A 1/4 Wavelength Antenna Explained & How to Place Ferrite Cores

Ferrite cores are often placed on cables to suppress the common mode noises so that radiation can be suppressed. Experienced engineers would know the suitable materials to do the job deeding on the frequency range of interests. But where to place a Ferrite core on a cable? This video demonstrates the theory behind it. For more demos like this, you can check our first video-recorded training course at mach1design-sh...

[@PhilsLab]

I normally don't comment on videos, but this was a great, practical demonstration - thanks for sharing!

[@MachOneDesignEMC]

Thanks Phil! That means a lot.

[@ininjad24]

Excellent practical demonstration - seeing is believing! The FFT analysis of square wave with fast switching could be included to demonstrate the harmonic spectrum. In real life, inserting ferrites is a post-facto treatment. Do consider making making videos where filters are introduced during the design stage.

[@stiaankruger2699]

Great demo thanks for sharing.

[@buckbrown823]

Really great video. Great effort and set-up makes it easy and practical to understand.

[@jd080657]

Great demonstration. Seeing is believing

[@sparqqling]

Great video, perfect demonstration of the principles at play and concise in just 7 minutes. Hats off, that's not easy! Click bait title would be nice, most people do this wrong 20dB attenuation with a ferrite bead.

[@MachOneDesignEMC]

Thanks Frank for the good words! Need to learn from you about the bait title, any suggestions? Won't be easy to change this now I guess, but for future for sure.

[@edinfific2576]

@@MachOneDesignEMC At the end of your current title you could add "to reduce EMI". That should help.

[@anderskihlelb3bj835]

Will this also reduce the TX of the signal also? Ore just reduce the Noise?

[@farshidtavakoli6107]

Very informative🌹🙏

[@GroundControl204]

Great video on CMC. Waving from my tech bench. 73👋🏻

[@woodskid4ever]

Beautifully explained! Thank you!

[@benoitraulin1264]

Thanx for this beautiful physics lesson. Great job, clear as cristal.

[@joestocker660]

Thank you so much for this. Fascinating subject, excellent teaching style. Learning 1 concept it a few minutes is ideal for many of us. Thanks again! 👍😃

[@tfhAlex94]

Very good demonstration. Keep up the good work.

[@fase144]

+A video!

[@TangoMikePapa]

I love this. RF should not be black magic. Keep up the good work!

[@joelevi9823]

Very good demonstration. Thanks

[@SheltonDCruz]

Excellent presentation!!!

[@EricTViking]

Excellent video - very interesting 👍

[@roberthopkins8089]

Very good job sir thank you

[@TwstedTV]

Wow very very cool.

[@m1geo]

Nice, thanks!

[@KevinStoriesTV]

Great tutorial

[@ANTALIFE]

Awesome demo :D

[@KP-dt9wq]

Great videos!!! Could you please demonstrate the functionality of a TEM cell ?

[@MachOneDesignEMC]

Hi, thanks. Yes, that's a terrific suggestion. Will certainly do that in a video soon.

[@KP-dt9wq]

@@MachOneDesignEMC thank you! I am currently thinking of buying a TEM cell however I am not sure how useful will be compared to the values given from an EMC chamber …

[@MrHotrod73]

Faboulos! l like your instruments !! 73 Raoul

[@chaolinshi1816]

专业👍学习了

[@nikolaivic4480]

Thank you for very simple and nice explanation, which can be very useful in EMC debugging. Could you please share more details about your setup: antenna specs, spectrum analyzer and the current probe? Thanks

[@MachOneDesignEMC]

Hi Nikola, antenna just normal telescopic/whip/rod antenna, those you find on a vintage radio basically. Spectrum Analyser is a siglent SSA 3000 series, Current probe is a Tekbox TBCP2-500 I believe.

[@nikolaivic4480]

@@MachOneDesignEMC Hi, thanks for the answer. I meant on the receiving bi-log antenna? :) Thanks

[@MachOneDesignEMC]

@@nikolaivic4480 that is a PCB antenna from Kent Electronics, see www.wa5vjb.com/products1.html, I bought the 36 USD one. Very good

[@nikolaivic4480]

@@MachOneDesignEMC thanks a lot for the details. Do you use some other like bi-conical to cover

[@MachOneDesignEMC]

That one I think you can use down to 200 MHz or even lower. Generally from 50MHz to 300 MHz, I use www.tekbox.com/product/tbma1-biconical-antenna/

[@arconeagain]

So therefore if you want to suppress CMC on a feedline to a quarter wave antenna, you would place the ferrite a half wavelength down from the feedpoint. Questions: do you take the velocity factor of the coaxial into consideration? Could you do a demonstration on this?

[@magnuspihl6974]

Great video! Looking forward for more! Very pedagogical. I would love to find someone who can show for electrical safety standard 62368-1. Have you seen any or can make such?

[@MachOneDesignEMC]

Thanks. I am not an electrical safety guy, though i understand the design and importance of HV safety. Will certainly have a look sometime later.

[@mabo4848]

Tnx

[@superdtp1630]

I am a bit confused about your claim. From the instrument the signal of interest(123MHz) is suppressed by the ferrite? Also how does 250KHz signal from signal generator gets out from the 60cm length(1/4 wave) antenna? 60cm length(1/4 wave) antenna resonate at around 125MHz but get suppressed by the ferrite but 250KHz is fed into the antenna. Could you explain?

[@stargazer7644]

He explained this at 1:29 . It isn't the repetition frequency of 250kHz that matters, it is the very fast rise time of the pulse. A 3ns pulse contains lots of harmonics much higher in frequency than 250 kHz. The 1/4 wave antenna is peferentially radiating those frequencies around 125 MHz. Higher and lower frequencies are coming out of the generator. If it was a 250 kHz sine wave instead of a pulse the rise time would be very slow and there wouldn't be any higher frequency components.

[@simonbaxter8001]

Nice explanation Min. How does that translate to PCB signal traces or interconnect cables. Should the filter be at the source end of the line or do you need to determine where the maximum impedance of the trace/cable is? Assuming that the impedance is nor constant along its length, in which case the placement of the filter is not critical?

[@MachOneDesignEMC]

Thanks Simon, for the positive feedback. Filters on PCB are different, they need to be placed on a quiet side, i.e. away from the noise source to be effective. This is to avoid close field coupling. I guess this video demonstrates the point for a system/product with cables.

[@roku6170]

Hi great video Thank you. I am by no means familiar with emi rfi noise, so I got a question. I am using ferrite cores to supress noise on my pc quipment. From your video I understand I should be placing them near the ports of my pc. Am I correct in assuming that? I have a few cores around my power supply, usb, dvi and hdmi cables

[@MachOneDesignEMC]

Hi Roku, yes, put ferrite close to your equipment end if you don't know what is going on, it is better than to put it in the middle of the wire. Hope it helps.

[@arcesioarbelaez4730]

Hi, pretty nice video but what happen to the antenna's gain when the ferrite core is used? Does it affect the radiation pattern? Thanks.

[@MachOneDesignEMC]

Hi, that's a great question. It would surely affect the gain, or what we call the AF. In terms of radiation pattern, yes, I am sure it is affected. You need some good Filed solver so see the impact.

[@SureshKumar-nk2ok]

thank u so much sir ,the signal generator frequency reading 250khz but spectrum reading 130 mhz ?

[@PSPS437]

Length = wavelength/4. Length=60cm or 0.6m so wavelength- 0.6x4= 2.4. F=C/wavelength. C-3e8(speed of light). Wavelength-2.4. F is 3e8/2.4 which is 125MHz.

[@dominicread797]

The 250kHz square wave from the signal generator has fast edges which gives lots of harmonic energy up into many MHz. The antenna resonates at a frequency that matches its length - as described in the previous reply. That's how the signal generator has a different frequency than the spectrum analyzer sees in the antenna.

[@anderskihlelb3bj835]

Will this also reduce the TX of the signal also? Ore just reduce the Noise?

[@MachOneDesignEMC]

Hi Anders, it is a demo, so more for information only. But the principle should be the same for both signal and noise. The critical thing here is understanding how a cable can act as an antenna. If you are an RF engineer, you would like to use this to benefit your design, if you are an electronics engineer, you need to avoid this to pass radiated emissions test.

[@tejeshwvardhan1165]

I need info on that CT

[@MachOneDesignEMC]

Hi Tejeshw, the CT is a Tekbox RF current probe, see www.tekbox.com/product/tbcp2-32mm-snap-on-rf-current-monitoring-probes/. I was using the TBCP2-750.