A Better Off-Centre Fed Dipole - Part 1 (4:1 Transformer & 1:1 Choke / Balun Build)
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- Опубликовано: 23 июл 2024
- This first video shows you how to build both a 4:1 transformer and a 1:1 choke current balun. Both are used in off-centre fed dipoles and loops (with a feed point impedance of around 200 ohms). The 1:1 choke can also be used on dipoles and doublets at the ATU.
For the design of the transformer and balun, my thanks go to Peter aka TRX Lab on RUclips. His comprehensive explanation of the build and testing in a professional lab can be found in part 3 of an excellent series of videos - Part 3: • #105 Balun PART 3: How... (I recommend you watch all three parts!). Peter's tests go to show that choosing the correct wire is one of the critical elements to success. Other transformers and baluns may look the part, but completely underperform when tested.
I sourced my handy-sized 2x1m (red & black) wire lengths here in the UK at: www.hobbyrc.co.uk/18-awg-flex...
The ferrites used in this build are type FT-240-43 from Fair-Rite. These can be sourced from various well-known suppliers, but ensure the mix is correct.
Part 2 shows the build of the 40m OCFD, plus on-air tests and tuning: • A Better Off-Centre Fe...
This is one of many amateur/ham radio related videos on my channel. Please feel free to browse 👀, like 👍🏻 and subscribe 🔔, remembering to hit the bell so you’re notified of new videos as soon as they're published and you don't miss out.
0:00 Introduction
1:34 Balun & Transformer Build
5:15 Summary Diagram
6:00 Bench Testing
8:30 Summing Up
Thanks.
Ian. 
Наука
Brilliant, looking forward to part two. Thanks Ian.
Thanks Brian!
Excellent presentation Ian, thanks.
Thanks Tim!
Very informative Ian. Great for loops as you say.
Thanks Tom!
Thanks Ian,great upload as always 👍🇬🇧
Thanks!
Hi Ian
Excellent build instructions. Looking forward to part 2. You and the family stay safe. 73 WJ3U
Roger that Don. You too 👍🏻
Cheers for the video, Ian.
Nice job 👍
Mick
Thanks Mick!
Great Video. Not full of crap about yourself like most others. Just facts on the subject. 10/10
Thanks Peter 😀
Super video Ian, I will have to try this. 👍
Thanks Mike! Part 2 coming soon 🎥
Excellent video - I really like your matter-of-fact presentation, and clear diagrams. Really good balance - already subscribed, and glad I did :)
Thanks Stephen! Nice to have good feedback 👍🏻
Thanks Ian, looking forward to part 2. I have an OCFD here with the shorter end from the chimney resulting in the feed point at a good location to the upstairs shack at the back of the house. I also use around 220W from a Yaesu FT1000 MP MKV , into a virtually identical arrangement based on Peters website. I have not noticed any heating effects although the winding insulation appears to have suffered all little from UV.
Thanks Adam. Yes, looking good so far 👍🏻
First class as per usual Ian! 73,
Thanks Tom!
The 1:1 choke would be much better if it was wound with 50 ohm, Teflon insulated coax. The double, bifilar windings like you have shown paralleled only have half the common mode impedance and a single bifilar winding much the same as putting 2 resistors in parallel. The coax is easier to wind and much more effective.
Thanks for the info! Note also that this choke doubles as a balun.
Awesome, thanks very much. 👍
Thanks!
Nice video. I have one question/concern about the design, though.... Would it be more appropriate to switch the order of the two cores so that the 1:1 current balun (choke) is attached at the antenna side of the pair to convert the 200-ohm BALanced load to 200-ohm UNbalanced, then with the second core, transform the 200-ohm UNbalanced to 50-ohm UNbalanced? (So that the order is 1:1 BALUN to 4:1 UNUN with the antenna side being 200-ohm BALanced and the transceiver side being 50-ohm UNbalanced.) The way it is presented, as I understand it, in the video, the 4:1 UNUN is being used on the BALanced side of the circuit rather than the UNbalanced side of the circuit. This doesn't seem quite right to me, but maybe you have a good reason for it that I am not seeing?
You could possibly reverse them as you suggest I suspect, but then the 1:1 would need to be designed for 200Ohm? This version copies the 1:1 50Ohm I’ve successfully used on my 80m dipole. After some years of use I can confirm that it performs really well and is my go-to main antenna 👍🏻 Thanks for commenting 😀
Hello sir.
I followed your video but I think I must have made a mistake, I am going to verify all the windings are correct. However I have one question, when using the analizer did use the radio coax or just a wip for the testing, and also what lenght of coax will be necessary to do the job right?
Thank you in advance
Kb1mlq
73's
Just a small patch-lead I had to hand Duarte, nothing special. Good luck with yours 👍🏻
Excellent presentation Ian. I appreciate that the proof of the pudding is in the eating, but were you able to measure the choking capacity of your 1:1 across the bands?………………
Or am I being too pedantic?
Good question! If you watch Peter’s video (link in description) you can see his full range of measurements, (including choking). If I recall it is over -30dB from 7MHz up 🤔
Do you have good instructions on a web page, not just the video? I'd like to print them off for reference when i build mine.
No sorry, just the video.
Is it required for black wire to be connected to center and red to shield?
Hi Sam, it’s the positioning of the wires and not the colours. This red/black combination mirrors a previous balun video and links back to Pater’s channel. I kept things the same so they match each other. Thanks.
Two questions: 1. How many watts is this design good for? 2. Could I fold them over and tuck them in a waterproof enclosure?
Hi. Good for UK 400w limit (you can stack for higher powers). Yes you can stack the two, but ensure there’s a breather hole in the box for condensation to escape due to heating and cooling of the air inside. No box is fine and less weight. Good luck 👍🏻
Hello! Would this work on a center fed dipole, the reason I ask os because I am limited on space to mount an off center fed dipole.
Thank you
kb1mlq
Hi Duarte. I’m not sure what you mean as both a centre-fed and off-centre fed are both the same length, but the off-centre gives you extra bands on the harmonics. A trap-dipole will be shorter if you’re tight on space. Good luck!
Hello and thank you for the reply, the reason is the off center fed dipole is longer on one leg and that is where I have the problem, the center fed it is equal on both sides and that gives me more room to install where I want the feed point at.
Thank you so much for your prompt response
Duarte Bettencourt
KB1MLQ
Hi, can anyone tell me please, does the 1:1 balun need exactly 12 turns? I can only fit about 10 turns per side on my ferrite core - does it matter?
Many thanks :)
Keep trying Dave. It stretches the outer cover when you’ve given it a go. Really tight and it’ll go on I’m sure….I had several attempts….
Cheers, will do :)
I just copied your examples the 4:1 is brilliant I don't think my 1:1 is as good but is useful up to 6 meter .👍
Great Michael 👍🏻