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Do I have the Math right?

P

Peter Walker

W9WDX Amateur Radio Club Member
Feb 23, 2011
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I am thinking about stretching a multi-band dipole up in my attic and just want to make sure I have the math right. The number I get by dividing 468 by frequency in megahertz is the total length of the antenna? For instance, for a 160 meter antenna at 1.8MHz I would need a wire approximately 22 feet, 11 feet on each side or do I need 22 feet each side?
 

Peter Walker said:
I am thinking about stretching a multi-band dipole up in my attic and just want to make sure I have the math right. The number I get by dividing 468 by frequency in megahertz is the total length of the antenna? For instance, for a 160 meter antenna at 1.8MHz I would need a wire approximately 22 feet, 11 feet on each side or do I need 22 feet each side?
Click to expand...

No, you've got it wrong by miles.

the way to work it out is:

300/1.9 mhz (centre of 160m band in the us) = 157.89m/2 (for halfwave dipole) = 78.94m x .97 (approx velocity factor of wire) = 76.57m/ 2 (for each side of dipole) = 38.28m each side x 3.28 (to convert to feet for you non metric chaps over there in we're gonna kill you all with big macs land,lol) = 125.58 feet or 125' 7" each side.

a good starting point would be 126 1/2 feet to allow for tuning in situation. remember to trim either side of the dipole equally and in small increments as its very easy to cut it short and not so easy to add too once you've scalped it.

i hope that helps.
 
Last edited:
Peter Walker said:
I am thinking about stretching a multi-band dipole up in my attic and just want to make sure I have the math right. The number I get by dividing 468 by frequency in megahertz is the total length of the antenna? For instance, for a 160 meter antenna at 1.8MHz I would need a wire approximately 22 feet, 11 feet on each side or do I need 22 feet each side?
Click to expand...

You have the basic standard formula correct but maybe fat fingered the input.

468/freq in Mhz

468/1.8Mhz = 260 ft total length

260/2 = 130 ft (length of each dipole leg.

Then trim or lengthen the antenna for lowest SWR at whatever center freq you chose to operate on. Better to start a little long and trim than try to add. This works for all bands.
 
RatsoW8 said:
You have the basic standard formula correct but maybe fat fingered the input.

468/freq in Mhz

468/1.8Mhz = 260 ft total length

260/2 = 130 ft (length of each dipole leg.

Then trim or lengthen the antenna for lowest SWR at whatever center freq you chose to operate on. Better to start a little long and trim than try to add. This works for all bands.
Click to expand...

you have it slightly too long because you used the lowest frequency instead of the band centre frequency, but as you say trimming it will bring it too resonance. it will just take a bit more trimming.
 
You've got the right idea, just the wrong decimal place, sort of. For 1.8 Mhz 260 feet total is about right, and for the lowest end of the band. Trimming to the right length for the actual frequency of use is also normal. All of the "magic" number, 468, 234, etc, result in slightly 'too long' lengths. And like was already said, it's better a little too long than a little too short, right? Just fold/wrap that extra length back onto the antenna, or if you'd rather, cut it off. [Wrap the extra back onto the antenna, makes going lower in frequency latter much easier.]
- 'Doc

(If you haven't made that kind of math mistake yet, it's just a matter of time, everybody does.)
 
Just remember: the formula you used (Length = 468/F) is a good approximation of a HALF wavelength at that frequency. A FULL wavelength at 1.8 MHz is about 520 feet.

The "468/F" formula works reasonably well; you're probably not going to be able to install a 160 meter dipole at the correct height (about 260 feet up -- or more), so the length of your actual antenna will be a bit shorter. Just HOW much depends on too many things to worry about right now.
 
I thought the 260 was in inches. 260/12=21.6 rounded up to 22? I thought I read in the ARRL book I studied with for my license it said the answer was in inches. 260 inches.

So what would be a good multiband antenna that will not break the bank and I won't have to build a tower for?
 
No - 468/F (in MHz) = ~1/2 wavelength in FEET.

468/1.8 = 260 FEET. Each side of the dipole would be 130 FEET long.

Multiband antenna? Try a doublet, as long and as high as you can make it, center fed with parallel line (window, ladder or other) through a matching network or "tuner".
 

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