diy 1-1 unun

[vk4ffab]

Thanks for your answer, I will try it in both a metal box ten a plastic one and compair, its just i have some eddystone type boxes but few plastic 73.

I am a big fan of using what you got and seeing if it actually makes a difference. Ashar Farhan use tap washers in one of his home brew builds instead of iron powdered toriods, because they was what he had on hand, it worked perfectly acceptably as inductors. Almost impossible to replicate by anyone else, but you get the point. Often best wisdom is often nothing more than old wives tales passed down from one ham to another as if its true.

I had a flick though the arrl books I have here and could not find any info that was usable. So there is that LOL If you are worried you could mount the toriod 10mm or so off the metal case using a block of something non conductive. That will reduce any capacitive coupling.

[A.Z.]

it isn't about the toroid, but about the windings and the coupling

http://www.arrl.org/files/file/History/History%2520of%2520QST%2520Volume%25201%2520-%2520Technology/AntComp1-Lewallen(1).pdf

using a metal box we'll defeat the effect of the current BalUn due to parasitic coupling between the windings and the metal box, see it now ?

And... no "old wives tale" here

[shabaz]

Could you highlight the relevant section in that 8-page doc if you don't mind?

For sure, parasitic-created elements will impact more for some use cases, or if you do very unusual things (like strap the balun right against the conductive metalwork, without any gap/insulating shim/insulating standoff, etc.).  Or if you didn't insulate the balanced end. But these are very solvable things, and once mentioned, it would be clear to a newcomer that the device should not be placed against metalwork, and that the balanced end needs insulating.

If it were an unsolvable problem, then Minicircuits (and others) would obviously not sell baluns in metal enclosures. Didn't every high-end Hi-Fi radio receiver (folded steel enclosures) also contain them, for converting the 300 ohm balanced antenna input to 75 ohm?

[vk4ffab]

And... no "old wives tale" here

I don't know AZ. I gave that document a quick skim over and it does not talk about cases for the balun or the effects of coupling. If i could be arsed one way or the other I could do the actual experiments here, but I doubt that adding a few puff is really going to make all that much difference. At least, not to the point of desegregating the choking and balancing ability.

We have all used antenna that are out of balance. I ran portable for 2 years doing WWFF etc with a homebrew balunless dipole where the setup was invariably some level of inverted V often with dog legs, and then bought a sota beams linked dipole with balun and continued doing the same and it made no difference at all, when with it lashed to a tree beneath the canopy of a rain forest and other assorted stupidities

I am still not convinced. I am not saying its not a fact, its just the proof offered so far is not compelling. If coupling was such a universal deal breaker just about every balanced tuner under the sun would not work in a metal box with caps and inductors millimeters away from the enclosure and toriods bolted to the chassis. Or other manufacturers that put baluns in metal cases. There is more to the story than don't do it and I still think its a be aware of the issues rather than a you cant type of thing.

Happy to be proven wrong. Because I do not know for fact.

[radiolistener]

thanks for a sensible answer from the vk4, is it ok mounted in a metal case as long as the banana posts for the ladderline are insulated from the box, Also can i wind it the same as in the pic i posted, Same number of turns etc?.

you can use metal box if you don't want to place it near antenna, but use connectors with ground isolated from metal box.
Also note that small box is not a good choice, because you're needs to put your circuit at some distance from metal box in order to reduce capacitive coupling. So, a large box with a lot of space is better choice for that.

Since you're planning to use it with ladder line it will be placed far away from antenna, so using metal box will be ok. I know old military antenna tuner which has metal shell and terminals for ladder line and it works very good.

Here is example of antenna tuner in metal box, as you can see it uses terminals with isolators for symmetric or asymmetric antenna connection:

[A.Z.]

the point is, a metal box will need enough clearance to avoid undesired parasite coupling (same goes for tuners), so it's easier to use a non conductive material box for the balun, instead of making things more complex.

[vk4ffab]

the point is, a metal box will need enough clearance to avoid undesired parasite coupling (same goes for tuners), so it's easier to use a non conductive material box for the balun, instead of making things more complex.

Even that, when i think about it, how much separation is enough and how big is the effect? Probably not a lot and the effect is small anyway. Think of a yagi or hex beam. Sure the balun is in a plastic box, but its securely attached to the boom which is made of metal. So 3mm of plastic is some separation, but it is still a capacitor. The bigger reason to use plastic outdoors is that its much easier to seal up and its non corrosive and low cost.

And even then, there is more capacitance in the windings, inherent in coax itself, or twisted pair, that i think a few more pf really wont make much difference. as long as the self resonant frequency it stays above the 30mhz in the OP's case, it should not suffer any performance costs.

That is something the OP can check himself with a VNA.

[A.Z.]

think at a classic guanella current balun, some turns of coax wound over a toroidal core; some portion of that coax will have current flowing on the outer surface of the shield... see it ?

I agree that it may be more a "belt and suspenders" approach, but if we can prevent issues, why not ?