[Moon-net] 1/2 balun question

[Leif Asbrink]

Hi Edi,

> I have now a folded dipole yagi on 200 Ohm and works very well. After 1/2
> classic balun I have true asymmetric 50 Ohm and measured SWR it’s all right,
> in fact very good, more than expected ( I grounded the upper centre of
> dipole… tnx so much Leif SM5BSZ for last minute useful advice ) Next project
> is to combine two 100 Ohm folded dipole yagi, with 1/2 balun also ( so 25
> ohm asym each ) and I plan to use two identically pieces of 50 Ohm coaxial
> between them. The question is what impedance should have the coaxial from
> 1/2 loop itself? I don’t understand quite well why the loop can be done
> either from 25, 50 or 75 Ohm coaxial and what is best !
Oooh!!

ANY cable of length 0.5 wl THAT HAS NO LOSSES will have exactly
the same impedance in one end as the load at the other end.
The phase will be shifted 180 degrees. That is all. And that is what
you want!!

Look from the point where your feeder ends. There are two ways for current to
go. It can go out to the dipole, the side that is connected, and there the
impedanze is Zd/2. For a 100 ohm dipole, you would see 50 ohms towards ground.

Current can also go through the coaxial cable. It is 0.5 wl, so the impedance will
also be 50 ohms because the other end of the dipole is also 50 ohms:-)

The only criterion for the cablöe is the losses. Choose the cable that gives
minimum losses. A 50 ohm cable will operate with SWR=1:1 so there will
be no losses due to SWR. It could happen that you have a 75 ohm cable
of much better quality, and it would be the best choice if losses would
be lower with SWR losses included as compared to the 50 ohm cable.
(But in such a case you should design everything for the best cable you 
have, i.e. for 75 ohm)

73

Leif