) just to have an orderly and organised writeup after this very interesting, but not organized thread.

To come back to our original work, uploaded below are the practical test results with the finally completed double tuned narrow band pre-selector, with a Qu=500 and Ql=200. This unit has a razor like bandpass at +/- 20 KHz and starts cutting significantly at +/-60 KHz already. If a signal is not within the bandpass, you do not even see it. The broadcasters above 7200 KHz are gone. I am posting two pics taken at night and one taken today with a QSO just inside the bandpass. The insertion loss for this narrow bandpass is of course high, in the order of 12 dB, but this seems not to worry you. The nighttime screens show the quiet output of Tuner #1. The daytime one shows the QSO in progress thru the pre-selector.

Yesterday evening I noticed a very strange and unexpected phenomenon: there was heavy QRN due to far away stormy weather and Tuner #2 showed it all right, both in pops in the audio and display base jumping up and down. Tuner #1, receiving tru the pre-selector, exhibited simply white noise, with no pops. I am sorry, but I cannot explain why this is happening and hope someone reading this will be able to explain. Eliminating QRN with front end selectivity? I did not drink beer, I promise...

go to next post.
Attachments
Narrowband preselect. night 1.png
Narrowband preselect. night 1.png (489.13 KiB) Viewed 817 times
Narrowband preselect. night 2.png
Narrowband preselect. night 2.png (490.05 KiB) Viewed 817 times
Narrowband preselect. day 3.png
Narrowband preselect. day 3.png (467.13 KiB) Viewed 817 times

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glovisol
Posts: 640
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: Receive Antenna & Impedance "Matching"

Postby glovisol » Wed Jun 19, 2019 3:37 pm

Another important point is that even with the new coils (20 uH with 36 turns) is is still impossible to use 50 Ohm links for coupling, so I resorted to existing binocular core transformers and therefore insertion loss is over the calculated 12 dB and is around 16 db, but this problem will be eliminated with ad hoc designed & built transformers. In any case my idea is that keeping the selective circuit at 800 Ohm and then use transformers is the way to go. Pse also note that in Tuner # 1 I did not use the HI Z input, but the 50 Ohm input (thanks to the binocular transformers) in order to have a completely balanced comparison with Tuner #2.

I am uploading a pic of the completed hig Q varicouple pre-selector, along with the two High Q coils. The breakthru is to space the turns , but this is not easy. In the first attempt I wound two parallel wires together, then shellacked the winding, cooked the shellack and, when dry, removed one winding leaving the other coil spaced. But my shellack was disappointingly lossy and Qu went out of the window. Next try, I stole thin sewing twine from the wife and wound the coils with twine along with the copper and, finally, I was able to measure Qu>500! When you space, inductance goes down for the same number of turns and this is an advantage for the coupling links. By the way the roast twine is not suitable because its diameter is too large.

I was forgetting a very important point! You MUST wind both coils and links in the same direction. In this way, when you set up the pre-selector, you will be certain of unopposed or opposed coupling flux and use one of the two modes, increasing far selectivity one side or the other.

So it is up to you now, build this contraption and show us real performance with WSPR!!!

Cheers,

Gianfranco
Attachments
DSC_1576.JPG
The two High Q coils.
DSC_1576.JPG (95.99 KiB) Viewed 811 times
DSC_1575.JPG
This is the High Q coil with stolen sewing twine.
DSC_1575.JPG (107.21 KiB) Viewed 811 times
DSC_1577.JPG
The completed narrow band pre-selector
DSC_1577.JPG (94.62 KiB) Viewed 811 times

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glovisol
Posts: 640
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: Receive Antenna & Impedance "Matching"

Postby glovisol » Thu Jun 20, 2019 4:38 am

Uploaded below narrow band pre-selector schematic & data.
Attachments
Antenna double tuned Pre-selector 7 MHz (3).jpg
Antenna double tuned Pre-selector 7 MHz (3).jpg (112.45 KiB) Viewed 740 times
The receiver & the panadaptor.JPG
The receiver & the panadaptor.JPG (126.35 KiB) Viewed 740 times

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vk7jj
Posts: 207
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Location: Tasmania

Re: Receive Antenna & Impedance "Matching"

Postby vk7jj » Thu Jun 20, 2019 8:40 am

Hello again Gianfranco!

Agreed wholeheartedly about spending on computer hardware I can't justify it for a hobby, you were most fortunate with your purchase. I'll muddle through until I find something though I'll have to find another rig from the "not so old" collection to do 30m to free up the Win10 virtual machine so I can actually do some preselector testing instead of standing around looking over your shoulder and being a general pest. I've been reluctant to take it offline because only by running 40m, 30m and 20m simultaneously can I top the WSPR Challenge distance list, it's the RSP-2 doing the job on 30m and you can bet its up against some pretty competitive receivers out there.

Screen Shot 2019-06-20 at 6.45.12 pm.png
Screen Shot 2019-06-20 at 6.45.12 pm.png (59.38 KiB) Viewed 705 times

Actually the bands are bad at the moment, the total count is usually more like > 340 unique with > 4.5million Km

Your experience with the higher Q coils was interesting to read about with two things standing out, one being the lack of pops and noise from the storm in the narrow mode and the other being the ± 20KHz narrow passband along with the really steep skirts, that's a real and pleasant surprise.

And yes it is stealing: asportation without consent and with intent to permanently deprive the owner and I'm probably an accomplice :-)

Congratulations again on your build and especially your presentation, it will be really good to see it in a dedicated thread, it's a fine piece of work and a credit to you, I believe it provides a really effective alternate solution for front-end filtering, anyone should feel confident about having a go and not even a soldering iron is required.

Regards, Phil

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glovisol
Posts: 640
Joined: Thu May 10, 2018 6:42 pm
Location: Piedmont, Italy

Re: Receive Antenna & Impedance "Matching"

Postby glovisol » Thu Jun 20, 2019 9:43 am

Hi Phil,

Just one more note on PC's. If you need, say two or three, the best buy of the world are IBM/LENOVO X61's. At the moment they come at around $ 100 /120 and you can find the docking pads (very useful with many USB ports & the DVD RW) for around $ 15. You stick one RSP to each, they handle W7 or W10 easily and SDRplay works great, provided you use one receiver only with moderate decimation. I have three around...

Thanks for your assistance and keep up the good work!

Gianfranco

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