Folded Dipoles
Well, I had a full wave loop for 20 Metres, which I tried to convert into a folded dipole (now this is proof that one must think before one acts) which was a failure as there wasn’t enough wire strangely enough, and to top it off after the strong winds in Melbourne the other night caused some damage to that failed antenna.
Now generally for a dipole we work on the theory that the full length of the dipole is ½λ so for example for the 20 meter band we could go for the following formula:
(300 ÷ 14.175) ÷ 2
where 14.175 is the centre frequency I’m trying to work with.
So from this we get the value of:
10.582 metres
As I’m working with dipoles we can simplify the formula above to just 150 ÷ λ where lamda is the frequency we’re working with. I’m wanting to get good coverage over the 20 meter band so I’m using the frequency of 14.175.
So for a normal run of the mill dipole our two “legs” of the dipole would be:
5.291 metres
Now a folded dipole has the characteristic of offering a better match over a wider band. Meaning if all goes well the folded dipole will work over the entire 20 metre band without having to use a tuner.
Though using a tuner and balanced feed line it is said that one can use the folded dipole on other bands where the half wavelength is at the lowest frequency you’re operating on.
Now for my folded dipole, the full length of it for ease of construction was 10.6 Metres, and the distance between the top of the folded dipole and the bottom (I’m sure there is a word for this but for the life of me I can’t think of it.) was .6 Metres.
Now the feed impedance of the folded dipole is somewhat higher than that of your standard dipole (which is around 72Ω) , and is close to 300Ω so instead of the 1:1 Balun that a standard dipole would use a 4:1 Balun to try and bring the impedance as close to 72Ω as we can.
Anyway, after building this, and putting it up in the air, I’ve checked the swr of the antenna, and it appears to be 1.5:1 across most of the 20 metre band. So I need to tune the antenna up a little, as it is just a touch too long. But it works very well compared to the full wave loop I had used previously, and much better than my first attempt of a folded dipole.