[Repeater-Builder] Re: Interference on VHF repeater
A year ago I had a low-band VHF interference problem that was caused by a solar charge controller. Switching from an MPPT style charge controller to a less efficient PWM charge controller cleared up the problem. Might be worth double checking by disconnecting the panels since the charge controller can, and did, create noise without a load. Brian --- In Repeater-Builder@yahoogroups.com, "brett" wrote: > > Thanks - no battery chargers on site as all solar powered. There are solar > charge controllers, though I think I have discounted these. > > No fluros either. > > All equipment was switched off for a test, so any SMPS should have been off. > > Cheers, > Brett > > --- In Repeater-Builder@yahoogroups.com, DCFluX wrote: > > > > Look around for a switch mode power supply that uses 600kHz as the > > switch frequency. SMPS Battery Chargers are popular for causing this. > > Also florescent twist lights are really good for making desense on > > VHF. > > >
[Repeater-Builder] Re: Interference on VHF repeater
Thanks - no battery chargers on site as all solar powered. There are solar charge controllers, though I think I have discounted these. No fluros either. All equipment was switched off for a test, so any SMPS should have been off. Cheers, Brett --- In Repeater-Builder@yahoogroups.com, DCFluX wrote: > > Look around for a switch mode power supply that uses 600kHz as the > switch frequency. SMPS Battery Chargers are popular for causing this. > Also florescent twist lights are really good for making desense on > VHF. >
[Repeater-Builder] Re: Interference on VHF repeater
Hi Mike, thanks for your post - I'll think about this a bit more. There is no isolator on the TX - will add one (and BP filter) when I next go to site this week. The radio is synthesised. Regards, Brett --- In Repeater-Builder@yahoogroups.com, "wb8vlc" wrote: > > "I have inserted a 6dB pad in the antenna port of the duplexer and found > that the IM products drop 12dB, and also curiously, the frequency of the > products change. Removing the pad reverses this effect." > > The above says that it's a 2nd order mix, F1+F2, F1-F2, 2F1 or 2F2. > > Since it looks to be a 2nd order product, proven by your 6 dB attenuator > causing a 12 dB drop, whereas a 3rd order IM product would drop the product > by 18 dB. > > Th 2nd order mix indicates that it could be combination of an internally > generated signal from your equipment F1, Probably in the receiver input stage > itself and an outside signal source F2 from an external transmitter, yours or > another adjacent one. > > A pure 3rd order IM product is typically an indication of an internally > generated source in the receiver input stages itself without any externally > generated sources, but not always. > > Also do you have an isolator on the TX output along with a Low Pass filter > after the isolator ? > > Is this a synthesized exciter or crystal controlled ? > > If you can identify all the signals present on Spectrum Analyzer then with > the above 2nd order formulas you could probably zero down the 2 signals > causing the problem. > > Mike > > > --- In Repeater-Builder@yahoogroups.com, "brett" wrote: > > > > Hi all, > > > > I have come across an interesting problem which you may be able to shed > > some light on. I have an intermod issue where my TX sometimes opens up my > > RX. I have the distinctive hollow pipe sound. Both TX and RX have the > > same CTCSS tone. The intermod product is however not always present, and > > after looking at the RX output from the duplexer with a SA I see a comb of > > products that move slowly in time. When one of the products in the comb > > falls within the RX bandwidth the RX opens, until it moves on. > > > > This is not a busy site, and I have been able to power down everything on > > site except my repeater. Problem remains unchanged. > > > > I have also disconnected feeders from all other RF equipment on site - > > still no change. > > > > The fact that the IM product frequency changes with time (drift rate is > > roughly a few kHz's an hour) makes me think that there is either another > > unknown source of RF on site which has poor freq stability (pretty > > unlikley), or somehow my TX freq is involved in producing this freq. > > > > I have inserted a 6dB pad in the antenna port of the duplexer and found > > that the IM products drop 12dB, and also curiously, the frequency of the > > products change. Removing the pad reverses this effect. I have repeated > > this many times and the result was always the same. It appears that the > > frequency of the IM product is dependent on the strength of the radiated > > field from my antenna. > > > > This is my question: I have read that it is possible for a strong EM field > > to excite metal (eg tower member) such that re-radiation will occur at a > > frequency which is different from that which excited it. Can anyone > > confirm they have seen this, or can anyone point me to a reference that > > talks about this? > > > > I should also mention there are multiple solar panels and associated > > regulators on site. The regulators have been discounted as possible > > sources, but the panels (given they may have bypass/blocking diodes) may be > > a mixing location, however the source of the drifting tone is still unclear. > > > > Thanks, > > > > Brett VK2CBD. > > >
[Repeater-Builder] Re: Interference on VHF repeater
Hi John, I thought we may have a dubious TX issue - either spurs or as you say something unstable or oscillating. I replaced the TX. It had no impact on the problem. The issue appears to be external to the TX. Also I have terminated the duplexer into a 50ohm load and looked at the RX port of the duplexer and found it to be clear of anything. Regards, Brett --- In Repeater-Builder@yahoogroups.com, "John J. Riddell" wrote: > > Bret, you might have your PA going in to oscillation creating the spurs due > to a highly > reactive duplexer. > > We had a similar problem here many years ago and fixed it with a simple tuner > on the TX > similar ot the GE Z matcher . The one that we used was "Home Brew". > > When the tuner was adjusted for minimum VSWR, the spurs went away. > > 73 John VE3AMZ > > > - Original Message - > From: "brett" > To: > Sent: Saturday, September 04, 2010 7:26 AM > Subject: [Repeater-Builder] Interference on VHF repeater > > > > Hi all, > > > > I have come across an interesting problem which you may be able to shed > > some light on. I have an intermod issue where my TX > > sometimes opens up my RX. I have the distinctive hollow pipe sound. Both > > TX and RX have the same CTCSS tone. The intermod > > product is however not always present, and after looking at the RX output > > from the duplexer with a SA I see a comb of products > > that move slowly in time. When one of the products in the comb falls > > within the RX bandwidth the RX opens, until it moves on. > > > > This is not a busy site, and I have been able to power down everything on > > site except my repeater. Problem remains unchanged. > > > > I have also disconnected feeders from all other RF equipment on site - > > still no change. > > > > The fact that the IM product frequency changes with time (drift rate is > > roughly a few kHz's an hour) makes me think that there is > > either another unknown source of RF on site which has poor freq stability > > (pretty unlikley), or somehow my TX freq is involved in > > producing this freq. > > > > I have inserted a 6dB pad in the antenna port of the duplexer and found > > that the IM products drop 12dB, and also curiously, the > > frequency of the products change. Removing the pad reverses this effect. > > I have repeated this many times and the result was > > always the same. It appears that the frequency of the IM product is > > dependent on the strength of the radiated field from my > > antenna. > > > > This is my question: I have read that it is possible for a strong EM field > > to excite metal (eg tower member) such that > > re-radiation will occur at a frequency which is different from that which > > excited it. Can anyone confirm they have seen this, or > > can anyone point me to a reference that talks about this? > > > > I should also mention there are multiple solar panels and associated > > regulators on site. The regulators have been discounted as > > possible sources, but the panels (given they may have bypass/blocking > > diodes) may be a mixing location, however the source of the > > drifting tone is still unclear. > > > > Thanks, > > > > Brett VK2CBD. > > > > > > > > > > > > > > > > > > Yahoo! Groups Links > > > > > > >
[Repeater-Builder] Re: Interference on VHF repeater
Paul, no fluro lights on site. Actually no lighting at all - the site is solar powered so little capacity for lighting. No repeater ocntroller - the RX is directly connected to TX via a very simple (passive) audio and PTT circuit. I believe it is an IM product because the RX requires a CTCSS tone to open and the only TX on site that has the correct tone is my TX. Also the interference is only present when my TX is active, and the hollow pipe sound on the audio does sound like audio feed back you'd expect if your own TX was part of the mix. With a spectrum analyser in the RX signal path I can see the intermod products appear when the TX is active. They are at approx 30kHz intervals across the RX half of the duplexer band pass. Cheers, Brett --- In Repeater-Builder@yahoogroups.com, "Paul Plack" wrote: > > Brett, > > How did you determine it's an IM product? > > What repeater/controller combination are you using? I'd try powering down the > controller and manually keying the transmitter. If that solves it, it could > be the controller's reference oscillator or divider outputs leaking onto the > PTT line or elsewhere. > > Any compact fluorescent lights nearby? > > 73, > Paul, AE4KR > > - Original Message - > From: brett > To: Repeater-Builder@yahoogroups.com > Sent: Saturday, September 04, 2010 5:26 AM > Subject: [Repeater-Builder] Interference on VHF repeater > > > I have the distinctive hollow pipe sound. Both TX and RX have the same > CTCSS tone. The intermod product is however not always present, and after > looking at the RX output from the duplexer with a SA I see a comb of products > that move slowly in time. When one of the products in the comb falls within > the RX bandwidth the RX opens, until it moves on. > > This is not a busy site, and I have been able to power down everything on > site except my repeater. Problem remains unchanged. >
[Repeater-Builder] Re: Interference on VHF repeater
Eric, all cables are double shielded. No adaptors are used. There is no isolator on the TX side. Connectors are silver plated, with gold pins. Antenna is a single folded dipole mounted about 15m above the equipment shelter. TX power into the antenna is approximately 15W. There are solar panels within 2m (below) of the anntenna. Putting the attenuator at the RX port of the duplexer reduces the IM products 1 for 1, ie 6dB of attenuation at the RX port reduces the IM by 6dB. BTW I used a 50W rated attenuator and Txed for short periods only so think overheating of the attenuator is not that likely. Yes TX freq is stable and not drifting. Also I have tried a second TX with smae results. Regards, Brett --- In Repeater-Builder@yahoogroups.com, "Eric Lemmon" wrote: > > Brett, > > Some additional information will be helpful. What makes/models of equipment > are in your repeater? Are all jumper cables and the antenna feedline > double-shielded? Are any of the connectors nickel-plated? Are there any > barrels or adapters in your jumpers? Is there an isolator/circulator > following the transmitter? What antenna are you using, and how far above > the repeater equipment is it located? > > Try putting your attenuator right at the RX input connector, and repeat your > IM test. Putting it at the antenna output is not a good idea, since the TX > output power can cause it to overheat. > > Your description of the IM product suggests that it might be a spur > generated within your PA, which could drift due to temperature changes. > Have you verified that your TX carrier frequency is stable, and not > drifting? > > 73, Eric Lemmon WB6FLY > > > > > -Original Message- > From: Repeater-Builder@yahoogroups.com > [mailto:repeater-buil...@yahoogroups.com] On Behalf Of brett > Sent: Saturday, September 04, 2010 4:27 AM > To: Repeater-Builder@yahoogroups.com > Subject: [Repeater-Builder] Interference on VHF repeater > > > > Hi all, > > I have come across an interesting problem which you may be able to shed some > light on. I have an intermod issue where my TX sometimes opens up my RX. I > have the distinctive hollow pipe sound. Both TX and RX have the same CTCSS > tone. The intermod product is however not always present, and after looking > at the RX output from the duplexer with a SA I see a comb of products that > move slowly in time. When one of the products in the comb falls within the > RX bandwidth the RX opens, until it moves on. > > This is not a busy site, and I have been able to power down everything on > site except my repeater. Problem remains unchanged. > > I have also disconnected feeders from all other RF equipment on site - still > no change. > > The fact that the IM product frequency changes with time (drift rate is > roughly a few kHz's an hour) makes me think that there is either another > unknown source of RF on site which has poor freq stability (pretty > unlikley), or somehow my TX freq is involved in producing this freq. > > I have inserted a 6dB pad in the antenna port of the duplexer and found that > the IM products drop 12dB, and also curiously, the frequency of the products > change. Removing the pad reverses this effect. I have repeated this many > times and the result was always the same. It appears that the frequency of > the IM product is dependent on the strength of the radiated field from my > antenna. > > This is my question: I have read that it is possible for a strong EM field > to excite metal (eg tower member) such that re-radiation will occur at a > frequency which is different from that which excited it. Can anyone confirm > they have seen this, or can anyone point me to a reference that talks about > this? > > I should also mention there are multiple solar panels and associated > regulators on site. The regulators have been discounted as possible sources, > but the panels (given they may have bypass/blocking diodes) may be a mixing > location, however the source of the drifting tone is still unclear. > > Thanks, > > Brett VK2CBD. >
Re: [Repeater-Builder] Re: Interference on VHF repeater
Look around for a switch mode power supply that uses 600kHz as the switch frequency. SMPS Battery Chargers are popular for causing this. Also florescent twist lights are really good for making desense on VHF.
[Repeater-Builder] Re: Interference on VHF repeater
"I have inserted a 6dB pad in the antenna port of the duplexer and found that the IM products drop 12dB, and also curiously, the frequency of the products change. Removing the pad reverses this effect." The above says that it's a 2nd order mix, F1+F2, F1-F2, 2F1 or 2F2. Since it looks to be a 2nd order product, proven by your 6 dB attenuator causing a 12 dB drop, whereas a 3rd order IM product would drop the product by 18 dB. Th 2nd order mix indicates that it could be combination of an internally generated signal from your equipment F1, Probably in the receiver input stage itself and an outside signal source F2 from an external transmitter, yours or another adjacent one. A pure 3rd order IM product is typically an indication of an internally generated source in the receiver input stages itself without any externally generated sources, but not always. Also do you have an isolator on the TX output along with a Low Pass filter after the isolator ? Is this a synthesized exciter or crystal controlled ? If you can identify all the signals present on Spectrum Analyzer then with the above 2nd order formulas you could probably zero down the 2 signals causing the problem. Mike --- In Repeater-Builder@yahoogroups.com, "brett" wrote: > > Hi all, > > I have come across an interesting problem which you may be able to shed some > light on. I have an intermod issue where my TX sometimes opens up my RX. I > have the distinctive hollow pipe sound. Both TX and RX have the same CTCSS > tone. The intermod product is however not always present, and after looking > at the RX output from the duplexer with a SA I see a comb of products that > move slowly in time. When one of the products in the comb falls within the > RX bandwidth the RX opens, until it moves on. > > This is not a busy site, and I have been able to power down everything on > site except my repeater. Problem remains unchanged. > > I have also disconnected feeders from all other RF equipment on site - still > no change. > > The fact that the IM product frequency changes with time (drift rate is > roughly a few kHz's an hour) makes me think that there is either another > unknown source of RF on site which has poor freq stability (pretty unlikley), > or somehow my TX freq is involved in producing this freq. > > I have inserted a 6dB pad in the antenna port of the duplexer and found that > the IM products drop 12dB, and also curiously, the frequency of the products > change. Removing the pad reverses this effect. I have repeated this many > times and the result was always the same. It appears that the frequency of > the IM product is dependent on the strength of the radiated field from my > antenna. > > This is my question: I have read that it is possible for a strong EM field > to excite metal (eg tower member) such that re-radiation will occur at a > frequency which is different from that which excited it. Can anyone confirm > they have seen this, or can anyone point me to a reference that talks about > this? > > I should also mention there are multiple solar panels and associated > regulators on site. The regulators have been discounted as possible sources, > but the panels (given they may have bypass/blocking diodes) may be a mixing > location, however the source of the drifting tone is still unclear. > > Thanks, > > Brett VK2CBD. >
