> Walt/K5YFW wrote: > On a 100 mile NVIS path (using the ionospheric heigth as 175 miles), > the first hop would needed to be radiated at 64 deg. The second hop > at 76 deg, the thrid hop at 81 deg and the forthe hop at 83 deg. > > If you are using a dipole or Inverted V antenna with the feed point > less than 1/4 wavelength, your antenna radiation pattern will have > maximum radiation between 60 and 90 deg. > > Thus, if you are working another station with the same antenna, you > may be receiving 1, 2 and 3 hop signals. How does this affect BPSK > and QPSK signals from for example PSK31/63/125? The assumptions is > that SNRs are sufficient that the signal strength is not a problme.
Hi Walt, As I understand it... The path loss for the ground bounce at HF is likely to attenuate the delayed single-bounce signal, and especially the multi-bounce signal. This attenuation from the ground bounce signals usually results in a significantly exalted no-bounce skywave direct signal for NVIS. The question then becomes: How much immunity is there with PSK decoding to the time shifted interfering signal's amplitude? Digital decoding of PSK may not be much of a problem, because there is somewhat of a threshold effect with decoders, but still, phase shift symbol rate becomes the important factor here. What we need is a relative figure of merit for comparing PSK at various symbol rates and various other signal methods in these sort of channel conditions. The flavor of noise in the channel can often be a factor... especially impulse noise. The RF reflectivity of the earth at the bounce point has a significant effect on the bounce's attenuation value. So, the attenuated bounce may be more of a problem on sea water (-3dB to -12dB?), than when the bounce is on land (-6dB to -30dB?). I'm guessing at these attenuation values, for the purpose of discussion, rather than looking up the real data :) Also, when operating freq is far below FoF2, or in propagation conditions that have QSB, with a rapidly varying sharp D layer absorbtion cutoff frequency knee, the multi-bounce signal may become larger than the primary NVIS signal when the lower angle incident skywave signal is attenuated. But, most of the time, this isn't seen much though, since normally the D layer absorbtion is a rather smooth curve vs frequency and doesn't have a sharp cutoff at such high angles. The bounced multipath interference signals for NVIS can be much stronger at MF frequencies than at HF, due to sometimes sharper ionospheric high angle critical incidence at MF and the increased reflectivity of the ground at MF in some areas. This is a common problem for AM broadcast DXing where phase cancellations often distort the signal at night. This was a significant factor in LORAN, especially when flying over the ocean, and if you have ever watched the old LORAN scopes in aircraft, it can sometimes be rather tricky to discern which is the real signal to use at certain times of the day... fortunately, most experienced navigators were pretty good at it! Well, enough rambling for now... I gotta get back to work. 73 Bonnie BA7/KQ6XA .
