Radio Frequency (RF) & Wireless Design

Most Radio Frequency (RF) dependent electronic equipment uses standardized input and output impedance (resistance plus reactance) in order to make it easier for the users to establish baselines for other equipment purchases and for compatibility when connecting devices together.

The typical input / output impedance for the vast majority of electronics RF hardware is set to 50 ohms impedance and this is typically in a coax connector format, so its easy to identify it. The reason for 50 ohms is that it was the 'sweet spot' for maximum power transferred versus maximum signal transferred. For more information please read this link : https://en.wikipedia.org/wiki/Coaxial_cable#Choice_of_impedance

However, the CATV (Community Access Television) cable industry has long established itself using 75 ohms impedance since it has less signal attenuation where power delivery is not as important as signal quality. This is especially true for video signals where there are many changes in the voltage variations as frequencies are over-layed (heterodyning).

ALL coax is considered an UNBALANCED feed line since the area of the inner conductor is smaller (and higher impedance) compared to the outer conductor which is larger (and smaller impedance). Therefore the input impedance is higher than the (return) output impedance - and therefore UNBALANCED.

A BALANCED set of wires are typical when they are identical in diameter, material, length and proximity. A typical BALANCED line cables are Ethernet (cat 6), USB, and HDMI where the input/return wiring pairs are twisted to each other as pairs and isolated from other pairs by conductive shields.

So if these standards are established, why are BalUns (Balanced-Unbalanced) transformers necessary ? Well, it turns out that many things such as antennas, certain transmission lines (open air feed lines, twin-lead) and electronic parts (amplifiers) are not (and cannot) be made to match. So its necessary to use a component that will make the two components match.

Well you might be asking why does all of this matching even matter ? - Two reasons; first in order to transfer the maximum power they must match; second in order to delivery the best signal quality they must match. So BalUns help deliver the maximum power with the best signal quality at the same time with a single device. Transmitters need maximum power to the antenna and Receivers need maximum signal strength to be understood (high signal to noise = S/N).

The beginning of any RF connection is called the 'feed point' and is usually where the signal enters the component, device, or cable (transmission line). This is the point at which the signals are tested and where the aggregate (lumped-element model) impedance for the component, device, or cable is measured and tested.

I will be working on a BALANCED 450 ohm ladder line to an UNBALANCED 50 ohm coax 9-to1 (9:1) Bal-Un for the adjustable impedance wire dipole model that I created, but here's a heads-up on what it will look like : https://vk6ysf.com/balun_9-1.htm

It's designed by VK6YSF and its the perfect kit for those who want to make their own !

This Balun would be used at the bottom of the 450 ladder line that rests on Earth Ground and then connects to 50 ohm coax (I will use an N-style coax connector). Although ladder line and open wire feed lines DO NOT like running along Earth Ground, but coax is great for it and helps suppress EMI / RFI on the outside of the shield braid.

In fact coax is better buried under the dirt in Earth Ground for maximum EMI rejection and suppression ! But bear in mind that coax has heavy power losses even for the best (LMR) types, so coax should be used a sparingly as possible to provide maximum RF power and therefore maximum signal to noise performance.