NVIS Conditions Update

[VA3KOT John]

A screen capture of the critical frequency (foF2) from the ionosonde at Alpena, Michigan taken today shows encouraging signs for renewed NVIS (Near Vertical Incidence Skywave) activity on 40m. Typically, reliable communications using NVIS are made at a frequency about 20% below the critical frequency. Although today's plot shows frequent peaks in the 40m range it is too early in the solar cycle for regular NVIS operation on that band.
   
Note that even 80m is only useful from 13:00 - 23:00 UTC (8:00 am - 6:00 pm local) for NVIS. The fact that we can still hold the GBARC HF net on 80m outside this window suggests that ground wave propagation is being used for stations in the GBARC region. Ground wave propagation is enhanced by using a vertically polarized antenna. Those of us with horizontally polarized antennas (like mine) are at a disadvantage.

Remember that the propagation pattern for a NVIS antenna shows most of the energy going straight up at 90 degrees to the horizon in a classic "cloud burner" manner. At the same time, a useful amount of energy is still available at lower angles for mid-range or ground wave communication. Thus a AS2259-style 40m/80m inverted-V dipole is still useful even though it will only work on 80m during the daytime for true NVIS propagation.
   

[Ve3dgy]

Thanks John, this was very interesting and easy to understand. I’ve seen the solar data on iDX but really don’t have a datum to say what’s good or bad, and they group 40 and 80m together. Your plot clearly shows they should not be grouped in the evening, that’s useful for me to know. I find 80m weird, 2 weeks back on the net the noise was S5 and I heard everyone clear, this past net I was getting 10-15 over, and I was apparently not strong. Now my IC7300 wasn’t tuned, not sure why it doesn’t remember, maybe cause I was fooling round in the digital frequencies and retuned. My experience on 40M is always positive, aided I think with a 1:2 SWR on my dipole across half the band.

I was using 20M as well but for reasons I don’t understand my dipole SWR has gone off the scale, I have to pull it down and see what’s going on. I have them all fanned from a single point of contact and have added 6 and 10m, maybe getting too crowded. 

I’ve tried using voacap but I find it’s usually showing a low % on most bands across the pond. Your chart is much more encouraging.

Anyway, thanks for taking the time.

[VA3KOT John]

Thanks for the feedback Doug. I generally get S6-7 noise in my shack. I have a lot of LED lighting and all those switching regulators emit a fair amount of wideband RF. Our washer and dryer also seem to be RF noisy. At this time of year a lot of people are putting out Christmas lights that may contribute to noise. Sometimes a nearby hydro pole or transformer starts misbehaving. A portable AM radio tuned between stations is a good tool for tracking down where the noise is coming from.

Fan dipoles can be very difficult to work with because of interactions between the different band elements. I have never used one for more than two bands because of that. Some bands are harmonically related, e.g. 15m and 40m, so you may be able to reduce the number of elements that way. It may be worth considering separating them and feeding them with different coaxes. Rob, VE3RWY, has at least four antennas at his station (I've lost count, maybe Rob could comment) and uses coax switches in his shack to select one.

[Rob_Walker]

A very good post John.  The timing of your post happened to coincide with some material I have been reading with regards to the arrival and departure angles of most DX signals.

The vast majority of DX signals arrive between 3 and 8 degrees off the horizon. Manufacturers do not tell you this.  They tell you MAXIMUM antenna gain, regardless of where the signal is going.  If you look at the specs on some Yagis that claim substantial gains, the specs might be at 16º which might be good for one-hop transmissions (4000km).  However, there might only be a 1db gain at 5º

Doug, I have several antennas in my back yard but three of them go to a remote antenna switch at the base of my tower.  From the antenna switch its one run of coax into the shack.  There is a bias-t in my shack that sends 12 vdc down the coax for switching the antennas.  In my shack is a small box that will allow me to select up to 4 antennas.  If you are interested, here's a link:

https://www.radioworld.ca/ame-rcs4

Do you use a separate tuner with your 7300? 

73 Rob

[Ve3dgy]

Hey Rob thanks for the tip, that switcher might be a good option for me. I notice when I put my antenna analyzer on the coax that the swr changes on the bands when its been raining, might be an indication that my spacers are “leaking”.

 The IC7300 has a built in tuner that is supposed to remember it’s tuning at different frequencies on the different bands, I have to experiment but it seems to me when I retune at the lower frequencies to play with digital that it might forget the higher frequencies. I just have to remember to push the  tune button. I have a separate tuner for my Yaesu 857D.

I would love to win a free shopping tour of Radioworld, could buy all kinds of stuff I want but don’t need.

[Rob_Walker]

Doug when any antenna element becomes wet from rain it will change the SWR. Do you use bare or insulated wire?

[Rob_Walker]

Doug,

Concerning your rig not remembering it's previous tuning settings, this may not be all that out of the ordinary after all, especially if you are trying to tune a higher frequency after being on a previously tuned 'lower' frequency.  In my case, I have an MFJ-998RT remote tuner that used to sit  at the base of my 43' ZeroFive Antennas and exhibited the same characteristics.  The only work-around was to power the tuner off and back on. 

I have noticed this can occur if the tuning solution for the previous band results in a very high SWR on the new band, and more often happens when going from a lower frequency band to a higher frequency band especially when using a highly reactive antenna. This very high SWR can reflect all input power from your transceiver, so the autotuner cannot sense enough RF input power. The solution was to simply cycle the power to the autotuner when changing bands. This drops the autotuner to bypass prior to tuning so some forward power is sensed and subsequently permits a tune to occur.

I am not using my autotuner at the moment but I wonder if the same thing (in theory) is happening to you?  You wouldn't have to power down in your case but you would have to force an auto tune by holding the tune button in on the 7300.

[Ve3dgy]

And I think that’s exactly what was happening. Tom mentioned on the net that perhaps something changed cause I was working digital on the low end of the band, I engaged the tuner there and probably lost the higher frequencies. Now I know to tune more often. I checked 80M this morning and was getting about S7 noise, much better than the 10-15 over I was getting last wed.

[Rob_Walker]

I'm not exactly sure how the tuning process works with the 7300 but with my remote tuner, sometimes I could overcome the difficulty tuning by using a little more power.  I would normally use 10 watts to tune but if I didn't get a tune, I bumped the power up to 20 watts.  It worked sometimes but I also had to power cycle in some cases too.