Author Topic: HF NVIS antenna systems for regional communication  (Read 17958 times)

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spacecase0

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Re: HF NVIS antenna systems for regional communication
« Reply #30 on: June 15, 2012, 04:52:08 PM »
here is another vertical HF antenna solution
http://www.ldgelectronics.com/c/252/products/16/62/1
they have shorter ones as well...

Lamewolf

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Re: HF NVIS antenna systems for regional communication
« Reply #31 on: October 16, 2015, 07:45:37 AM »
here is another vertical HF antenna solution
http://www.ldgelectronics.com/c/252/products/16/62/1
they have shorter ones as well...

Yes, but a vertical does not do well for NVIS work....

spacecase0

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Re: HF NVIS antenna systems for regional communication
« Reply #32 on: October 16, 2015, 03:36:05 PM »
Yes, but a vertical does not do well for NVIS work....
that is handy to know,
I have had a hard time checking into local nets with the vertical antennas, but I had never made the connection as to why.


AD

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Re: HF NVIS antenna systems for regional communication
« Reply #34 on: October 16, 2015, 08:57:01 PM »
Thanks FERDoc.  Great find.  I may have to make one up. I have two full sets of the FG poles.
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Tempstar

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Re: HF NVIS antenna systems for regional communication
« Reply #35 on: October 17, 2015, 06:08:57 AM »
After playing with various designs like the G5RV and the Chameleon vertical, I have settled on a homebrew 20-40-80 fan dipole hung at each end at 45', I used 1/8" stainless cable for the elements and feed the down line with 8X then it changes to 213 for another 50' to the shack. This is thus far the best set up I have had. I work the East Coast easily and also work a buddy 53 miles away and  a net control for the Coastal Carolina Emergency Net (Shameless plug: 3.907 daily at 1900 eastern time) at 31 miles.
 I learned by studying military and ship set up that band selection is as important as antenna selection. Yeah, 10 meters is great in the morning, and 20 talks around the world during the day, but 40 and 80 will talk anytime the bands are anywhere approaching mediocre to decent.
 NVIS performance with a dipole is best determined by height above ground, the lower you go giving more vertical signal. It is also highly dependent on soil conductivity to provide  a suitable "backboard" to shape the RF. This can be overcome by using radials. A technique I stumbled on with radials under a dipole was to earth ground but not RF ground them. The other side of the RF is the dipole, and hooking the braid to a ground radial changes the equation of the antenna.
 And yes, verticals will do NVIS. Just pull 'em over like a Humvee or Andy Griffiths squad car.
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Lamewolf

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Re: HF NVIS antenna systems for regional communication
« Reply #36 on: October 17, 2015, 07:36:56 AM »
that is handy to know,
I have had a hard time checking into local nets with the vertical antennas, but I had never made the connection as to why.

Vertical antennas radiate at low angles that can be blocked by terrain like hills, mountains, buildings etc.  For NVIS work, you need a low horizontal antenna where it radiates almost straight up and causes the signals to bounce back down to Earth close in for your local region comms.  Imagine this scenario:  You are one side of a mountain and the station you want to contact is on the other side of the mountain 5 miles away.  Since a vertical radiates at low angles your signal is trying to go through the mountain but doesn't make it.  Now, use a low horizontal antenna such as a dipole and your signal goes basically straight up bouncing off the ionosphere and comes back down on the station on the other side of the mountain.  Kind of like shining a flashlight into a mirror on the ceiling at a slight angle and you can make it shine on the floor just a few inches from where the flashlight is sitting.  For NVIS work, try something like a dipole at 1/4 wavelength or less above ground and you can enhance the upward radiation by laying a counterpoise wire that is about 5% longer then the dipole on the ground directly under it (parallel to it on the ground).  Kind of like building a yagi that is aimed at the sky. And for example, an 80 meter dipole should be less than 65 feet above ground and works real well at around 30 feet for NVIS work.  I have even use dipole as low as 7 feet and worked stations 150 miles away like I was sitting in their back yard !

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Re: HF NVIS antenna systems for regional communication
« Reply #37 on: October 17, 2015, 02:42:20 PM »
There are two kinds of ships.  Submarines and targets.
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spacecase0

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Re: HF NVIS antenna systems for regional communication
« Reply #38 on: October 17, 2015, 05:36:13 PM »
from my tests, nothing works under oak trees,
and I have tried antennas that work perfect for contacts across the world set in a 20 foot gap between buildings, but under oak trees they just fail. (my choices are under oak trees or just over metal...)
my current antenna is average about 4 foot over a metal roof and it works well
the verticals I have tried were out in the clear and still failed, even if there was nothing around

I have a few radios, and they do not act the same with the same antenna
strangely enough, the alinco dx77 wins and the ft-857 is almost a total fail
but it seems that I don't have enough test hardware to tell what is really going on
a grid dip meter would be quite handy about now


Lamewolf

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Re: HF NVIS antenna systems for regional communication
« Reply #39 on: October 19, 2015, 06:52:42 AM »
from my tests, nothing works under oak trees,
and I have tried antennas that work perfect for contacts across the world set in a 20 foot gap between buildings, but under oak trees they just fail. (my choices are under oak trees or just over metal...)
my current antenna is average about 4 foot over a metal roof and it works well
the verticals I have tried were out in the clear and still failed, even if there was nothing around

I have a few radios, and they do not act the same with the same antenna
strangely enough, the alinco dx77 wins and the ft-857 is almost a total fail
but it seems that I don't have enough test hardware to tell what is really going on
a grid dip meter would be quite handy about now

If you want to do NVIS, put up a horizontal wire about 4' above that metal roof and feed it via a tuner wit the ground of the tuner connected to the metal roof.

DragoSapien

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Re: HF NVIS antenna systems for regional communication
« Reply #40 on: October 19, 2015, 09:33:31 PM »
I like reading these antenna posts.

spacecase0

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Re: HF NVIS antenna systems for regional communication
« Reply #41 on: October 20, 2015, 12:20:33 PM »
If you want to do NVIS, put up a horizontal wire about 4' above that metal roof and feed it via a tuner wit the ground of the tuner connected to the metal roof.
connected the tuner ground to the metal right as the cable leaves the window (it is connected to the metal roof through metal walls) to keep out the RF from the shack (it was an issue), and it works perfect, contacts all around a few local states
I can here stations better than the net control for a local net, and more critical is that they can he me.

now all I want is a grid dip meter, or some other way to check resonance separate from impedance so I can dial it in closer