In last December's ARRL 10M Contest, I set a goal to make 100 SSB QSOs and 100 CW QSOs. Things certainly were better this last December compared to December of 1996, so it wasn't too hard achieving those goals.

One event stood out, trying to work a W4 on CW. He had a pretty decent signal - the S-meter on my TS-180S hovered around S4. But I just couldn't raise him. He'd stand-by after his CQ and I'd call, but he'd go right on CQing again. That was frustrating. 

My first reaction was to make sure I had the amplifier on (yep, it was) and make sure I didn't have the VFO SPLIT on. (Been there, done that). In fact, once I had the SPLIT on while trying to work someone on SSB, and the other VFO was down in the CW band - that's a BIG oops. After all those checked out OK, I tried some more but finally gave up and went on my way looking for others to work. I figured the W4's receiver needed some help in the sensitivity department. 
I didn't think too much about it after that. But then I received an e-mail from a contester in Florida on the Monday after the contest. He recently moved there, and was in the process of setting up his new station. All he could put up for the 10M contest was a TA-33 at low height fixed on a northerly heading. He said he heard me CQing on CW, and tried to call. But I'd just go on calling CQ. Hmmm, the tables are turned.

Maybe the W4's problem wasn't his receiver - maybe other things were at work here. This episode also reminded me of one of W9EGQ's Amateur Radio columns in the old Popular Electronics in the early '60s. He discussed one-way skip on 15M, but I don't remember the specifics - anybody out there have any of these old issues of Popular Electronics? 

Over the years I've seen several explanations of one-way skip, so this month's column is devoted to reviewing these explanations. 

One possible explanation is what I alluded to earlier - receiver performance. Receiver sensitivity is measured in terms of a minimum discernible signal (MDS) - this tells where the noise floor of a receiver is, which defines how weak a signal can be heard. Some receivers are better than others in this aspect. For example, ON4UN's book Low Band DXing has a chapter devoted to receiver performance. The MDS of many receivers is tabulated. The best sensitivity is around -140dBm, and the worst sensitivity is around -120dBm. 20dB is quite a difference. This might explain a one-way path.

Transmitter power is another factor. Even if comparable receivers are used at both ends, signals could be weak enough to be at the receiver's noise floor so that the 100W station hears the 1kW station, but the 1kW station can't hear the 100w station. A 10dB difference in power can also be significant. Maybe not by itself, but throw in antenna gain and the difference could be even more significant. I figure this was most of the problem with the contester in Florida who couldn't work me - he was barefoot at 100W due to a broken amp, and I was at 1kW. Along these lines, think about the problems QRPers may have working high power stations, especially under marginal conditions. 

Another explanation, and the one that is most favored for one-way propagation, is QRN. The station who can't hear may have more atmospheric and/or man-made noise than the other station. A good example of different atmospheric noise levels being a problem is the northern hemisphere trying to work the southern hemisphere on the low bands. When one hemisphere is in winter (atmospheric noise relatively low), the other is in summer (atmospheric noise relatively high). 

A good example of man-made noise being a problem is a rural environment compared to a city environment. Being in a rural environment doesn't necessarily guarantee low noise, though - only one offending noise source can create havoc, like a security light (been there, done that, too). 

Speaking of the low bands, a somewhat unique situation can occur due to the use of low noise receiving antennas (Beverages, EWEs, loops, etc). Many stations use verticals for the transmit antenna to get most of their energy concentrated at low elevation angles (there's another subtle reason why a vertical is the way to go on the low bands for those of us in North America - it's tied to magneto-ionic theory and coupling energy into the ionosphere - more on that next month).    Using a single vertical for transmit means an omni-directional pattern. Thus the transmitted signal goes in all directions.

But the receive antenna, being designed for low noise, usually has a null in some direction. A good example of this is an East Coast station transmitting on a vertical, but listening on a Beverage pointed to Europe. The West Coast may hear the East Coaster fine, but the East Coast can't hear the West Coast very well because of the front-to-back ratio of the Beverage. This specific topic has surfaced several times on the Internet reflector devoted to 160M (the topband reflector) in the form of West Coasters telling East Coasters to please listen to the west. 

When a band is just opening is another explanation. The best example I can think of is the CQ World Wide DX Contest. When the higher bands are just opening up from the Midwest to Europe, we can hear them. But the bands are wide open in Europe. They are receiving much stronger signals from other Europeans, from Africa, from Asia, from the Mideast, etc. They just can't hear us yet among all the other stations due to QRM. We have to wait for the band to get better.

There's one other mechanism that I thought might produce one-way propagation, and it's tied to the ionosphere itself. Normally we tend to think of the ionosphere as somewhat constant between the transmitter and receiver. But this is the exception rather than the rule, especially for longer paths. Usually the E region critical frequency foE, the F2 region critical frequency foF2, and the F2 region height of maximum ionization hmF2 vary along the path. This would be especially pronounced when one end of the path was near sunrise or sunset. Figure 1 shows the parameter foF2 (foE varies similarly and hmF2 is essentially constant) for an example I looked at - the path from Ft. Wayne to Croatia 9A for November at 1200Z (7 a.m. here in Ft. Wayne) at a smoothed solar flux of 155. 

I figured that the my energy going to 9A would see a dramatically different ionosphere than the 9A energy going to W9, and that would show up as significant elevation angle differences for the incoming and outgoing energy at each end. Couple that with an antenna pattern with peaks and nulls, and one-way propagation could happen. The result of ray tracing at 14MHz with the above example requires the following elevation angles:

No significant difference exists at each end with respect to transmit and receive. If my antenna is good enough to receive the required low angle, then it would be good enough to transmit the required low angle. And likewise for 9A. This is only half the story, of course. I also looked at the signal strength in the off chance that it would be different going one way compared to the other. But that didn't give a hint of one-way propagation, either. This was interesting, so I tried another path, W9 to KH6, with the same result - no hint of one-way propagation. Oh well, at least I had fun going through the calculations (yeah, right).

But remember the above analysis was done at 14MHz. On the lower frequencies (specifically 160M), non-reciprocal propagation is fairly well documented in the technical literature. This shows up as a stronger signal in one direction as opposed to the other direction - usually East-West. This is due to how propagation at frequencies near the gyro-frequency is affected by the earth's magnetic field. So those who work DX on 160M have this to contend with. 

In summary, I hope I've shed some light on one-way propagation. Some explanations you can do something about. Buy an amplifier, make sure your receiver is sensitive enough (without compromising its ability to handle strong signals), and eliminate man-made noise sources. The others, atmospheric noise, the band just opening, and the ionosphere itself on the lower frequencies are things you don't have much control over. All I can say is keep trying, and good luck.