2m balun - how big ferrite is needed for 5 Watts - Page 1
Forum section: electronics / rf-microwave

author: szoftveres
author_rank: Regular Contributor
timestamp: June 22, 2020, 05:43:33 am
content: I'm using a tiny dual-aperture core (something like thishttp://toroids.info/BN-61-1502.php) as a 1:1 balun to drive a 3 element yagi from my Baofeng UV-82 at 5 Watts.Wondering if this core can handle the power level. Both the primary and secondary have 1.5 turns.Can someone shed some light on (given the above described windings) how much TX power can this core handle before going into saturation at 146MHz? Assume 50 ohms. Thanks!

author: radiolistener
author_rank: Super Contributor
timestamp: June 22, 2020, 11:03:46 am
content: You can determine the maximum flux density Bmax of a core with thefollowing equation:Bmax = (E * 10^2) / (4.44 * f * N * Ae) + (N * Idc * AL) / 10 * AeWhereE - applied RMS voltagef - frequency in MHzN - number of turnsAe - cross sectional area of the magnetic path in cm^2Idc - bias current in AmpsAL - core inductance indexIn your case there is no DC, so you can neglect terms after +.For 50 Ohm load and 5W, E = sqrt(5*50) = 15.811 Vrms.You can use peak voltage to have some margin, in such case for 50 Ohm load and 5W, Epk = sqrt(5*50*2) = 22.36 VpkBN-61-1502 has height=B=6.6 mm and width=(C-H)/2=(7.5-1.7)/2=2.9 mm.So, Ae = 6.6 * 2.9 = 19.14 mm^2 / 100 = 0.1914 cm^2.Bmax = (22.36 * 100) / (4.44 * 146 * 1.5 * 0.1914) = 12.015 GaussI'm not sure how to get flux density limit for ferrite core with 61 material, but according tothis amidon table, and extrapolation it will be about 13 Gauss for 146 MHz. This is higher than 12 Gauss. So, at a glance it looks like your core is ok for that power, but there is no margin, so it needs to checkAlso you canestimate core loss:Core loss (mW / cm^3) = (8.86 * 10^-10) * (f^1.14) * (B^2.19)Wheref - frequency in HzIn your case:Core loss = (8.86 * 10^-10) * ((146*10^6)^1.14 * (12.015^2.19) = 416 mW/cm^3Also look at this article:https://www.vkham.com/Info/ferro/tut_3.html

author: TheMG
author_rank: Frequent Contributor
timestamp: June 23, 2020, 03:14:14 pm
content: Any particular reason for using a ferrite core transformer? At VHF frequencies and above, a transmission line balun using a quarter wavelength coax is easily achieved and can handle much higher power levels without a problem. Simple and cheap to make too.

author: ardf
author_rank: Contributor
timestamp: June 23, 2020, 05:01:08 pm
content: There is an excellent 1:1 compact VHF balun construction by G4UNU. With 0.4 mm enameled wire the loss is less than 0.3 dB.

author: profdc9
author_rank: Frequent Contributor
timestamp: June 23, 2020, 11:25:01 pm
content: I've used a few turns of RG-316 or RG-174 through a FT82-43 core. 61 core is a bit better but should be plenty of impedance at VHF. Maybe this is bigger than what you want?

author: szoftveres
author_rank: Regular Contributor
timestamp: June 24, 2020, 01:30:15 am
content: Quote from: TheMG on June 23, 2020, 03:14:14 pmAs far as I know, a Yagi has the impedance in the 25 ohm range, so my idea was to use the transformer for the balun as well as for impedance matching (to 50ohms). AFAIK the current balun doesn't do the impedance matching, but in the past few days I've also heard about the so-called 'hairpin-match' so maybe I'll use all these ideas and give them a shot.

author: iMo
author_rank: Super Contributor
timestamp: July 05, 2020, 04:23:07 pm
content: Quote from: szoftveres on June 24, 2020, 01:30:15 amIt depends.. It could be from 12.5ohm to 300ohm based on construction.Afaik BN-61-202 can handle 5W continuous (you talk FM I suppose).You may check your core with a 50ohm dummy load (ie. made of 5pcs 250ohm 1W resistors) - simply push 5W via the core and measure its temperature.