AM Broadcast vs Shortwave propagation

[sparklehorse]

Hello,
I'm sure I knew the answer to this question at one time, but my aging brain is failing me at the moment. I'm wanting to know why AM Broadcast (Medium Wave) stations typically propagate over much shorter distances at night than Shortwave broadcasts. In my experience AM radio stations can often be heard at night at distances of 700 to maybe 1200 miles or a bit more. I don't recall ever hearing a Mexican AM broadcast station here in northern Oregon at night, or a station from the US East Coast, but I have heard KGO in San Francisco many times, as well as radio stations in Utah, Nevada, Colorado, etc. Yet with Shortwave broadcast I've often heard New Zealand, China, the BBC, and others from far flung corners of the globe. I know Medium Wave frequencies typically refract off the F layer, the same layer as HF frequencies, so one would think the propagation distances would be similar. I also know that with the right conditions a 100 watt Ham rig can make contacts on the opposite side of the globe, so I don't think a difference in ERP is the reason. Maybe MW frequencies don't reflect off the earth as well as HF, making multiple hops unlikely? I'm really not sure, and the answer has eluded my Google-fu. Can someone here answer this question please? I'm sure someone knows.

Thanks for the help!

.

 

[ka3jjz]

MW frequencies pretty much obey the same laws as the lower part of HF; dead during the day, they wake up at night. They can and do travel great distances, when conditions allow it. Otherwise Trans Pacific(TP) and Trans Atlantic (TA) catches wouldn't exist.

Now there are quite a few things that can hinder MW reception; many ham transceivers, and some HF receivers, deliberately added attenuation below 1 Mhz to prevent the possibility of overload and other problems. I haven't been dabbling in this area for a long time, but I know there were a lot of discussions on how you can bypass this attenuation, depending on the radio

Another thing to keep in mind is that mountains are not your friend. If the majority of the signal you want to hear happens to run into mountains (remember, there is no one path that a signal takes when it bends off the F layer), chances are you aren't going to hear it.

And of course, don't forget the antenna. While your typical G5RV or even an inverted L will certainly hear stations on MW, it isn't very efficient. Not many of us live in areas where you can put up 1000 foot plus antennas, so the next best thing is a loop. And I'm not even talking about a Pixel, Wellbrook or the W6LVP designs. There are many homebrew loop designs on the net, and we even have a few in the wiki.

Some things to think about...Mike

 

[majoco]

I believe that in the US, BC band broadcasters have to reduce their power and make radiation pattern changes at night - this will reduce the DX effect. Also broadcasters aren't interested in DX, they're only after the local market, so - only sufficient power for the job. Most BC band b'cast antennas are vertical masts perhaps with loading coils so their vertical radiation pattern is mostly horizontal - there's not a lot going up in the air to be refracted back down at some remote distance. Most of the propagation is via ground wave as Marconi found out when carrying out his early experiments, so he had massive amounts of power into huge antennas at what we now call VLF - very low frequencies - which is why you can ( I can't! ) often hear European LW broadcasts in the US. One of the good results of that is that they gave all the shortwave frequencies to the hams to play with, who pretty soon found out you could get worldwide communications with modest power and small antennas. Then they wanted it back!

 

[ka3jjz]

Some do, yes, reduce power and / or change their radiation patterns at night - the graveyard channels are frequencies where this happens, and that sometimes opens up DX possibilities. And while lower power makes for tougher catches, it doesn't take all that much to get heard - just ask any QRP DXer and s/he will tell you the same (altho that's HF, the same rules apply...)

Mike

 

[a29zuk]

Also remember the max power of a broadcast station in the US and Canada is 50,000 watts. I believe the max for a shortwave station is 500,000 watts.
As Mike was saying in his earlier post most outside antennas for shortwave frequencies are closer to 1/2 or full wavelength where a 468' antenna is the 1/2 wavelength length for 1000khz on your broadcast dial. Doubling this length or more will make a beam off the end of the antenna(beverage). This would make it easier to receive a transoceanic station.

Jim

 

[ka3jjz]

Well, not all HF listening antennas are based on a 1/2 wave or multiples. There are several that are untuned, and certainly not 500 foot long (heh). While that is the optimum, listening doesn't require such stringent constraints, unlike antennas used, say, in the amateur service, where matching and such becomes much more critical and necessary for the transmitter to stay in service.

Look at loops - some are based on a 1/10 wavelength diameter.

Mike

 

[ka3jjz]

Since I mentioned loops in an earlier message, here's our wiki on the subject - it's certainly nowhere near comprehensive, but it gives you a start as to what's out there....

Loops - The RadioReference Wiki

Undoubtedly clubs like the IRCA and the NRC have much more to say on that topic...Mike

 

[Boombox]

My unscientific observation (I've read up on propagation a long time ago when I was building listening and CB wire loop and beam antennas) is that if *all* radio waves refracted off of the ionosphere, even microwaves would be going across the oceans. But they don't. I think ionospheric refraction has something to do with the physics of how it's made up, and it is more receptive to refracting certain radio wavelengths, and less apt to refract others -- all because of the physics of the ionosphere itself.

But that's just distant memory combined with nothing more than a guess.

 

[ka3jjz]

Actually the phenomenon of refraction off the ionosphere becomes a lot more difficult as you leave the HF spectrum and go higher. Yes, it can and does happen - just ask any ham that works 6m, 2m or 432 SSB - but weather related DX is far and away more common. And as we get into 700-800 Mhz and higher - it hardly happens at all.

Back to topic - keep in mind that Mexican MW stations have radiation patterns just like their cousins in the US. So if their pattern doesn't favor in your direction, it will be harder to hear them - and much more of a DX challenge.

MW DXing is, in many ways, something more of a challenge if you get bored with HF stuff. Mike

 

[Boombox]

RE: Mexican MW stations in the NW US:
Sparklehorse, I live in the next major metro about 200 km north of you, and nearly every night lately you can hear XERF on 1570. You may have to null out KGAL 1580. XERF plays old style ranchero music.

Another station you may hear would be XECL 990 in Mexicali Mexico. They play Spanish language version of American oldies music. XEUT Tijuana is often audible here on 1630 -- your 1640 station may be a problem for that one, though. XEUT is a college station that plays a variety of cool music, ranging from electronic jazz to baroque.

If you hear ESPN sports on 1700 chances are high that it's XEPE, Tecate, Mexico.

XEWW 690 should be audible -- it's audible up here. They have a variety of Mexican programming. So is XEPRS 1090, which is an ESPN outlet aimed at Southern Cal from just across the border. You may have to null the Portland sports station on 1080 to hear that one, though.

 

[sparklehorse]

RE: Mexican MW stations in the NW US:
Sparklehorse, I live in the next major metro about 200 km north of you, and nearly every night lately you can hear XERF on 1570. You may have to null out KGAL 1580. XERF plays old style ranchero music.

Another station you may hear would be XECL 990 in Mexicali Mexico. They play Spanish language version of American oldies music. XEUT Tijuana is often audible here on 1630 -- your 1640 station may be a problem for that one, though. XEUT is a college station that plays a variety of cool music, ranging from electronic jazz to baroque.

If you hear ESPN sports on 1700 chances are high that it's XEPE, Tecate, Mexico.

XEWW 690 should be audible -- it's audible up here. They have a variety of Mexican programming. So is XEPRS 1090, which is an ESPN outlet aimed at Southern Cal from just across the border. You may have to null the Portland sports station on 1080 to hear that one, though.

Thank you for the info. I'm not surprised you've heard these Mexican stations as they are only about 1100 miles from Seattle. This is well within the AM propagation distance range I've experienced. It's also within the range of a single bounce off the F-Layer. I personally have not heard such stations, but I have not sought them out either as I am not an AM DXer. I just tune around the band occasionally to see what's there. What would surprise me is if you have received AM broadcast stations from the US East Coast, or Guatemala, or any location whose distance would require the signal to bounce off the ground. Maybe East Coast AM DXers log European stations because an ocean bounce doesn't attenuate AM signals as much as a ground bounce?
.

 

[ka3jjz]

Well, yes, from what little I recall, an ocean path is definitely a help for those that want to do TA (Trans Atlantic) DXing here on the East Coast. You would be in a fair position to hear TP (Trans Pacific) stations, even tho you are well inland from the coast. There's been a number of Pacific MW DXpeditions to prove that theory. Just keep in mind that much of the rest of the world uses a 9 khz spacing; we in the Americas use 10 khz. So anytime you start hearing a 1 khz het on a MW station, it's time to start digging. Hawaii and the various US territories would be obvious targets, not just Central America (altho that would be interesting, too).

Trans continental MW DX is possible, but it's not easy by any means. I've even heard of folks going for a 50 state award on MW DX. As you might expect, this type of DX uses nice long antennas in quiet neighborhoods, away from the urban sprawl.

Mike

 

[ka3jjz]

Sparkle, have you performed a great circle path chart from Portland to Central America? This would at least give you some of the most probable paths RF would take between those 2 points. There are utilities on the net, I'm sure, that can do this?

Mike

 

[sparklehorse]

Sparkle, have you performed a great circle path chart from Portland to Central America? This would at least give you some of the most probable paths RF would take between those 2 points. There are utilities on the net, I'm sure, that can do this?

Mike

Yes, there's this one someone posted the other day:

"As The Crow Flies" Distance Calculator

I should probably re-iterate that I'm not really interested in DXing MW signals. I was only curious as to why HF signals seem to propagate over much greater distances than MW. The consensus here seems to be that MW can propagate over vast distances, but AM broadcast signals are not generally broadcast at the same high ERP as Shortwave broadcasters, and also one must use an appropriate receiving antenna. I've never used anything ambitious for AM reception as far as an antenna, so that likely explains a lot. Thanks folks, for helping me understand!
.

 

[Boombox]

Thank you for the info. I'm not surprised you've heard these Mexican stations as they are only about 1100 miles from Seattle. This is well within the AM propagation distance range I've experienced. It's also within the range of a single bounce off the F-Layer. I personally have not heard such stations, but I have not sought them out either as I am not an AM DXer. I just tune around the band occasionally to see what's there. What would surprise me is if you have received AM broadcast stations from the US East Coast, or Guatemala, or any location whose distance would require the signal to bounce off the ground. Maybe East Coast AM DXers log European stations because an ocean bounce doesn't attenuate AM signals as much as a ground bounce?
.

I've heard Cuba on MW, and a station on 1700 from New York. I've heard South Korea, on a portable once. I've heard Brownsville TX (as several NW DXers have, many of them only using a portable and maybe an external loop). I've heard Mexico City and a long time ago I heard Colombia (on a boombox, with aid of a loop). Those are all farther than 1100 miles. Other guys have heard more than I have. So long distance MW skywave is probably more common than you think. And I think you're correct, salt water reflects better than dry land does.

I still think the difference between SW and MW skywave propagation depends on the physics of radio waves and the makeup of the ionosphere. Another example would be VLF and ELF penetrating deep into the water, whereas HF and VHF won't. It's because of the physics.

 

[wyShack]

Medium wave and lower are more affected by D layer absorption. Also, the noise level is much higher as frequencies are 'reused' by nearer stations. There are still some 'clear channel' stations which have 'exclusive' use of a given 'channel' (like WCCO in Minneapolis at 830 khz). 'Clear channels' are established by each country so what may be a 'clear channel' in Mexico could be used multiple times here in the US. Also the domestic broad cast band may be slightly different in different countries -Canada used to have (maybe still does) several stations 'below' the typical US broadcast band.

Hope this helps fill in the blanks for your answer.

73

 

[tyleman]

Sparklehorse: You should be able to get signals from the East Coast in Portland. My recollection goes back to the 1960s, but I used to regularly hear WWL in New Orleans, Chicago, Winnipeg and Saskatoon. Probably others. A lot depended on the time, too. Early morning seemed to be better.

I also remember hearing Spanish speaking stations. I don't remember details, but it seems to me there was a station in Mexico, Tijuana maybe, that had 100 kw.

It used to be some of the AM stations would be off the air on Sunday evenings/Monday mornings, for work on the transmitters.

 

[K5MPH]

I've heard Cuba on MW, and a station on 1700 from New York. I've heard South Korea, on a portable once. I've heard Brownsville TX (as several NW DXers have, many of them only using a portable and maybe an external loop). I've heard Mexico City and a long time ago I heard Colombia (on a boombox, with aid of a loop). Those are all farther than 1100 miles. Other guys have heard more than I have. So long distance MW skywave is probably more common than you think. And I think you're correct, salt water reflects better than dry land does.

I still think the difference between SW and MW skywave propagation depends on the physics of radio waves and the makeup of the ionosphere. Another example would be VLF and ELF penetrating deep into the water, whereas HF and VHF won't. It's because of the physics.

What radio stations are you hearing from Brownsville,Texas......

 

[Boombox]

^^^^^^ I believe it's KVNS, 1700 kHz. They used to play classic hits, but now are sports. I haven't heard them much over the past year or so, but in 2012-2013, I heard them frequently. Sometimes they would overpower XEPE which is an ESPN station just over the border from San Diego County, California.

I also heard that Tejano station on 1700 from the DFW area somewhere.