Friday, July 29, 2011


Making the Balun
Refering to Photos. , take a length of coaxial cable (Coax) about 400mm long. Buy sufficient ferrite tubes (reference 1.) to make an assembly of tubes at least 300mm long. The ferrite tubes need to be a snug fit over the outside of the coax. Thread the tubes over the coax. Secure each end of the assembly with a zip-tie such that the tubes fit tightly together. Fit appropriate connectors to each end of the coax. There! You have completed your sleeve or choke Balun. Easy wasn’t it?
I’ve made two of these Baluns. The first was fitted with a dipole centre on one end and a female "N" connector on the other. The second Balun has a male "N" on one end and a female "N" on the other. The assembly of tubes should be as close to the antenna feed-point as possible. I’ve enclosed the ferrites with heatshrink.
as you will see from the photos, I used some double shielded teflon coax, simply because I had some. This has a pinkish translucent outer insulation. Jaycar stock several sizes of ferrite tubes. RG-58 coax will fit through the smaller tubes. If you use thicker coax, you may have to strip off the outer insulation to get it through the ferrite tubes. This is good thing as a closer fit will increase the efficiency of the Balun. However it does introduce the problem of effectively waterproofing the assembly. This can be achieved with heatshrink tubing, self-amalgamating tape, silicon sealant, etc
Why Do I Need a Balun?
The vast majority of modern HF rigs have "unbalanced" outputs. They are designed to feed an antenna via coax. The shield of the coax is grounded to the rig and thence to station ground. The RF signal goes up the inner conductor of the coax and due to skin effect, up the inside of the coax shield.
Since most antennas (eg, a dipole) are balanced systems, the transition from unbalanced to balanced at the antenna feed-point can cause some of the RF to appear on the outside of the coax shield. This is undesirable on several accounts. Firstly some power will be lost if part of the signal returns to the rig on the outside of the coax. Secondly, RF can appear in the shack with undesirable results. Thirdly, the RF flowing on the outside of the coax can cause interference to other services (TVI).
So unless you want to win the WAN award (Worked All Neighbours), you might consider using a Balun.
How Does This Balun Work?
Well, we have already discussed the undesirability of RF flowing on the outside of the coax antenna feeder. If we introduce a very high impedance at RF frequencies on the outside of the coax shield, then all of the RF current will flow out into the antenna, and none will flow on the outside of the coax. This is what the specified high initial permeability ferrite sleeves or tubes achieve. Thus ensuring the transition from an unbalanced coax to a balance antenna. Hence BAlanced to UNbalanced (BALUN). The advantage of this design of Balun is that it cannot saturate regardless of power levels. The limiting factor is the melting point of the coax insulation.
How Can We Prove That This Balun Works?
It is relatively simple to prove this (Photo). We take our balun and lay it on a ground plane (a sheet of copper or aluminium) and ground the shield at the input end. Then we connect two 25 Ohm resistors at the output end (assuming 50 Ohm coax), one from the centre of the coax to the ground plane and the other from the shield to the ground plane. Then we feed an RF signal into the input of the Balun. If the amplitude of the RF signal is equal across both resistors then we have proved that all of the RF is flowing out of the balun into the load and none is flowing back to the source on the outside of the coax. I have conducted this test on the Baluns that I constructed and the SWR is flat across the HF spectrum and the voltage is equal across each resistor.
Well, I hope you have found this cheap and easy project of interest. My Baluns are working well on both my 40M resonant dipole and my G5RV antennas.
Reference 1. Jaycar sells several types of ferrite sleeves. I used LF-1258 which have an inside diameter of 5.6mm. LF-1260 has an ID of 8mm. Ideally the sleeves should be a snug fit over the coax. Don’t forget to use the Club account!


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