This is the talk that I presented at the RSGB Convention in October 2018.
Update: RSGB have now released the video at https://thersgb.org/members/resources/?id=5762 (member login required; it will probably be several more months before the video is transferred to YouTube).
I always cringe to hear myself talking, but the sound track does add some context to the previously released slide pack ( still available here ).
Even without the voiceover, the important points are clear:
- Today, “baluns” are about controlling interference, reducing noise
- Choke baluns are best for doing this.
So we should aim to:
- Minimize common-mode RF currents on the outside of the coax
and along the boom.
- Don’t upset the good feedpoint balance that VHF-UHF Yagis already have.
Comments and discussion, please! This is new information for VHF-UHF, and certainly not the last words to be said about it. It’s still very much ‘work in progress’.
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Technical slide content © 2018 Ian White, GM3SEK
Slide template © 2018 RSGB
Images as attributed (except where that would be embarrassing).
Hello Ian, this is very interesting indeed and very timely. I am building quite a few systems at the moment and I was going to just put a string of ferrite cylinders on the coax. On my dual band 4m and 6m antenna I used ferrite without shorting to the boom. However on my new 2m and 70cms antennas I will do exactly what you have described.
It is a minefield of misinformation! Thanks very much for taking this to on.
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Kudos on your excellent piece on VHF chokes. Another good solution for 2M is a string of #31 solid or 0.5-in i.d. clamp-ons. Each is good for about 280 ohms Rs. For 6M, each multiple 2-turn choke through the 1-in i.d. solid or clamp-on core provides about 900 ohms Rs. Smaller diameter cores can be used if RG400 is used for the length of line needed for the choke.
I have watched with interest your 2018 Convention talk on VHF baluns. You mentioned the quarter wave ‘hump’ between the feedpoint and where the coax feed meets the boom. Also you mentioned that at the end of this ‘balun’ away from the feedpoint you did not connect the coax shield to the boom due to the hassle involved. Would there be anything to be gained by doing so? Would the effect of the balun be more stable to connect the shield to the boom? Also, is it better to route the coax, via such a ‘hump’, backwards in the direction of the reflector to exit the aantenna assembly rather than forwards past the first director? Sorry for all the questions but I would like to get it right before installing high up on a mast.
There would be some advantage in connecting the ‘cold’ end of the quarter-wave loop directly to the boom, because a lower impedance at that point will create a higher impedance at the open end. But living here on the Wet Coast, I am always very wary of creating new points where water can get in (especially into the braid, where corrosion can ruin the coax in a matter of days) unless it is absolutely necessary. However, your conditions may be different so it really is your call.
Regarding the best direction for the loop (forward or backward)… it isn’t an easy choice. It depends on the specific Yagi design, and whether the parasitic elements will get in the way. Many Yagi designs are quite crowded ahead of the driven element, while many others position the reflector about 0.25wl behind the driven element – which is exactly where you need to attach the coax to the boom. All of that makes it very difficult to give general advice; you need to look very carefully at each individual case.
I am just building a 70cm Yagi which may offer a good worked example in a few weeks time.
Thank you very much for your talk and the insight. I am also very interested in your developing thinking here. Did you progress your assessment of The impact of velocity factor on the quarter wavelength mechanical distance ?
Hi Ian, in your lecture you rejected the use of ferrites for baluns at VHF and up. For 4 metres and up I’ve made a trial ‘Guanella’ type core. This consists of 6 turns of 2 x 1.5mm enamelled copper wire carefully laid out side by side to form the transmission line section. The 6 turns are wound on on a FT140-61 core. The choking impedance appears to be around 250 ohms, a bit low really, but the SWR measured into a 50 ohm load is better than 1.1:1 at 71MHz. The SWR is around 1.5:1 at 160MHz. To increase the choking impedance, I’m going to have to add more turns, which will I expect kill the upper frequency response.
Making the 1/4 wave stub section from your lecture is equally not a problem for me to manufacture – just wondering whats wrong with the ‘Guanella’ solution?
Hi Ian, in your lecture you rejected the use of ferrites for baluns at VHF and up. For 4 metres I’ve made a trial ‘Guanella’ type core. This consists of 6 turns of 2 x 1.5mm enamelled copper wire carefully laid out side by side to form the transmission line section. The 6 turns are wound on on a FT140-61 core. The choking impedance appears to be around 250 ohms, a bit low really, but the SWR measured into a 50 ohm load is better than 1.1:1 at 71MHz. The SWR is around 1.5:1 at 160MHz. To increase the choking impedance, I’m going to have to add more turns, which will I expect kill the upper frequency response.
Making the 1/4 wave stub section from your lecture is not a problem for me to manufacture – just wondering whats wrong with the ‘Guanella’ solution?
I didn’t reject ferrite chokes in general – only the HF style chokes with multiple turns on a toroid. These chokes are unsuitable for VHF because of the physical size of the coax and the toroid, which will make the resonant frequency too low. Adding more turns will only make that even worse.
For VHF I would recommend multiple ferrite beads threaded onto the coax. These are simple, non-critical and broadband, the drawback being that you need several beads. For ferrite materials like Fair-Rite 43, the impedance of each bead is typically around 100-200 ohms. I would recommend at least 500 ohms as a minimum target impedance for this particular application, so in order to achieve that you would need 3, 4 or maybe 5 beads in series (depending on the specification of the bead).
73 from Ian GM3SEK