{"id":6091,"date":"2013-12-15T18:32:00","date_gmt":"2013-12-15T07:32:00","guid":{"rendered":"https:\/\/www.homebrewradio.us\/blog\/2013\/12\/15\/20-40-bitx-build-update-1-vfo-success\/"},"modified":"2025-07-21T20:20:04","modified_gmt":"2025-07-21T10:20:04","slug":"20-40-bitx-build-update-1-vfo-success","status":"publish","type":"post","link":"https:\/\/www.homebrewradio.us\/blog\/2013\/12\/15\/20-40-bitx-build-update-1-vfo-success\/","title":{"rendered":"20\/40 BITX Build Update #1: VFO Success"},"content":{"rendered":"
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I’ve started construction on my second BITX transceiver, this one for 20 and 40 meters. As with the previous rig, I decided to take the VFO first, but this time I pledged not to surrender, not to wimp out with a VXO or (worse!) a DDS. No, this time it would be a real LC VFO. <\/strong>
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I took seriously all the admonitions in the tech literature about the fickle permeability of ferrite and iron powder, so this time I used an old-fashioned air-core coil. It is wound around a cardboard tube. The tube was previously the bottom portion of a coat-hanger from the dry cleaner. I wound 40 or so turns on this core, then measured the inductance: 5.6 uH — that looked about right.<\/strong>
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For the oscillator stage I used a Colpitts circuit very similar to the one in the original BITX schematic (but I am hoping I won’t need the varactor diode fine tune mechanism). I had on hand a nice Heathkit 19-146 pf variable cap with an internal 4:1 reduction drive. Not wanting to pluck rotor places out of this beautiful part, I had to calculate the series capacitance that would yield a frequency spread of about 175 kHz. It turned out to be 40 pf. Then I had to figure out how much capacitance to put in parallel with the variable. Well, it all ended up like this:<\/strong> <\/p>\n

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There was a bit of trial and error in the process of getting the VFO to cover the desired range. A big help in all this was an on-line reactance calculator. I found this one to be very useful: <\/strong><\/div>\n

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http:\/\/www.1728.org\/resfreq.htm<\/strong><\/a>
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I found that the VFO is more stable if I reduce the voltage from 12 volts down to about 8. Also, I found that when evaluating the stability, it is better (psychologically!) just to use a stable superhet receiver instead of the frequency counter. The counter may appear to be jumping around a lot, especially if the signal you are monitoring is not very strong. Just listening to it at zero beat on the superhet is very reassuring. <\/strong>
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I followed the Colpitts oscillator with the FET buffer and two stage BJT amplifiers from page 50 of Doug DeMaw’s QRP Notebook (page 50). I now have the requisite 7 dbm signal. And it appears to be quite stable.<\/strong> <\/p>\n

I plan using this with an 11 MHz IF, with the VFO running at around 3.875–3.700 for 40 meters and <\/strong>3.175 — 3.355 for 20.<\/strong> I plan to use a small relay to switch in some additional capacitance to move the VFO down to the range for 20 meters. <\/strong><\/p>\n

Our book: “SolderSmoke — Global Adventures in Wireless Electronics” http:\/\/soldersmoke.com\/book.htm<\/a> Our coffee mugs, T-Shirts, bumper stickers: http:\/\/www.cafepress.com\/SolderSmoke<\/a> Our Book Store: http:\/\/astore.amazon.com\/contracross-20<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

I’ve started construction on my second BITX transceiver, this one for 20 and 40 meters. As with the previous rig, I decided to take the VFO first, but this time I pledged not to surrender, not to wimp out with a VXO or (worse!) a DDS. No, this time it would be a real LC … Continue reading “20\/40 BITX Build Update #1: VFO Success”<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":6092,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-6091","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-uncategorised"],"_links":{"self":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/6091","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/comments?post=6091"}],"version-history":[{"count":1,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/6091\/revisions"}],"predecessor-version":[{"id":6094,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/posts\/6091\/revisions\/6094"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/media\/6092"}],"wp:attachment":[{"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/media?parent=6091"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/categories?post=6091"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.homebrewradio.us\/blog\/wp-json\/wp\/v2\/tags?post=6091"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}