Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

donate to eham
   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: AM Radio Harmonics Interference  (Read 2020 times)
K6VML
Member

Posts: 6




Ignore
« on: September 16, 2016, 03:25:01 AM »

Hello all,

I regularly check in with several other hams at 1700 Pacific time on 3983 kHz. I have a 134-ft OCF dipole. Occasionally on the frequency I will encounter interference from two different local AM stations' harmonics, and sometimes maybe even a third, located between about 1290 and 1350 on the AM dial.

Interference from AM station harmonics is nothing new of course. What intrigues me is I long ago discovered that if I simply keyed the mic briefly, or pushed the auto tune button, the AM interference would often subside or disappear completely. Unfortunately, it can also work the other way and bring on the interference when there was none just before. The same can happen of course after I make a normal transmission. This can happen repeatedly in the course of our QSO.

Why does transmitting, by means of a tuner carrier, keying the mic, or making a transmission, cancel or bring on this interference? It happens with both my FT-890AT and my TS-570SG on the same antenna. It reminds me of the FM-station capture phenomenon and I wonder if the physics are somewhat similar here.

By the way, the SWR on my OCF dipole without the tuner is not ideal--3:1 or thereabouts on most bands--in case that is a factor.

Thanks for any insights on this particular question!

Dave K6VML
Logged
WA8ZTZ
Member

Posts: 164




Ignore
« Reply #1 on: September 16, 2016, 05:14:12 AM »

My guess would be a bad, loose, or corroded connection somewhere in the antenna system causing some rectification and mixing.  This would especially be the case if the local AM stations are nearby.  Not sure if the frequencies mentioned in your post are exact as you mentioned "about 1290 and 1350 on the AM dial", but 1350 x 2= 2700 + 1290= 3990.  Momentarily transmitting may cause the condition to clear itself.
Logged
WA8ZTZ
Member

Posts: 164




Ignore
« Reply #2 on: September 16, 2016, 05:44:33 AM »

OK, did some internet searching and it looks like KTDD 1350 is located about 6-7 mi SSW of your QTH and KKDD 1290 is located about 4 mi SE of your QTH.  Both run 5000watts daytime.  KTDD and KKDD signal strength at your QTH probably around 60 mV/m or greater.  These are quite strong signals and any corroded or oxidized connection can cause the mixing problems referenced in my first reply.
Logged
KE2EE
Member

Posts: 27




Ignore
« Reply #3 on: September 16, 2016, 05:56:12 PM »

Dave, well I have some ideas- I know at times when using an auto tuner, after the tune function has been set, at times without changing freq. and when retuning you may hear the tuner relays chattering some more trying for a better match. Perhaps these different LC combinations that come up may add or subtract to the incoming AM noise.
Also I have witnessed a local AM station's RF riding in on a wire antenna and effecting the auto tuners ability to find a good match. The tuner uses diodes to sample forward and reflected power
for the match. The tuner trys different LC combinations until the reflected power is at a minimum. The Broadcast Rf rides in and prevents a low reflected power reading which can upset the tuners micropressers ability to find a good match.

Mike
« Last Edit: September 16, 2016, 06:05:21 PM by KE2EE » Logged
K6VML
Member

Posts: 6




Ignore
« Reply #4 on: September 16, 2016, 09:27:50 PM »

Hey thanks Carl and Mike for very interesting responses. The PL-259 going into the shack does have a small layer of oxidation that could conceivably cause the problem as you describe. I'll check all my jumpers too. Hopefully any possible bad connection is not further up the antenna.

Thanks Carl for the research and microvolt estimate. I was incorrect to say "about." Those are indeed the exact frequencies.

And interesting too about the AM stations possibly interfering with a proper match. Both radios keep the settings unchanged in memory, but another button push could possibly change the LC combination for better or worse regarding the AM signal path. I could haul out my old Dentron manual tuner for comparison.

I could have a better ground than my water pipe. I have a proper 8-ft ground rod ready to install outside the shack window. Maybe that would help too as a friend also suggested.

Hopefully I can get back to you with good news after trying these out.
Logged
AC7CW
Member

Posts: 675




Ignore
« Reply #5 on: September 16, 2016, 10:30:07 PM »

Tuning an antenna changes the radiation pattern. You might be putting the offending signal in a nulled direction.
Logged

Novice 1958, 20WPM Extra now... (and get off my lawn)
WA8ZTZ
Member

Posts: 164




Ignore
« Reply #6 on: September 17, 2016, 01:59:03 AM »



Hopefully I can get back to you with good news after trying these out.


OK, keep us posted,  it will be interesting to see how this turns out.   Wink

btw,  as a kid growing up in Detroit, had 5000 watt WWJ 950 about 1 1/2 mi from my then QTH,  came booming in on various of my crystal sets and foxhole radios,  could receive it with a razor blade and piece of pencil lead,  the problem was trying to separate it from much weaker WKMH 1310 (which later became legendary rocker WKNR "Keener 13") so as to listen to Ernie Harwell do the Tigers play by play  Smiley...  anyway, a bit of Detroit radio history here but the point is that a strong local AM BCB signal can easily get into the mix   

Logged
WA8ZTZ
Member

Posts: 164




Ignore
« Reply #7 on: September 17, 2016, 09:14:13 AM »

One other thought,   if this problem has come on recently and you haven't moved, changed, modified anything in the shack then the problem may likely be outdoors  where connections are exposed to weather. 
Logged
NA6DF
Member

Posts: 30


WWW

Ignore
« Reply #8 on: September 23, 2016, 10:23:38 AM »

Thought to consider: It may not be in your system at all. Years ago, I ran into a similar problem at my old QTH. Mixes and such from a 50 KW blowtorch on 1530 about 5 miles away, as well as a 10 kw 1690 less than 1 mile away. I finally tracked it down to all the rusty and crusty joints in the wire mesh fencing around the property that were being excited by the high RF levels. Rusty metal equals diodes in a high RF envoronment. After everything had been snugged up, tightened, etc my issues went away. BTW, I was the engineer then, and am now that blowtorch on 1530 (KFBK)

73, NA6DF

Hello all,

I regularly check in with several other hams at 1700 Pacific time on 3983 kHz. I have a 134-ft OCF dipole. Occasionally on the frequency I will encounter interference from two different local AM stations' harmonics, and sometimes maybe even a third, located between about 1290 and 1350 on the AM dial.

Interference from AM station harmonics is nothing new of course. What intrigues me is I long ago discovered that if I simply keyed the mic briefly, or pushed the auto tune button, the AM interference would often subside or disappear completely. Unfortunately, it can also work the other way and bring on the interference when there was none just before. The same can happen of course after I make a normal transmission. This can happen repeatedly in the course of our QSO.

Why does transmitting, by means of a tuner carrier, keying the mic, or making a transmission, cancel or bring on this interference? It happens with both my FT-890AT and my TS-570SG on the same antenna. It reminds me of the FM-station capture phenomenon and I wonder if the physics are somewhat similar here.

By the way, the SWR on my OCF dipole without the tuner is not ideal--3:1 or thereabouts on most bands--in case that is a factor.

Thanks for any insights on this particular question!

Dave K6VML

Logged
K6VML
Member

Posts: 6




Ignore
« Reply #9 on: September 23, 2016, 06:35:04 PM »

Thanks for the additional responses and radio station history/info! It's not a recent problem. And I was careful to weatherproof the connections when I homebrewed the antenna.

Regarding NA6DF's reply, that got me to thinking. The home water heater is located outside, immediately adjacent to the window next to the shack, inside a galvanized housing "box" that sits on a plastic base. Perhaps I should connect all the housing panels electrically together with wire, connectors, sheet metal screws, and star washers. Then connect the housing to ground, and maybe even the water heater itself if it isn't already. NA6DF or anyone, does that sound like a plan?

Logged
NA6DF
Member

Posts: 30


WWW

Ignore
« Reply #10 on: September 25, 2016, 06:25:54 AM »

Maybe, but first, I would try using a portable battery powered receiver if you have one, to try and locate the source of the signals. One other issue that I have witnessed in modern solid state AM broadcast transmitters is that unless they have sufficient bandpass filtering RF mixes can be created within the RF amplifiers of these transmitters from other nearby AM stations. We saw that in the SF bay area long ago, when a station put in a new solid state rig. Nearly every station around the bay within about 300 khz was mixing in that stations amplifier. It got the attention of one of my buddies, as he was a very serious 160 meter guy. He offered to the station to solve the problem for them, and I supplied some high powered caps and coils from my own stock and he built them a filter which solved it. Possibly you could check with the local stations engineers and see if they have an AM field strength meter. Most of the modern ones go to 5 MHZ, and have highly directional loop antennas in them. Makes it a lot easy to track down a signal source. Good luck.
Logged
WA8ZTZ
Member

Posts: 164




Ignore
« Reply #11 on: September 25, 2016, 09:05:42 AM »

The fact that you had mentioned that transmitting can cause the condition to change still leads me to suspect something in the rig, tuner, feedline, antenna system.  Hopefully, troubleshooting these by substitution and process of elimination will find the culprit.

If not, then the idea of tracking down the source with a battery portable RX is a good idea.
Logged
NA6DF
Member

Posts: 30


WWW

Ignore
« Reply #12 on: September 26, 2016, 10:42:18 AM »

another possibility that I have seen is that the mix can be caused right in the front end of the receiver, either by first mixer overload or by any front end protection diodes that go into full saturation.  Back when I lived near the RF flamethrowers, I also used a 1.8 MHz high pass filter to prevent that from happening. Mine was an ICE filter, not sure who makes them now.

dave
Logged
WA8ZTZ
Member

Posts: 164




Ignore
« Reply #13 on: September 26, 2016, 04:57:20 PM »

front end protection diodes that go into full saturation


Take a look at QST June 2015 page 65  "Accidental Mixers" for an example of how various diodes in a strong RF environment can behave (or misbehave).  Sad

Interestingly enough, this article was the text in the W1AW code practice tonight...  reminded me of this thread.     
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!