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Re: Receive Antenna & Impedance "Matching"

Posted: Thu Jun 20, 2019 8:28 pm
by glovisol
With the double tuned pre-selector on Tuner #1 set at 7150 KHz this evening I received JA5CJZ (Japan) on 20:00 GMT in QSO with IK6BGJ on 7,175 KHz. This confirms that providing extra front end selectivity & attenuating the strong broadcasters above 7,200 KHz really pays a big benefit.

Re: Receive Antenna & Impedance "Matching"

Posted: Fri Jun 21, 2019 7:43 am
by vk7jj
Version 2 works well, as you can see with beehive trimmers. They used to be so common, looking on eBay it seems they're made of unobtainium. Yesterday's reference to eBay was in regard to some pistons priced at over $400 each.

Testing version 2 for insertion loss was easy using WSPR. Looking at some local stations with the preselector inline and then bypassing it, repeated several times, showed an average of 6dB. As per the photo the coax pigtails are soldered straight to the loop couplings, keeps it simple.

It's currently inline and live on-air. Be interesting to see what happens to the day's spot count; the data is always a day behind though because it's not available for download until a full day following reception.

Version 2:
preselectorv2.jpg (75.02 KiB) Viewed 1460 times
Version 3 is part constructed, it uses parallel coils vertically mounted, about 40mm apart. Very rough tests showed there may be enough coupling, if not I'll try variations on the gimmick capacitor thing.

Regards, Phil

Re: Receive Antenna & Impedance "Matching"

Posted: Fri Jun 21, 2019 12:12 pm
by glovisol
Very good, Phil, I look forward to see what results yo get by the additional selectivity!

Best regards,


Re: Receive Antenna & Impedance "Matching"

Posted: Fri Jun 21, 2019 3:25 pm
by glovisol

It is true that the larger diameter twine is not good for RF coils, but it can be used for other HI "Q" coils all the same. Have a nice weekend!


Re: Receive Antenna & Impedance "Matching"

Posted: Fri Jun 21, 2019 6:30 pm
by nOtko
Mmmm... looks great)))) and just the right impedance

Re: Receive Antenna & Impedance "Matching"

Posted: Sat Jun 22, 2019 5:48 am
by Paul
Wow! There's something I can relate to...
Buon apetitio!

Re: Receive Antenna & Impedance "Matching"

Posted: Sat Jun 22, 2019 7:52 am
by vk7jj
Wow indeed, you got that right Paul!

Nicely wound Gianfranco, a very clever coil it is ... instead of you feeding it, it feeds you :-)

A very pleasant scene indeed, good on you for posting it. There is no way I can compete!


Here is my version 3, working really nicely. 7.0386 is the 40m WSPR frequency. Next free time will be a 30m effort.
preselector3swr.jpg (182 KiB) Viewed 1368 times
Perhaps Gianfranco, a single stage would be interesting to other users, I've been using a toroidal single stage up till now but ferrites present an availability stumbling block for some people so a simple PVC coil with switch and small variable capacitor and taps for different bands or maybe separate coils might be worth trying. I suspect more selectivity than one single stage can provide is needed on LF bands though, LF seems popular and could do with some help with the broadcast band problem as you are aware. Any thoughts?



Re: Receive Antenna & Impedance "Matching"

Posted: Sat Jun 22, 2019 9:41 am
by glovisol
Hi Paul, it is easier to make real HI Q coils....Will go in the wood oven outside and cook for 5 hours, so it is early morning tomorrow, 5 o'clock.

- Phil, Very ood show! I am glad of having contributed. I note you did not use the sewing twine. Remember you almost double Qu if you space the turns and with a Qu=500 you can decrease the loading and get more selectivity with the same insertion loss.

- In my experience (and also according to theory) you get insufficient Qu with toroids, unless you stay below 2 - 3 MHz. Why use them, expensive & hard to find, when an air core coil is much better and built in minutes? This is true if you aim at BANDPASS filtering. If you use larger bandwith highpass or lowpass polynomial filters it is all another story.

- Multiple tap coils are a compromise and you end up with 5% or more 3 db bandwith. If you require selectivity, you must use the highest Q possible, without compromise.

- I have already done extensive work for cleaning out the LF band below Medium Wave. Look here: LOW PASS FILTER FOR OPTIMUM LOW FREQUENCY RECEPTION. ... f=5&t=3943

...and for cleaning HF above Medium Wave: HIGH PASS FILTER FOR OPTIMUM HF RECEPTION ... f=5&t=3973

These filters are very easy & inexpensive to build and really blot out MW.

Finally, if you wish to protect the 30 m band with a double tuned, just let me know and I will test the highest Qu coils for that frequency. I need the exact frequency (or range) you use.

Have a nice weekend,


Re: Receive Antenna & Impedance "Matching"

Posted: Sun Jun 23, 2019 9:13 am
by vk7jj
It is a nice weekend thank you, trust yours has been too. White frost in the mornings with perfectly still sunny days and the brightest blue sky.

Happy solstice :-)

Your double spacing lesson was not lost Gianfranco, I need to firm on a few other things first. I'd also like to try to do away with PVC formers, there are some interesting techniques that others have used eg.

Your filter work is terrific. The current arrangement hopefully complements that by providing a narrower and reasonably easily adjustable bandwidth on a roll-your-own-frequency basis for users whose listening interests take them out of conventional bands.

My daily WSPR results are in with the preselector, they are fine but within the normal day to day tolerance based on the monthly moving average which tracks the ups and downs of propagation.

What I really need to do is run two identical 40m systems side by side but I don't have enough gear to do that and keep everything else going at the same time at the moment. At least there was no sudden plunge with the 6dB insertion loss!

I did some careful listening using the preselector today on 40m SSB with the IF AGC turned off. Increasing the IF gain manually by +6dB seemed to provide better SNR figures with the preselector in circuit than with it out of circuit and with the IF 6dB lower. The RF gain was on max for all tests.

In other words the RF band noise being reduced by the preselector appeared to be greater than any extra noise generated by higher IF gain. As it happened the band was quiet, hopefully that simple evaluation points to overall improved performance with the current preselector arrangement.

Regards, Phil

Re: Receive Antenna & Impedance "Matching"

Posted: Mon Jun 24, 2019 10:40 am
by glovisol
Hi Phil,

Just recovering from the Solstice party (35 of us) with roast and all....

Your sensitivity & SNR checking & comparing scheme looks very interesting and I look forward to your results. This of noise minimisation is really the most imortant issue (and always has been) in radio communications (not to say all).

Interesting ideas in coil buliding in your reference.