Subject: Preselector for SW portables From: bmayr191@fbch.tuwien.ac.at (Bernd Mayr) Date: 1995/09/04 Message-ID: Newsgroups: rec.radio.shortwave [More Headers] Referring to a request by someone I decided to post here the design of the preselector I made for my portable SW receiver (Grundig S 500). **Please choose a fixed space font (Courier, Fixedsys) to display it properly** ** +++ width = 80 characters +++ ** ref.: "Kurzwellen-Booster" by Axel Blechschmidt (DL 6 NCV) in "funk" 8/91 Verlag Technik und Handwerk, Postfach 2274, D-76492 Baden-Baden Germany ------------------------------------------------------------------------------- | LAYOUT FOR PRESELECTOR (principle) | | | | ext ant (optional, not necessary) | | | | | |-----o---o-----------------------------o------<< rcvr telescopic antenna | | I I | | Z-----. I | | Z---. I/ | | coil Z--. <-----o switch ====/==== capacitor | | Z -. I /I tuneable 200-500pF | | Z --. I I | | I I I | | |---------o----------o------------------o------oo rcvr ground | | | | | gnd (e.g. water pipe, optional) | | | | | ------------------------------------------------------------------------------- What you need: -tuneable capacitor, max about 200-500 pF -step switch with at least 5 connections (the author says 1x12) (it is round, you switch by turning). -core for coil (any solid plastic tube diameter 15 mm = 0,6 inch, length abt 60 mm = 2 1/2 inches)) -copper wire (thin, max 1 mm diam., insulated), about 10 metres. I made both the coil and the connections from the same wire. (10 m ~30 ft.). Do not use any coaxial cable. -crocodile clamp to connect wire to telescopic antenna -banana pin -two sockets for banana pins -any plastic housing for the whole thing, Before you mount it into the housing you can first try it out by fixing it on a piece of cardboard (I did so) -soldering equipment for electric connections Assembly The most difficult step is the making of the coil. Fix the wire on one end of the plastic tube and then wind it onto it. Make 5 taps on the coil by letting small "loop" of a few inches which you twist afterwards. The taps should be made at the 4th, 6th, 10th, 18th, 30th turn. The coil should have 50 turns. Then you place all parts as shown above on a board or in a housing. Alle connections are soldered. Be careful to make proof connections to guaran- tee good performance of the preselector. The two banana sockets are placed on the left side in the drawing, they serve for optional ground and/or wire ant. connection. The banana plug is placed where you see the "oo", the clamp is on the "<<". That's all. Operating Connect everything to the receiver. The receiver must have a ground (e.g. on the antenna socket, if there isn't one try other like the DC input socket). Tune in a station on SW and try to find a signal maximun by first switching the step switch. Then you make the fine tuning with the capacitor. If you can't find any maximum at all try to pull/push the coil so that its length changes. You can also remove or add some turns from/to it. Results I tried the preselector with various types of SW portables: my S 500, the Siemens RK 661 (presumably the same as the Sangean ATS 808) and another no name type. In all cases the preselector enhanced reception significantly on the desired frequency. Any 3rd IM products and image frequencies wich had been there DISAPPEARED. The only thing which can happen is that electro- static noise on the same frequency also gets stronger with the preselector. Generally... This article reflects *ONLY* my personal experience. The thing is that I do not know if that recipe always works perfectly, although I succeeded. I just made it according to the particular article. I think that people less experienced in home-brewing should also manage to make it (believe me I'm not really an expert). Maybe you can ask someone you know to help you. Some two years ago the ADDX (Germany) offered such a type of preselector ready for use. The offer was not restricted to members but I do not know whether they still have any for sale. The price was 50 German Marks (abt 35$US). Other solutions... There are various types of commercial preselectors on the market, but they often require a 50 Ohm socket (some radios as the S 500 or the 700 have one) and an outdoor or indoor wire. Some of them are quite expensive. Another interesting possibility are magnetic loop antennas. Requirements are similar to those mentioned above (except for the wire), as well as are prices. I wouldn't recommend active antennas because they can overload especially small portable receivers. They also can amplify noise that comes from electric house- hold goods, fluorescent lamps or TV sets (for this reason they should be placed outdoors). I cannot discard, however, that for some one of these solutions proves better... Good luck! :-)