[Elecraft] RF into K3 via serial port?

[Matt Zilmer]

Hi Bill,

I've had RF problems with powered speakers and with USB.  In my case,
the antenna is more than 50 feet from the shack location.  Some RF was
getting into the speaker input or circuitry anyway.  Since it also got
into the USB, I also isolated the computer.

The antenna is not resonant except on 80m, so there is some amount of
RF on the feedline even though it's isolated using a BL-2 balun before
a transition to 450 ohm ladder line.  That transition point is about
12 feet from the shack.

Using a few Laird Technologies toroid cores solved the problem.  In
case it matters, I used cores that are most effective at 5-10 MHz and
large enough to put 4-5 turns on each core.  This gives a few hundred
ohms of isolation.  The Laird cores I used were Laird part number
LFB360230-300.  You can buy these through Digikey using their part
number 240-2287-ND.

73,
matt W6NIA



On Sat, 5 Apr 2014 17:10:01 -0700 (PDT), you wrote:

>  In my transition to Win7, I added a 2 port serial PCI card (NewEgg purchase -- Sunix 5037A) to my Dell PC. I was going to use one of these 2 ports to communicate with my K3 Utility terminal program. Very handy CW / RTTY application. Shortly after installation I made a nice contact on 30m RTTY and then moved to 80m RTTY. SURPRISE!!  During TX the K3 hung in TX and only a couple of characters (if that) were ever transmitted. I was attempting to run about 70watts or so. Only by reducing power to a few watts would the RTTY transmission seemingly be successful.
>
>  The 80m antenna is resonate and very low SWR even without the KXPA100 tuner. (My K3/10 is for VHF/UHF/Microwave IF use mostly, hence the external PA/Tuner).
>
>  So RF is getting back into the K3 .. odd ... nothing is "hot" in the shack at all. My workaround is to use the new serial ports for other hardware/applications and use the one serial port in the Dell Dimension 4700 for the K3 / terminal program. Things are happy that way.
>
>  I may investigate and try a ferrite core on the serial cable. But I will need to get an "HF" core since all of mine are for higher freq use. I just thought this was really odd. My serial cable is "homebrew" minimum conductor cable and I don't recall if it is shielded or not.
>
>
>  Anyone else had any similar experiences??
>
>73 Bill  K0AWU
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Matt Zilmer, W6NIA
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