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Author Topic: one-way propagation  (Read 2441 times)
WI7B
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« on: April 10, 2005, 08:43:23 PM »

Dear Elmer,

After working numerous contacts from the East Coast on 40m, I've noticed in the early evening that propagation seems to favor East Coast signals to the West.  I can constantly read NY stations 59, while they hear me only 22 or 33.  What's up with that?

---* a poor West Coast OM
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W8JI
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« Reply #1 on: April 11, 2005, 03:41:15 AM »

There are some cases of "one way" propagation, but not many. Virtually all or certainly most of the time it is local conditions causing this effect.

By the way, you can't really go by S meter readings since they vary wildly from radio model to radio model and by how the operator is setting up his radio. They are generally NOT good absolute indicators.

In your example the east coast stations are LONG past their sunset, and the European BC stations are pounding in very strong. There is also more noise propagating in for them, since they have darkness in all directions.

You on the other hand are further from the BC stations, and primarily have strong noise propagating only from dark areas.

I see this effect on 160 meters and 40 meters, where I can hear Europeans long before my sunset but have no hope of being heard.

It's a common rumor that a good HF transmitting antenna is a good receiving antenna, but absolutely false. Another dominant player in this effect is your antenna efficiency and your power. An inefficient, very low gain, or even negative gain antenna will receive quite well, since directivity sets receive capability and NOT gain. When transmitting, only absolute gain at the desired angle and direction matters.

Also signal quality makes a difference. Too much bass or enhanced highs and you waste power and become hard to copy on SSB when signals are weak.

73 Tom  
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AA4PB
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« Reply #2 on: April 11, 2005, 04:37:33 AM »

Tom, I assume you are referring to the S/N issue. Some antennas can be better for receive because they reduce the noise level more than they reduce the signal level and its the S/N that determines the ability to copy. As far as I know, antennas have exactly the same gain and directional pattern on receive as they do on transmit.
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K5DVW
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« Reply #3 on: April 11, 2005, 05:52:04 AM »

It's most likely the antenna. Consider a vertical antenna with a poor ground system as an example. On receive and on transmit you still have a pretty lossy antenna, say the loss is 10 dB, which can be typical of a 1/4 wave vertical without enough radials.

On receive, say you have a very sensitive modern day receiver that has an HF noise floor about 15 dB BELOW atmospheric noise. Again, very typical. The result is that you can hear stations pretty well and you have margin before you notice any S/N degradation. In other words, sounds like signals are coming in strong!

On transmit, say you have 100W, or 50 dBm. Everything is equal transmit vs receive so you loose the same 10 dB. You're now transmitting 40 dBm, or 10 Watts... and this is about 2 S units loss.

I believe that one way propagation is exceptionally rare if it exists at all at HF. I've seen it happen on a microwave duplex channel during dispersive fading conditions, but never seen it on HF.

What kind of antenna do you have?
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WI7B
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« Reply #4 on: April 11, 2005, 09:00:33 AM »


ThanX guys! You've help me understand better what I am
experiencing by viewing the rig, antenna, and ground
as a whole system.  

Indeed, K5DVW, I have  a vertical with 40m counterpoise(see http://WI7B.org/), but it has been supremely effective in rountine contact to Japan, Asiatic Russia, and the South Pacific.  The difference is my skips west are to the "bright side",
and begin and end over the Pacific. My transmissions
and first skip to the "dark side" east begin on dry,
rocky, high desert (bad ground).  Now I am thinking
incoming RF is little impacted by the high desert
topography. However, low-angle transmissions must be
greatly affected by poor reflection over dry, rocky
ground for a few wavelengths?

73,

---* Ken
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W5GNB
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« Reply #5 on: April 11, 2005, 10:40:50 AM »

I have experienced this same problem when using the "FAN DIPOLE"  but it was much more pronounced when operating with a "G5RV" antenna.  

HIHIHI!!

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WB2WIK
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« Reply #6 on: April 11, 2005, 01:56:17 PM »

Here's another vote to skip the "one way propagation" theory for two reasons:

1.  It doesn't make a lot of sense, and I'm not sure I've ever actually witnessed this in 40 years of hamming; and

2.  Even if it did exist, like all other things involving propagation, we're powerless to do anything about it, so why worry?

Part of operating the HF bands is how many things beyond our control contribute to or detract from a contact.  And, as has been said, how much your own transmitting antenna contributes to the mix, irrespective of receiving.

On 10-12-15m, where I have a competitive antenna, anybody I can hear, I can work, usually first call -- with rare DX, maybe 2-3 calls, but it's extremely unusual to not complete a contact with anybody I can hear.  Often, I receive reports far stronger than I could honestly give out.

On 17m, it's almost the same, but a little bit more balanced, and I occasionally receive a report that's worse than the one I can honestly give.

On 20m, where my antenna is sub-par compared to many big-gun DXers (I'm using a beam, it's just not 46' long, nor stacked, nor up 120 feet), it's very common that I hear stations stronger than they hear me.  I can get through most of the time, but it's not a slam-dunk.

All those different results, using the same rig, the same power, the same tower, the same everything except boom length vs. wavelength.  I have the "longest boom" (compared to wavelength) antenna on 10m, and the "shortest boom" (v. wavelength) antenna on 20m.

Funny how that works, eh?

On 40m, I become almost competitive again, even without a beam, simply because I have a good wire antenna that's up high enough to work well.   There, I can receive and send equal reports with almost anybody, and usually get through in a pile-up unless I'm against a bunch of contesters running beams.

The "aiming into the dark" vs. "aiming into the light" has merit, too.  If I'm aimed into a noisy sky, signals have to be very strong to *sound* 59.  Aimed into a quiet sky, even S1 signals can sound 59.  Same rig, same S-meter, different noise level.

Another reason why I give reports with my eyes closed...it's about the only way to give an accurate report.

WB2WIK/6
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X-WB1AUW
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« Reply #7 on: April 11, 2005, 02:51:17 PM »

My theory is that many people give 59 reports when 47 or 46 is more accurate (on ten meters Q5, S2).  On low bands Q3, S2 when the noise level is S7.

Reports on low bands of 59, when the noise level is 7 or 8 are miss leading.  Some OTs on low bands give reports indicating how much out of the noise a signal is.  

Also, those east of us on the low bands have strong openings N-S, and to their east---lots more QRM than we have when the band opens.

Have FUN workin’ ‘em.
73
Bob
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AA4PB
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« Reply #8 on: April 11, 2005, 05:21:35 PM »

Also consider that the other guy might have a higher local noise level than you do. That could easily account for you copying him much better than he copies you, even though the actual absolute signal strength is the same - and that probably gets attributed to "one-way propagation" in a lot of cases.
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W8JI
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« Reply #9 on: April 12, 2005, 03:09:48 AM »

I looked at your antenna, and if it has only one counterpoise wire it probably has terrible efficiency. It also would be somewhat directional.

If it has a large ground system, perhaps 10 or more evenly spaced counterpoise wires that were 1/4 wl long, efficiency would be much better. It would not be nearly as good as a full size antenna on lower bands, however.

I think the antenna might explain a good part of the frustration. Your receiver has enough gain to overcome the poor efficiency (20dB noise headroom is typical). The transmitter does not, unless you ran 10,000 watts (20dB gain over 100W).

73 Tom
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WB2WIK
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« Reply #10 on: April 12, 2005, 08:00:09 AM »

That was a really good catch by Tom W8JI re the antenna.

Is the "bugcatcher" vertical whip what you're using on 40 meters??

If so, I imagine you know the efficiency of these things at 7 MHz is very, very small.  Almost anything works for receiving at 7 MHz (to improve S/N at my home station, I sometimes toss a 50' roll of wire out the window and let it lay on the ground, use that for receiving, and can hear some stations better with that than I can with my transmitting antenna), but not so for transmitting.

This may be the entire explanation.

WB2WIK/6
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