Receive Antenna & Impedance "Matching"

Useful information regarding antennas for SDR products.
glovisol
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Re: Receive Antenna & Impedance "Matching"

Post by glovisol » Mon Apr 29, 2019 7:14 am

Hi Mike,

I could not agree more with your advice on how to cope with LOCAL NOISE. Generally speaking, one must cope with local noise and with atmospheric noise. Then one must cope with small aperture antennas and big aperture antennas. Finally there are magnetic loops, small area amplified active antennas and passive antennas.

Active antennas trade small dimensions with gain and the built in amplifier, placed near the antenna, can fall prey not only to local noise overload, but also to local signal overload. Of course one must cope with the limitations of the location he is in, so the fight against local disturbance is vital and your advice commendable.

But, if you are lucky enough to live in a place where it is possible to use passive, medium to large aperture antennas and with low local noise, then the main problem is atmospheric noise and in this case the use of attenation and attenuators becomes essential. Here the observance and control of system impedances is very important, also for getting as much dynamic range as possible. So, when atmospheric noise is higher, you attenuate more, when it lower (it happens, yes, it does) then you must use any ounce of signal coming out of the antenna terminal.

Cheers,

glovisol

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vk7jj
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Re: Receive Antenna & Impedance "Matching"

Post by vk7jj » Thu Jun 06, 2019 7:29 am

Hi glovisol and others on this thread.

glovisol, I wonder if I could interest you in single frequency preselectors.

Putting aside the point that RF SNR is the ratio of the power of one's tiny little narrow wanted signal v's the power of everything else the front end of your radio is listening to across whatever bandwidth it can hear, preselectors play an overlooked role (IMHO of course) in preventing the AGC of the radio from responding to high power unwanted signals and having the unhappy effect of reducing the gain of that tiny little narrow signal first mentioned.

The above applies particularly to SDRs as their obvious design goal is wide bandwidth, a popular and extreme example being the KiwiSDR. It's good that SDRPlay devices offer a choice that includes a fairly narrow bandwidth with 64x decimation in LIF mode but even then I've found a preselector is essential when chasing every last dB of performance.

My use case is mainly for WSPR using their fixed frequencies so I've been using 8.5-100pF ceramic trimmers readily available from the local parts supplier.

Up to now I've used smallish toroids with three windings as per the rough diagram; 1 x winding to match the antenna/feedline, 1 x tuned adjustable resonant winding and 1 x output winding
toroid sample.png
toroid sample.png (27.46 KiB) Viewed 2757 times
I've been working on a non-toroidal version using tapped coils wound on a 2.7cm PVC tube. I spent some time calculating values and improving Q though not to extremes. I'm planning to cascade two, coupling them via a 'gimmick' twisted wire capacitor of a pF or two at the tops of the two coils.

A single tap (a simple twisted pigtail) situated two turns from the bottom seems to work OK for coupling antenna and radio.

The frequencies and values I have operating now are:

WSPR frequencies

40m = 7.0386
30m = 10.1387
20m = 14.0956

values below selected for a tuned circuit reactance of 1000 ohms

20m => 11.3uH, 11.3pF
30m => 15.7uH, 15.7pF
40m => 22.6uH, 22.6pF

coil diameter = 2.7cm, wire = 0.4mm enamel copper

20m => 17 turns, coil length approx 0.71cm
30m => 21.3 turns, coil length approx 0.87cm
40m => 27 turns, coil length approx 1.11cm

Your thoughts would be muchly appreciated.

Phil VK7JJ

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glovisol
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Re: Receive Antenna & Impedance "Matching"

Post by glovisol » Thu Jun 06, 2019 9:31 am

Hi Phil, I am glad you bring up this subject. With AGC behaviour I completely agree and some time ago started this thread: "HF weak signal reception in presence of noise".

https://www.sdrplay.com/community/viewt ... AGC#p11634

where is is shown that, without external preselectors, the best defence is to keep the AGC level as low as possible and get the desired output by adjusting the volume control: an old timer's trick already used in the fifties.

Of course the "best" solution is to pre-select, increasing the front end selectivity: thanks for having mentioned this (forgive the malice, but another sore point in another of the cancelled posts of Noulluser00). I think you are aware my HP "Q" meter was finally repaired some time ago and furthermore I now have, not only an ample collection of Iron Powder toroid cores, but also the multi-coil techique, as described in the filter threads, so in the next few days I shall try my hand on the preselector business.

As shown in the Antenna Termination Advantage thread I have also obtained a noteworthy improvement in the Beverage noise output and in a few days I shall post comparative results over 15 days.
Attachments
LOW AGC LEVEL.jpg
LOW AGC LEVEL.jpg (42.08 KiB) Viewed 2741 times

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vk7jj
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Re: Receive Antenna & Impedance "Matching"

Post by vk7jj » Fri Jun 07, 2019 12:04 am

Thank you for the reminder "HF weak signal reception in presence of noise" which was an excellent re-read, isn't it annoying how easily one tends to overlook things one has read in the past or can half remember and then not find them again.

Yes, one of the many good things about the SDRplay hardware and SDRuno software combination is the extensive control over AGC and RF and IF gain and an appreciation of the benefits of RF v IF with the RSP hardware. Thank you guys!

I've been using WSPR for quite some time originally to model and then build the best possible antennas within my means for gain and directionality, then focus on feedlines, then focus on antenna and feeline noise minimisation, then shack noise elimination, then focus on different radio hardware, then AGC/IF/RF interactivity, then prefiltering and preselectors, then computer operating system and application software stability for long term 24/7 operation and so on down the chain.

WSPR provides an amazingly good testing ground for all those things and I've learned and come to appreciate heaps of things that I could not otherwise have done.

You've been exploring similar chains down through the various possibilities and enjoying it :-)

I guess we are all little islands of specialist knowledge. Some really good conclusions can be reached can't they.

BTW on the subject in hand - preselectors - I did experiment with using single quartz crystals in series with the antenna and they work really well for WSPR as they give a bandwidth of around 300 to 800 Hz and rejection of up to 70dB. So simple and so effective though they do need a little tweaking with small values of C to correct self resonance problems and get rid of small spurs.

I mention them in the hope that anyone reading this may know of an affordable source for crystals at WSPR frequencies. I managed to pull a 10.14MHz crystal from my junkbox down to WSPR's 10.1387MHz for the testing but the price of having crystals ground is prohibitive for me.

Here's a test with a naked 7MHz crystal from the junk box as an example. Most testing was done with Steve Andrew's great software, thank you Steve! I just happened to be running something else when I took this screen shot. The insertion loss is negligible by the way. Pity we can't achieve the same with L and C.
series wide.png
series wide.png (235.92 KiB) Viewed 2678 times

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glovisol
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Re: Receive Antenna & Impedance "Matching"

Post by glovisol » Sat Jun 08, 2019 7:03 am

Hi Phil, I analysed your single tuned circuit design for 7.0386 Mhz.

Using crystal preselectors is of course a dream, difficult to be realised. Your screen with this filter is fantastic! However, coming back to the "real" world, here we are.

Using a coil former with 27 mm dia. and 0.4 mm dia. enamelled copper wire, you obtain the following results:

1) CLOSED SPACED TURNS (your coil)
Turns N: 28
Coil length: 12 mm
Inductance: 22.6 uH
Resonating capacitance: 22.6 pF
Equivalent series resistance: 4.9 Ohm
Calculated unloaded Qu = 207
Measured unloaded Qu = 235
Self capacitance: 0.86 pf

2) SPACED TURNS
Pitch: 0.6 mm
Turns N: 32
Coil length: 19 mm
Inductance: 22.6 uH
Resonating capacitance: 22.6 pF
Equivalent series resistance: 3 Ohm
Calculated unloaded Qu = 335
Measured unloaded Qu = 360
Self capacitance: 0.83 pf

Here below is the graph for predicting insertion loss, assuming you use a high Q air variable capacitor. Assuming a ratio of 4 for an insertion loss of 3 dB, the 3 dB bandwith (with some margin) you can get from the close wound coil is:
Ql = 235/4 = 59
BW3= 7,0386/59 = 120 KHz; e.g: +/- 60 KHz

If you use the coil with a 0.6 mm pitch:
Ql= 335/4 = 84
BW = 7,0386/84 = 84 KHz; e.g: +/- 42 KHz

If you change the coil former to a smaller diameter, trying to get diameter = length, you might get a Qu = 400, then:
Ql = 400/4 = 100
BW = 7,0386 / 100 = 70 KHz; e.g: +/- 35KHz

Using a double tuned circuit the calculated insertion loss increases by 1.41, but skirt selectivity becomes much better. Finally with a Qu = 400, a ratio of 2 and a 15 dB low noise amplifier to recover the 14 dB loss of the double tuned circuit, you could end up with a selectivity of +/- 17.5 KHz, rivalling the crystal filter. There would be no concern for amplifier overload as it would be protected by the pre-selector, although some loss in sensitivity is to be expected as noise figure loss due to attenuation cannot be fully recovered.

Kind regards,

Gianfranco
Attachments
Qu Vs.Ql ratio.jpg
Qu Vs.Ql ratio.jpg (63.35 KiB) Viewed 2608 times

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glovisol
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Re: Receive Antenna & Impedance "Matching"

Post by glovisol » Sat Jun 08, 2019 7:29 pm

THE SATURDAY AFTERNOON SPECIAL - ASSESSING A HOME MADE PRE-SELECTOR @ 7 MHz

Hi Phil,

This afternoon I quickly prepared a 7 MHz single tuned pre-selector according to the input you provided, just to smell the wind. Pre-selector data as follows:

Former diameter: 20 mm PVC.
Wire: 0.355 mm enamelled copper.
Winding: close wound.
Inductance: 22 uH.
Turns: 39.
Measured Qu: 300.
Input coupling from Beverage: 1.5 turns link (450 Ohm nominal).
Output coupling to RSPduo: 2 turns link ( 1 KOhm nominal).
Resonance capacitor, fixed: 15 pF ceramic.
Resonance capacitor, tuning: 0.5 - 15 pF miniature air trimmer.
Resonating capacitance: 15 + 7 = 22 pF approx.

Unloaded Qu was too low, but I was in a hurry to see how it worked. As was to be expected, insertion loss is a disappointing 12 dB (partially confirming theory, but the Qu of the 15 pF capacitor must not be that high) and tuning is very broad. All this can be seen in the screen uploaded below. The up side is that the preselector can efficiently attenuate the broadcasters above 7,200 KHz. For instance, the level of strongest broadcaster at 7,265 KHz is attenuated from -45 dBm to -80 dbM: this is extremely useful for RSP operation. If you watch closely you see that ADC OVERLOAD is on with no pre-selector!

From this experience I doubt that Qu's better than 300/350 can be obtained at this frequency, so the way forward is not tuneable pre-selectors, but multiple coil fixed tuned filters covering the entire band. At 7 MHz, for instance, I would design a fixed tuned bandpass with steep skirts, passing 7,000 to 7,200 KHz. If you think about it, it is nearly impossible a ham station will overload the RSP or overpower the AGC, while the strong broadcasters would be kept well down. Another advantage is that the inductive links, well away from the main inductor, reduce the parasitic, noise conducting capacitance, to an extremely low value and a ferrite isolation transformer is no longer necessary.

But we are just at the beginning and many ideas come in the night....

Kind regards,

Gianfranco
Attachments
Pre-selector compare.jpg
Pre-selector compare.jpg (131 KiB) Viewed 2564 times
The experimental pre-selector.JPG
The experimental pre-selector.JPG (169.17 KiB) Viewed 2564 times

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glovisol
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Re: Receive Antenna & Impedance "Matching"

Post by glovisol » Sat Jun 08, 2019 8:39 pm

Phil,

I am a fool, I made judgement too early!! Now it is 20:30 GMt (22:30 local time) and the advantage given by this little pipsqueak of pre-selector is amazing! At this hour propagation goes up, the Broadcasters rise to between -40 to - 30 dBm and I must keep the RF gain at position 3 (20 dB gain reduction, because otherwise I have ADC Overload) and of course with this condition S/N suffers, because the IF noise comes up.

With the pre-selector I can keep the gain at position 0 (maximum gain). At present, on the pre-selector peak, I can read signals at S1 to S3 strength with no problem at all. With this difference, who cares for 12 dB insertion loss? Well, we have a big room for improvement!!

Gianfranco
Attachments
Screenshot (11).png
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vk7jj
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Re: Receive Antenna & Impedance "Matching"

Post by vk7jj » Sun Jun 09, 2019 12:11 am

"But we are just at the beginning..."

You've made more than a very good good beginning :-)

Yes, the lightbulb moment for anyone using SDRuno should be when they slide the RF gain up and down with AGC on and off and notice a greater than 35dB change in system gain. If at the same time they are fretting about a 3dB loss in an antenna splitter or gloating that their Wombat Loop antenna has a couple of dB better performance than a Zingbatt Array then they have cognitive dissonance.

If their AGC has the ability to reduce their marginal-in-the-noise wanted signal by 35dB all because their RF front end is lovely and wide and happily drawing a bunch of strong active stations in their waterfall they should want to do something about it. And it's not just local or high power broadcasters that can produce strong signals as it's not unusual for me to see +30dB readings from low power WSPR stations that are a thousand or more kilometres distant.

As an aside re. antenna splitters I wonder if you've seen this one, I'm using the 2 port version at the moment, it shows up really well on the test bench and works really well in practice: http://www.dxing.info/equipment/rolling ... _bryant.dx

Thank you very much for your efforts. My need for dedicated single frequency preselectors is more easily realisable than a tuneable system but the benefits from my own limited work to date has convinced me that very many people would benefit from a tuneable preselector as a way to mitigate the effects of not just strong stations but also noise more generally.

SDRs present a very obvious dichotomy, on the one hand we want them to be as wide as possible to view the entire band of interest and on the other hand as soon as we find something of interest we want the exact reverse, we want to get rid of everything except that little narrow single frequency we are trying to listen to.

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glovisol
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Re: Receive Antenna & Impedance "Matching"

Post by glovisol » Sun Jun 09, 2019 4:33 am

Yes, you could not state the problem better. Today I am going to sweep the prototype preselector on the spectrum analyser, then make plans. With regard to the splitter story, thanks for the reference, going to study it. Perhaps you missed my work on the subject:

https://www.sdrplay.com/community/viewt ... f=5&t=4364

and

https://www.sdrplay.com/community/viewt ... f=5&t=4434

This ideas you brought up deserve a separate thread and I am going to start one.

Have a happy Sunday!

Gianfranco

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glovisol
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Re: Receive Antenna & Impedance "Matching"

Post by glovisol » Sun Jun 09, 2019 7:26 am

TESTING THE SINGLE TUNED PRE-SELECTOR - 1

Spectrum Analyser tests are much better then expected and fully confirm preliminary calculations. Here below test setup and one sweep pic showing 10 dB insertion loss at the resonance peak, while on the receive test I had estimated 12 dB. Full analysis of the test results in the next post.
Attachments
Pre-selector test setup.JPG
Pre-selector test setup. Transformers are needed to scale down to 50 Ohm
Pre-selector test setup.JPG (235.45 KiB) Viewed 2485 times
Notch 3.jpg
Pre-selector sweep @+/- 500 KHz
Notch 3.jpg (195.47 KiB) Viewed 2485 times

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