Home |
Search |
Today's Posts |
|
#1
![]() |
|||
|
|||
![]() From: Jeff Seale Organization: Insight Broadband Newsgroups: rec.radio.shortwave Date: Sat, 08 May 2004 21:16:15 GMT Subject: MARS? Arthur Harris wrote: "Greg" wrote: Is anyone hearing MARS activity (the military affiliates, not the planet)? I never hear anything on the freq's I have. I hear the MARS nets most often between 4.0 and 4.1 MHz mornings and evenings. Art N2AH The last MARS net I heard was on 4003 kHz at about 8:00 EDT. Jeff Seale Louisville, KY There was already activity on that freq when I tuned in around 7:45 EDT. Also, I have heard the special Armed Forces Day traffic on 13985, 13996, and 14467 USA earlier and 14467 is still active. Greg |
#2
![]() |
|||
|
|||
![]()
Greg wrote:
There was already activity on that freq when I tuned in around 7:45 EDT. Also, I have heard the special Armed Forces Day traffic on 13985, 13996, and 14467 USA earlier and 14467 is still active. Greg That's cool, I don't do too much daytime/morning listening though except on the weekends when I don't have to work. The earliest you'll find me hitting the radio is about 3:00 PM EDT. Jeff Seale Louisville, KY |
#3
![]() |
|||
|
|||
![]()
On Sun, 09 May 2004 02:31:59 GMT, Jeff Seale
wrote: Greg wrote: There was already activity on that freq when I tuned in around 7:45 EDT. Also, I have heard the special Armed Forces Day traffic on 13985, 13996, and 14467 USA earlier and 14467 is still active. Greg That's cool, I don't do too much daytime/morning listening though except on the weekends when I don't have to work. The earliest you'll find me hitting the radio is about 3:00 PM EDT. Jeff Seale Louisville, KY I've been wondering: If a shortwave transmitter was put on Mars could I pick it up - at night I mean? Would I need an external antenna? BTW - I tune down. |
#4
![]() |
|||
|
|||
![]() "Bill Everhart" wrote in message ... I've been wondering: If a shortwave transmitter was put on Mars could I pick it up - at night I mean? Probably. The ionosphere would refract the received signals, but I think it would still come in if it hit the ionosphere at a the correct angle. If the signals were directly overhead, absorbtion would be the main problem. There's a couple of small bands in SW allocated to radio astronomy. One is around 13MHz and the other is around 26MHz. Would I need an external antenna? Only if the signal from Mars is weak when it gets to Earth. As long as this is hypothetical, let's give the Martians a terawatt transmitter and a steerable parabolic dish a mile across. In the real world, radio astronomy needs good antennas. BTW - I tune down. Never up? Frank Dresser |
#5
![]() |
|||
|
|||
![]()
"Bill Everhart" wrote:
I've been wondering: If a shortwave transmitter was put on Mars could I pick it up - at night I mean? Would I need an external antenna? Interesting question. When Mars is at its closest point to Earth, it's still about 35 million miles away. Only a tiny portion of the transmitted power would arrive on Earth (the rest would "miss" us and go out into space in all directions). The "free space path loss" between Earth and Mars at 15 MHz would be 211 dB. That's a HUGE loss. At UHF and microwave frequencies the path loss is even greater, BUT the use of very high gain dish antennas both on Earth and Mars, as well a low noise figure receivers, makes communication possible. At HF, antenna gain of more than about 10 dB is hard to obtain. And the atmosphereic noise at HF is a killer for weak signal reception. Plus, you'd have to be listening at a time when your side of the Earth was facing "their" side of Mars, and the E and F layers of the ionosphere were NOT refracting signals. BTW - I tune down. Huh? Art N2AH |
#6
![]() |
|||
|
|||
![]()
In article ,
"Arthur Harris" wrote: snip The "free space path loss" between Earth and Mars at 15 MHz would be 211 dB. That's a HUGE loss. At UHF and microwave frequencies the path loss is even greater, BUT the use of very high gain dish antennas both on Earth and Mars, as well a low noise figure receivers, makes communication possible. At HF, antenna gain of more than about 10 dB is hard to obtain. And the atmosphereic noise at HF is a killer for weak signal reception. snip How did you calculate this loss? I'm assuming you mean the 211 dB to be an absorptive loss? Maybe you are considering the antenna on Mars to be a point source off a 180 degree ground plane so the loss figure is power distributed over a half sphere with the Earth - Mars distance? -- Telamon Ventura, California |
#7
![]() |
|||
|
|||
![]()
"Telamon" wrote"
"Arthur Harris" wrote: The "free space path loss" between Earth and Mars at 15 MHz would be 211 dB. That's a HUGE loss. At UHF and microwave frequencies the path loss is even greater, BUT the use of very high gain dish antennas both on Earth and Mars, as well a low noise figure receivers, makes communication possible. At HF, antenna gain of more than about 10 dB is hard to obtain. And the atmosphereic noise at HF is a killer for weak signal reception. snip How did you calculate this loss? I'm assuming you mean the 211 dB to be an absorptive loss? Maybe you are considering the antenna on Mars to be a point source off a 180 degree ground plane so the loss figure is power distributed over a half sphere with the Earth - Mars distance? The Free Space Path Loss Equation is: Path Loss (dB) = 36.6 + 20 Log F + 20 Log D Where F is freq in MHz and D is distance in miles. This assumes isotropic antennas as both ends. The "path loss" represents the portion of the transmitted signal that is NOT captured by the receiving antenna. It does not include absorptive losses (which should be negligible in free space). Some will argue that this is not a true dissipative loss, and that is correct. But by knowing the "path loss" you can determine your needs as to transmit power, receiver sensitivity, and antenna gain in order to assure successful communication. Art N2AH |
#8
![]() |
|||
|
|||
![]()
Arthur Harris wrote:
The Free Space Path Loss Equation is: Path Loss (dB) = 36.6 + 20 Log F + 20 Log D Where F is freq in MHz and D is distance in miles. This assumes isotropic antennas as both ends. The "path loss" represents the portion of the transmitted signal that is NOT captured by the receiving antenna. It does not include absorptive losses (which should be negligible in free space). Some will argue that this is not a true dissipative loss, and that is correct. But by knowing the "path loss" you can determine your needs as to transmit power, receiver sensitivity, and antenna gain in order to assure successful communication. The key here is a theorem that shows that the "capture area" of a perfectly efficient isotropic antenna is (wavelength)^2/(4 Pi). -jpd |
#9
![]() |
|||
|
|||
![]()
for all of you who were speaking about what times you listen, here is what
happened to me when I was real young. I heard an advertisement on a shortwave station for a program I wanted to hear, but at the time, I didn't understand 24 hour time since I had never heard of it before. However, when I asked about it, either my uncle or my grandpa (maybe both) explained it to me. and I then understood it. But I STILL missed the program when I tuned in at the time said to hear it. I didn't find out until later that the reason why I missed it is even though I understood 24 hour time, I wasn't aware of different time zones in different parts of the world. The time given was in GMT and I was tuning in according to Eastern Time instead of GMT. |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
![]() |
||||
Thread | Forum | |||
Attention MARS & Ex MARS Operators | Policy | |||
What is the deal with the MARS stuff? | Policy | |||
Attention MARS & Ex MARS Operators | Antenna | |||
Attention MARS & Ex MARS Operators | Dx | |||
Attention MARS & ex MARS Operators | Boatanchors |