RSGB: Useful practical skills videos - simple Balun

for this construction project we're going to build and solder our balloon you may recall in your foundation training discussing balance a ballon which is a contraction of the words balanced to unbalanced is used on antennas such as dipole to convert the signals generated on both sides of the dipole into a single signal which is passed down the center pin of the coax cable and into the antenna port of your transceiver i'm joined now by leigh mike seven mic uniform tango and we're going to build a ballon there's several different styles of balum that you can build and the idea of all of them is the same it's to stop any rf current flowing down the outside of your coax when you have an unbalanced feeder so let's say that you have a simple dipole antenna this is a balanced antenna that means that the two sides look the same if we were to use something like a ladder line feeder we could connect that straight to the antenna but instead if we want to use coax that's an unbalanced feeder it looks like a center inner core and then a shield around the outside and we want to connect this up to our dipole we want to do that in a way that means that no rf current is going to flow on the outside of that shield it's all going to flow on the inside instead and we do that by putting a ballon here at the feed point there's lots of different designs of balan and you make most of them very easily yourself one really easy way of doing this if you're just in a bit of a tricky situation is to use one of these large split ferrite cores this is two lumps of a sort of iron plus other materials mix and you can snap it together like that and it forms a bit of a doughnut if you've got your coax you can push that through the center and it's great being split because it means even if you've got a connector on the end of your coax you can very easily just do this with around it with it in place and i'm just going to wrap as many turns as i can get through that center it's important that it's going to be able to close right up so there's no gaps between it and that's a really simple balance but today we're going to build quite a simple design based on one single ferrite core now it's important when you buy these cores you buy the correct type of core for the design and a core is made up of two uh the chord type is two numbers for example 120-63 now the first number is how big it is and the second number is the type of material so this is a type 63 ferrite that's generally pretty useful for a lot of hf frequencies so if a design for a ballon that you want to build says you need a 120-63 core make sure you do get that type very similar to the the simple clip on ferrite we're going to grab our coax and of course this one doesn't split in half so you not need to not have a connector on the end of this stage and you're going to pass it through the center of the ferrite core so you probably want about a meter of coax sticking through the center of your turret once you've done that you might find it useful to just use a cable tie to secure that in place at the start now each time you pass that coax through the center that counts as one turn and we're going to go around just on one half at the moment and we're going to put probably five turns on so we've got one already i'm going to put another one on that's now two turns and keep these nice and tight as you go so two turns three turns four and five so i've gone through the center five times and spread those around about a half of that torrid now once you've done that this is the slightly tricky bit you've got your free end here going through the bottom you want to bring that across the back near to where that first turn came in so you haven't put it through the center anymore you've kind of bought it diagonally across and then i'm going to bring that up and then back down again okay so i've just now come across the center and i've got my first turn on the other side and now you wind in the opposite direction another five another four times so five turns total on that side uh with the original wire i'm in it at the moment um yep so you should this is this is the sort of the long bit of coax and we should be waiting to come back and ideally you want all of these terms nice and evenly spread none of them overlapping i'm just going to trim off part of my cable tight that looks great so what we've got here is we've got our coax coming in at the bottom around this side crossing over and around the other side and then coming out and that will then connect onto our antenna you now secure the other end this end of it that's loose at the moment secure that with another cable toy it should look a bit like that great so you've nailed made a bellum and that's all it is all there is to it the reason by the way that we did that crossover in the middle is if we'd gone all the way around and come out right next to the input then sometimes those two bits can couple together and you don't get quite as good an effect as if you bring them out on opposite sides so that crossover in the middle can be a little bit tricky to to wind but it does give a much better result you

Channel: Radio Society of Great Britain

Views: 2,235

Rating: 5 out of 5

Keywords: RSGB, Radio Society of Great Britain, amateur radio, radio amateur, amateur radio construction, balun, antenna, aerial

Id: WIwelWoDT7w

Length: 7min 49sec (469 seconds)

Published: Mon Mar 08 2021