I received one of my monthly Ham Rags today. As usual I flipped back to the light reading section to see how other folks make HF antennas work in bad locations, with low power or invisibly. Instead of the usual fare I found a chunk of what I considered incomplete info that I just had to illustrate and hopefully correct.
The box of info I am referring to is in March 2009 CQ magazine page 51. In this box the author is suggesting the use of ferrite material to reduce RFI in the home. So far so good. What I object to is the use of meaningless numbers to make him sound like he knows what he is talking about.
First he says “Nearly all cases of RFI occur when an affected device is in the antennas induction field”, and then proceeds to give power levels and distances in feet from the sample antenna. Who measured that, for what kind of antenna and at what frequency? Near Field varies as the frequency changes. Near Field is larger for 160 M signals than say 10 M signals. There is no definition of frequency at all.
Further along, mention is made of choking affected devices (TV’s, Stereo’s etc. etc.) by winding as many turns through the ferrite material as possible. This sentence is what got my blood boiling.
Ferrite is great stuff. In today’s noisy home electrical environment it almost a necessity. However it is often applied by many Hams that don’t understand 1) the material, 2) where to apply what type, 3) the effects frequency and stray capacitance have on the application being successful. So what we end up with is a bunch of folks buying material, applying it and then experiencing problems. They get frustrated and toss the material and forever have a tainted view of ferrite and its ability to reduce or eliminate RFI. I believe RFI is one of the major contributors to Hams abandoning HF and or leaving the hobby entirely. Putting up with ticked off neighbors and family members and not being able to effectively manage their RFI issues frightens off many a Ham.
Lets take a short tour of what happens when you wind a ferrite with wire-lets say a speaker wire. In all of the below examples I used 16 gauge speaker wire. I did this experiment to see what impedance I could achieve with a set of cores I had laying around the shack. The core type and size isn’t really important for this experiment since the stray capacity mechanism applies to all windings at RF frequencies. To be specific, this is Type 43 Material and the part number from Fair Rite Corp is embedded in the plot. This particular series of plots is describing what occurs when using two ferrite cores in binocular configuration while increasing the turns through those same cores. Simply stated binocular is placing two identical cores next to themselves such that they look like a miniature set of binoculars. One normally winds this way to get N squared impedance. I use binocular configurations to use less ferrite and cabling per unit of impedance achieved.
Here is a sample of a couple of binocular wound cores.
Someone much brighter than I once calculated that any choke you would apply should have a minimum impedance of 1000 ohms. This is how I measuring what happens to chokes as frequency changes and the turns count goes up. I need my chokes to work with greater than 1K ohm impedance from 3.5 Mhz to 29 Mhz. These are sample plots. These curves happened to be a complete family and were convenient.
The important curves to watch are the Green curve: total impedance and the Orange curve: Resistive impedance. Remember that it is resistive values that do the work of reducing RFI, not total impedance which is composed of both resistive and reactive components. The impedance scale on all plots below is in Black and located at the top of the right hand vertical axis on the graph just inside the working area on the plot just below the green Z mag label.
First a single turn plot.
1.5 Turns – Watch the right side scale labeling as it changes as impedance goes up.
As turns increase, the plot gets humpy, rolls over and at higher frequencies the resistive component drops as a percentage of total impedance. This last example is a poor choke. It rolls off too much. With this core, wire size and frequency range of 1-31 Mhz, the best we can hope for is at 2 or 2.5 turns. Remember-more turns = more interwinding capacitance = poorer high frequency performance in these RFI filters.
If you are going to apply ferrite, use enough material to provide 1 K ohms at the range of frequencies that you are concerned with. Adding more impedance is better, less impedance could be useless. The only alternatives to deploying ferrite in your home is to erect your antenna such that it is located far enough away from your house wiring so that near field radiation is not engulfing your home on the lowest frequency that you expect to use; or live with RFI. I for one like to be able to show an irate neighbor that my TV’s and electronics are unaffected during transmissions.
I believe I have shown convincingly that adding turns thinking that it only gets better with more turns is a fallacious argument.
Thanks for reading my Blog. Best, Chas W7MAP/5