[TenTec] RF gets into computer

[Dr. Gerald N. Johnson]

On Sun, 2007-04-22 at 13:10 -0400, Martin AA6E wrote:
> Dr. Gerald N. Johnson wrote:
> > On Sun, 2007-04-22 at 16:22 +0000, John B. Egger wrote:
> >> John K Gotwals wrote:
> >>
> >>> When I operate on 75 meter SSB with my Orion, my computer beeps and 
> >>> disconnects/reconnects my USB connected keyboard. Note that I do not have 
> >>> this problem on 40 meter SSB. My computer is located on the second floor of 
> >>> a brick house, and one end of my dipole is about 15 feet from my shack.
> >>>
> >>> >From comments made by others on this list, I may have to put RF filters on 
> >>> the cables and wires connected to my computer. I am contemplating the use of 
> >>> Amidon FT-240-77 ferrite toroids, which have an I.D. of 1.4 inches and cost 
> >>> $9 each. If I buy in quantities of 10, is there a cheaper source for this 
> >>> toroid?
> >>>
> >>> Any comments or suggestions on this problem would be appreciated. 
> >>>
> > 
> >>>  
> >>>
> >> Old thread, but something is exacerbating my problem recently. On 40m CW 
> >> the Orion II at full power is now QRM'ing my computer; I had to copy 
> >> some traffic by pen and paper this morning because of it.
> >>
> >> Two experiments, after wrapping the keyboard cord in three Radio Shack 
> >> snap-on ferrite filters: (1) Moving the keyboard about 3' from the 
> >> Orion, instead of its usual 1', allows full power keying without 
> >> interference, and (2) using a dummy load on the Orion, and then on the 
> >> output of the MFJ 993B autotuner, allowed full power without 
> >> interference, with the keyboard in its normal place. (Reattaching the 
> >> antenna produced the QRM again.)
> >>
> >> Any thoughts? I thought perhaps the RFI was from the Orion front panel,  
> >> which might explain the moved-keyboard success, and I was surprised to 
> >> see the problem disappear with the dummy load.
> >>
> >> --John K3GHH
> >> Registered Linux User #291592
> >>
> > Seeing the problem cured with the dummy load points to the antenna. You
> > need to either reduce power or move the antenna. You are getting plenty
> > of RF exposure that close to the antenna as the computer is detecting.
> > USB bandwidth includes 40 meters so a choke on the outside of the cable
> > may not have effect.
> > 
> > Look to Mouser, www.mouser.com for more toroids.
> > 
> > 73, Jerry, K0CQ
> >
> 
> Not so much the antenna, though that could be the source if it is close 
> by,  but more likely the feed line.

Antenna at 15' I call close. If the feed line is radiating that makes it
worse and even with a balun at the antenna, if the feed line isn't at
right angles to the antenna RF will be coupled to the outside of the
feedline.

> Is your antenna properly balanced 
> so that there is no current flowing on the outside of the coax -- 
> assuming you are using coax?  If not, you may need a balun at the feed 
> point or possibly a "1:1 current balun" ("unun") in the shack.
> 
> Ferrites on the computer wiring may help -- I generally use 6-8 turns on 
> type 43 cores.  This helps to suppress computer to receiver interference 
> also.
> 
> I haven't had a problem with interference to the computer, even running 
> QRO. But you've got minimize RF in the shack.
> 
> 73 Martin AA6E

73, Jerry, K0CQ