RE: [Repeater-Builder] Can the 4th harmonic of 1250 AM keep UHF repeaters locked up?
I know of one amateur repeater where a distant AM broadcast signal mixed with a commercial Tx operating on the same tower to produce interference on the repeater input. The problem was tracked to a rust joint between the galvanised iron roof and the guttering of the equipment hut. The broadcast station was more than 30 miles away, so rusty joints can work as pretty efficient broad band mixers! The only solution was to fix the roof as moving frequencies of the other players wasn't possible :-) Cheers, Mark VK3BYY -Original Message- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Tony KT9AC Sent: Monday, 22 February 2010 08:53 AM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Can the 4th harmonic of 1250 AM keep UHF repeaters locked up? Thanks Jeff. The AM station has the same power both day and night, just goes from 2 towers to 4 to change the pattern. Rusty bolt or fence line etc seems the most likely. The problem does seem to disappear when its raining out, which helps verify this theory. It might be a needle in a haystack trying to find this, so maybe remoting the receiver might be the easiest. Thanks and I'll continue to investigate. I can try temporarily moving the frequencies apart about 100Khz and see if my 5Mhz theory holds water. Tony Jeff DePolo wrote: > > > > So, if we take the 1250Khz signal or 1.25Mhz x 4 = 5Mhz. I > > realize that > > the 4th harmonic of a 5KW broadcast station isn't very powerful > > Well...it *shouldn't* be very strong. It has to be attenuated 43 + 10 > * log > (Pwatts) as measured in the field (not at the transmitter output > terminals). > If you have access to a field intensity meter that covers up to 5 MHz, > or a > spectrum analyzer and a calibrated antenna, you can measure it yourself. > > AM stations that change power and/or pattern at night sometimes use a > different transmitter between day and night depending on the power levels. > Some stations also have pre-sunrise, post-sunset, or critical hours > authorizations that are intermediate power levels between day and night > power levels, or as an adjunct to daytime-only authorization. Bottom > line - > the 4th harmonic content may vary due to a combination of pattern, > transmitter power output, or even different transmitters. > > > but > > being in its nearfield might be enough to cause a mix with the UHF > > transmit output. > > Well, 1 or 2 miles isn't really near-field, but in any case, the field > intensity may be relatively high depending on all of the other variables > (power, pattern, etc.). > > Usually interference to VHF/UHF involving mixes with AM broadcast occur > somewhere at or near the VHF/UHF site, not at the AM site. In some cases, > the problem can actually be caused within the equipment on the ground > rather > than externally at the antenna or on the tower. If it's an in-the-cabinet > mix, it could be caused by inadequate RF shielding. Before going on a wild > good chase, I'd ensure that everything is properly RF-shielded, shielded > cables are used for interconnects, grounding is good, all shields are in > place, all mechanical connections (e.g. screws) are tight, no oxidizes or > corroded connectors, etc. > > To rule out a lot of AM coming down the coax (which is fairly unlikely for > most VHF/UHF antenna designs), install a high-pass filter. Even a shorted > quarter-wave stub should give a fair amount of attenuation down in the MW > range. If you have any in-line lightning protection (Polyphasers, et al), > try removing them. > > But, more than likely, if in fact the AM station is the cause (either its > fundamental or a harmonic), you have a passive intermodulation mix, > the old > "rusty bolt" problem. It could be in your antenna, on your tower, in your > duplexer, in a corroded connector, who knows where. Divide and conquer is > the only way to try to isolate it. > > --- Jeff WN3A > > Yahoo! Groups Links
Re: [Repeater-Builder] Can the 4th harmonic of 1250 AM keep UHF repeaters locked up?
If you have a spectrum analyzer, look at the receiver pass side of the duplexer, you should see the garbage come up as soon as the transmitter keys. Also if you do not modulate the repeater the garbage should be decode able with a radio such as the Kenwood TH-F6 or Yaesu VX-7 set to AM mode tuned to the repeaters input frequency. You will also see garbage on the opposite offset. if it is a 5MHz mix. You should be able to walk around with the handheld and look at the S-meter to find the mixer.
Re: [Repeater-Builder] Can the 4th harmonic of 1250 AM keep UHF repeaters locked up?
Thanks Jeff. The AM station has the same power both day and night, just goes from 2 towers to 4 to change the pattern. Rusty bolt or fence line etc seems the most likely. The problem does seem to disappear when its raining out, which helps verify this theory. It might be a needle in a haystack trying to find this, so maybe remoting the receiver might be the easiest. Thanks and I'll continue to investigate. I can try temporarily moving the frequencies apart about 100Khz and see if my 5Mhz theory holds water. Tony Jeff DePolo wrote: > > > > So, if we take the 1250Khz signal or 1.25Mhz x 4 = 5Mhz. I > > realize that > > the 4th harmonic of a 5KW broadcast station isn't very powerful > > Well...it *shouldn't* be very strong. It has to be attenuated 43 + 10 > * log > (Pwatts) as measured in the field (not at the transmitter output > terminals). > If you have access to a field intensity meter that covers up to 5 MHz, > or a > spectrum analyzer and a calibrated antenna, you can measure it yourself. > > AM stations that change power and/or pattern at night sometimes use a > different transmitter between day and night depending on the power levels. > Some stations also have pre-sunrise, post-sunset, or critical hours > authorizations that are intermediate power levels between day and night > power levels, or as an adjunct to daytime-only authorization. Bottom > line - > the 4th harmonic content may vary due to a combination of pattern, > transmitter power output, or even different transmitters. > > > but > > being in its nearfield might be enough to cause a mix with the UHF > > transmit output. > > Well, 1 or 2 miles isn't really near-field, but in any case, the field > intensity may be relatively high depending on all of the other variables > (power, pattern, etc.). > > Usually interference to VHF/UHF involving mixes with AM broadcast occur > somewhere at or near the VHF/UHF site, not at the AM site. In some cases, > the problem can actually be caused within the equipment on the ground > rather > than externally at the antenna or on the tower. If it's an in-the-cabinet > mix, it could be caused by inadequate RF shielding. Before going on a wild > good chase, I'd ensure that everything is properly RF-shielded, shielded > cables are used for interconnects, grounding is good, all shields are in > place, all mechanical connections (e.g. screws) are tight, no oxidizes or > corroded connectors, etc. > > To rule out a lot of AM coming down the coax (which is fairly unlikely for > most VHF/UHF antenna designs), install a high-pass filter. Even a shorted > quarter-wave stub should give a fair amount of attenuation down in the MW > range. If you have any in-line lightning protection (Polyphasers, et al), > try removing them. > > But, more than likely, if in fact the AM station is the cause (either its > fundamental or a harmonic), you have a passive intermodulation mix, > the old > "rusty bolt" problem. It could be in your antenna, on your tower, in your > duplexer, in a corroded connector, who knows where. Divide and conquer is > the only way to try to isolate it. > > --- Jeff WN3A > > Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> Your email settings: Individual Email | Traditional <*> To change settings online go to: http://groups.yahoo.com/group/Repeater-Builder/join (Yahoo! ID required) <*> To change settings via email: repeater-builder-dig...@yahoogroups.com repeater-builder-fullfeatu...@yahoogroups.com <*> To unsubscribe from this group, send an email to: repeater-builder-unsubscr...@yahoogroups.com <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
RE: [Repeater-Builder] Can the 4th harmonic of 1250 AM keep UHF repeaters locked up?
> So, if we take the 1250Khz signal or 1.25Mhz x 4 = 5Mhz. I > realize that > the 4th harmonic of a 5KW broadcast station isn't very powerful Well...it *shouldn't* be very strong. It has to be attenuated 43 + 10 * log (Pwatts) as measured in the field (not at the transmitter output terminals). If you have access to a field intensity meter that covers up to 5 MHz, or a spectrum analyzer and a calibrated antenna, you can measure it yourself. AM stations that change power and/or pattern at night sometimes use a different transmitter between day and night depending on the power levels. Some stations also have pre-sunrise, post-sunset, or critical hours authorizations that are intermediate power levels between day and night power levels, or as an adjunct to daytime-only authorization. Bottom line - the 4th harmonic content may vary due to a combination of pattern, transmitter power output, or even different transmitters. > but > being in its nearfield might be enough to cause a mix with the UHF > transmit output. Well, 1 or 2 miles isn't really near-field, but in any case, the field intensity may be relatively high depending on all of the other variables (power, pattern, etc.). Usually interference to VHF/UHF involving mixes with AM broadcast occur somewhere at or near the VHF/UHF site, not at the AM site. In some cases, the problem can actually be caused within the equipment on the ground rather than externally at the antenna or on the tower. If it's an in-the-cabinet mix, it could be caused by inadequate RF shielding. Before going on a wild good chase, I'd ensure that everything is properly RF-shielded, shielded cables are used for interconnects, grounding is good, all shields are in place, all mechanical connections (e.g. screws) are tight, no oxidizes or corroded connectors, etc. To rule out a lot of AM coming down the coax (which is fairly unlikely for most VHF/UHF antenna designs), install a high-pass filter. Even a shorted quarter-wave stub should give a fair amount of attenuation down in the MW range. If you have any in-line lightning protection (Polyphasers, et al), try removing them. But, more than likely, if in fact the AM station is the cause (either its fundamental or a harmonic), you have a passive intermodulation mix, the old "rusty bolt" problem. It could be in your antenna, on your tower, in your duplexer, in a corroded connector, who knows where. Divide and conquer is the only way to try to isolate it. --- Jeff WN3A
