RF into the computer during TX on PSK31

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wbswetnam

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I am quite new with the digital modes. I'm having probs with the computer freezing up during transmit on PSK31. Maybe it would be helpful to describe my equipment:

I operate a Kenwood TS480-SAT at 25W (on PSK31), the modem is a SignaLink USB using the USB cable which came with it, an Acer laptop computer running Windows 8 and Fldigi. The antenna is a fan dipole and yes, it uses a 1:1 current balun.

I suspect that RF is getting from the SignaLink modem into the computer via the USB cable. During transmit, it often freezes the program and gets stuck in TX. I have to physically turn off the modem, close Fldigi, then restart it. Then a few minutes later it may do it again.

How do I fix this problem? Is there such a thing as a shielded USB cable?
 

wbswetnam

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popnokick

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They MIGHT work... depends on the frequency and amplitude of the interference. But they're inexpensive to try. There are quite a few sources for them. I've used them to reduce interference in my mobile setup.
 

KE5MC

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I agree with you that RF is getting into 'things'. As commented snap-ons do work, but they usually are the last thing attempted as they don't solve the problem at the origin, but at the device effected.

What kind of SWR do you have? Beyond the 1:1 current balun and fan dipole what other material conditions are you working with?

Mike
 

wbswetnam

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What kind of SWR do you have? Beyond the 1:1 current balun and fan dipole what other material conditions are you working with?

Mike
It is a rather broadband fan dipole, that's probably most of the problem. I have four sets of elements, cut for 40m/15m, 20m, 17m, and 10m. I've discovered that I can work 12m and 6m as well, all with SWR below 2.5 (well, for the lower half of 6m anyway). My radio has a built-in antenna tuner which can adjust for SWR as high as 3.0 but not above.

So, I guess the problem is the antenna... (?) Is there anything I can do about that?
 

ko6jw_2

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It's probably not the antenna itself. After all, it is supposed to radiate RF. There are a number of issues to consider. My first question is do you a good earth ground (not the building's electrical ground) and is everything grounded at a common point? The second thing is that the tuner in the radio does not tune the antenna. It attempts to match the complex impedance of the antenna to the 50 ohms the radio wants to see. That means that the radio sees 50 ohms, but there could be standing waves in the feed line which means that the feed line may be radiating. This brings RF back into the shack. Certainly isolating the USB cables with chokes is a possible solution, but if there is a ground loop this will not help. A ground loop results from a situation where not all equipment is grounded to the same point and a current can flow though the ground (shields) because of these potential differences. You're probably looking at several steps to fix the issues.
 

wbswetnam

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It's probably not the antenna itself. After all, it is supposed to radiate RF. There are a number of issues to consider. My first question is do you a good earth ground (not the building's electrical ground) and is everything grounded at a common point? The second thing is that the tuner in the radio does not tune the antenna. It attempts to match the complex impedance of the antenna to the 50 ohms the radio wants to see. That means that the radio sees 50 ohms, but there could be standing waves in the feed line which means that the feed line may be radiating. This brings RF back into the shack. Certainly isolating the USB cables with chokes is a possible solution, but if there is a ground loop this will not help. A ground loop results from a situation where not all equipment is grounded to the same point and a current can flow though the ground (shields) because of these potential differences. You're probably looking at several steps to fix the issues.
Ummm... ya got me, podner! I have no grounding system at all. Actually, I thought that with a dipole antenna using a current balun, a ground wasn't necessary. The antenna is mounted in the attic so there's no (or little) danger of a lightening strike to the antenna system.
 

jim202

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Another place to consider is the coax cable your using coming out of the radio. Using cheap coax that has limited shielding can cause RF to be in the area of the radio. Take a look at the percent of shielding your coax has. You get what you pay for in coax. The lower cost cable is generally due to the percent of braid used as the shield is less than decent coax cable.

Another point to consider is if you have old LMR brand coax cable. As this coax ages, it can get moisture under the outer jacket and wick into the braid. With the LMR coax, the first shield is an aluminum foil. Then there is a copper braid over the foil. The dis similar material can oxidize and cause all sorts of problems, noise and intermod with transmitter power applied.

Most people think that because there is a ground rod under the electrical power meter, it is a good ground. Point to keep in mind here is the age of that ground rod. If the house is over 10 to 15 years, I would suggest that you install a new ground rod to replace the one that is there. Use a new bonding clamp also. These ground rods corrode with age and loose their ability to provide a low resistance ground.

Reason I say this about the electrical ground rod is I had a lightning strike to a tree near the house I was living at. It did considerable damage to electronic equipment all over the house. I had surge protectors on most of the items that got damaged. This started me thinking as to why all the damage with the surge protectors installed. The only common thing is the electrical ground. The surge protectors use the electrical ground as part of their protection.

I brought home a ground tester and checked the ground rod at the electrical meter. I did pull the main breaker that fed the house. Didn't want to chance of getting an electrical shock when I pulled the meter ground off the ground rod. Connected up the ground tester and did a 4 point ground measurement. It tested to about 200 Ohms. That is a very poor ground for any electrical connection. I put in a new ground rod and measured the ground resistance on the new ground rod. It measured about 3 Ohms. So I found the reason the surge protectors didn't do their job.

This is kind of a long winded comment. But I am just trying to point out that that it may take several steps to solve your problem.

The snap on ferrite cores do work. I had a VHF portable used as a transmitter on a radio interoperability gateway that crashed the computer that had the USB connection going to the computer from the gateway. The ferrite core solved the problem. Just remember that there is different material for different frequencies. You will have to find a core that will work at the frequency range your using. At the HF frequencies your using, it will take a good sized core with the correct type of ferrite material to do much good.
 

wbswetnam

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Another place to consider is the coax cable your using coming out of the radio. Using cheap coax that has limited shielding can cause RF to be in the area of the radio. Take a look at the percent of shielding your coax has. You get what you pay for in coax. The lower cost cable is generally due to the percent of braid used as the shield is less than decent coax cable.
I used brand-new RG8X coax.

I guess I'm wrong about not needing a grounding system, huh? I had a conversation about HF antennas with an Extra class ham and he told me not to worry about a grounding system since the antenna is mounted in the attic. Bad advice?
 

ko6jw_2

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It's true that a dipole antenna does not require a ground in the sense that a vertical antenna does. A vertical requires a ground place or a counterpoise. This a different kind of ground from the one we are talking about. The objective is to have the ground potential of all parts of the system at the same point so that no current flows in the grounds.

Here's an example. I have an Icom 706MkIIG in my car. The antennas are securely mounted and grounded to the car body, yet I had intermittent problems on transmit. I thought it was a power supply problem. The 706 is a bit finicky about seeing 13.8V. I ran new power cables, I bought a battery booster and still no luck. Then one day I decided to run a short length of braid from a piece of old RG8 from the car body to the ground lug on the radio. Problem solved. The radio did not have a good RF ground. The coax from the radio to the antenna mounts was not sufficient.

A final point. There are a lot of reasons for buying good coax, but your issues are not being caused by cheap cable. If the currents between the shield and the center conductor are not exactly the same the coax will radiate. The antenna tuner only matches from the transmitter to the tuner. The situation from the tuner to the antenna is a different story. There can be high SWR in that part of the system and that means that power will be reflected back into the shack.
 

KE5MC

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One of the your comments spoke of your antenna in the attic. Nothing wrong with that, but it does bring into play how balanced is your antenna. Not all current 1:1 are created equal and keeping current off the feedline will be more challenging. Wiring, ductwork and various construction material can easily upset the balanced characteristic we would expect for a fan dipole in the open. End result is common mode current on the feedline beyond what your balun might handle. I use Balun Designs products and he has a good webpage describing his product line. They are not cheap because of the quality of material used and his workmanship is excellent.

Lot of comments on grounds from others. I would focus on making sure all devices plugged into AC outlets are on the same branch of the ground connection in the outlet. I make it easy by only using one outlet and a power strip. Making sure I don't overload the outlets 15A capacity. This is only to make sure AC hum is as low as possible on any interconnecting cables, audio or control. Next is to connect each of your pieces of radio equipment that has a ground stud with a short strap to a common tie point. For me I take that common point to a stake outside the window. I don't expect it to be a robust RF ground and only a modest electrical ground for lighting strikes in the area. A direct strike and I am toast!

Likely that your issue is a combination that adds together and any one change will not make a big change. Also go over you current antenna system connection with a close eye to details. For FD we were having issues with one antenna on 20m. It turns out the chassis connector on the balun had a less than tight connection from the shell/shield side to one side of the balun. Center conductor side was good other was intermittent. Only discovered after FD. No telling how much better that station would have worked if all was well.
KE5MC, Mike

It is a rather broadband fan dipole, that's probably most of the problem. I have four sets of elements, cut for 40m/15m, 20m, 17m, and 10m. I've discovered that I can work 12m and 6m as well, all with SWR below 2.5 (well, for the lower half of 6m anyway). My radio has a built-in antenna tuner which can adjust for SWR as high as 3.0 but not above.

So, I guess the problem is the antenna... (?) Is there anything I can do about that?
 

wtp

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another thought.

by broadcasting in the attic you are right next to the power lines that go into the radio and computer. feedback.
 

wbswetnam

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Problem solved!

I went to Radio Shack and I bought the last two of the ferrite beads they had for sale. I brought them home and I put both of them on the data line to the USB port of the computer, fired up the radio and sent a test message on PSK31. Nope, same problem. Then I put one of the ferrite beads on the power supply input to the computer, right at the DC input jack. I tried it again. Taaaa-Daaaa! That was the culprit! Problem solved.... at least as far as the RF leakage into the computer. But now I have a sneaking suspicion that my antenna is irradiating the house with RF... good thing I'm only running barefoot, and usually just 25 watts on PSK31.
 

khooke

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But now I have a sneaking suspicion that my antenna is irradiating the house with RF... good thing I'm only running barefoot, and usually just 25 watts on PSK31.
Yes, but given you're in close proximity to your antenna, this shouldn't be a surprise? The good news is you're most likely far enough away (several feet, even if you're in a room immediately below the antenna) to still be safe limits for Maximum Permitted Exposure, even if you're running 100w. Couple of good links for reference (the second table is very useful for a quick check):
FCC RF-Exposure Regulations -- the Station Evaluation
http://www.arrl.org/files/file/Technology/tis/info/pdf/Table4567.pdf

If you're worried about the impact to other electronic equipment, then you'll just have to observe and see if anything 'acts odd'. Some things will be more obvious than others... for example one of my hardwired smoke alarms starts chirping with just 10w on 10m for some reason specifically around the JT65 freq of 28.076 but not higher freqs and is ok at 100w on 15m and 20m. My coax to my wire dipole in my attic was running directly over the detector, which I didn't know initially because the detector is under all the insulation. Moving the coax just a couple of feet solved the problem for 15m, but not so far for 10m. But knowing that, I just avoid doing JT65 on 10m.

Some experimenting on different bands and different power levels might be worthwhile?
 

KR0SIV

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I have fried a motherboard/processor with both a UV-5R and a Kenwood HF rig....
Good GROUNDING and Shielding is a MUST
 
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