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04272004 12:58 AM 13 years ago  Post 1 
flyoz rrApprentice Melbourne, Australia  Hi guys someone must be a techo out there who can help?.
But can anyone remember the formula or can they advise the Inductor value (or antenna min Length) for 36Mhz??
Some vague memory rattling about in me grey matter suggests 2X wire length (most likely due to longer wavelength) or better 1/2??? the inductance?? now my brain hurts... doh  

04272004 02:04 AM 13 years ago  Post 2 
Phil Cole rrVeteran Menlo Park CA  If you just want to scale from 72 MHz to 36 MHz, then you'll have to change both the inductor and the wire.Double the length of the wire, due to the wavelength being doubled. This will get you to the same impedance at the feedpoint. To tune the antenna, the loading coil reactance has to have the same value as the reactance of the antenna wire. You have halved the frequency, so you need to double the inductance of the coil to get the same reactance. Don't forget the wire pigtail on the receiver counts in the length of the antenna. So, at 72 MHz you have 6 in. of wire + 4 in. (say) of lead, giving 10 in. total. Double this to 20 in, then subtact your 4 in. lead. So, in this example, your 36 MHz antenna wire needs to be 16 in. This is all pretty rough, since the shorter you make your antenna, the more critical the tuning. The best way is to get a bunch of inductors of different values, from say half to double the calculated value, and try them one at a time until you get the best range. I'd only bother with this if you wanted the best possible antenna, instead of one that's good enough to get the job done. If "good enough" is good enough, try the values calculated from the above, and if you get satisfactory range then you are done.  

04272004 04:50 AM 13 years ago  Post 3 
flyoz rrApprentice Melbourne, Australia  Phil waht can I say? Your a legend! thanks for the fast replyIve been searching the web for hours  

04272004 10:02 AM 13 years ago  Post 4 
flyoz rrApprentice Melbourne, Australia  Hi Phil using your advice + a formula I get Formula for Base loaded antennaH (inductor)=350/ 2Pi x Freq X Tan of 360X AE length / wavelength (AE=antenna lentgth in meters, wavelength = 300000000 speed of light / 36000000 freq) 350/6.2856X36000000 X Tan 21.94646 (20"or 0.508Mtrs/8.33) = 350/91414000 approx (the last 700 changed to 000 doesnt effect result) =0.00000038Henries or in micro Henries (uH)below ***3.8uH base load inductor with a 12" whip and an 8" feeder section from RX for a tuned whip at 36MHz.*** If anyone gets a different result please let me know. By Phils Rule of thumb? advice the 72MHz whip 6" long +1.5uH coil and a 4" leader would yeild a result at 36Mhz of 12" whip, 8" feeder as above but 3uH inductor so I guess 3.8uH is probably the exact result???  

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