Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KC0LWN, Feb 8, 2015.
Earlier in the thread, G3TXQ posted some graphs based on some of my measurement data for K material (Steve doesn't have an FT240-K toroid). My measurement data was inaccurate because I used the wrong calibration file. I have sent the corrected measurement data to Steve and maybe he will post another of his nice looking graphs. There was nothing wrong with my AIM4170D or its software. Here's a corrected graph for 2 turns on each toroid. The K-material that I got from Amidon is superior to the 52-material. I apologize for being responsible for the earlier inaccurate data.
My chart is identical to Cecil's, so there's not much point in repeating it.
If you compare Cecil's latest results for #52 material with those I measured independently in the Post#33 chart, you'll see that they are now very similar - certainly within the tolerance of ferrite material variations.
I use single FT240-K on my 10/15m stacked monobanders as BALUNs because narrow and high Zcm. On the other side of coax I prefer CMCs with Resistive Zcm over wide frequency range wih #31 43...to kill the background noise and induced noise from my other antennas.
With towers close to each other I use a lot of CMCs in order to operate SO2R in contests.
As GM3SEK said earlier, there is a ferrite for every application, you just have to pick the right one.
Here is an interesting link comparing different materials for matching non-choke use.
Regarding 52 material and plots presented by Steve, G3TXQ, my measurements are very different and show peak of Zcm higher in HF spectrum. Materials like K, 52,61,67,68 are very touchy and will show a lot difference with winding style and leads lenght.
Most common mistake when measuring Chokes, Traps etc. is to leave long leads which add series inductance and lowers the resonant point and peak of Zcm.
For example, with 12 turns RG58 on 2 cores 240-52, extending only one lead by 1/2" to the fixture, moves resonant peak from 21 MHz to 18MHz.
BTW 12 Turns of RG58 or RG142 will make great BALUN for 20-10m, Resistive with Zcm peaking 13kOhms.
Still, Steves table for winding Chokes is great help for home brewing a BALUN or RF Choke.
73 Danny E73M
I think Steve will agree with me here. Steve and I made measurements as others have and the peak of Zcm at self-resonance is very sensitive to distributed C.
Even 1 pF can move the peak many MHz. I don't have the data at hand here at work but I can show you the arithmetic later if Steve doesn't beat me to it
An extra 1pF equivalent parallel capacitance can drop the resonant frequency of a 12tx2xFT240-52 choke from 14MHz to 9MHz; on the other hand a few inches extra on the "pigtails" has negligible effect. So changes in stray capacitance due to changes in winding layout are much more likely to be the cause of any resonant frequency changes.
Just to put some numbers on the pigtail inductance - every inch extra on each tail adds about 13.5nH to the equivalent series inductance; given that the basic choke equivalent series inductance is around 110uH, you can see that the length of the pigtails can't have any significant effect.
For most practical purposes it's not an issue, because that choke still has >10,000 Ohms Zcm +/-3MHz either side of resonance.
Interesting observation abot "pigtails", can you explain me why am I seeing move from 15 to 21MHz by shortening the pigtails by an inch or so to the edge of the toroid.
I do S21 with VNWA by DG8SAQ, don't you see it with your VNA?
Ups Forgot to note, mine is wound in cross style aka W1JR style, which usually moves resonant point higher, but still wonder why we have different results.
Thanks, Danny E73M
I just made measurements on a choke wound on two stacked FT240-52 cores. It had 8" long tails and a resonant frequency of 10.4MHz. When I shortened the tails to 1" the resonant frequency rose to 10.9MHz.
How far apart are your "pigtails"? If they are not several inches apart it's likely that the resonance shift you are seeing is due to the change in parallel capacitance rather that any change in inductance.
If I add some pigtails to my jig - but with no choke present - I can easily see a 1pF change by moving the tails to within 1" of each other.
I would stress again that the important factor is the parallel capacitance added by the jig or the pigtails - not the series inductance. At resonance these chokes typically have an equivalent circuit comprising a series inductance in excess of 100uH and a parallel capacitance of a few pF. Even long pigtails will only add a comparatively small inductance, but they can add significant capacitance.
Try some measurements with identical length pigtails, but varying their spacing.