# 'Basic Antenna' Project

SM0VPO Forum :: SM0VPO :: Questions

## Re: 'Basic Antenna' Project

Hello Graham,

You are more than welcome, any time.

I do apologise for not responding sooner, but I have been fully occupied in other areas, such as teaching. I am off home now (23:30 PM !!). Left the hotel at 21:30PM. Not much time for social media today :-(

Very best regards from Harry - SM0VPO

You are more than welcome, any time.

I do apologise for not responding sooner, but I have been fully occupied in other areas, such as teaching. I am off home now (23:30 PM !!). Left the hotel at 21:30PM. Not much time for social media today :-(

Very best regards from Harry - SM0VPO

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**Admin**- Admin
- Posts : 480

Join date : 2012-11-24

Age : 66

Location : Märsta, Sweden

## Re: 'Basic Antenna' Project

Thank you Harry. This is exactly what I am after.

**GrahamH**- Posts : 2

Join date : 2017-09-19

Location : Australia

## Re: 'Basic Antenna' Project

Hello Graham,

Yes, your statements are 100% correct. The impedance does vary from 72Ω at the centre to a few thousand at the end. The impedance at any point in the dipole is given by the formula for radiation resistance:

R = 72Ω / sin(2xPi / λ)²

where

x = distance from the end of the dipole

λ = wavelength (a full wavelength, the dipole is 1/2 wavelength)

Pi = 3.1415927

So you can pick a point and calculate the impedance at that point.

Hope this helps you.

BR Harry

Yes, your statements are 100% correct. The impedance does vary from 72Ω at the centre to a few thousand at the end. The impedance at any point in the dipole is given by the formula for radiation resistance:

R = 72Ω / sin(2xPi / λ)²

where

x = distance from the end of the dipole

λ = wavelength (a full wavelength, the dipole is 1/2 wavelength)

Pi = 3.1415927

So you can pick a point and calculate the impedance at that point.

*Transpose the formula and you can find the feed point for any specific impedance.*Hope this helps you.

BR Harry

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**Admin**- Admin
- Posts : 480

Join date : 2012-11-24

Age : 66

Location : Märsta, Sweden

## 'Basic Antenna' Project

Thinking about the 'Basic Antenna' Project which uses a 4:1 balun to feed a dipole off-centre, is there a formula / graph / nomogram which shows the impedance in a resonant dipole over its length? Not linear, As in I know the centre is ~70 ohms, I've read that the end is ~3000 and this project implies ~200 - 300 at 1/3 length. Knowing this could ease HF antenna construction at my home.

**GrahamH**- Posts : 2

Join date : 2017-09-19

Location : Australia

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