Receive Antenna & Impedance "Matching"

Useful information regarding antennas for SDR products.
glovisol
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Location: Piedmont, Italy

Re: Receive Antenna & Impedance "Matching"

Post by glovisol » Tue Jun 11, 2019 8:44 am

Hi Phil, calculated and verified coil data (which correct the difficult measurement of coil lengths) is as follows:

40 m - 7.10 MHz - Number of turns: 27.0 - Length: 12.1 mm - Qu = 204 - L = 21.6 uH - Loss resistance r=4.7 Ohm.

30 m - 10.1 MHz - Number of turns: 21.3 - Length: 8.70 mm - Qu = 209 - L = 12.7 uH - Loss resistance r =3.85 Ohm.

20 m - 14.1 MHz - Number of turns: 17.0 - Length 7.80 mm - Qu = 222 - L = 10.5 uH - Loss resistance r=4.2 Ohm.

I am now going to build the coils & do the Qu measurement.

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glovisol
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Re: Receive Antenna & Impedance "Matching"

Post by glovisol » Tue Jun 11, 2019 11:03 am

Phil,

I prepared a former with dia. 27 mm by winding cloth tape over my standard 25 mm PVC form. Measurement results are as follows.

7.1 MHz - 27 turns - L = 20.8 uH - Qu = 350*0.8 = 280 (calculated was 204).

10.1 MHz - 21 turns - L = 12.6 uH - Qu = 300 * 0.8 = 240 (calculated was 209).

14.1 MHz - 15 turns - L = 12 uH. Qu measurement cannot be done because the coil requires less tha 12 pF to resonate @ 14.1 MHz and the minimum tuning capacitamce of the Q meter is 12 pF. In any case, in my opinion, for building a pre-selector the value of 12 uH @ 14.1 MHz is too high, because, if the tuned circuit resonates with less than 10 PF in parallel, the coupling capacitance will be too low and difficult to control.

Please also consider that Qu increases significantly by SPACING the turns. This is easily achieved by winding meat roast twine (dia. 0.65 mm) along with the copper wire (an old German trick of WW II). As an example, 21 turns spaced give an inductance of 6 uH and deliver a Qu > 400 @ 10.1 MHz. See the pictures below.

If you are interested in building spaced coils, I can provide the calculations & predicted Qu for all of them.

Best regards,

Gianfranco
Attachments
Qu measure 1.JPG
7.1 MHz close wound coil under test on the Q Meter.
Qu measure 1.JPG (89.28 KiB) Viewed 1739 times
Qu measure 2.JPG
10.1 MHZ space wound coil under test.
Qu measure 2.JPG (86.38 KiB) Viewed 1739 times

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glovisol
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Location: Piedmont, Italy

Re: Receive Antenna & Impedance "Matching"

Post by glovisol » Tue Jun 11, 2019 11:29 am

OK Phil,

Here are the calcs. for "spaced" coils.

For diameter: 27 mm - Wire diameter: 0.4 mm - spacing (pitch): 0.65 mm.

40 m - 7.1 MHz - N=30 turns - L=20.4 uH - C resonance: 24pF - Qu=351 - Predicted Qu=386 - Loss r=2.59 Ohm.

30 m - 10.1 MHz - N=18 turns - L=9.7 uH - C resonance: 25.6 pF - Qu=345 - Predicted Qu=379 - Loss r = 1.8 Ohm.

20 m - 14.1 MHz - N=14 turns - L=6.6 uH - C resonance: 19 pF - Qu=359 - Predicted Qu=395 - Loss r = 1.6 Ohm.

Qu is the calculated value. Predicted Qu is the Qu you would get on the Q meter.

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vk7jj
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Re: Receive Antenna & Impedance "Matching"

Post by vk7jj » Wed Jun 12, 2019 12:59 am

Hi Gianfranco, thank you so much for going to the trouble of doing the 27mm enlargement, that was appreciated. Great to have direct instrumental reading of Q, I wonder that you have such a device!

The loaded Q is something I'm going to have to live with or not on a real time trial basis and the insertion loss is secondary to the sharpness of the rolloff for me but of course there is a threshold where the combination of good antenna gain and RF gain in the RSP fall short so it is a balancing act.

Re. the 20m coil, yes I did notice I was close to the end of travel for the trimmer, I may have less stray C than you because of the close mounting of the trimmer but the L could lose a few turns or better still based on your comments the coil turn spacing should be increased as a worthwhile Q improvement in itself.

The next question I suppose is how the toroidal solution might compare in terms of Q? That's what I first built and its easier for coupling, just add another winding as per my first pic. Seeing you are well versed and in these measurements you are just the man to ask!

We haven't finished pruning and other winter work yet so my soldering and programming time has been cut drastically, please don't feel you should pursue these things at the expense of your other work and interests as I'm sure dragging the chain this end.

Phil

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glovisol
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Re: Receive Antenna & Impedance "Matching"

Post by glovisol » Wed Jun 12, 2019 8:13 am

Hi Phil,

Glad to be of help. Do not worry, I am glad when I am working in the lab.

Iron powder toroids are not suitable for high Qu applications. They provide (fantastic for their size!) Qu's of 250 max. in the low HF and 150 max. in the High HF. If you are really good, you can squeeze a maximum of Qu=170 @ 7 MHz. Toroids are good for building multiple section filters (the "more stuff" approach) with steep shoulders, but with relatively large & flat band-pass, the contrary of what you are looking for. See pic. below. For more data look here, this is the best info, to date.

https://52ebad10ee97eea25d5e-d7d4081925 ... res_TN.pdf

So back to high Qu air core coils.....perhaps what is shown below could be of interest. It is the first double tuned section of the N=4 Chebyshev pre-selector I am developing. If you undercouple, narrowband selectivity will go further up, with more loss, of course. Details in next post as the wife is hollering I must go for shopping....
Attachments
IRON POWDER Q's.jpg
IRON POWDER Q's.jpg (38.91 KiB) Viewed 1675 times
Double Tuned 2.jpg
Double Tuned 2.jpg (196.37 KiB) Viewed 1675 times
Double tuned.JPG
Double tuned.JPG (92.39 KiB) Viewed 1675 times

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glovisol
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Re: Receive Antenna & Impedance "Matching"

Post by glovisol » Wed Jun 12, 2019 6:50 pm

Hi Phil,

Today I have done fantastic progress, just look at these three screens....The light will come to you when you look at the noise ratios. It is the RSPduo reading the same antenna at the same time with the two different tuners. The main tuner thru the double tuned pre-selector, the slave tuner straight thru. Even with the 6/8 dB loss in the pre-selector the S/N is consistently higher in very many photos taken at random, of which these two are just specimens. Not only all man made residual noise coupled from the antenna is gone, but the strong broadcasters are gone as well.

I think I have also solved the problem of the tuneable pre-selector, I hope to your satisfaction, but I need two/three days to uveil this eye opener.

Cheers,

Gianfranco
Attachments
Level Compare.png
This shows antenna input balance in both tuners
Level Compare.png (475.88 KiB) Viewed 1639 times
Screenshot (20).png
Here is S/N, LEFT 17.6 - RIGHT 8.8 dB
Screenshot (20).png (504.55 KiB) Viewed 1639 times
Screenshot (28).png
Here is S/N, LEFT 26.5 - RIGHT 17.5 dB
Screenshot (28).png (484.41 KiB) Viewed 1639 times

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glovisol
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Re: Receive Antenna & Impedance "Matching"

Post by glovisol » Wed Jun 12, 2019 7:18 pm

Typical situation: On left you copy with no trouble, on right you hear NOTHING in the garbage!
Attachments
Typical situation (Large).png
S/N: LEFT 9.6 dB - RIGHT: 0 dB
Typical situation (Large).png (480.88 KiB) Viewed 1636 times

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vk7jj
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Re: Receive Antenna & Impedance "Matching"

Post by vk7jj » Thu Jun 13, 2019 9:00 am

Well done Gianfranco! Isn't that good, I can see where you are heading with the Duo, the Duo is really well suited to the idea you first mentioned you were working on.

You've had quite a few hours now since you got this setup going, I wonder if you've found it as useful as you hoped as the propagation varied?

I'm not happy with my trimmers either, will source something better. Also one of my Linux laptops developed the annoying habit of wiping out it's boot blocks and stuffed two consecutive days of WSPR spots on 40m despite quick fixes so today I wiped it clean and installed Mint instead of Xubutu hoping for a change in behaviour, it's a shame because it had been stable for months before the problems suddenly appeared.

Good luck with your progress towards your eye-opener.

Regards Phil

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glovisol
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Re: Receive Antenna & Impedance "Matching"

Post by glovisol » Thu Jun 13, 2019 9:19 am

RECAP OF PRE-SELECTOR WORK DONE UP TO NOW

Interesting results have been obtained from the pre-selector work, data for the single tuned and the double tuned prototypes are uploaded below. The only unsolved problem regards the insertion loss of the double tuned unit: the insertion loss as measured on the Spectrum Analyser is in the order of 8 dB, while when connected to the RSP the insertion loss appears to be over 15 dB. This difference will be investigated. In any case tests have shown that an antenna pre-selector is extremely useful in crowded frequency bands, surrounded by extremely powerful broadcast signals which tend to de-sensitise the RSP through different mechanisms:

a) Overload the system, as shown by the ADC OVERLOAD ALARM.
b) Compel the operator to reduce RF gain, thus increasing noise generated by the IF amplification.
c) Sometimes cause the generation of second order or third order products which interfere with weak wanted signals.
d) Take control of the AGC attenuating the system gain to the detriment of weaker wanted signals.

The pre-selector not only attenuates unwanted near band extra strong broadcast signals (for example in 40 m from 7.2 MHz up) but also strong interference far away in frequency, thereby helping the already very effective action of the RSP fixed input filters.

The RSPduo is an invaluable instrument in assessing the efficiency of the pre-selectors under examination, because it provides an instantaneous, real time comparison between the protected (by pre-selector) and the unprotected tuner. In fact one can use the main tuner as the "receiver" and the slave tuner as "panadaptor".
Attachments
Antenna single tuned Pre-selector 7 MHz .jpg
Antenna single tuned Pre-selector 7 MHz .jpg (55.38 KiB) Viewed 1585 times
Antenna double tuned Pre-selector 7 MHz.jpg
Antenna double tuned Pre-selector 7 MHz.jpg (94.87 KiB) Viewed 1585 times
Double tuned compare 14.jpg
Single tuned & double tuned frequency responses compared.
Double tuned compare 14.jpg (144.43 KiB) Viewed 1585 times

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glovisol
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Re: Receive Antenna & Impedance "Matching"

Post by glovisol » Thu Jun 13, 2019 9:30 am

MORE FREQUENCY RESPONSES FOR THE DOUBLE TUNED PRE-SELECTOR

Markers are at 25 KHz, when shown.
Attachments
Double tuned 6.jpg
Double tuned pre-selector pass band
Double tuned 6.jpg (189.12 KiB) Viewed 1582 times
Double tuned 7.jpg
Double tuned pre-selector insertion loss & stopband
Double tuned 7.jpg (191.35 KiB) Viewed 1582 times
Double tuned 9.jpg
Double tuned pre-selector ultimate attenuation
Double tuned 9.jpg (189.03 KiB) Viewed 1582 times

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