# ARRL

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## homebrew variable coil

Oct 10th 2012, 20:04

#### Roland

Joined: Nov 5th 2011, 19:24
Total Topics: 0
Total Posts: 0
I am trying to make a coil for a portable vertical antenna like the superantenna mp1.
I will be using 1 1/4 inch pvc...and 16 awg bare copper wire..with a slider to change bands....the pvc is threaded so the spacing will be close wound....
what I am trying to find out is how many turns and how long of a pvc pipe will I need to be able to work from 80 thru 10...also how much wire if possible. I dont know how to figure out the inductance of a coil..I know there are formulas but very confusing...to much math for me.
I was thinking about maybe an 18 inch piece of 1 1/4 inch pvc ...
Roland// rleon1@att.net
w5rjl
Oct 11th 2012, 00:05

#### W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
The ARRL Antenna book, back to at least the 14th edition, has a chart and formula for calculating the inductance required to load a dipole. Since a dipole is two verticals back to back, you can use this chart to figure out the inductance required to resonate your antenna on each band.

http://www.66pacific.com/calculators/coil_calc.aspx
Here is an inductance calculator you can use to to determine how many turns you need for a particular inductance value.

Zack Lau W1VT
ARRL Senior Lab Engineer
Oct 11th 2012, 02:37

#### Roland

Joined: Nov 5th 2011, 19:24
Total Topics: 0
Total Posts: 0
Thanks for the calculator Zack..I will try it out..
I do have and arrl antenna book but its a 13th edition second print.
Havent found anything in it like you were talking about...with the dipoles...
Guess I will try and google some more....i have tried to find it on the internet but so for nothing...maybe Im asking the wrong question on google.ha
Anyway thanks again for the info...
Roland w5rjl
Oct 11th 2012, 13:29

#### W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0

I don't have the 13th edition handy, but the 12th edition, end of Chapter 6, multiband antennas, suggests a coil for a 25 ft multiband vertical that might be useful as a starting point for your design.

Zack Lau W1VT
ARRL Senior Lab Engineer