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Author Topic: NVIS Mobil Antenna  (Read 10481 times)

Posts: 116

« on: December 22, 2011, 11:03:21 PM »

I am interested in HF commutation between vehicles while off roading in the Southwest.  No line of sight. No repeater. From the bottom of a canyon to the bottom of another canyon 30 miles apart.  Communicate between two vehicles 50 miles apart with a 10,000 foot mountain in between. The NVIS antenna is what we need. I have read about the military vehicles with their 20 foot whips bent into a large inverted U that they claim provide good results in areas like I just described.  I have found extensive information on the web related to the portable wire NVIS military and civilian antennas but nothing definitive on the mobil antenna. Communication while in motion. If this mobil antenna exists and works as advertised can anyone help point me in the right direction on some 411 about it.

Thanks 73
« Last Edit: December 23, 2011, 12:09:49 AM by KF7ITG » Logged

Posts: 29

« Reply #1 on: December 23, 2011, 07:55:21 AM »,or.r_gc.r_pw.r_cp.,cf.osb&fp=a1bc1589ec616832&biw=1111&bih=548

Posts: 7718

« Reply #2 on: December 23, 2011, 09:00:39 AM »

The take-off-angle link is about 80 degrees. NEC shows a vertical mobile whip producing about the same gain as a low, short dipole over average ground. The band for this link is 80 or possibly 40 meters (or 60 meters) during the day and 160 or possibly 80 meters at night.

An experiment would be to compare an 80 meter Hamstick against a dipole made with two Hamsticks. Repeated A-B testing will average out the differences in polarization shift through the ionospheric path. Receive testing with the other station transmitting is sufficient. Plot the results of each A-B measurement and see what it looks like.

Posts: 17480

« Reply #3 on: December 23, 2011, 09:13:13 AM »

Bending over a standard mobile whip doesn't help a lot, as maximum radiation is from the
lower portion which is still primarily vertical.  To make it work you need to use some sort
of end loading so the horizontal portion carries significant current.  One possibility is a
mobile whip sticking horizontally off the back of the vehicle (up as high as possible off
the ground.)  Or if you use a long bent antenna running up from the front bumper and
horizontally over the vehicle, putting a Hustler mobile resonator (or other adjustable
mobile antenna) on the very end will work better than base loading.

You could try a "hamstick dipole" above the vehicle and running lengthwise.  We've never
gotten the 80m version to work because the tuning is so touchy and efficiency is very
low, but it may be practical on 40m when the band is open.

A "magnetic loop" is another approach, often employed in military systems.  I've seen one
installation that used a loop running from the front bumper to the rear (though you also need
good bonding to join the ends under the vehicle - don't rely on the frame and bumper
mounting hardware.)  This was fed at one end with a shunt capacitor for matching, and
a variable capacitor was used in series with the other end for tuning.  Tuning is very sharp
with such an antenna, but if you are using a fixed frequency it may give better signal
strengths than the other methods if you can keep losses low.

Here are come commercial / military solutions:

Posts: 10248


« Reply #4 on: December 23, 2011, 09:59:00 AM »

This whole business of NVIS using a mobile antenna with the whip bend over is junk science. Anyone with the wherewithal to model same, would know that. What's more, you often see references of using NVIS at high frequencies. The truth is, it is difficult to do at any frequency over about 5 MHz, and even then it requires antennas specifically for the purpose. It is indeed unfortunate that the internet allows this sort of thing to propagate (pun intended).


Posts: 17480

« Reply #5 on: December 23, 2011, 01:50:26 PM »

Quote from: K0BG
...The truth is, it is difficult to do at any frequency over about 5 MHz, and even then it requires antennas specifically for the purpose...

Actually NVIS propagation is common on 40m during the day when conditions are good.  They
haven't been for several years, but it is getting better as the sunspots increase.  As a simple
rule of thumb the MUF is roughly 4 times the Critical Frequency, so when 10m is open, 40m may
support NVIS work.

Posts: 8

« Reply #6 on: December 28, 2011, 10:32:03 AM »

NVIS from a mobile can work pretty well if you are willing to pull over and spend a few minutes etting up.  A 40m dipole mounted 6-10 feet off the ground using either tree limbs or portable poles (that come in various section legths for easy portability) will work during the day.  The same setup with an 80m dipole will work at night.  I have used this sort of arrangement myself and can get the whole thing set up in under 10 minutes easy.  I camp a lot up here in Idaho, and one thing I enjoy is taking along an HF rig and talk to my buddies back in Boise, usually 50-200 miles and several 8,000 ft+ mountain ranges away, no problem.  But if you are talking about using NVIS while actually moving down the road, I doubt it can be done without a very elaborate (and expensive) antenna system.

Posts: 121

« Reply #7 on: December 29, 2011, 05:38:36 AM »

 >>>> "NVIS from a mobile can work pretty well if you are willing to pull over and spend a few minutes etting up" <<<<

Are we confusing Mobile and Portable here?
Doesn't Mobile indicates while traveling down the highway/road?

(((73))) Milverton.

Posts: 1454

« Reply #8 on: January 02, 2012, 11:32:21 AM »

Communications would be a lot more reliable in this situation by just placing a temporary 2 meter handheld repeater at the top of the mountain.  Wink

Posts: 17480

« Reply #9 on: January 02, 2012, 01:26:43 PM »

Quote from: KI4SDY
Communications would be a lot more reliable in this situation by just placing a temporary 2 meter handheld repeater at the top of the mountain.  Wink

I used to work with a group that had a portable 2m repeater using a pair of hand-helds.
It required two people to lift it and had to be hauled around in the bed of a pickup truck.
So remember that a "handheld" repeater doesn't mean that the repeater can be held in
the hand, only that the radios used in it can.  The required duplexor is much larger than
the radios.

But that also assumes that you have access to the mountain tops where you will be
driving - that isn't always the case.

Posts: 1454

« Reply #10 on: January 02, 2012, 06:13:08 PM »

Your example is too complex for a temporary situation out in the boonies. I was thinking of the MFJ-662 Simplex Pocket Repeater. It only requires one handheld and solar panels for recharging during battery idle periods. Cheap fast, and easy!  Grin
« Last Edit: January 03, 2012, 06:36:50 AM by KI4SDY » Logged

Posts: 17480

« Reply #11 on: January 04, 2012, 11:40:49 AM »

This still makes several assumptions that may not be warranted in the situation described:

(1) that the top of the 10,000' hill between the two vehicles is accessible without spending
two days scaling it.

(2) that, if it is accessible, it isn't already filled with radio equipment that will desense
an HT placed nearby, especially one with an antenna sufficient for reaching into a canyon
30 miles away.

(3) that there is enough time in a weekend to place the radio initially and pick it up
afterwards and still have time for the planned outing.

(4) that a "simplex repeater" actually provides a usable communications solution.  We tried
on for Search and Rescue - it seemed like a good idea, but in practice it caused a horrible
confusion.  (Not to mention interfering with stations 250 miles away.)  We gave up and
used a manual relay instead.


Posts: 1454

« Reply #12 on: January 05, 2012, 12:27:42 AM »

Sending a man-ual relay up to the top of the mountain with a handheld, some beer and snacks for the day would be more reliable than mobile NVIS! Is he experimenting or is he serious about communicating? We don't know because he hasn't responded.  Wink

If you had used the device before, why didn't you mention it as an option along with the drawbacks? Obviously it worked well if it was interfering with stations 250 miles away! Like any piece of ham equipment, it just has to be used appropriately. That includes informing prospective users how it works and how to interface with the system for maximum efficiency. I am surprised that your group did not check the repeater guide to make sure any interference would be eliminated. That is what licensed hams are supposed to know to do.  Undecided    
« Last Edit: January 05, 2012, 05:36:27 AM by KI4SDY » Logged

Posts: 17480

« Reply #13 on: January 05, 2012, 07:03:29 AM »

This was on SAR channels, not the ham bands.  And the stations were interfering with
might not have wanted to publish the frequencies they were going to use.  There
were a number of poor choices involved, but they weren't made by hams.

But the important piece that relates to this specific situation was that, even from a
hilltop where we were causing interference 250 miles away, it didn't give us
reliable communications into the wilderness area close by where we were working.
It was better to send a team up along a ridge where they could change their position
to suite the location of the teams in the field.

Again, that assumes you have the time and resources to deploy a relay team, and
a convenient place that they can access that covers the area in question.  That
often won't be the case when a couple families go 4-wheeling together for a
weekend and you may cover a couple hundred miles through steep canyons and
around 10,000 foot hills.

Is mobile NVIS easy?  No, certainly not.  Can it be done?  Yes, it can, as is shown
by the number of military antennas designed for this purpose.

Probably the final solution will require careful assessment of the actual communications
needs and what is practical.  For example, it may be that using standard mobile whips
will work much of the time, with a protocol to stop and set up a quick dipole when
there is a critical message and signals are too weak, or at set times (such as lunch
breaks).  A horizontal Hamstick off the top rear of the cab may work for some vehicles
but not as well for pickup trucks perhaps.  In desert conditions in the SW a horizontal
Hamstick dipole in line with the vehicle may work, while it would be unsuitable in more
wooded areas.  A self-supporting loop (possibly with side stays) from the front of the
vehicle to the rear will have sharp tuning, but could be one of the more efficient
approaches if the needs made it worth the investment for building such a thing.
In any case, a careful study of the propagation forecasts would be required to know
which bands were going to give the most reliable coverage.

But we don't know a lot of the specifics, so the best we can do is to suggest a number
of options and let the OP decide what best fits his needs - it probably will require
some experimenting over time to find a solution that handles the mechanical stresses
of mobile operation in the desert while providing somewhat reliable communications.
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