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Author Topic: mobile NVIS  (Read 658 times)
KK7UE
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Posts: 9




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« on: April 25, 2003, 04:30:46 AM »

I am interested in operating mobile NVIS (75 & 40) from my 95 Nissan 4x4 pu. Not portable - I want to be able to operate on the fly with NVIS. Any input is appreciated, thanks!
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K5LXP
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« Reply #1 on: April 25, 2003, 09:17:03 AM »

With an HF mobile antenna on a vehicle, especially 40 and 80M, you're *always* NVIS!  Most antennas less than a half wave above ground have a significant high-angle lobe.

Mark K5LXP
Albuquerque, NM
k5lxp@arrl.net
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K0BG
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« Reply #2 on: April 25, 2003, 09:22:10 AM »

Mark is correct except for the lobe part. It's more a elongated vertical globe. In other words, just about the best NVIS money can buy.

Alan, KØBG
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WB2WIK
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« Reply #3 on: April 25, 2003, 12:48:45 PM »

I sure agree, on 40 & 75.

For the higher bands, what I've done (copied from the US Marines, who does the same thing) is use a spring mounted long whip on the rear of the vehicle, pulled over the vehicle with its tip clipped to the corner of the windshield, using a rain gutter clip.  Makes a vertical whip nearly horizontal, and does seem to improve very short skip on 10-12-15 etc.  But on 40 & 75, shouldn't be necessary at all.

WB2WIK/6
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KK7UE
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« Reply #4 on: April 26, 2003, 04:34:42 PM »

Thanks guys for the input! My (perceived?) problem is that a screwdriver vertical is not true NVIS. I am leaning more towards the bent whip idea. I wish those army humvees would slow down a little so I could get a close look at their systems <grin>. Tnx, 73
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AB8JC
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« Reply #5 on: May 28, 2003, 03:05:17 PM »

What exactly about the antenna makes it "true" NVIS?

The antenna has little to do with it.  What matters is where the radiated energy goes.  If it goes pretty much vertically and bounces back down, then it's NVIS.

If you want to get serious about it, check out www.tactical-link.com
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N3EOP
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« Reply #6 on: September 03, 2003, 01:14:35 AM »

> What exactly about the antenna makes it "true" NVIS?

Well, if an antenna has a vertical null, then it's pretty much *not* an NVIS antenna...
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KS4TY
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« Reply #7 on: October 30, 2003, 10:11:50 AM »

This is something I am trying to accomplish also.  There certainly isn't as much info on mobile NVIS as about other things.

I have tried screwing two hamsticks together end to end into a mobile antenna roof rack mount and tuning with an SGC-230, then bending the long hamstick over my van.  This requires a strong spring. Smiley  This works at least mobile to base, it just doesn't get much WAF (Wife Acceptance Factor) and isn't very stealthy.

I have also tried wrapping an insulated roofrack with 80ft of wire.  This tunes up but seems a bit deaf.

My next attempt is to do a shorted loop with a 1in copper pipe, and maybe even silver plate it for conductivity.

The best place for mobile info (that I know about) is:
https://www.tactical-link.com/nvis3.htm

and the David Fielder book:
http://www.qsl.net/g3cwi/nvisbook.html

For non moving use, simply connecting ~1/4 wave of wire to an antenna mount on your vehicle and throwing it over a few bushes or somehow suspending it <1/4 above ground and tuning up works well.

There are also several yahoo email groups related to this:
NVIS, hfpack, hflink.

Ed
KS4TY
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KE4SKY
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« Reply #8 on: December 31, 2003, 02:04:16 PM »

I use a Diamond K400 mount on the rear hatch of my Jeep Cherokee with a Pro-Am / Valor hamstick knockoff.

When driving at Interstate highway speeds the whip is heeled over 45 degrees anyway, so that you have high angle radiation.

If operating stationary, attach a 1/4 wavelength of insulated AWG14 wire via a quick disconnect to the mobile mount, have the dogbone on the other end tied to a length of construction twine with a heavy surf casting sinker on the other end, heave it up 25 feet or so into a tree, then pull the vehicle forward just  enough to tension the wire as a sloper and then match as needed with the tuner.  With a smaller vehicle than full sized pickup, van or SUV you may need to add some counterpoise by using your jumper cables to connect from the vehicle frame to a metal guard rail, wire fence or ground stake.  Much more effective than the loaded whip.  
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K8KAS
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Posts: 569




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« Reply #9 on: January 13, 2004, 12:14:49 PM »

The NVIS improvement below 8Mhz by bending the whip is TOKEN, only tenths of a Db improvement in high angle radiation. I was present durning open sight testing for military vehicle application and only a marketing type could be impressed with the improvements in signal. Above 8 Mhz there was 6 to 8 tenths of a Db.
No magic at all. The bending of the whip has more to do with vehicle profile/hitting things with the whip than NVIS performance.
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