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[N1AEP]

I'm near Boston and do a lot of MW and 160m listening in the evening and through the night. Typically I can hear many broadcast stations from Hartford, Washington, DC, Rochester, NY, Buffalo, Albany, NY etc. pretty reliably. Also hams in New England and mid-Atlantic.

Would these stations be coming in on ground wave or surface wave at these distances, at night?  Or am I receiving sky-waves off the F layer? Aren't these rather short distances for an F-layer sky-wave? Especially considering that the transmit antennas are vertical and thus transmitting at very low angles? (Not NVIS.) I would expect that I'd be inside the "skip zone" and wouldn't be able to hear them. I've also read that groundeave coverage extends further at night, which may or may not come into play.

Meanwhile, I also often hear broadcasters (and Top Band hams) much further out...NC, SC, GA, MO, MI, IN, IL, etc. and I presume these are strictly skywave, off the nighttime F layer, given their longer distances away.

So I wonder what's going on in these cases. One thing I have definitely noticed while driving at night from Boston to southern CT, NYC or out to central NY, tuned to WBZ 1030 kc., Boston on the car radio, I tend to lose the signal after about 150? miles, it is inaudible for quite some time, and then I can start hearing it again at about 300 miles. This would suggest that I am indeed passing through a skip-zone: I lose the signal for an extended time, after which I can hear it once again.

Just not sure how a low angle MW signal from a vertical MW ant. (i.e., with very little vertical signal component) reflecting off the F layer could actually return to earth only a few hundred miles from the source. Further distances I would expect, several hundred miles anyway, with F layer involved. But can the closer-in dx also be coming off same F layer?

...If anyone could pls comment or explain these obsevations / phenomena.  Many thanks!
73

[K9RJ]

K9LA's website has excellent information. See http://k9la.us/html/160m.html

Harris K9RJ

[W8JX]

During the day D layer absorption is very high on 160 and high on 80 to which limits range on 160 to ground wave and on 80 to a few hundred miles. 40m is effected less by it and can work up to 1000 miles or so.  20m and above is not really effected by it and is effected by E and F layer conditions/MUF. At night D layer dissipates and  E and F layer propagation is possible on 160 greatly extending its range. This same loss of D layer opens 80m and 40 meters up to mush greater ranges and DX at night.

[W1VT]

Perhaps the local terrain is reflecting the signal upward, so you are getting short skip not predicted by simple propagation and antenna models that don't take the local ground conditions into account.  This short skip can interfere with ground wave signals, resulting in a zone where reception is poor.  This short skip would only be noticeable at night, when the D layer absorption is minimal.

Zack W1VT

[N1AEP]

I am grateful for the explanations. Thanks!
73

[WB6BYU]

Meanwhile, remember that your mobile antenna will have a null overhead (as will the transmitting antenna).
So even if the ionosphere will support high angle radiation over shorter distances, neither antenna is optimized
for a high angle path.

Also, just to add to the confusion, a recent study of NVIS propagation showed that occasionally the normal
NVIS paths on the low end of 80m were via the E layer rather than the F layer, requiring a relatively lower
angle of radiation to cover the same distance.


That's part of the fun of HF operation:  just when you think you have it figured out, the ionosphere decides
to do something different...

[WA8NVW]

To add a few more comments about your BCDX, WBZ is usually good in Cleveland at night and from there heading west well past Chicago.  The biggest listening problem is sideband 'hash' from adjacent and second adjacent channel assignments.  If you compare the 'big guns' of NYC and Chicago you'll find Chicago has the slot 10 kHz up. Example 760 Detroit MI, 770 WABC NYC, 780 WBBM Chicago, 790 WSGW Saginaw MI, 800 CKLW Windsor Ont. Also I've noticed the new blade style automotive antennas display much less vertically polarized on the AM band.

[KH6AQ]

If it was ground wave propagation the stations would come in during the day.

[RFRY]

...  This short skip can interfere with ground wave signals, resulting in a zone where reception is poor.  This short skip would only be noticeable at night, when the D layer absorption is minimal.

Sorry for the late post, but below is a study of the day/night fields of WBZ and some others on 1030 kHz.

As noted by W1VT, there is a zone of nighttime self-interference common with "Class A" 50 kW 24/7 AM broadcast stations where their groundwave and nighttime skywave have roughly the same field intensities.

For receive locations further away, the groundwave gets weaker while the skywave gets stronger, and the effect is reduced to unnoticeable levels (mostly).