Lets start a project to build an HF preselector? - Page 1
Forum section: electronics / rf-microwave

author: cdev
author_rank: Super Contributor
timestamp: March 23, 2021, 09:54:53 pm
content: I like the feeling of peaking a signal and cutting through the noise.This is kind of what I want to make. The peak is similar to the peak you get with a magnetic loop.I'd like to join in a project with other people to get the benefit of an open source shared design.Here is an older preselectorIt might be possible to incorporate the yaesu FT-101 preselector assembly (which is available used for around $15 from vendors on ebay ) to add selectivity to the preselector.And last there is a high end Harris preselector. Probably used in Cold War spy radios to listen to the Eastern Bloc. The golden era of shortwave listening.This video explores the pros and cons of different active gain devices and illustrates some novel coupling techniques from old radios.This guy is trying to help us out. with his Litz wire loop. Nothing can get rid of the bible thumping stations.

author: coppercone2
author_rank: Super Contributor
timestamp: March 24, 2021, 03:05:12 am
content: I have an old antenna that has a preselector built in from ~300KHz to 30MHz in a few steps. I never reverse engineered the circuit but it looks like it has 3 transformers or inductors in there (I want to say they are transformers), the base is a brass can, the antenna is a shielded loop.Not exactly sure how it does it, but it does work when tested with a function generator transmitter in the near field. I only cleaned up the switches because I was scared something will break. I found the schematic representation in there and if I recall it shows a transformer as the circuit element and when you take it apart its 3 ferrite slugs with some really thin wires coming out of it.http://www.introni.it/pdf/NF105.pdfDoes it allow me to listen to cuba yammering inside of my living room from any location with slightly better fidelity? yes. have I turned on my shortwave since january? noYou might want to consider using ferrites if you can, they are more popular here then air coils, i.e. the lisn thread took off when someone started using 'modern' ferrite materials rather then air cores (no one cares about saturation, its more about how easy it is and compact).if tested with a function generator emitting in a loop of wire closed off it gives a few db of rejection. I do think its an elegant and creative circuit, notice how they used the primaries on the last few taps as series inductors rather then switches.What I would do though is for the top circuit that you listed, if you build it, you might want to invest in a dremel stand so you can grind flats on the springs you can solder to so you can make nice solder joints at some angle rather then having stuff soldered together willy nilly by grinding flats on the coils after they are oriented correctly. I have a feeling few people will build torroids hanging off the sidewhat would be popular I think is if you made 3d print models of the support structures for the coils and such, the sheet metal work scares people off (too complex)A nice upright coil is built like a spring, you orient it and grind a flat on it so it sits nicely upright. That would be worthwhile.

author: A.Z.
author_rank: Super Contributor
timestamp: March 24, 2021, 08:57:56 am
content: Hi "cdev", no time to follow a project like that at the moment, but I hope to be of some help by giving you some ideashttps://ik4auy.xoom.it/pdf%20files/IK4AUY%20QEX%20article%20%203-03%20.pdfthe above paper carries the infos about building a full blown preselector unit for the Ham bands and may be a good starting point for your project, willing to further improve it, you may then consider the idea of adding a "phase" noise canceller, I mean something like thishttps://www.vk5tm.com/homebrew/noisecancel/noisecancel.phpthe canceller should be switchable at will so that if there's no need for it, it will be excludedFurther informations about the preselector can be found herehttps://ik4auy.xoom.it/qex_march_april_03.htmhttps://ik4auy.xoom.it/qex__errata.htm