++++++++++++++++++++++++++++++++++++++ Date: Wed, 2 May 2001 09:45:08 -0100 From: "Julian" Subject: Re: [Elecraft] RF into computer A fully isolated interface with opto-isolator in the PTT line and 1:1 transformers in the audio lines would probably fix the problem, as would a proper ground. - -- Julian, G4ILO. (RSGB, ARRL, K2 #392 ++++++++++++++++++++++++++++++++++++++++++++++++++++ Date: Mon, 30 Apr 2001 09:41:37 -0700 From: Vic Rosenthal Subject: Re: [Elecraft] RF into computer. Alex wrote: > > I seem to be getting RF into the PC on PSK. I am running shielded audio > cable, but still causing lots of horrible interference on the transmitted > signal.. > > This problem only occurs when using the balanced dipole and tuner. Coax fed > dipoles are ok. Can anyone help explain this also. If you are connecting a balanced feedline to the output of the KAT2, the problem is that RF is flowing on the outside of the K2 case and all over your shack, including cables, computers, etc. The length of your feedline is probably presenting a high impedance on the band in question. The difference in capacitance to ground of the two conductors (one of which goes to the chassis) is making the chassis (etc.) hot. A balun probably will NOT help in this situation, because most baluns are designed to work over a limited range of (low) impedances. In addition to the RFI problem, you are wasting power in radiation from the unbalanced currents in the feedline, and from the shack wiring. You can eliminate the interference by connecting a 1/4 wavelength wire to the chassis. This will force a voltage node at this point. However, it will not reduce the radiation losses from the feedline, and the wire will radiate itself. The best solutions are 1) change the feedline length so that the KAT2 will see a low impedance AND use a balun, or 2) use a true balanced antenna tuner (like an old Johnson Matchbox) instead of the built-in tuner. 73, Vic, K2VCO Fresno CA ++++++++++++++++++++++++++++++++ Date: Tue, 1 May 2001 08:56:28 -0500 From: "Joe Wittmer" Subject: Re: [Elecraft] RF into computer. Hi Alex, I am using a 949E and a Squirt antenna. My feedline runs out of the house from the shack. I found that the antenna would not work well if I ran parallel with the house to the apex of the Squirt. I had to run the feedline up and out away from the house to the apex of the Squirt. So, I believe that the feedline was resonate with something in the house (maybe the siding? the ac lines? ) when running parallel with the house. I was really amazed at the difference... The antenna worked ok at 14070 but I could not move up and work RTTY at all... I can now work a wide section of the band without much adjustment in the tuner. So, I thought it might be possible you are having a similar situation. I know my antenna is not the best... But, I have had pretty good luck with it so far from my QTH. BTW, I know this is off topic... But, I quite enjoy hearing how other K2 owners are handling antenna issues. 73 Joe KB9SIZ - ----- Original Message ----- From: "Alex" To: "Stuart Rohre" ; "ZOOM" ; Sent: Monday, April 30, 2001 7:36 PM Subject: Re: [Elecraft] RF into computer. > Thanks to all who have volunteered help with my problems. Here is some > clarification and more information. > I'm not using the KAT2,I'm using the MFJ-949E and also have the MFJ 931 > artificial ground with counterpoise wires attached to it. This is feeding > 450 ohm ladder line to a dipole in the attic which roughly 40 ft long. There > is not an earth ground. The counterpoise run very close to the computer and > could be the source of most of the interference under the right conditions. > Hope this answers a few questions. > > The PSK interface is pretty much the basic i/f ; has audio from the K2 > through a shielded cable to the sound card and output from the sound card is > fed to the mike input through a radio shack transformer and a voltage > divider. > > Thanks > Alex Turner, N4BYJ ++++++++++++++++++++++++++++++++++ Date: Tue, 1 May 2001 14:17:02 -0500 From: "Stuart Rohre" Subject: Re: [Elecraft] RF into computer. Alex's Computer RFI was much more understandable when he explained his use of an external transmatch, and a counterpoise tuner, and wire counterpoise near the computer. Such a set up would introduce imbalance, or at least radiation from the tuned counterpoise wire. Just as we say to try a tuned counterpoise or quarter wave radials from the rig to eliminate RF in the shack, we also need to be aware of balanced (center fed) antennas or loops and variants not needing other RF grounds. And, we need to be aware that if the shack is close to the antenna, as on an upper floor, direct radiation may re-enter the rig. Another thought, Most tuners like the MFJ, need an external jumper wire to properly complete the use of their built in 4:1 balun, and this is sometimes indicated on the back panel by a dotted line between two of the insulated output standoffs. Hope you had that in use with the 450 ohm line. A balanced antenna, or dipole (center fed), should not need the counterpoise, nor an earth ground. Earth ground serves only as a AC safety ground, or static discharge ground. Many hams and the older antenna books tend to confuse the RF and AC /safety/static ground functions. A loop, beam, delta, dipole center fed, etc. do not need anything else, they are self contained radiating and receiving systems. Folks, when relating an antenna problem, or RFI problem, include details of all transmatches used, RF grounds, (or not), radials, counterpoises, distance from the antenna to the rig, bands affected, power at which the problem is noticed, etc. to aid in the group understanding what is likely going on. Be aware most built in transmatches are unbalanced L networks, and thus not directly intended to feed a balanced line/antenna, unless special steps are taken, such as the use of the LDG balun with the K2. 73, Stuart K5KVH +++++++++++++++++++++++++++++++++++++++++++++++ Date: Wed, 20 Nov 2002 23:12:57 -0800 From: "John, KI6WX" Subject: Re: [Elecraft] OT - Toroids for Stereo RFI? To: Elecraft Mailing List Hi; For blocking HF into cables, you want the highest mu ferrite material you can get. I often use #73 material for this purpose, but there are higher mu ferrites available. As for size, you have two choices. You can buy a big toroid and wind a number of turns on it, or you can buy one that just barely fits over the wire (this sometimes requires removing connectors). If you do the second, you do want a tight fit around the wire. Also, some manufacturers make split ferrites which can be clamped around the cables using the plastic housing that contains the ferrite material. These work great when you don't want to remove the connector when sliding the ferrite over the cable. -John KI6WX +++++++++++++++++