+- Australian Ham Radio Discussion Forum ( AHRDF ) (https://www.ahrdf.net/forum)
+-- Forum: BAND BY BAND (https://www.ahrdf.net/forum/forum-28.html)
+--- Forum: LF, HF, MF (https://www.ahrdf.net/forum/forum-4.html)
+--- Thread: MW Broadcast QRM on 160m (/thread-530.html)
MW Broadcast QRM on 160m - VK4AQJ - 17-03-2018
I have a problem with this one local broadcaster on 927kHz AM. it's about time they phased out MW AM like they are doing in Europe. The sound quality is abysmal anyway. (rant over). That's the only one that causes problems for me. Being local, the strength of the signal is obviously swamping the bandpass filters in the rig.
I have had thoughts about using an L-C series resonant circuit at 927 kHz. So the 927kHz signal would be intercepted and sent to ground.
I've seen examples of high pass filters being used, but I also operate on 630m, and don't want to have to switch the filter in and out every time I change bands.
Has anybody else had experience with eliminating AM broadcast QRM? I will probably have to use a relay circuit to bypass the filter on transmit.
The other concern is that even though the second harmonic from the broadcaster is reported as 50dB down, that may not be enough, and no amount of filtering at 927kHz will address a second harmonic at 1854kHz.
- Jim
RE: MW Broadcast QRM on 160m - VK3QI - 17-03-2018
(17-03-2018, 08:59 AM)VK4AQJ Wrote: I have a problem with this one local broadcaster on 927kHz AM. it's about time they phased out MW AM like they are doing in Europe. The sound quality is abysmal anyway. (rant over). That's the only one that causes problems for me. Being local, the strength of the signal is obviously swamping the bandpass filters in the rig.
I have had thoughts about using an L-C series resonant circuit at 927 kHz. So the 927kHz signal would be intercepted and sent to ground.
I've seen examples of high pass filters being used, but I also operate on 630m, and don't want to have to switch the filter in and out every time I change bands.
Has anybody else had experience with eliminating AM broadcast QRM? I will probably have to use a relay circuit to bypass the filter on transmit.
The other concern is that even though the second harmonic from the broadcaster is reported as 50dB down, that may not be enough, and no amount of filtering at 927kHz will address a second harmonic at 1854kHz.
- Jim
Jim,
Before you get too carried away with filters etc. do a couple of other things:
(1) do a distance/attenuation calculation of what level the second harmonic may be at your qth from the broadcast station
(2) do a search in your neighbourhood for any possible "rusty bolt" type of rectification that might be reinforcing the harmonic at your qth.
Also ask yourself is the level of interference is weather or temperature dependent? That may give you a clue as to whether the problem is external to your qth or not.
Cheers
Peter VK3QI
(17-03-2018, 08:59 AM)VK4AQJ Wrote: I have a problem with this one local broadcaster on 927kHz AM. it's about time they phased out MW AM like they are doing in Europe. The sound quality is abysmal anyway. (rant over). That's the only one that causes problems for me. Being local, the strength of the signal is obviously swamping the bandpass filters in the rig.
I have had thoughts about using an L-C series resonant circuit at 927 kHz. So the 927kHz signal would be intercepted and sent to ground.
I've seen examples of high pass filters being used, but I also operate on 630m, and don't want to have to switch the filter in and out every time I change bands.
Has anybody else had experience with eliminating AM broadcast QRM? I will probably have to use a relay circuit to bypass the filter on transmit.
The other concern is that even though the second harmonic from the broadcaster is reported as 50dB down, that may not be enough, and no amount of filtering at 927kHz will address a second harmonic at 1854kHz.
- Jim
Jim,
Before you get too carried away with filters etc. do a couple of other things:
(1) do a distance/attenuation calculation of what level the second harmonic may be at your qth from the broadcast station
(2) do a search in your neighbourhood for any possible "rusty bolt" type of rectification that might be reinforcing the harmonic at your qth.
Also ask yourself is the level of interference is weather or temperature dependent? That may give you a clue as to whether the problem is external to your qth or not.
Cheers
Peter VK3QI
(17-03-2018, 08:59 AM)VK4AQJ Wrote: I have a problem with this one local broadcaster on 927kHz AM. it's about time they phased out MW AM like they are doing in Europe. The sound quality is abysmal anyway. (rant over). That's the only one that causes problems for me. Being local, the strength of the signal is obviously swamping the bandpass filters in the rig.
I have had thoughts about using an L-C series resonant circuit at 927 kHz. So the 927kHz signal would be intercepted and sent to ground.
I've seen examples of high pass filters being used, but I also operate on 630m, and don't want to have to switch the filter in and out every time I change bands.
Has anybody else had experience with eliminating AM broadcast QRM? I will probably have to use a relay circuit to bypass the filter on transmit.
The other concern is that even though the second harmonic from the broadcaster is reported as 50dB down, that may not be enough, and no amount of filtering at 927kHz will address a second harmonic at 1854kHz.
- Jim
Jim,
Before you get too carried away with filters etc. do a couple of other things:
(1) do a distance/attenuation calculation of what level the second harmonic may be at your qth from the broadcast station
(2) do a search in your neighbourhood for any possible "rusty bolt" type of rectification that might be reinforcing the harmonic at your qth.
Also ask yourself is the level of interference is weather or temperature dependent? That may give you a clue as to whether the problem is external to your qth or not.
Cheers
Peter VK3QI
RE: MW Broadcast QRM on 160m - VK3RX - 17-03-2018
(17-03-2018, 08:59 AM)VK4AQJ Wrote: <snip>
The other concern is that even though the second harmonic from the broadcaster is reported as 50dB down, that may not be enough, and no amount of filtering at 927kHz will address a second harmonic at 1854kHz.
- Jim
That may end up being your problem.
Some years back I worked at Mildura airport, and 3MA one of the local AM stations was on 1467kHz. Aviation changed from HF AM to SSB, and at the same time the frequencies used changed.
Our 2MHz frequency became 2944kHz, and the second harmonic of 3MA being 2934kHz caused interference from day one. I recall a report was filed with the then P&T Dept. and correspondence went back and forth with a reminder about interference to air navigation etc.
The 3MA site was about 10km from our airport HF receiver site, and I think it was determined at the time that the station could legitimately radiate something like 5 Watts second harmonic and still be in compliance with the regs. That 5 Watts was enough to cause the problem.
The final outcome was that naturally no way was 3MA going to change their frequency, and so the aviation network changed the 2MHz frequency from 2944kHz to 2869kHz and all was then OK.
RE: MW Broadcast QRM on 160m - VK4AQJ - 17-03-2018
Damien,
Thanks for your reply. The - 50dB figure came from correspondance from the Radio Station engineer and another local amateur. It reinforces what I was thinking. I will check the power out of the broadcast station from the ACMA site and work out what erp that translates to. We have a few hills between us, but the distance is only about 5km. It's about time the MW band was put to better use. As the listeners die off, it will eventually become too expensive to run. It's a much better option for them to use 88-108MHz FM. The running cost is so much lower.
I just checked - 50dB attentuation is something like 50mW for a 5kW transmitter. My gut feeling is that it should be low enough, especially considering that the second harmonic would not match very well into a resonant antenna. The mast height is 75 metres according to ACMA, which is almost exactly a quarter wave ( roughly 77 m).
For 5km, assuming an optimistic antenna gain of -10db at both ends, the free space path loss is about 65dB. The signal I'm getting is 20dB over 9, day and night regardless of WX conditions.
Jim
RE: MW Broadcast QRM on 160m - VK4AQJ - 30-03-2018
Well, I tried a few things while our midweek 160m net was on. First of all, the precise distance to the MW broadcast antenna is 2.9km. I had this very strong Broadcast QRM on the net frequency, and I could just make out the strongest station R-3. I had made up a series trap for 927kHz.
I had tried it on transmit previously, and it didn't affect the SWR. When I instered the trap, the strongest station went up to R5, but the QRM was still there. I tried the attenuator. This gave a marginal improvement.
So I then decided to try the 40m dipole. It's oriented with the end pointed at the Broadcast station, but way off resonance for top band.
It was a Eureka moment. The Broadcast QRM disappeared completely, and I was able to copy all stations on the net. All were complaining of QRN. I couldn't hear any QRN. One faint station joined the net, and the others were having problems copying him. I could read him R-5 even though he was further from me that the others.
In conclusion, I don't think we're dealing with a second harmonic, but just front end overload. I know that some 160m operators use a separate antenna for receive, but it hadn't occured to me to try the 40m dipole.
Ihave had a QSO with VK5ASF on 160m. He uses a highly sophisticated (and expensive) TMLA antenna, and from what I saw, it worked very well.
I may opt for a magnetic loop for receive only, which will probably cut down the cost by 90% compared to a transmit antenna, and use a relay to switch to my current 160m antenna on transmit.
RE: MW Broadcast QRM on 160m - VK3RX - 30-03-2018
Interesting.
You earlier said:
Quote:- 50dB attenuation is something like 50mW for a 5kW transmitter
So I'm surprised that would cause such overload at 3km, but then I've never had a BC station so close. Can you hear the second harmonic on a shortwave portable?
I wonder if the station engineer actually measured and proved the station's compliance. I know with the Mildura situation I described the station had to for the then P&T, but in that situation the interference involved the aviation authority so there was a bit more clout.
RE: MW Broadcast QRM on 160m - VK4ADC - 31-03-2018
(30-03-2018, 05:22 PM)VK4AQJ Wrote: I had made up a series trap for 927kHz.
I had tried it on transmit previously, and it didn't affect the SWR. When I instered the trap, the strongest station went up to R5, but the QRM was still there. I tried the attenuator. This gave a marginal improvement.
You didn't confirm whether you had placed the series trap across the coax inner to ground... which is what I would expect you did.
Another thing to try is to change those same two components into a parallel LC, still resonant at 927, but place it in series to the antenna port. If your L has a high enough Q, the impedance at resonance, and therefore signal rejection, will be magnitudes higher than you can achieve by placing a series LC across a 50 ohms line.
Use a 12V relay off the PTT line with a set of contacts to short the parallel LC circuit on transmit to minimise losses.
RE: MW Broadcast QRM on 160m - VK4AQJ - 03-04-2018
Thanks Doug, I'm going to go with a separate receive antenna for now, and that will have to be my 40m dipole until I get something better.
"You didn't confirm whether you had placed the series trap across the coax inner to ground... which is what I would expect you did."
That's exactly what I did. I'll file away your suggestion of a parallel LC circuit for future reference.
Jim
(30-03-2018, 06:12 PM)VK3RX Wrote: Interesting.
You earlier said:
Quote:- 50dB attenuation is something like 50mW for a 5kW transmitter
So I'm surprised that would cause such overload at 3km, but then I've never had a BC station so close. Can you hear the second harmonic on a shortwave portable?
I wonder if the station engineer actually measured and proved the station's compliance. I know with the Mildura situation I described the station had to for the then P&T, but in that situation the interference involved the aviation authority so there was a bit more clout.
1 I can't hear the second harmonic on a Shortwave portable.
2. I doubt if the engineer actually checked the compliance, but I could be wrong. He actually sent a plan for a trap the size of a 200 L drum, which was overkill to put it mildly. As I said, I now think we're dealing with front end overload anyway, which would explain why there was no QRM with a 40m dipole.
- Jim
RE: MW Broadcast QRM on 160m - VK2ZIW - 04-04-2018
Hi all,
Here in Emu Plains NSW 2750 I have a mixing product on 1854KHz, probably a rusty joint somewhere.
1825 to 1845 is clear and 1860 to 1875 is OK also.
So, listening to the guys on 1859 is difficult at 6am in the morning. 1857 is even harder.
VK8RC port 4 missed the DaylightSaving. and didn't have to get up an hour earlier.
Antenna here: the four wires supporting my TV mast, balun at the mast.
Alan VK2ZIW