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Re: Receive Antenna & Impedance "Matching"

Posted: Thu Jun 13, 2019 9:42 am
by glovisol
Hi Phil,

your last post crossed with mine. I am sorry about your PC problems. I eventually shelled out $300 and got an used Lenovo T540p that handles SDRplay as a bicycle and I can use both tuners of the RSPduo as a dream setup.

Yes, I did not really believe a pre-selector could be as useful as you claimed until yesterday evening, when I had a ball on 40 m with the double-tuned from late afternoon (no propagation) to full propagation later on, but the results showed you were completely right on the point!


Re: Receive Antenna & Impedance "Matching"

Posted: Fri Jun 14, 2019 1:02 am
by vk7jj
What a great effort Gianfranco, not just your construction and measurements but also your presentation, you do it very well indeed!

Re. PCs, working with really old gear is fun until it isn't. :-)

I'm relieved and really pleased you were able to establish for yourself the worth of the project. Thank you so much.

Preselectors are even more effective for weak signal work, which is sort of intuitive, sorry for pushing WSPR down your throat but it has sufficient participants and the right tools to prove some fundamental truths after having pursued it and analysed the results methodically for the last 12 months:

- it's not only possible but almost guaranteed that 10 or 100 milliwatts of RF on an appropriate band is still at sufficient strength to be heard right round the globe (skip distances are band dependent as we know, so that is a limitation)

- the only two things that prevent it from being heard are 1) noise, 2) a decent antenna.

Noise: if only the wanted signal is present with no noise and no AGC effect, any good receiver can hear it. So we need to get rid of the noise.

Antenna: simple antennas can gather sufficient RF for any reasonable receiver.

What we have to do is help isolate the signal from all the rest of the RF gathered by that good antenna before it gets to the receiver, it's that simple.


Re: Receive Antenna & Impedance "Matching"

Posted: Fri Jun 14, 2019 6:20 am
by Paul
Thank you both for your posts on this topic, which I have been following with interest.
Whilst I lack the knowledge, capabilities and equipment to exploit your findings and conclusions properly, it has been most interesting, informative and the dialogue understandable even for the likes of me.
The graphics and imagery were very helpful to me in understanding at least most of the information.
I am a "listener" only, have a very modest setup - still work in progress and an even "pipsqueakier" (off the shelf) preselector, which I have used to some effect.

Re: Receive Antenna & Impedance "Matching"

Posted: Fri Jun 14, 2019 7:05 am
by glovisol
Hi Phil,

Not every year is a cherry year, but this is. When you have a cherry year, you can pick up as many as you wish, but it is like emptying the desert picking up thimbles of sand. No matter how many friends & chilren you call to pick up the bounty for themselves, still the trees look full, a bonanza for all birds! But I have my store of computer distilled 65° proof grappa (from the grapes of last year) and we fill the glass jars with cherries, grappa and a few spoonfuls of sugar. They will make us happy in wintertime, by the fire.

Ceramic trimmers should be avoided in Hi Q resonator networks: they bring you back the losses you had removed with the high Qu coils. If you cannot find the Hi Qu (& stable) air trimmers, use mica compression: despite they lowly appearance, if they are clean (no dust or dirt, I mean) their Qu is quite good.

Here below pictures of the "eye opener". It is a very simple idea really. You take a piece of 25 mm dia. PVC pipe and you drill 3 holes 1 mm da. on one edge and 2 holes 1 mm dia on the other edge. You glue with cyanoacrilate a 3 way miniature terminal on one side and a 2 way terminal on the other side. Then you wind your coil and the link & you solder the trimmer: here is your sliding bandpass section. Build a second one and slide the two sections on a 20 mm dia PVC pipe used as a guide. The pointed pins of the glued terminals act as brakes against the guide pipe and the sliding action feels "solid" & is really smooth.

Now if you want the "Phil style pre-selector" you pull the two sections far apart and you tune both of them for a very sharp peak. Then you bring them nearer a little, until you reach the insertion loss of your desires.

If you want the "Gianfranco style pre-selector" you place the two sections nearer and stagger tune a little to get a beautiful "critically coupled" bandpass response with a very, very low insertion loss, of course. This is the ideal test setup for all pre-selectors you fancy, just build the BP sections for every frequency you need. Once you have the coupling you like, you can build a fixed one. You can also mount the end links on sliding sections, then you have the patent, all-variable, pre-selector lab.

Poetry today, full data tomorrow.

Re: Receive Antenna & Impedance "Matching"

Posted: Fri Jun 14, 2019 7:07 am
by glovisol
Hi Paul,

It is comforting to know there are guys, out there, who read our ramblings and perhaps get out some useful idea out of them, thanks.


Re: Receive Antenna & Impedance "Matching"

Posted: Fri Jun 14, 2019 7:33 am
by vk7jj
Hi Paul,

glad to know of your interest, I'm sure Gianfranco will comment too.

Winding coils and playing with things as per Gianfranco's screen shots is physically easy once you get started, it's just a matter of making that start

One of the things that used to hold me back in the years BC (Before Computers) was working out how many turns of what thickness wire on what diameter former would resonate on what frequency with what size capacitor.

For anyone else following along who might like to try building a preselector on their own favourite frequency here is the quick and easy calculator I use.

1. Download the Windows application MMANA from here
- the download link is 3/4 way down the page, - (v. - 2.6 MB)

2. It's lightweight and runs on any version of Windows from XP up and it's used to model antennas, BUT
- without doing any modelling at all it has the calculator tool available from the menu as an accessory
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Sorry about the crappy pics I'm not on my home computer. The illegible writing on the bottom pic says "That's it, you've done it, now you can build it."


Last but not least, if you go to the File menu of MMANA you can find and open a whole bunch of interesting antennas that people have built that came with the MMANA installation, open them and have a play!


Re: Receive Antenna & Impedance "Matching"

Posted: Fri Jun 14, 2019 11:56 am
by glovisol
Phil, thank you for the info about the MMANA software, I am going to look it up for antenna modeling. Here below pre-selector comparisons which could be obtained in a few seconds by just shifting the elements. So you now have "Phil style" and Gianfranco's style preselectors.

Seriously now, I am going to demonstrate whether maximum selectivity obtainable (e.g. +/- 20 KHz) is going to make a difference or not in a crowded environment at night time. This is a very important issue: in fact if we could get away with, say, +/-80 KHz, then the 40 m band could be protected with an untuned preselector, which would make things far easier for the operator.

Re: Receive Antenna & Impedance "Matching"

Posted: Fri Jun 14, 2019 11:58 am
by glovisol
Now for Gianfranco's style...In the reference below there is data & formulas for calculating it, but the sliding coil trick makes it easier.... I was forgetting, if you make a third element assembly without link, you quickly end up with a triple tuned preselector. I would not dare to do 4, as perhaps this would be going too far.

REFERENCE: REFERENCE DATA FOR RADIO ENGINEERS, 5th EDITION, ITT - Chapter 9, Filters, simple bandpass design

Re: Receive Antenna & Impedance "Matching"

Posted: Fri Jun 14, 2019 2:52 pm
by sdrom33
Very interesting thread, Vk7jj & glovisol! The screens showing the s/n improvement are impressive and also the idea to compare the two tuners of the RSPduo is very useful. However I noticed that in the varicouple 3 pic the spectrum analyser shows "overload". In fact the pic shows gain and not loss.

Re: Receive Antenna & Impedance "Matching"

Posted: Sat Jun 15, 2019 12:27 am
by vk7jj
Hello again Gianfranco.

Splendid to hear of your cherry year, it stands in stark contrast to our gardens here white with frost with the winter solstice only a handful of days away. Yesterday we celebrated my wife's birthday and being rock and roll addicts danced until early this morning. Not so good for radio though. Or my brain, I'm a bit subdued right now!

Your adjustable coupling is a real winner as it provides a preselector for all use cases.

Strong in-band signals can be a real problem with a good antenna, eg. in the last hour with only local propagation and only within the 200Hz wide WSPR passband we see SNRs of +14dB from 200 milliwatts at a distance greater than 1000K, you can imagine how it is with the signals from some of the American SSB stations with their kilowatts of power.
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In many instances their close in transmitter spurious ie. within 100Hz of the fundamental, show up as duplicate signals only a handful of dB weaker. Sometimes their power supply ripple produces spurious a few dB down (in this case 50Hz full wave rectification) eg:
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My own systems run on 1KW batteries, solar powered with MMP regulators running at high frequency which I've had to deal with as another issue, which makes it easier for me to point the finger at mains originating noise and intermod.

So, as I'm sure you appreciate, each of us with our different antennas and locations is going to see altogether different signals across our bands of interest, making the adjustable coupling a very elegant solution indeed.


One remaining issue is coupling to the radio and antenna, as you're aware I'm doing that using the preselector itself while you're using separate ferrites. Any thoughts?

Regards, Phil