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HelicopterMain Discussion › Help re techo Antenna question :)
04-27-2004 12:58 AM  13 years agoPost 1
flyoz

rrApprentice

Melbourne, Australia

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Hi guys -someone must be a techo out there who can help?.
(Ive forgotton most of my theory)
Re a base loaded antenna: I read a 6" piece of Piano wire with a 1.5uHenry (microHenry) inductor is right for 72Mhz.

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

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04-27-2004 02:04 AM  13 years agoPost 2
Phil Cole

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Menlo Park CA

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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.

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04-27-2004 04:50 AM  13 years agoPost 3
flyoz

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Melbourne, Australia

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Phil waht can I say?

Your a legend! thanks for the fast reply
Ive been searching the web for hours

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04-27-2004 10:02 AM  13 years agoPost 4
flyoz

rrApprentice

Melbourne, Australia

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Hi Phil using your advice + a formula I get

Formula for Base loaded antenna
H (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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