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

[Jim Lux]

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-----
>From: towertalk-bounces at contesting.com
>[mailto:towertalk-bounces at contesting.com] On Behalf Of Rob Atkinson, K5UJ
>Sent: Friday, November 12, 2004 11:40 AM
>To: towertalk at contesting.com
>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.
>
>tnx
>rob/K5UJ
>
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