RF issues with logging computer / internal keyer

[Julius Fazekas n2wn]

I couldn't find the original post for this, but did get to see some neat videos.

I've been toying with mobile/portable operations for a couple years now using mostly a K2. Did use my K3 in the WVQP mobile this year.

My first attempts were OK, but laced with problems. Often I had to drop from 5 watts to 2 to keep my logging computer from locking up in a key down state. I was using the K2 with the KAT2, either just the internal battery or a combo of internal and the truck battery. The antenna was often non-resonant wire on a fishing pole.

At times I thought I had the RF problems solved, but the next contest would prove me wrong. Things improved with various changes, like adding a radial. Might get all of 3.5 watts out before lockup.

I finally ironed out the problems I was having... Nothing radically new, I eliminated the problem at the source: the antenna. At one hamfest, I picked up a group of mobile antenna parts Hustler and Hamstick. Not putting down the fishing pole concept, it works (and could have worked a lot better) but it's not a driving around antenna. I guess this stuff made me think. What do I want to accomplish? Get out better, get rid of RFI and leave the antenna up moving from point A to B.

What have other done? What have I learned from the home station and endless reading about and working on 160M verticals?

First, make sure you're working against a decent ground. All vehicles are not created equally when it comes to ground. Some of my buddies have had to bond body parts, engine parts and exhaust system to get their systems to work right. I use the rear bumper trailer hitch hole as my mount and ground. It's done great.

Second, use a balun at the base of the vertical. This assumes you have a reasonable SWR to begin with for the frequency(s) of interest. I used part of an Elecraft balun kit inside of a plastic electrical box.

Third, use a shunt coil. This really helped with 80/75/40. 40 was the worst band in my setup for RFI problems.

No longer do I need to crank down the power. I can run a full five watts (or the max for a QRP K2 and K3) problem free. I'll be testing it for 100 watts, but don't think I'll have an issue, at least close to the tuned frequency of the antenna. I'm not sure how well it will behave trying to run CW and SSB from antenna tuned to the CW portion of the bands.

Too, the K3 was a better battery state indicator than my "real" battery tester. It dropped out when I started the vehicle... What do you know, turned out my battery was on it's last legs! New battery, no low voltage dropout by the K3.

73,
Julius
 

[Jim Brown-10]

On Tue, 30 Jun 2009 11:15:50 -0700 (PDT), Julius Fazekas n2wn wrote:

>My first attempts were OK, but laced with problems. Often I had to drop from
>5 watts to 2 to keep my logging computer from locking up in a key down

The fundamental cause of this problem is the improperly designed serial cable
for the K2. It uses parallel wires, and is shielded. A proper cable designed
for RF immunity requires TWISTED PAIRS.

I ran into exactly the same problem you did in Chicago, when my antenna for
80M and 160M was a long wire that ended in the shack and was fed against a
counterpoise. That put a current maxima in the station, right next to the
computer, and the associated magnetic field coupled into the serial cable and
locked up the computer (in my case, at about 8 watts).

The fix was quite simple. I replaced the parallel wire cable between the K2
and the computer with CAT5, using one pair for each signal circuit. I also
terminated the return of each pair to the SHELL of the DB9 rather than pin 5,
fixing a pin 1 problem that also couples RFI. With that simple fix, I could
run my K2 to my Ten Tec Titan at full power with no RFI.

The wiring of a conventional serial cable is documented in

http://audiosystemsgroup.com/HamInterfacing.pdf 

Note that the K2 serial cable is NOT a standard serial cable, so the wiring in
the applications note applies only the wiring between the K2 and the computer.
No change is required for the wiring between the K2 and KPA100.

Note also that a cable shield provides NO magnetic shielding, only ELECTRIC
shielding. Twisting is required to prevent magnetic coupling, and also greatly
reduces electric field coupling. Nearly all coupling in the near field of an
antenna is magnetic, except at higher HF frequencies. It would only be
necessary to shield this cable if you are using high power above 15MHz with an
antenna that is within a few feet of the rig.

73,

Jim Brown K9YC



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[Julius Fazekas n2wn]

In this case, the cable was made from twisted shielded pairs. This was after one of the discussions on the K2 cables on this reflector. The cable I built also did not have the dogleg, just rig to K2.

I, too, had problems on 160 at one point in time... The base of my vertical, and max current, is still about 30 feet from the operating position. A new cable and better antenna interface cleared those problems up. Not sure if that would hold true if I ran an amp, sometimes LP/QRP is a real blessing.

As I recall, I tied the drains together, but left them floating (no connection to pin 5 or the connector shell).

I am sorry I didn't pick up a couple reels of CAT5 cable that were being auctioned at the lab, went really cheap. That was before I knew the benefit of the stuff in control applications. Oh well, live and learn.

Pleased to say no RFI issues in the shack, even though it is on the second floor. Your documents have helped.

73,
Julius

Jim Brown-10 wrote

On Tue, 30 Jun 2009 11:15:50 -0700 (PDT), Julius Fazekas n2wn wrote:

>My first attempts were OK, but laced with problems. Often I had to drop from
>5 watts to 2 to keep my logging computer from locking up in a key down

The fundamental cause of this problem is the improperly designed serial cable
for the K2. It uses parallel wires, and is shielded. A proper cable designed
for RF immunity requires TWISTED PAIRS.

I ran into exactly the same problem you did in Chicago, when my antenna for
80M and 160M was a long wire that ended in the shack and was fed against a
counterpoise. That put a current maxima in the station, right next to the
computer, and the associated magnetic field coupled into the serial cable and
locked up the computer (in my case, at about 8 watts).

The fix was quite simple. I replaced the parallel wire cable between the K2
and the computer with CAT5, using one pair for each signal circuit. I also
terminated the return of each pair to the SHELL of the DB9 rather than pin 5,
fixing a pin 1 problem that also couples RFI. With that simple fix, I could
run my K2 to my Ten Tec Titan at full power with no RFI.

The wiring of a conventional serial cable is documented in

http://audiosystemsgroup.com/HamInterfacing.pdf 

Note that the K2 serial cable is NOT a standard serial cable, so the wiring in
the applications note applies only the wiring between the K2 and the computer.
No change is required for the wiring between the K2 and KPA100.

Note also that a cable shield provides NO magnetic shielding, only ELECTRIC
shielding. Twisting is required to prevent magnetic coupling, and also greatly
reduces electric field coupling. Nearly all coupling in the near field of an
antenna is magnetic, except at higher HF frequencies. It would only be
necessary to shield this cable if you are using high power above 15MHz with an
antenna that is within a few feet of the rig.

73,

Jim Brown K9YC



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[Elliott Lawrence-3]

The mike jack connections as shown on page 13 are pin 1 audio, pin 7 and 8
ground.  I assume that either or both ground pins can be used and their is
no need to use one versus the other.  My experience with other radios using
multiple ground pins is that one had be associated with the audio input and
other for control purposes.  The isolation was necessary to prevent noise on
the audio input.

Thanks,
Elliott WA6TLA

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