|Subject:||Re: [TowerTalk] I have a really stupid question (baluns and ununs)|
|From:||"Rob Atkinson, K5UJ" <firstname.lastname@example.org>|
|Date:||Fri, 12 Nov 2004 19:44:31 +0000|
Okay Thanks Tom and Jerry-- the key apparently is to keep the feed real
short in terms of wavelength, on the ant. side of the xformer. This won't
be difficult to do on 1.9 mhz. Then the characteristic Z of the feed
becomes insignificant as you are approaching 0 x 1/2 w. Therefore, in
theory, i could get away with any unbal. feed (provided it handled the
power) if short enough, i.e. i could make something with disks holding a
center rod inside a copper pipe if I could machine a proper xfer from the
uhf female to the feed, and not even know the Z of the homebrew line. Not
gg to do that, but just wanted to try to expand on the point for my
I was thinking about this purely in terms of the mechanical aspect of the connection; not wavelength. Tower Talk comes through again and yes, the male is always perfect.
> available. unless someone is working with some sort of complex arrangement > of 50 ohm feeds in parallel, these ununs are usually placed at the > feedpoints, as in for example a vertical that has a f.p. Z of 10 or 15 ohms, > right? I'd appreciate it if someone would explain to me why a 50 to 20 ohm > unun has a UHF female on the 20 ohm side, since I don't know of any 20 ohm > coax, instead of a pair of lugs. If at the feedpoint, do you simply defeat > the purpose of the UHF connector by clamping the counterpoise to the threads > and stick a short jumper to the vertical in the center? There's something > wrong with this picture I'm missing.
In order to transform impedance the bump has to occupy some distance in terms of wavelength. In order to have a large effect, the bump also has to be big.
The bigger the bump and the longer the bump, the more effect it has on the system.
In a UHF connector, just like in my marriage, the male is always perfect. It is the female that causes all the problems. The impedance bump in the female UHF chassis connector is roughly about 1/2 inch long. They also typically are around 30 ohms or so Z0. That's UHF connectors work fine all the way to lower UHF. The mismatch isn't big, and it is VERY short.
There are indeed 20 ohm transmission lines. I even have so 15 ohms and lower Z transmission lines. But they are not needed if leads are short. Most lug or wire connections are in the hundreds of ohms!! Even they are not a problem.
Think of it this way, if the line is really short in terms of wavelength, the standing waves have no place to stand. Just keep the leads short.
+++++++++++++++++++++++++++++++++++++++++++++++++++++ Rob Seems most people using a UNUN (myself included) are using it to transform the 50 ohm feedline to match a low or high Z (but resonant) antenna such as a ground mounted vertical or inverted L. I used a short (12" long) section of rg213 from the UNUN to the antenna (about 20 ohms at the feedpoint of my 160m inverted L). Impedence at the end of a coax cable depends not only on the impedence of the cable but the frequency versus wavelength. Remember the old saying: 1/4 wave transforms, 1/2 wave repeats. Anything in between will depend on the wavelength. Don't worry about stupid questions....watch out for stupid answers (like mine)..HI..HI. JerryC KC8TES
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