RF in the shack? - Page 1
Forum section: electronics / rf-microwave

author: djacobow
author_rank: Super Contributor
timestamp: February 16, 2017, 06:30:17 pm
content: I've got a basic 100W transceiver with a USB soundcard built in. When I try to use a digital mode above 10W or so, the USB hiccups and my software (FLDIGI, WSJT, etc) crashes, and usually, the radio is left in xmit. :-(I'm getting to my wits end trying to deal with this.Setup:* IC7300 connected by USB to desktop computer* feeding separate 20m and 40m dipoles through RG8/U and MFJ949-E* Antennas are mounted in a tree in a vee, the 40m is about 30 feet up, the 20m is about 25 feet up. Because of space limitations the conductors are not 180 degrees opposite from each other in azimuth. It's more like 150 degrees.* Antenna feedpoint is almost directly over the shackThings I have tried:* replaced my old antenna (G5RV hacked various ways) to these bog-simple dipoles* small fortune in mix-31 ferrites. Have them on the AC cable to the rig and computer PSUs. Have them on the DC between the rig PSU and rig. Have on the USB at the rig end and at the computer end. Have an MFJ915 choke on the feedline.* taking tuner out of the eqn (SWRs are fine anyway, and there is an internal tuner in my rig that can handle them fine. I got the tuner to deal with my old antenna)* I don't have a great grounding setup. I have tried floating everything, and I have tried grounding everything together to regular electric service ground (which is only about ~10 feet of solid #12 and a few more feet of solid #6 to the ground rod)So far, no luck. Still the same problem. Radio keys up, maybe a second or two of modulation, then USB and fldigi die and radio is left in xmit (but with no modulation).I *think* this may be a classic "RF in the shack" situation, but I am not entirely sure it's because of feedline currents. It could be from that fact that the intentionally radiating part of the antenna is so close. How do can I find out the source?I'm thinking of pounding a new ground rod just for the station, along with that expensive ribbon braided grounding conductor to reach it. I have no confidence it will make a difference, and it is more money and effort just to find out. I would think that in a dipole grounding shouldn't be such a concern, anyway.I can use the radio at full power with a key or the hand mic. The radio seems perfectly happy. Furthermore, the computer itself doesn't crash. I'm assuming it's the USB connection. I guess it could even be the USB chip in the radio.Sorry for the long post.

author: mmagin
author_rank: Frequent Contributor
timestamp: February 16, 2017, 07:25:07 pm
content: How is the feedline (coax?) coupled to the dipole? If you're not using some form of balun, you're putting a balanced load on an unbalanced transmission line, which at a minimum will change the antenna pattern.

author: worsthorse
author_rank: Super Contributor
timestamp: February 16, 2017, 08:09:45 pm
content: definitely sounds like RF in the shack. is it happening on both bands? does it happen as you run the power down from 100W to 5W?one potential problem is the antennas being directly over the shack. take at a look at the radiation pattern for a dipole, especially in a vee configuration to see why. another is common mode current on the coax. how are you feeding the dipoles? are they symmetrical? do you have a 1:1 choke balun on the coax feedline? 73 - bill

author: djacobow
author_rank: Super Contributor
timestamp: February 16, 2017, 08:10:04 pm
content: Antennas are commercial ones I got at HRO. Radio Oasis. Feedpoint is a "W2AU ANsulator", which perhaps I naively assumed includes a balun.My old antenna had a ladder line feed line into my tuner's built in balun. Same issues.

author: worsthorse
author_rank: Super Contributor
timestamp: February 16, 2017, 08:49:00 pm
content: You say this all works fine with voice modes. Do you mean SSB? FM? I honestly don't understand how. And I am not sure but I don't think that the Ansulator is a choke or even a balun. You may want to look up the specs to see what it is designed to do. If it isn't a choke, first thing to try is adding one, maybe more than one. If you aren't going to make it yourself, buy a good quality, heavy duty choke. Ferrites will help but they, too, have to be good quality. If you bought them on ebay or aliexpress, they probably aren't. The first thing to try is turning the power down and seeing at what power the problems start.At the minimum you need a transforming balun from the dipole to the coax feedline and a choke on the coax itself. Without the first, you have an impedance mismatch of significant size at the feedpoint, which is definitely creating a common mode current problem. Without the second, any common mode current on the coax is getting into your equipment. Unless you've trimmed the antenna to be resonant on 40M and aren't using it on other bands, you also need an antenna tuner, but you didn't mention one. What kind of SWR readings are you seeing?I don't know what digital modes you are using but none of them require 100W. Have you followed the instructions for setting up your rig for digital modes operation? Same question for the Signalink setup, including how to set the transmit audio so you don't splatter.

author: djacobow
author_rank: Super Contributor
timestamp: February 16, 2017, 09:40:46 pm
content: Thanks for thinking about this.To clarify:* Does it happen on 20m and 40m both? I think so, but at different power levels. I will check again.* I should have said that if I am not using the USB, the radio works fine at any power level. That can be SSB voice using the mic, or CW with a key.* I should have said that the 40m antenna does have a choke on it. The 20m does not. The choke is the MFJ deally sealed inside PVC. I think it's just a length of coax with a bunch of ferrite beads on it. I don't notice a significant different in behavior whether its there or not.* I guess I don't know if the center insulator of the antenna has a balun or not. I guess for $50 it probably doesn't. I can order a proper balun, of course. Getting the antenna down and up is not something I look forward to, but...* The ferrites I have are Mix-31 from Palomar Engineering, so I think they're the real deal.I play around with various digital modes. For JT65 I usually use 5W and generally won't go over 15W. I like to use PSK31, RTTY, and I want to play with others like Olivia. I like to keep the power dialed down, but I will occasionally crank it to try to get heard by a call I really want in the logbook.

author: worsthorse
author_rank: Super Contributor
timestamp: February 17, 2017, 04:07:07 am
content: okay... then the problem is not the antenna or feed system, it is in your digital mode setup. that said, the suggestions i made about the antenna, choke, etc are still good ideas...the problem happens with fldigi. are you running flrig? can you use flrig in other modes without problems? have you tried other rig control programs? other digital programs?if i were you, i would start from scratch. start with the receive side of the signalink and computer. do the setup from scratch. make sure the signalink jumpers (JP1, JP2, JP3) are not shorted. test the receive side on multiple modes. can you receive and decode reliably? then move to the transmit side. go through every relevant setting on your rig, and follow the instructions for signalink and software transmit meticulously. i suggest using a program like digipan or soundmodem with a "tune" function to test and set the rig ALC. if the setup works with the tune function, it should work for modes, too.if the problem happens when you are trying to set up the transmit side of the chain, it may be a problem with the signalink PTT function.good luck!

author: djacobow
author_rank: Super Contributor
timestamp: February 17, 2017, 05:30:36 am
content: Thanks for the ideas, wch.I was not using flrig, just the built in rigcat in fldigi.I just switched from my shack desktop computer to a laptop, though, and it seems to be working reasonably well. In fact, I just had a couple of PSK31 QSO's -- huzzah!So maybe it's something with that computer. I also think it might the USB hub I was using. (I'm plugged right into the laptop right now.)I would like to get back to using that computer, but at least I know it's not my rig and, if my antenna system is causing RFI, it is not so excessive that it makes *any* computer malfunction. :-) That said, I think I will put proper transformer type baluns on both my antennas.Anyway, I am happy to have some progress, finally!

author: borjam
author_rank: Unknown
timestamp: February 17, 2017, 07:16:28 am
content: Using an Icom IC-7200 I had problems with the CI-V baudrate set to "auto". Try fixing it at, say, 19200 bps. You need to configure that both on the radio and the control program you're using.

author: voltz
author_rank: Frequent Contributor
timestamp: February 17, 2017, 08:11:13 am
content: As a quick test, try the rig into a dummy load and run at full power. This will confirm its RF energy related from the antenna or feeder. If the problem persists, there could be something local like an earth loop via the USB/PC connection to ground.Assuming this is antenna related (and it probably is), try looping the coax a few turns say 4 inches in diameter and holding it together with a couple of cable ties. The loops should be close to the rig/tuner ANT socket. Alternatively try using ferrite rings to do the same job. This forms an RF choke and will reduce RF from the coax shield getting back into the rig.

author: djacobow
author_rank: Super Contributor
timestamp: February 17, 2017, 06:55:17 pm
content: Quote from: borjam on February 17, 2017, 07:16:28 amI am not sure how this could be related, since it works fine under low power. But it is certainly easy enough to try. I'll let report back.

author: djacobow
author_rank: Super Contributor
timestamp: February 17, 2017, 06:58:17 pm
content: Quote from: voltz on February 17, 2017, 08:11:13 amYes, I have no problems into a dummy load. I'm sure that RF is the source of the issue. But I'm not sure if I have "excessive" RF in the shack or just an "excessively sensitive" computer/USB setup. The fact that the laptop connected over USB works when the desktop connected through a USB hub does not suggests that it is likely a little of both.Quote from: voltz on February 17, 2017, 08:11:13 amI have these loop chokes at the antenna feedpoints, but not in the shack at the rig. I'll try that, too. Thanks!

author: djacobow
author_rank: Super Contributor
timestamp: February 23, 2017, 04:48:22 pm
content: So, the latest update is that I could repro this issue pretty reliably even transmitting into the dummy load in my tuner. I'm not entirely certain what that means -- there can certainly still be RF in the shack, but it can't be that great.But when I operate the rig from my laptop, I get no issues.So, it is either: the desktop PC, the cables going into it, the usb cable, the usb hub, or something else plugged into the usb hub (keyboard, mouse, etc). That's a lot of permutations to try to find the exact item.So far i've just been lazy, operating from the laptop. I'm also going to order a different USB hub.

author: voltz
author_rank: Frequent Contributor
timestamp: February 24, 2017, 08:36:28 am
content: So its not antenna or feeder related. That rules out a major chunk of your system.The only difference i can think of between PC and laptop is the mains earth.The PC should be earthed at the mains (and possibly via peripherals plugged in) and the laptop wont be. There is a chance this is earth loop related. Or it could be the USB filtering is better on the laptop rather than the PC. Not easy to say exactly what to try next and made worse by not being able to see your actual cable layout.Have you tried a ferrite filter or looped cable filter on the USB cable itself?. Its worth a try in case this is RF from the radio itself back feeding along the USB cable to the PC.Or try connecting a proper external ground to the radio.Into a dummy load, there should be next to no RF anywhere. If there is, then it suggests RF is leaking from the radio via the USB connector or your feeder to the MFJ tuner may have a bad shield connection. Sorry its hard to be more precise. Just ideas what it could be.edit: another quick thought, have you tried removing the Hub from the USB link and go straight to PC?. Thats another potential problem area.

author: Fank1
author_rank: Regular Contributor
timestamp: February 27, 2017, 01:08:58 am
content: How good of a ground connection do you have ??

author: djacobow
author_rank: Super Contributor
timestamp: February 27, 2017, 01:18:30 am
content: My ground connection is "just ok." It is a solid copper (not braid) connection to a ground rod about 15 feet away.Anyway, the latest is that I replaced the USB hub with a new one and instead of using the USB "upstream" cable it came with, I used a quality one with factory attached ferrites on both ends. I also wrapped the power for the hub 10 times in a ferrite toroid and the USB cable going to the radio in a similar toroid.It is working! I am not getting any hiccups.All that said, there *is* definitely RF in my shack. It's easy enough to observe on an oscilloscope with a wire attached to the end of the probe.Which now leaves me wondering, not if I have an RF in the shack, but when do I know if it is a problem? Clearly, if it causes my equipment to malfunction, it is a problem for me. But now that I can at least not observe a malfunction, does that mean that I do not have "too much" RF in the shack?I think it's not that simple. With calibrated equipment I could determine if I am within safety limits, but with primitive means of detection... how does one know? It's not like I was getting RF burns/zaps.Maybe I should just try not to worry about it as long as my station "works."Hmm.

author: Electro Fan
author_rank: Super Contributor
timestamp: February 28, 2017, 05:01:10 am
content: Quote from: djacobow on February 27, 2017, 01:18:30 amGlad to hear it is working.When you say that you can observe the RF on your oscilloscope with a wire attached to the end of the probe presumably that is when you hit transmit, right?I think it's going to be normal to detect RF when you are transmitting. I sometimes use a spectrum analyzer when operating and there is no doubt that RF transmitted from the antenna (which is about 80 feet away outside) reaches inside and is perfectly visible on the spectrum analyzer (I have a small antenna attached to the spectrum analyzer). Certainly, if your signal is going to be heard around the country and maybe around the world, it's going to be heard in your shack. So in that sense, I think it's normal, but maybe someone here can describe how non-normal RF can be distinguished from normal RF.

author: voltz
author_rank: Frequent Contributor
timestamp: February 28, 2017, 11:52:57 am
content: Glad its working.Into a dedicated dummy load, and with your feeder and connections all good, you should have little RF in the shack. There will always by some leakage though and that can be detected on a SA or receiver near by. Thats normal. Whats not normal is for that small level of RF to make equipment malfunction.Try removing the MFJ ATU and replace with a proper dedicated dummy load, ideally with no feeder between them. Thats the best reduction you should get.My MFJ's dummy load 'works' but is leaky. Yours probably is the same. It comes down to things like the case screws not being secure enough and radiation through the large plastic meter. Rig grounding as already mentioned makes a big difference too.Good luck with it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 28, 2017, 01:22:53 pm
content: USB is quite sensitive to RF, as you've discovered. I've seen no shortage of horribly put together devices. Many seemingly intentionally so: impedance added to the shield connection!Poorly shielded cables, and poorly designed hubs, devices, etc. are the suspect.Looks like you've discovered the culprit, so... there you go.This is, of course, after you've shown the shack isn't the problem -- consumer stuff is usually tested around 3V/m field strength. Whatever that works out to, at your frequency(ies) of operation and with whatever sniffing gadgets you have on hand. Properly shielded USB should handle much more than that, really; other parts in parts in the system will probably drop first.Tim

author: djacobow
author_rank: Super Contributor
timestamp: March 03, 2017, 03:18:39 am
content: Quote from: Electro Fan on February 28, 2017, 05:01:10 amYes, only when transmitting. I have no calibrated way to measure it, but it seems not insignificant. If I short out the probe tip to the probe ground, I can still see signal on the scope if the gain is turned up high enough. I don't know if the signal is getting in through the probe, the probe cable, or maybe the scope itself.There is another strange symptom when transmitting, too. If my lab power supply (a Mastech) is on, if I tx with the power high enough, I will see its voltage monitor go up a bit and I can hear the transformer winding selection relays click on and off. This happens with the PSU totally unloaded. If I loaded up it doesn't happen.Anyway, I seldom use that PSU when I'm doing radio, so it doesn't matter much.All that said, i'll continue to play around to try to reduce the RF in the shack. I think I will put baluns or chokes on my dipoles. After that, I'll just relax and not worry about it.

author: Nemo1956
author_rank: Contributor
timestamp: March 07, 2017, 07:18:51 am
content: Put a length of wire that will resonate on 20 and 40 inside your shack and connect it to the ground of you ATUThat will get rid of your problem.