NVIS (Near Vertical Incidence Skywave) depends on the ionosphere reflecting
near vertical signals back to earth. However this happens primarily at low
frequencies and at a certain critical frequency the signals penetrate the
ionosphere and go out into space rather than being reflected. This critical
frequency varies depending on ionospheric conditions. What Allison is
saying is that over the last several years ionospheric conditions have been
such that the critical frequency has been below 7 mHz. except for rare
circumstances thus precluding NVIS communications on 40 meters.
Mike
K5ESS
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-----Original Message-----
From:
[email protected] [mailto:
[email protected]] On Behalf Of Alan Jones
Sent: Monday, June 17, 2019 7:09 AM
To:
[email protected]Subject: Re: [qrp-tech] QRPGuys EFHW 40m-15m Mini Tuner
Allison, can you explain what you mean by "critical frequency" ?
N8WQ
On Sun, 16 Jun 2019 22:56:03 -0400, ajparent1/kb1gmx <kb1gmx@...>
wrote:
Low antennas for 60M and down to 160 are useful for short range.
Unfortunately NVIS on 40M is way oversold as the critical frequency
hasn't been near 7mhz in years except on rare days.
I know as i have a low dipole for 40 and 75 also be default 160.
The 75M dipole is the antenna for ranges to about 400 miles and
occasionally further. That antenna is barely 12Ft up. Its also a
low noise antenna as its pattern is UP and houses with their noise
are not that direction. Same for the 160M even thought he center
is at 35ft (.25wave on 160 is on the order of 100-120 feet!).
However trying to use the 40M low dipole for close in work confirms
no NVIS. However on 40M a vertical was often god for close in
because of its low takeoff angle.
When I say higher we are definitely talking more than .25Wave and
more like .5 wave. On 40M getting 40-50ft up is not that hard for
most (note ,5wave is 66ft). But at 80M that's more than 60ft
for a .25wave up and .5 wave is 120ft.
HIgher is better but understanding what High means as a function
of wavelength (frequency) takes on dimensions, sometimes large.
The other part is propagation, we have had a lot of solar and geomagnetic
activity where 20M and even 40M was dead and 75M had very high
absorption and people running 1KW 160 miles away were hard to hear
with any antenna. I have a fairly decent location. During conditions
like that I've had better results at 6 and 2M with only 100W to the
same stations. Often no one antenna is totally the solution for any
band and conditions.
Allison
