RF interference on the VHF Business High band - Page 1
Forum section: electronics / rf-microwave

author: videobruce
author_rank: Frequent Contributor
timestamp: September 18, 2023, 04:30:15 pm
content: This is in the US market.This has been going on for some time now, probably 9 months or so and it beyond annoying. This is using more than one 'scanner receiver', one having 'digital' capabilities (Uniden BCD996P2) which is less susceptible to interference than the Radio Shack Pro 2004 & Pro 2006 (made back by GRE in the late 80's and early 90's both very popular here.). This happens on the SAME exact frequencies on all receivers, considering they are completely different designs, it's not intermod as far as I can tell.Hard to explain, but here goes. I have been getting 'bombed' with what I assume is some type of 'digital' RF interference that only seems to hit selective VHF high band frequencies in the assigned Railroad band centered around 161MHz which happens to be my most important service I have monitored for decades.I tried doing numerous Spectrum Analyzer scans for the frequency source of the interference to no avail. I tried to narrow the source down based on the frequency and hours that it was/is present and my 1st guess was ATV (Amateur TV) since it is not typical 2-way voice communications, it's somewhat different than most 'digital' UHF business channels (NXDN for example) with the 'buzz like sounding audio (for lack of a better term). It was more like what I remember hearing the sidebands of analog NTSC transmission when close to a xmitter sounded like.It slacked off for a couple of months, I thought it was maybe gone, but it returned in the past few weeks, but a different 'sound' to it, but the same exact frequencies are affected. This time the audio has a 'motorboat' sound to it. Definitely different than before, but the exact same frequencies are affected. About 5 different channels, again all receivers at the same time. Each receiver has it's own roof antenna.My question is;Since this is the VHF high band (150-170 MHz), NOT the overly crowded UHF business band where most all the 'digital business channels are, where on earth can this be coming from? A sub-harmonic of UHF on the high band? What and where below the high band would harmonic into this band??Any other specific questions please ask.

author: videobruce
author_rank: Frequent Contributor
timestamp: September 18, 2023, 04:41:54 pm
content: Attached are a audio sample and screen shots of my Siglent SSA 3021X.The spike w/ the marker is the interference. It bounces around within the bandwidth shown on the scope. It's anything but stable, either in signal strength or frequency. There no real pattern to it. It appears and disappears through the day and evening. It's definitely not in the immediate neighborhood, I'm guessing less than a mile away, but that is just an initial guess.I also have a RF Explorer, but I have found inconsistent readings between the two SA's. Signals showing on one, but not the other.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 18, 2023, 09:21:11 pm
content: Have you tried FM demod on it yet? A narrow peak "bouncing" around implies stable frequency within an acquisition period (that's a DSA isn't it, i.e. acquire a huge gulp of samples, FFT it, repeat), but changing from period to period, implying frequency modulation at a rate comparable to the acquisition period; or there are large gaps in time between acquisitions.So, perhaps you see something different if you change the sweep rate / display refresh / buffer size (number of samples) / etc. If the modulation is periodic, aliasing it against the acq rate should be possible.I suppose also possible, it's FM over such a range that you're not seeing the sidebands (or they're too low to receive), or more than one (as in the second spectrum). But that seems unlikely, if it's 60Hz or thereabouts.It could well be switching noise, perhaps from a newer GaN based device that has higher harmonics than ever, but 160MHz is still within the range of silicon devices as well. The 60Hz-ish modulation would be typical for mains-powered equipment.You haven't seen anything by sweeping a dipole/loop (or better) and looking for nulls? Or check for polarization as well, perhaps it's mixed by multipath.Tim

author: videobruce
author_rank: Frequent Contributor
timestamp: September 18, 2023, 10:36:02 pm
content: I'm afraid that most of that is somewhat over my head (above my pay grade).But I did use the demod function. With nothing to compare it with, I didn't know what to expect. I'm not good at making home made antennas.

author: CaptDon
author_rank: Super Contributor
timestamp: September 20, 2023, 02:09:22 am
content: All of your scanners and other NBFM demodulator radios probably have a 10.7MHz I.F. strip so places to look would be 10.7MHz higher or lower than the channel frequency you are trying to receive. There are no 'subharmonics' however you may be hearing a 2nd or 3rd harmonic perhaps looking at anything in your area transmitting in the 80.5MHz or 53.6MHz range. The old varactor tripler units have gone out of favor with the F.C.C. for commercial products and we had to scrap a bunch of Moseley Microwave (900MHz) studio transmitter link stuff because it was outlawed. Truthfully the stuff sucked to begin with and would break into spurs that looked like a forest on a spectrum analyzer. 161-10.7 is 150.3 and 161 +10.7 is 171.7 perhaps pocsag pager interference or digital voice data for public services. Doubt very much it is amateur radio related as the bands and harmonics don't line up at all. Another possibility is leakage from the CATV neigborhood network. They may be polling water meters or electrical meters on oddball frequencies unique to the cable backhaul networking. If it interferes with your 'train channels' why aren't the railroads troubled by it?? Seems they would be having interference problems also and be gunning for bear to find the culprit? B.T.W., I retired from the G.E. locomotive factory so I have lots of train radio experience. Also worked for a short line that got gobbled up by G&W.

author: TheSteve
author_rank: Unknown
timestamp: September 20, 2023, 03:55:50 am
content: That does not sound like any paging protocol. (GSC, POCSAG, FLEX).It is likely a mixing product from something, or some ones transmitter/amp is failing.

author: videobruce
author_rank: Frequent Contributor
timestamp: September 20, 2023, 07:45:25 am
content: CaptDon;Thanks for the reply, how on earth did you wind up in Iceland??Interesting about looking 10.7 above and below that if frequency. I have now confirmed that I can see this signal on my SA which with that other interference last year that wasn't the case. And the fact that handheld Moto does sometimes receives it, thou nowhere as strong as the 'scanners'.The problem is whatever this is is not always there, it's very inconsistent, sometimes it's only there say 10, 15 or 20 minutes. The limited driving around hasen't really produced any results other than I've gone over 2 miles from home and it's still there.

author: TheSteve
author_rank: Unknown
timestamp: September 21, 2023, 12:06:44 am
content: Build yourself a 3 element yagi and an offset attenuator and hunt it down. (I like the 3 element yagi's made with tape measuring material for the elements).

author: rfclown
author_rank: Frequent Contributor
timestamp: September 21, 2023, 12:12:56 am
content: Many years ago when I worked on VHF land mobile radios, we had fits testing radios in production on some frequencies due to the new fangled electronic ballasts that they had put in the fluorescent light fixtures.

author: videobruce
author_rank: Frequent Contributor
timestamp: September 21, 2023, 08:56:28 am
content: But the distance spread is too great for anything this 'local'.

author: PA0PBZ
author_rank: Super Contributor
timestamp: September 21, 2023, 10:24:05 am
content: I looked at the audio and it has a very stable 30mS period, whatever that means. It reminds me of the former Russian shortwave 'over the horizon' radar, aka the woodpecker (https://en.wikipedia.org/wiki/Duga_radar), it does not sound like SMPS interference to me. Your best option would be trying to locate the signal with something portable that can receive the signal, and the easiest to do that would be to use a directive antenna, either a Yagi or just a simple open dipole which you can create yourself and aim for the weakest signal.

author: videobruce
author_rank: Frequent Contributor
timestamp: September 21, 2023, 12:54:16 pm
content: Quote from: TheSteve on September 21, 2023, 12:06:44 am"Tape measuring material" ??Link to some plans? I'm not really into antenna building thou.

author: bingo600
author_rank: Super Contributor
timestamp: September 21, 2023, 01:27:36 pm
content: Quote from: videobruce on September 21, 2023, 12:54:16 pmMy guess is that Steve meant a "Steel measure tape" , those 3..5m meaurement retractable bands , usually white or yellow painted steel band w. CM/Inches marking.https://w5nor.org/tape/https://www.jpole-antenna.com/2017/02/07/build-it-2-meter-tape-measure-yagi-beam-antenna/https://www.instructables.com/The-Tape-Measure-Antenna/

author: PA0PBZ
author_rank: Super Contributor
timestamp: September 21, 2023, 01:33:54 pm
content: 2 pieces of conductive material strong enough not to bend by itself (installation wire or if you want to cut your tape measure), each 1/4 wavelength opposite to each other on a piece of wood and a piece of coax cable soldered where the 2 pieces meet to your receiver.Ignore the fact that it should be 75 Ohm, 50 is also fine and in your case (160 MHz) the total length should be 90 cm so 2 x 45 cm. Do the inch conversion yourselfNote that this is the simple version without a front/back difference, if you want that you have to add more elements like in the pictures above.

author: videobruce
author_rank: Frequent Contributor
timestamp: September 21, 2023, 01:45:15 pm
content: I didn't think he meant actually using the tape itself.Other then the lack of flexibility, why not just use copper wire (18-14 gauge) in those pipes?? Is there a issue with the wider tape vs the smaller diameter wire as to bandwidth?I'm surprised that the tape would remain stiff enough when one starts handling the antenna.

author: Uunoctium
author_rank: Frequent Contributor
timestamp: September 21, 2023, 02:49:12 pm
content: Maybe AIS frequencies?Channel A - 161.975 MHzChannel B - 162.025 MHzhttp://www.allaboutais.com/index.php/en/technical-info/technical-fundamentalshttps://www.sigidwiki.com/wiki/Automatic_Identification_System_(AIS)( w audio sample )

author: videobruce
author_rank: Frequent Contributor
timestamp: September 21, 2023, 08:09:21 pm
content: No, that definitively isn't it.

author: donmr
author_rank: Regular Contributor
timestamp: September 21, 2023, 11:55:44 pm
content: Hams often do make these from tape measures. The material like to stay straight but is springy and can be folded or rolled up for storage. It won't care if you bump on a tree branch while using it either. Don't use your good one, buy a cheap one somewhere.

author: videobruce
author_rank: Frequent Contributor
timestamp: October 03, 2023, 10:06:27 am
content: I want to build that 3 element Yagi tape measure antenna, but I don't know which calculator to use as far as the dimensions and spacing of the elements.http://n5dux.com/ham/tape-measure-yagi/Here are some of the choices;http://www.mapleleafcom.com/calculator_3element_yagi.shtmlhttps://www.steeman.org/Antenna/Yagi-Antenna-Calculatorhttps://www.changpuak.ch/electronics/yagi_uda_antenna_DL6WU.phphttp://www.csgnetwork.com/antennae3ycalc.htmlhttps://3g-aerial.biz/en/online-calculations/antenna-calculations/3-elemets-yagi-uda-online-calculatorThe problem is some of these show the spacing the same between the 3 elements which I believe isn't correct. Also element diameter comes into play, but how does one determine that with a 'flat tape measure'??

author: videobruce
author_rank: Frequent Contributor
timestamp: October 03, 2023, 09:47:57 pm
content: This is where I'm at now. Note the corrected dimensions for 161MHz and comments/questions in red;

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 04, 2023, 07:17:34 am
content: I'm just awake and still on my first coffee but I can't imagine why the director - dipole distance increased with the increased frequency, maybe you should have a second look at the calculations.

author: radiolistener
author_rank: Super Contributor
timestamp: October 04, 2023, 07:34:56 am
content: Quote from: PA0PBZ on September 21, 2023, 01:33:54 pmNote that coax cable is not symmetric transmission line and since your schematic doesn't have balun, the cable will be a part of antenna...

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 04, 2023, 07:48:05 am
content: Quote from: radiolistener on October 04, 2023, 07:34:56 amTrue, it was a simple picture I just took from the internet to show the general idea, it would be better with a balun or gamma match.

author: videobruce
author_rank: Frequent Contributor
timestamp: October 04, 2023, 01:49:23 pm
content: Quote from: PA0PBZ on October 04, 2023, 07:17:34 amSomething to do with design favoring using the 'null' off the back of the antenna for direction finding as opposed to using it as a normal antenna and looking for max signal??Those numbers in red are what I was given elsewhere by someone that has reportedly built one. I'll have to take another look.

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 04, 2023, 03:13:57 pm
content: Yes, you can change directivity with the size of the director/reflector and the distance to the dipole, but the change seems rather big. If after a second check it still seems fine just go for it.

author: videobruce
author_rank: Frequent Contributor
timestamp: October 04, 2023, 03:22:01 pm
content: Any other comments on this??I'm still wondering about the 'U' matching stub and the balun?

author: videobruce
author_rank: Frequent Contributor
timestamp: October 06, 2023, 07:38:39 pm
content: Quote from: PA0PBZ on October 04, 2023, 07:17:34 amThat was a error on my part, I misread someones else's response on another forum. That dimension should be 10", not 17".

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 06, 2023, 08:01:58 pm
content: Quote from: videobruce on October 06, 2023, 07:38:39 pmThat makes more sense, now build it and go fox hunting!

author: A.Z.
author_rank: Super Contributor
timestamp: October 07, 2023, 07:13:47 am
content: instead of a 3 elementshttps://www.essexham.co.uk/gordon-dick-bird-g4zu-informationcheck the "jungle job" antenna near bottom of page, notice that calc formulas are for meters, the structure can be built using wood or pvc if wanted and using wires for dipole and reflector

author: videobruce
author_rank: Frequent Contributor
timestamp: October 07, 2023, 12:24:37 pm
content: Ok, I did.BUT,Using a RF Explorer which I felt would be the best choice for portability and still give me a 'spectrum' to scan since the interference is not set on a single frequency. The 'receive' results areverypoor compared to the stock telescoping 'whip' antenna that is supplied with the unit. I barely get any readings including even baseline noise that doesn't register. It's just a flat line for the most part with a few minor peaks here and there. I noticed this right away inside.This iswithoutany balun or matching 'U', just a straight run of RG-58 cable to a BNC fitting and a BNC to SMA adapter.I first confirmed the jumper cable and the solder points, both are good. I rechecked the Explorer with their antenna and all is well. The test was done outdoors on a 2nd floor porch moving around and doing a 360 circle. This is aiming the antenna for max signal, not off the rear for a null which in no way would work unless I was less than a few hundred feet from the source.I'm going to run some more tests and report back.

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 07, 2023, 12:41:48 pm
content: QuoteAnd there's probably your problem, the outer conductor of the coax becomes part of the antenna and it's just rolling a dice if it will add or substract from the antenna signal, and there will not be much left of the directivity either, as we discussed earlier in this thread. Can you see any variation when using the original whip, holding it horizontal and make a 360? There should be a minimum when the whip is pointing to and away from the source. maybe it's enough to locate it.

author: videobruce
author_rank: Frequent Contributor
timestamp: October 07, 2023, 01:12:39 pm
content: I'll have to wait until the interference is there. Of course it's not now.The baseband noise level is about as strong in the horizontal plane as it is vertically.I was going to make one of those 'U' sorting loops to see if that makes any difference.

author: videobruce
author_rank: Frequent Contributor
timestamp: October 07, 2023, 04:55:24 pm
content: Well, I added that 'U' and 7 turns of the cable per those other designs and it made matters worse (if that could even happen). I removed the 'U', no change still almost nothing. I un-wound the cable and it seems to be back the way it was, some signals, still well less then the included whip.Ok, I'll bite, how could that cable wound 7 turns do that?

author: A.Z.
author_rank: Super Contributor
timestamp: October 08, 2023, 08:01:13 am
content: I'll refrain, try the antenna I suggested, route the coax feeder along the boom and down the support, add a Maxwell (W2DU) choke on the coax right after the feedpoint; that antenna will suffice and the feedpoint impedance will just match the coax, plus building it is much easier than that tape measure yagisee here and remember that the math gives meters not feet (F is in MHz)https://www.essexham.co.uk/images/g4zu-junglejob-01.jpgthe boom and reflector supports may be built using pvc pipes or broomsticks, in this case the reflector will be made using a wire just like the dipole, performance will suffer a bit, but not too much

author: videobruce
author_rank: Frequent Contributor
timestamp: October 08, 2023, 01:02:30 pm
content: Whenever I make up most any 'patch cords' especially for RF, I almost always check resistance of the center conductor, then the shield/ground and lastly for a short between the center and shield.Note I said "almost".There was a dead short across the BNC fitting.All is apparent well.I did some initial tests using the local US Weather Service (162.55) as a reference signal since it's continuous and found differences with and w/o the 7 turn coil and/or tat 'U' shaped shorting jumper. It seems sensitivity is slightly better w/ the U jumper in place. The 'coil' mod did seem to change things, but not apparently for the better.

author: videobruce
author_rank: Frequent Contributor
timestamp: October 11, 2023, 07:29:02 am
content: It was asked if I noticed changes in the pattern of the interference with any weather changes. Up to a couple of weeks or so ago, there wasn't since there weren't any real changes in the weather,but,from last weekend to now the interference has completely stopped, or at least drifted outside of the frequency area of interest. Nothing as far as I can tell. The temperature has dropped 20+ degrees overall 4 or 5 days ago and we have had some, but not much rain. Before, we were in a dry spell and in the upper 80's during the day. Now it's mid fifties and mid 40's at night.That can't just be coincidence. Any thoughts on this??

author: videobruce
author_rank: Frequent Contributor
timestamp: October 11, 2023, 08:36:24 pm
content: Attached are scans of the corrected yagi with shortened leads from the feed, a trimmed director length (less than 1/4" was trimmed off and remounted driven elements (my 1st cut was 1/2" which was too much, I cut two new pieces). I also spaced those further apart on the 'cross' fitting per the original design (see pics).I tested on a rear porch securing to to a wooden railing, later having my XYL hold the antenna in different positions away from the porch roof as much as possible while I watched the SA to see if the notch varied.This behaved differently as to the level settings that I'm use to using. I included scans w/o a load (after normalizing w/ a load), another scan with the load, and two scans w/ the antenna attached (via a 15-18' RG-58 cable), the last pic was w/ the marker table showing.