locked Re: Hamlib 4.5 testing #hamlib


Reino Talarmo
 

Hi All,
When you have RFI problems, then Jim's K9YC net pages provides a lot of useful information. Most probably the discussed case is so called Pin 1 problem.
The shield cutting and ferrites may help as just a few dB less RF current in the victim wire could (marginally) solve the problem. Both help, if there is a low impedance to a common ground at both sides of the USB cable.
If an end fed antenna is used, then the "missing half" of the antenna i.e. a low impedance connection to ground needs to be arranges somehow. An end fed antenna is actually more or less a vertical antenna unless all station equipment are hanging in the air, including power supply, PC and operator such as in The Zeppelin case. RF always finds a way to ground (or to other wires or structures that form a suitable RF impedance). If that impedance is high, then voltage will be high as some current in flowing.
Typically that "missing" part of the antenna is outer shield of the feedline, rig enclosure, any wires connected to rig including USB cable, PC and any wires connected to PC including operator. A good symptom is mouse or touchpad that goes wild during transmission.
How we can divert RF current from entering into rig's or PC's sensitive circuits? (The sensitive circuit need not to be the actual USB chip, but some other related circuitry). The first should be a proper grounding or counterpoise at antenna feed point, next is a low impedance grounding of the enclosure of the rig. Now the remaining RF current flows on the USB cable as common mode. Best would be to by bypassing USB data and power lines by a low impedance. That means a good shielded cable, where the cable shield is only connected to the outer shells of connector and the shells are connected to enclosures outside or rig and PC. The latter requirement may be impossible due to various reason how connectors are wired to enclosure and circuit board ground. Another or additional method is to connect enclosures together by a short heavy wire. This may be impossible especially at the PC end. I have had some success by using VGA connector shell for that purpose.
The next method is to add common mode RF impedance of the UBSB cable by adding ferrites. That helps, if the PC happens to be a low impedance to ground. If the impedance of the PC to ground is high say hundreds of ohms, then a typical one turn ferrite clam with tens of ohms impedance has very little effect.
The shield cutting may help as the Pin 1 problem is minimized. We should remember that there are still data and power lines inside the USB cable and those will then curry the common mode current. The common mode RF current distribution inside the equipment may be less disturbing due to different impedances and well balanced data line construction. The shield cutting may as well make situation worse.

Better to stop now. In short it would be better to divert the common mode current to ground before it reaches the USB or some other RF sensitive connection cable.

73, Reino OH3mA

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