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Dave wrote:
"starman" wrote in message Hmmm. A tank circuit between the antenna input and ground, to improve reception? I believe you, but I don't understand how it works. Could you enlighten me a little? Does it resonate at the selected frequency (chosen by adjusting the variable cap) and thereby "select" that signal for the input to the radio? (This is all I can figure.) I could do that real easy. I have several variable caps of that general size, and can pick up whatever size inductor I need. I even have the charts and formulas for calculating the appropriate value of inductance, just have to sit down with them. Thanks for the suggestion. PS: Haven't checked out the links yet, but will do so asap. Dave The parallel tuned (tank) circuit causes all frequencies above and below the tuned one, to be shunted to ground while passing the desired frequency range to the radio's antenna input. It makes up for the lack of good preselection in the receivers front-end. This is the main problem when connecting a good antenna to a portable radio. Be sure to use coils with ferrite cores for good selectivity. Gotcha. That was the only way I could figure it. I saw something almost similar (is that like almost pregnant?) in my Practical Antenna Hanbook (Joe Carr) last night, but suspect it is a printing error. It was an series inductor following behind a capacitor shunt to ground. Only that would (I think) choke off RF and shunt it to ground. (Am I wrong?) Have been trying to figure that one out all day. Last night I fixed my big solder gun, and can now solder a ground wire to the grounding rod outside my bedroom window. Hope to do that tomorrow. I am definetly going to set this up. Have already tested several of my small capacitors, but they do not appear stable enough for anything serious. Just to familiarize myself with the mathematics, I have already calculated the inductors I would need for the first one I tested. Just finished removing a better variable cap from an old junk shortwave radio (a Luke, I think) and am going to try to use it. May have a problem with my meter though. May have to take the variable cap to the shop and have it tested on a known good LCR meter (gotta get one of those. I a currently using a DMM with limited capacitance capability.) If I do that I'll take my calculator and notepad with me, so I can figure out and write down the values of inductance I need. Damn I'm having fun. Thank you very much for this suggestion. It is much appreciated. Dave If you can find a large enough variable capacitor, like 10-600 pf, you can tune the entire shortwave spectrum (3-30 Mhz) with one coil. One way to do this is to connect the individual gangs of a multi ganged capacitor together in parallel. However there is a practical limit to this because the high end (fully opened) range of the cap' will also get larger and this will prevent the circuit from tuning all the way up to 30-Mhz. That's why it's usually easier to use a standard capacitor like 10-365 pf with at least two coils and a switch. It's also best to use coils with an adjustable ferrite core (slug) so you can fine tune the circuit's tracking on the low end of the range. You can buy such coils from suppliers like Amidon and Miller. They come either wound for a particular value of inductance or without any windings, if you want to wind your own. The ferrite core material should be the type made for HF use, such as T-43. I used to get my coil forms from old televisions that used vacuum tubes. These sets had several coils in the I.F. and chroma circuits which worked well for shortwave purposes. I would remove the windings and rewind them for the value I needed in the preselector. Let us know how your project goes. -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 100,000 Newsgroups - 19 Different Servers! =----- |
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