We receive many emails about our products and recommended solutions. Here are some of the most typical questions and their answers.
Ham Station RFI Recommendations
Hello:
I read an article which brought me to your site
http://nk7z.net/rebuilding-the-shack/
I have a Amateur radio Yaesu 950 and a Yaesu 897 with a power supply and trying to understand how/when to use the ferrite beads.
The frequencies used for the antenna are 1.9mhz 3.8mhz, 7.200mhz, 14,200 mhz, 17mhz, 21 mhz, 28 mhz, so it looks like the mix 31 are the right choice. Are these clamp on beads? should I use more than one for each antenna?
Are they needed on the dc power supply? Anything you can help me with would be great so I can improve my radio operating conditions.
I have 2 antennas, an end fed and a dipole with similar frequencies as stated above.
Lou
KC2YNC
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Hi Lou
Sounds like a very nice station you have. I have used both rigs and they work great!
Now as to ferrites, the NK7Z is correct in using ferrite mix 31 for his station. In general mix 31 is effective from 1-250 Mhz, but the choking impedance varies as the frequency (like an inductor). More ferrite is required on lower bands (160 meters, 80 meters) than on 10 meters to get the same amount of choking.
For 50 ohm feedlines you generally need at least 10 times the feedline impedance or 500 ohms with 1000 ohms being better if you have a lot of outer shield common mode current. For reference, a typical ferrite snap on has about 50 ohms of choking impedance for a single turn, but if you can put more than 1 turn thru the center, the Z increases as the square of the number of turns (e.g. 2 turn = 4*Z, 3 turns = 9*Z, etc). For your coax feedlines, a minimum would be 5 beads such as our BA-8 kit (clip on beads) or Kit 105 (Snap on beads). These groups of ferrites chokes are called line isolators, 1:1 baluns, or feedline chokes depending on who you buy them from.
The form factor of the choke depends on the size of coax feedline you have. For RG-8X size you can use simple ring toroids with multiple turns which are very effective (Palomar part # F240-31). For 1/2″ coax like LMR400 or RG-213, you can use larger snap ons like our FSB-1 (1″ in hole) for 3 turns of a 1/2″ cable – very effective!
If you use an antenna switch then I will suggest another, more effective choke. Run all you antennas to the switch and put a CB-1-5000 or CB-1-1500 CUBE choke between the antenna switch and the antenna tuner/amp/transceiver. This choke has SO-239 (Coax connectors) on the input and output and just installs in series in the coax line. It has a typical choking impedance of 3K-10K ohms (depending on frequency) AND with this high value of choking impedance it may significantly reduce you receiver noise due to common mode current (neighborhood noise). I have seen testimonials of 1-4 “S” units noise reduction using this RFI choke. I use 1 on the output of my Alpha amplifier which then feeds the antenna tuner with a multi-antenna switch.
OK now for the AC, DC and interconnect lines. In general most RFI issues are on 160-20 meters and are caused by the near field antenna radiation OR the feedline shield acting as an “antenna”. Radiation is picked up by the longer “antennas” in your house like the AC wiring, Phone/DSL lines and in some cases satellite/cable coax shields. These “antennas” couple RFI into you radios (or computers or stereo system) via the power cords and you hear yourself coming through the speakers, or the device freezes, or otherwise malfunctions. The “Cure” is choking all lines going into the Victim device – that includes AC or DC power lines, all I/O lines. If you experience RFI at high frequencies like 10-20 meters, then look for shorter “antennas” like USB camera lines, long mic cables, etc that are < 1/4 wavelength long at the interfering frequency and choke these off.
For AC/DC power lines the most effective choke is the ring toroid with multiple turns (Palomar part # F240-31) – use on one every AC power cord and DC power cord – 3-6 turns can go thru with the plug attached. For most stations, 10-20 of these ring toroids should get the job done. I would suggest a couple of 10 packs of F240-31 and a couple of 10 packs of FSB-1/2 (1/2″ hole) ferrite snap on for the interconnect cables, and a CB-1-1500/5000 for the common feedline. That package of ferrite chokes should significantly reduce your common mode RFI issues or prevent them from occurring in the future as you add/change your gear and antennas.
For additional info click on any of the presentations that I have given for a Power Point™ slide show presentation of the above details with specific example of how to use various different forms of ferrite chokes. Here is the link for the presentations: https://palomar-engineers.com/speaker-presentations
Hope that helps
Bob Brehm, AK6R
Chief Engineer