Diapole Antenna length

[n4dog]

I need to have the correct length for a Wire diapole antenna to cover 75 meters around 3950, 40 meters around 7250, and 20 meters around 14300.
Thanks,
Griff
N4DOG

PS: Thanks for any info. I am also going to try and put these antenna together as a tri-band.

 

[traumacop]

Check out this link, it should be helpfull. Anyway you decide to build the antenna, the calculation is the same.

http://www.hamuniverse.com/multidipole.html

Darrin KB8ECF

 

[n4dog]

Thanks for the info. I will save that website for the future of this project. I still would like to know how to figure the lengths of each run.

 

[MarkEagleUSA]

I still would like to know how to figure the lengths of each run.

As it says in the link: 468/freq mhz = total length for each band.

 

[n4dog]

Thanks for pointing it out. I guess I should have read the whole site first. LOL Thanks
Griff

 

[N_Jay]

I think it goes someything like this.

A 1/2 wave dipole at 40 METERS is (oh about) 20 Meters.

That would be (Hmmm, let me do some ciphering) real close to 10 METERS.

(Darn, that metric stuff is tough!) :twisted: :twisted:

 

[Al42]

I think it goes someything like this.

A 1/2 wave dipole at 40 METERS is (oh about) 20 Meters.

That would be (Hmmm, let me do some ciphering) real close to 10 METERS.

(Darn, that metric stuff is tough!) :twisted: :twisted:

I think it's the "half" stuff that's tough. What's metric half in real numbers?

(Darn, that tongue is really stuck in there.)

 

[Pro-95]

(((984 / freq in Mhz) * .3018) * 1000) = Meter

Meter * 3.28084 = feet

Decimal feet can be converted to inches by multiplying by 12.

So what about harmonics?

A dipole cut to 80meter is also ~= to a 40meter(2x), 30meter(3x) and 20meter(4x). An antenna cut for 10 meter has a 6 meter in it, as a 6meter x 5 is 10 meter.

 

[ReceiverBeaver]

And you will find that you'll still need to TEST such antennas for usage after initial construction. The math is fine but this calculation only works exactly for so-called isotropic free-space antennas in a labratory.

Every dipole and length-dependant wire antenna I've ever put up always actually tested as coming in either above or below the targeted frequencies due to the "real time" installation differences such as height above ground, type of feedline used, proximity to nearby objects ect........so I always had to lengthen or shorten the wire a bit to achieve the exact center freq. I wanted. The formula is the correct starting point, but you'll likely need to adjust it.

The MFJ family of Antenna Analyzers work great for this like their 259B and 269 models. Best radio tool you'll ever own.

 

[loumaag]

So what about harmonics?

Opps, time to go back to the book. Yes, you can use an antenna on odd harmonics, although it isn't as good as one cut for the correct frequency. In practical terms a 40 meter antenna works fair on 15 meters. Remember to use the design frequency to see what the odd mulitple is, not the wave-length.

 

[Pro-95]

Thanks Lou,

I have only glanced at harmonics, which is why I posed the question, what about harmonics? But I intend to delve into the subject but at the moment my studying scheule is a little packed.

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[loumaag]

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