Ferrite Core Inside a compact Magnetic Loop Antenna - Page 1
Forum section: electronics / rf-microwave

author: jonovid
author_rank: Super Contributor
timestamp: January 14, 2023, 06:17:50 pm
content: I have been thinking . recycling old CRT TV ferrite deflection yokes.can the used ferrite from old CRT TV ferrite deflection yokes add performance to a transmitting loop antenna?see imagesee linkhttp://loop-antennas.ceyhunsezenoglu.com/en/latest/magnetic-core-loops/introduction.htmlwhat is the frequency limit of the ferrite used in an old CRT TV ferrite deflection yoke?will it work for a transmitting loop antenna?ham radio experiment to try-is there a market for compact HF aerial design for Amateur radio in densely populated urban areas.I can only make antenna of receiving signals.never seen any one use ferrite from old CRT TV ferrite deflection yokes for an antenna design.can it be work for transmitting loop antennas too.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 14, 2023, 06:50:41 pm
content: magnetic loop antennas are not very popular for transmitting as the voltage gets quite high. So it mainly works at low power.There are quite different ferrites. Chances are the yokes from the TV don't work to much higher frequencies than they are used in the TV. So maybe 100 kHz.The shape of the ferrite elements is also not very suitable for an antenna.I could imaging one could use the TV yokes in SMPS.

author: iMo
author_rank: Super Contributor
timestamp: January 14, 2023, 07:09:08 pm
content: The ferrite material - it depends on the bands you target - for 80-10m the material 43 is an optimal choice, for higher bands like 10-4m I would use material 61.How to check you ferrite material - made 2-3 turns over it and do measure the inductance, calculate the AL, material 43 is around 900nH/turn2, material 61 is around 100nH/turn2. There are other parameters to consider as well, but most probably the deflection ferrites are not intended for HF, imho.The ferrite rods from old radios are also not suitable, as those work till couple of MHz only. There are companies selling big ferrite rods for such antennas, like 10cm diameter and 1m long (heavy, expensive), search for it..PS: You can make S11 measurement (with 1-2 turns) from freq1 to freq2 (I did with nanovna) and make difficult calcs in matlab to get mu' and mu'' graphs which do represent inductance and core losses vs frequency and are usually depicted in the datasheets.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 14, 2023, 07:46:15 pm
content: No. The shape isn't especially useful (the effective size of the antenna is about the bounding sphere of the core half), and the property that makes it work for receive, doesn't work for transmit: antenna gain is extremely low (how much depends on what size antenna and center frequency), which is acceptable as atmospheric noise dominates the LF and MF bands, but this means transmit efficiency is nonexistent.AFAIK, at HF, the levels are such that a fairly compact antenna with no core and whatever type (E or H based) will do. Preferably it should be tuned for the band of interest, and bigger the better (but a good receiver won't need too much).Put another way, stretching out the wire in a yoke will do a far, far better job than leaving it wrapped around the core.Tim

author: Bud
author_rank: Super Contributor
timestamp: January 14, 2023, 09:43:16 pm
content: TV deflection yoke cores would have high loss at HF frequencies, meaning if you put one in transformer configuration, much power would be dissipated in the core as heat.A practical useful application for them on HF is antenna RF current baluns/chokes. Here is one I constructed and measured time ago. It has 6 turns of RG-58 coax. As can be seen from the chart, it has 500Ohm and higher impedance from 3.5MHz to 30 MHz, covering most of HF amateur bands. A couple more turns and it should also be good for 75/80 meter band.

author: jonovid
author_rank: Super Contributor
timestamp: January 14, 2023, 11:20:37 pm
content: Thank you all for your response. now got a better understanding of what is possible.be interesting to know if ferrite deflection yoke collars can be milled or grind down as with stone, to fit to into a large collar size more suitablefor an receiving antenna or metal detector experiment. of many graded for the same Hz ferrite blocks set into a large ring of ferrite blocks glued together.Thank you all

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: January 15, 2023, 12:56:12 am
content: Ferrite cores or any core will contain the radiated signal. You want to radiate the signal, not contain it.

author: jonpaul
author_rank: Super Contributor
timestamp: January 15, 2023, 03:06:50 am
content: yoke, flyback optimize for lowest cost, best paramètres at CRT flyback freq eg 15,750 Hztotally different ferrite than for RF use.Shape also wrong.Cores are not costly.design and buy the core materials and shape.Machining ferrite requires diamonds charged toolsJon

author: Kalvin
author_rank: Super Contributor
timestamp: January 15, 2023, 11:50:25 am
content: Quote from: Kleinstein on January 14, 2023, 06:50:41 pmYes, the voltage rating of the tuning capacitor in the transmitting magnetic loop antenna needs careful consideration.Although the transmitting magnetic loop antennas require relative expensive tuning capacitor, the magnetic loop antennas are quite popular among the amateur radio operators in the areas where it is not feasible/possible to build full-size antennas.About the tuning capacitor voltage ratings and construction types used in transmitting magnetic loop antennas:http://webclass.org/k5ijb/antennas/Small-magnetic-loops-FAQ.htmQuoteUsing a low-loss air variable tuning capacitor with 1kV - 2kV voltage rating it is possible to construct a low-power transmitting magnetic loop antenna that can handle power up to 10W - 20W without arching.It is quite possible to use a low-loss air variable tuning capacitor up to 100W transmit power. The voltage rating of the tuning capacitor should to be at least 6kV - 8kV. At these high transmit power levels and voltage ratings a vacuum variable capacitor starts making sense moneywise, though. For example:https://www.hamradioandvision.com/100-watt-loop-antennaBy using a vacuum variable tuning capacitor, it is possible to construct a magnetic loop antenna that can be used with 100W - 1000W transmitter. The voltage rating of the vacuum capacitor should to be at least 8kV - 10kV when running at 100W, and much more when running at 1000W. For example:http://www.na0tc.org/lib/exe/fetch.php?media=technical:magloop1_n0cu_2109_2.pdfA nice article about transmitting magnetic loop antennas for HF bands:https://www.nonstopsystems.com/radio/pdf-ant/article-antenna-mag-loop-2.pdf