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RE: [TowerTalk] I have a really stupid question (baluns and ununs)

To:, "TowerTalk" <>
Subject: RE: [TowerTalk] I have a really stupid question (baluns and ununs)
From: Jim Lux <>
Date: Fri, 12 Nov 2004 11:40:33 -0800
List-post: <>
At 12:44 PM 11/12/2004 -0600, Keith Dutson wrote:
What method other than coax is there to connect an unbalanced signal?  Does
this answer the question?

Keith NM5G

There are lots of transmission line configurations that are unbalanced. A single wire above ground might be one. Another might be a microstripline on a PC board. A notionally balanced twinlead might be unbalanced if installed too close to something else.

One might want to distinguish between (un)balanced transmission lines and (un)balanced signals.

What we usually mean is whether the (assumed) impedance to "ground" is the same for a balanced signal/line. In the case of coax, the shielding effect of the outer shield means that the impedance of the center conductor to the outside cannot be the same as the impedance of the outer conductor to ground: ecause the center connector cannot "see" the outside world, it's impedance to ground is undefined (or maybe, infinite... I have to think about it)).

As far as the commercially available devices go.. You just pick a connector and go with it. At HF frequencies, the connector impedance bump problem is pretty small, because the connector is physically small (in wavelength terms), and the dimensions and dielectrics aren't "too different" from what ever you're hooking it to. Imagine, if you will, a coaxial connector with very closely spaced inner and outer conductors insulated by alumina (dielectric constant=10)... it might look like a lumped capacitor across the line. With a UHF connector, at least you're using a standard piece of hardware that is inexpensive and everybody has. Bear in mind that you might want the outside of the UHF connector isolated from "ground" (as at the feedpoint of a dipole). Dual Banana jacks or lugs have mechanical problems (particularly through many mate/demate cycles). Twinax connectors are a bit exotic and expensive, and I don't know that they'd handle much power.

Oddly, you could probably use a conventional 110VAC dual prong plug with RF. Good clamping forces, they're available in a locking configuration, etc. Here, though, there's a huge safety hazard. It's just a bad idea to use anything that looks like a power connector for anything other than the signal it's designed for.

Anderson Powerpole connectors might be another good possibility. Low contact resistance, easy mate/demate, self wiping contacts. However, they have the same problem with accidental intermating of incompatible signals.

As far as low impedances go... a lot of Yagi designs have very low feedpoint resistances (because of the high mutual coupling). You can either do the transformation with a matching network on the antenna (i.e. a gamma match, or delta match, or....), which is really like using a lumped LC network, and might(!) be narrow band. Or, you can use a transformer (which will tend to be broader band).

Physically short radiators will also have low feedpoint impedances. A good example would be a coil loaded vertical against a good ground plane. The radiation resistance is low, the losses are low, the coil has been trimmed to compensate for the reactive part of the feedpoint impedance, but the resistive part is quite small. Again, you have a choice between using some LC network (lots of those in the ARRL handbook in connection with mobile whips) or a broadband transformer. Screwdriver antennas use the transfomer approach because it IS broadband.

-----Original Message-----
[] On Behalf Of Rob Atkinson, K5UJ
Sent: Friday, November 12, 2004 11:40 AM
Subject: [TowerTalk] I have a really stupid question (baluns and ununs)

Okay, I have what must be a stupid question.  I say that because I have
looked for an answer and have been unable to find one, so the answer must be
so obvious that I'm not seeing it and everyone else knows it.  Well, since I
have embarrassed myself before and surely will again and therefore don't
care about my reputation since I never had one anyway, I am going to ask
this question:

We commonly use ununs to transform one feed impedence to another, or
transform an unbalanced feed impedence to the feedpoint Z of an unbalanced
antenna.  The commercially available ununs have UHF females on each side of
them, which makes sense in the case of the 1:1 ununs and the ones that are
designed to transform 50 to 75 ohms which are common unbal. feed impedences.

I see a few ununs on the market that seem to be intended to work with very
low unbalanced impedences such as 5, 10 or 20 ohms, and they too all have
UHF females.  This does not make sense to me since as far as I know, there
are no 5, 10 or 20 ohm coax feed products out there that are commonly
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.


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