|To:||email@example.com, "TowerTalk" <firstname.lastname@example.org>|
|Subject:||RE: [TowerTalk] I have a really stupid question (baluns and ununs)|
|From:||Jim Lux <email@example.com>|
|Date:||Fri, 12 Nov 2004 11:40:33 -0800|
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?
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: firstname.lastname@example.org [mailto:email@example.com] On Behalf Of Rob Atkinson, K5UJ Sent: Friday, November 12, 2004 11:40 AM To: firstname.lastname@example.org Subject: [TowerTalk] I have a really stupid question (baluns and ununs)
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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