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Message from vitesse at Saturday, 03-Oct-09 00:23:36 GMT
I was asking a question on the French Canadian radio ham web site (www.radioamateur.ca) and a guy came to me with a great answer. Never heard of this propagation mode, but it seem to be working very well as an Ham in Quebec is using this mode. This could sometime explain weird HF reception.

Here a link about the article, really great but in French.
http://www.radioamateur.ca/techniques/nvis.html

Here another one took from www.radioamateur.ca I haven't read this one but it's in English.
http://www.qsl.net/w/wb5ude//nvis/
Message from norave at Saturday, 03-Oct-09 04:17:52 GMT
Hi Luc,

WB5UDE wrote "to establish reliable communications over a radius of 0-200 miles or so". I'd use VHF.

Just my $0.02
Sven
Message from vitesse at Saturday, 03-Oct-09 12:20:30 GMT
I haven't read WB5UDE, but VA2-JOT in french explain many situation where it more useful and easy to use HF and NVIS than VHF
Message from tkruczek at Monday, 05-Oct-09 21:34:10 GMT
yep, some hams on 75/80M use a low (relative to the ground) loop or dipole to achieve NVIS. It does work and I've worked stations that had excellent signals within a couple hundred miles. During the daylight hours and early to mid-evenings seemed to provide the best performance.

Tom / W1TXT /tkruczek
Message from Calico at Tuesday, 06-Oct-09 09:05:21 GMT
NVIS ?

Maybe the most popular everyday transmission in the low bands! :)

Without really intending to, most of us when we transmit on the low bands, we use NVIS if our antennas are, dipoles, long wires, OFC dipoles etc.

Given the high take-off angle of the radiation lobes if the antenna height is less than lamda/2 (half wavelength), most of us practise NVIS every time we transmit on the 7, 3, and 1.8 MHz bands.

To start having a good-ish take off angle for example at 3.700 MHz our dipole center would have to be about 40 meters (+/- 130 ft) above the nearest ground, for 1.85 MHz would have to be about 80 meters (+/- 260 ft), rather unachieveable heights.

If the antenna height is (as usual) less than half a wavelength (λ/2), the signal takes off at steep angles, (instead of the low desired for dx) and is bounced off by the ionosphere very close to where it originated in the first place, hence jokingly we call such antennas, cloud warmers, they don't do much more than...warming clouds! ;)

The ARRL Handbook, ARRL Antenna Book, etc have got details about the relationship between take off angle lobes and height of the antenna.

In brief here is what makes the difference between NVIS and Dx:

http://www.astrosurf.com/luxorion/Radio/dipole-height.gif

The Forces are especially interested in concentrating their signal close to their tactical field, in regional operations, it would be ugly if they used Dx antennas and their target corresponding stations fell into a skip (no-signal) zone! Hence they use NVIS antennas especially in mountainous regions.

Also, broadcasters, like Radio 700 on 6.005 MHz who want to reach audiences in German speaking areas of Europe, use similar antenna systems (Δ-loops at only 5m height above the ground thus resulting into about 80 degrees of radiation angle!) so that they transmnit most of the energy where their audiences are, close by and not far away.

Vertical antennas, can achieve very low angle of radiation at low bands, but have other limitations.

73
Tim (Dorset Radio)

Last edited by Calico at Tuesday, 06-Oct-09 14:00:32 UTC
Message from Anthony at Tuesday, 06-Oct-09 13:24:49 GMT
If you listen particularly in and near to UK, 5680 kHz usb is used by Kinloss search and rescue using NVIS. That's how HF is used with such good effect to communicate with helicopters that can often be on the ground and an incident scene, or flying low in a valley.
Message from okhoonz at Wednesday, 07-Oct-09 06:14:04 GMT
I use a modified version of a supernvis , it's a dual band 40/80m with corosponding? reflector wires also.

7ft high

The signal to noise ratio is quite good , for TX purposes above 10mhz would be a waste of time due to the muf at vertical at any given day (lately around 5mhz) , but as a RX antenna it still performs quite well as at angle of 30 odd degress most signals arrive at most times anyway .

It are very quite .

Near vertical means just that , near but not even close , so how far is Near!.

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