HFU HF Underground
Technical Topics => Equipment => Topic started by: ChrisSmolinski on December 27, 2016, 2153 UTC
-
Once again, a giant ferrite toroid coil saves the day. I have a random wire antenna running into the basement workshop, and reception was horrible, I could barely hear anything, even SWBC stations. I considered that maybe it wasn't a lack of signal problem so much a signal to noise problem, so I located a large ferrite toroid coil from the junkbox, wrapped as many turns around it as I could (about a dozen), on the end at the radio, and voila, the noise/hash was gone.
I have these on the sky loop antenna feed, and should put one on the beverage. Actually, I think that's what this one was meant for. Guess I need to buy some more :)
It was a Fair-Rite 5943003801, 61 mm toroid, type 43 ferrite. I buy mine from Mouser for about $4 each: http://www.mouser.com/ProductDetail/Fair-Rite/5943003801
-
I wonder if these would help me with my lower frequency listening. You get a little way below 6 MHz and everything becomes staticky, regardless of atmospheric conditions, weather, etc. I've finally hypothesized that my suburban environment is to blame, but need a way to test it.
-
Probably worth a try, especially at around $4 each (they seem to sell for more anywhere other than Mouser). I noticed only one cheaper large ferrite toroid at Mouser, a TDK core: http://www.mouser.com/ProductDetail/EPCOS-TDK/B64290A0048X830/
It's not much less, but much larger in size, you could get a lot more turns on it. The composition is material N30, whatever that is. The AL value is 5500, vs 1075 for the type 43 Fair Rite core. Not sure how it would work for this application... paging jFarley to the Electromagnetics Courtesy Telephone...
-
Don't know nothing about N30, and never used it. High mu mixes such as 43 and 77 work wonders at MW to lower HF in cleanup chokes.
At lower freqs, I have used a number of FT-140A-W for chokes and transformers, and I think this mix has worked a little better for me. You can get a dozen turns of a USB cable through the aperture of this core.
-
I found a data sheet for N30: https://de.tdk.eu/download/187204/d0c6f095380215e7cb282c0c66ddf5d5/pdf-n30.pdf
-
I got a pile o' toroids from Amidon. I've tried them, but they don't help. Considering my loop, when rotated, changes RFI, I can pretty much say that my RFI is coming in from not the coax.
<rant>
Dear Power Company & Valued Neighbor - KNOCK IT OFF.
</rant>
-
You put a wire antenna around a ferrite core?
-
Sorry, I should have been more clear, it's the coax cable that is wrapped around the core. Like this:
(http://www.radiohobbyist.org/blog/mypics/RFI_chokes.jpg)
-
Ah yes. Ferrite can do miracles.
-
Here's a short before and after video, showing the improvement: https://www.youtube.com/watch?v=iZeJgI0mi5o&feature=youtu.be
-
That is very considerable improvement.
-
John Bryant wrote an excellent article on the use of RF chokes to "sanitize" your coax:
http://www.dxing.info/equipment/coax_leadin_bryant.pdf (http://www.dxing.info/equipment/coax_leadin_bryant.pdf)
The article discusses different core materials, and provides equations on designing chokes for your frequency of interest.
I have a similar installation as Chris on all my coax lead-ins, except that I use a smaller FT-140 type J core and RG-174 coax. The RG-174 coax is terminated on each end with a PL-259 connector. I then use a female-female coax adaptor to connect the choke in between the coax lead-in and my antenna switch.
It's a worthwhile experiment to see just how much unwanted signal your coax will pickup on it's own, with a short between the center and braid at the remote end. In my case I found a significant improvement with only a single choke near my radio, as the choke outside at the antenna didn't make much of a difference. Also, I didn't bother with the separate ground on the shield remote to the radio.
If you use coax as a lead-in for your longwire, you will also get a huge improvement if you use an RF matching isolation transformer to connect the coax to the longwire. This is a lot easier to do than it sounds. John Bryant also wrote an article on this topic: http://www.dxing.info/equipment/impedance_matching_bryant.pdf (http://www.dxing.info/equipment/impedance_matching_bryant.pdf)
You can also use Minicircuits transformers to do the same thing, with no need to wind anything. http://www.minicircuits.com/products/transformers_pic_a.shtml (http://www.minicircuits.com/products/transformers_pic_a.shtml)
Bruce Conti has an excellent tutorial on the construction of these isolation transformers. I've built several for DXers using the Rubbermaid baby food container as the housing, and this made the difference between hearing mostly noise to actually logging some real DX.
http://www.bamlog.com/diyxfmr.htm (http://www.bamlog.com/diyxfmr.htm)
-
Thanks for the great links, Curious George!
-
Excellent post, thanks Chris
-
The coax shield is acting as the other half of the antenna. AC current needs 2 wires. Attach the shield to some ground laid radials where it enters the shack. Ideally one would want a single point ground which is bonded to the house AC mains at this location. Most of my noise floor was lowered with counterpoise and bonding. Ferrite was added icing on the cake.
Counterpoise, counterpoise, counterpoise...the other half of your antenna. 73