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Author Topic: Vehicle-to-vehicle comms using 6m  (Read 1076 times)
K3IMW
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Posts: 80




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« on: December 20, 2001, 10:37:16 AM »

A Tech friend and ourselves do some off-roading several times a year.  We would like to be able to maintain communications up to a distance of several miles, when line-of-sight doesn't exist (and there are no accessible repeaters nearby).

It seems to me that we should be able to do more with 6m than with 2m, since 6m has some 'HF' characteristics.  Our current setup uses 1/4 wavelength vertical for 6m.  This would seem far from optimal for short range work.

The best setup that I can think of would have the following characteristics:

  - horizontal polarization to minimize QRM
  - have high takeoff angle if there is any chance of NVIS effects on this frequency

Presumably it would be useful to make use of local scatter and groundwave also.

What's the best (mobile) antenna for this?  A roof-mounted dipole seems like the most likely candidate.  A squalo would have low takeoff angles, which isn't what we are looking for, correct?

Thanks for any suggestions, experimental ideas will be researched over the next few months in the local deserts Smiley

73 de ian, K3IMW/6

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WB2WIK
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« Reply #1 on: December 20, 2001, 01:43:41 PM »

What local deserts are you going to where no repeaters are available?  On 6m, repeaters on Santiago Peak, Palomar Mountain, and Frazier Peak alone can cover about 90% of southern California (including thousands of square miles of nothingness) if you have a well-equipped mobile.

I think your questions and comments, while sounding reasonable, are made before actually getting on 6m and seeing what the band's like.  1/4-wave vertical whips for mobile-to-mobile coverage will likely provide better coverage than anything else you could use.  

Horizontal polarization "to minimize QRM" really doesn't make a lot of sense, since 6m is so underutilized that I'd defy one to find QRM!  During the largest band openings of the decade, November 17-18 when 6m was open to the entire world, there wasn't QRM.  Under normal conditions, it's hard to find anyone to talk to, let alone finding QRM.

A high takeoff angle is exactly what you _don't_ want, for extended mobile-to-mobile coverage.  NVIS doesn't exist on 6m, at least not 99% of the time.  NVIS is an F-layer mode, and the m.u.f. is well below 6 meters about 330 days a year, so the likelihood of being able to use this mode on six is pretty slim.

Groundwave is an LF-MF phenomenon which does not occur on six meters, at all.  Hams use this term improperly, quite often, so it's easy to understand where this comes from; however, ground wave above about 3 MHz just ain't.  Local propagation on VHF is either "direct wave" (line of sight) or via one of the tropospheric scatter modes.

If you're looking for extended m-to-m coverage over wide distances, without repeaters, I'd recommend you simply switch from FM to SSB.  The mode change, alone, without doing anything else, will improve weak signal readibility about 20 dB, and extend the range of mobile communications quite substantially.  I note my m-to-m range on 6m FM is ~20 miles when both stations are running 1/4-wave whips and 25W rigs.  It extends to about 60-80 miles by simply flipping the switch to SSB.  That might do it!

73 de Steve, WB2WIK/6
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K3IMW
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Posts: 80




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« Reply #2 on: December 20, 2001, 02:20:33 PM »

Hi Steve!

> Posted By WB2WIK
>
> What local deserts are you going to where no repeaters are available?  On
6m, repeaters on Santiago Peak, Palomar Mountain, and Frazier Peak alone can
cover about 90% of southern California (including thousands of square miles
of nothingness) if you have a well-equipped mobile.

Our usual haunts are Anza Borrego and the Mojave, with occasional forays
further afield.

We have good topo maps, etc, with which to see which repeaters should be
visible on our various
trips.  The problem is that you can be very effectively screened from a
repeater just by driving into
a canyon with high walls.  If you were a few hundred feet vertically higher
up, no problem - but as it is,
blackout.

>
> I think your questions and comments, while sounding reasonable, are made
before actually getting on 6m and seeing what the band's like.  1/4-wave
vertical whips for mobile-to-mobile coverage will likely provide better
coverage than anything else you could use.

Actually, no, we have been out on a number of trips since getting the rigs
and my questions are a
result of observing the results that we've had and thinking about how we
might be able to achieve
better communications in the type of terrain that we inhabit.

In fact, we usually have better communications using 2m than 6m, even though
we have twice the
power o/p on 6m than we do on 2m.  I suspect this is partly because there
are more 2m repeaters
so the average distance to a repeater is lower; and also because the 2m
antennas are siginficantly
more efficient (5/8 lambda, and the roof is a better ground plane for 2m
than for 6m).

>
> Horizontal polarization "to minimize QRM" really doesn't make a lot of
sense, since 6m is so underutilized that I'd defy one to find QRM!  During
the largest band openings of the decade, November 17-18 when 6m was open to
the entire world, there wasn't QRM.  Under normal conditions, it's hard to
find anyone to talk to, let alone finding QRM.

Point taken.

>
> A high takeoff angle is exactly what you _don't_ want, for extended
mobile-to-mobile coverage.  NVIS doesn't exist on 6m, at least not 99% of
the time.  NVIS is an F-layer mode, and the m.u.f. is well below 6 meters
about 330 days a year, so the likelihood of being able to use this mode on
six is pretty slim.

Since we are only looking for an effective radius of say 10 miles from the
transmitter, I was thinking that
even a highly inefficient backscatter mechanism would be usable, if one were
transmitting/receiving in the
right direction.  I guess if the MUF is at 15MHz then what's coming back at
50MHz is probably too many
orders of magnitude down to be usable.

>
> Groundwave is an LF-MF phenomenon which does not occur on six meters, at
all.  Hams use this term improperly, quite often, so it's easy to understand
where this comes from; however, ground wave above about 3 MHz just ain't.
Local propagation on VHF is either "direct wave" (line of sight) or via one
of the tropospheric scatter modes.
>
> If you're looking for extended m-to-m coverage over wide distances,
without repeaters, I'd recommend you simply switch from FM to SSB.  The mode
change, alone, without doing anything else, will improve weak signal
readibility about 20 dB, and extend the range of mobile communications quite
substantially.  I note my m-to-m range on 6m FM is ~20 miles when both
stations are running 1/4-wave whips and 25W rigs.  It extends to about 60-80
miles by simply flipping the switch to SSB.  That might do it!

Interesting.  We have done some experiments with line-of-sight comms at
distances up to about 40-50
miles (topo maps to the rescue again).  We didn't find a significant
difference between FM simplex and
SSB, even reducing the power signifcantly from the 50W we started at.  Over
what sort of terrain have
you had 80 mile mobile-to-mobile communications?

Once you get out of line-of-sight switching mode didn't help us at all.  I
wonder if this is related to the
local noise level (audio and/or electrical).

Anyone have recent experience with squalos?  I have one built but since it's
horizontally polarized I
seem to need a pair in order to conduct comparitive tests.

Thanks for the interesting feedback,

--ian K3IMW/6 somewhere up a canyon Smiley

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WB2WIK
Member

Posts: 20672




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« Reply #3 on: December 20, 2001, 07:41:23 PM »

Hi Ian,

I don't even know where Anza Borrego is, so you've certainly got me there.  The Mojave, of course, is huge; however the parts of the Mojave I know about and have been to aren't particularly canyon-filled, it's mostly high desert.  In fact, Pat, N6RMJ, who works 6m mobile daily and is well-equipped, operates from all over the Antelope Valley, just west of the Mojave and also high desert, and we work each other just fine, with strong signals over the 60-to-100 mile path, any old time.  (N6RMJ is one guy you might want to discuss the "how to's" of 6m mobile operation with.  His signal is absolutely outstanding, always, and he does drive an RV.)

I'd surely agree that driving down into a canyon with high walls is not a good way to work anything on the ham bands, let alone 2m or 6m.  I do believe you about having better results on 2m than six, frequently, and I've noted the same thing.  Higher frequencies can penetrate non-conductive obstructions (dirt, rock, etc) better than lower ones, depending upon composition, thickness and placement of those materials.  In fact, 70cm (440 MHz) often does better than 50 or 146 MHz under some conditions.

That switching to SSB did not yield an apparent improvement in communications is weird.  The signal-to-noise advantages of linear detectors, and narrower bandwidth, used in SSB communications are clearly established and very obvious, which is the entire reason the amateur community switched to mostly SSB operation in the early 1960's and never looked back.  I'm guessing perhaps you have ignition noise or other automotive noise issues that are limiting your abilities to hear weaker signals, and until those are resolved you won't have good luck working stuff in the sub-microvolt range.

I've used squalos, they're just square halos: 1/2-wave horizontal loop with a matching device to step impedance up to 50 Ohms.  A 6m squalo on a car roof (metal) definitely has a high angle of radiation; the same squalo mounted on a 7' mast behind the car, 3-4' above the deck and several feet away from the roof, will have a lower radiation angle.  M2, Par and others make a variety of 6m loop-type antennas you might wish to investigate, but again, unless both you and your friend are using horizontal polarization, going horizontal for just one of you won't help.  

You might try the SSB vs. FM test one time when you and your friend are at the "fringe" of FM communications.  Stop your vehicles, switch to SSB and turn your engines off.  I'll bet you hear each other just fine.  If so, start your engines again.  If that ends the contact, you have some work to do on the vehicles themselves, before you can take advantage of the benefits of SSB mobile.

73 de Steve, WB2WIK/6

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K3IMW
Member

Posts: 80




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« Reply #4 on: December 20, 2001, 08:35:47 PM »

The Anza-Borrego state desert park is a large area North and East of San Diego.  From LA you take 5 South and head inland on the 76 (fewer narrow windy parts than the more picturesque 78 through Julian).  It ranges from sea level to 8000 feet and takes you all the way E to the Salton Sea.  Truly worth visiting, particularly in the spring when the desert wildflowers are out (contact the ranger station for forecasts).

The lower areas are deeply eroded badlands and have some entertaining off-road driving and primitive camping opportunities.

Still on the search for more efficient antenna alternatives, had any luck with longer mobile antennas?  I don't know if a 6m J-pole can be mounted ruggedly enough for off-road use.

--ian K3IMW/6
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K9FV
Member

Posts: 480




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« Reply #5 on: December 21, 2001, 10:28:37 AM »

Ian and Steve, both of you have provided an interesting reading.  Ian, good luck - Steve has always been very helpful and knowledgeable.  Ian, please post when you try the test Steve mentioned about getting to the fringe of FM (line of sight on a hilltop?), then turning engines off and trying SSB.

Thanks agian to both of you for excellent posts,

Ken H>
K9FV
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WB2WIK
Member

Posts: 20672




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« Reply #6 on: December 21, 2001, 12:07:35 PM »

Hi Ian,

I don't think you'd really want a mobile j-pole; it will have a lower angle of radiation than the 1/4-wave whip, and only about 2dB gain over the whip...no huge improvement available.  The improvement available by switching to SSB in a quiet environment is far greater.

Running higher power on both ends might help, too.  If you're running 50W now, and go to 150W using a pair of Mirage A1015 amps (commonly available, and not expensive), you'll pick up ~4.5dB, which is a greater improvement than could be had with nearly any reasonable mobile antenna.

Again, I'd recommend you contact Pat, N6RMJ, to see what his secret is.  It seems no matter where he goes, I can hear him on 6m SSB...I think Pat's using stacked halos or something, and they're mounted up pretty high above ground.

73 de Steve, WB2WIK/6
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AA4PB
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Posts: 13037




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« Reply #7 on: December 27, 2001, 08:12:51 AM »

6 meters is pretty much line of sight (a little more) most of the time. Generally, you want low take off angles. The only way to get gain from an antenna is to concentrate the energy in the desired direction at the desired take off angle. Omni directional gain antennas work by reducing the take off angle.

Antenna gain will get you a little increase in range as will increased power or going to SSB where you can copy weaker signals. I'm afraid however that the only thing that will get any *major* range increases is to raise the antennas to increase the line of sight distance. Another band such as 75 meters or 40 meters will probably have a much greater effect.
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KD7PKO
Member

Posts: 7




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« Reply #8 on: January 04, 2002, 02:07:14 AM »

  Hi Ian and Steve,

  This is proving to be a very intersting topic. Having installed a fair amount of Motorola and GE gear in public saftey vehicles, I have to agree with Steve about the 1/4 wave whip. Here in Nevada the Highway Patrol operates on 46 megs.A lot of the volunteer Search and Rescue folks in Washington State operate at 47.50. Given the right topography, 40 mile contacts are common, I would imagine the same is possible at 50-54. The only other item I can offer to the discussion is the old mantra: grounds, grounds, grounds..... I have had to add grounds/bonds all over variuos 4x's to eliminate noise and improve the general signal quality. Good luck and I will keep up with you to see what solutions you come up with.

73, Chris KD7KPO

p.s. You might try on of the commercial low band antennas from Larsen or Maxrad if your rigs have all metal roofs.
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N4ZYV
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Posts: 90




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« Reply #9 on: February 07, 2002, 09:51:02 PM »

How about phasing two antennas for a directional pattern? couldn't this be done with a couple coax switches to change the pattern? I am no antenna genius here but it seems to me this could work. I guess the roof would need to be bigger than a Jeep CJ has.
 Is this viable? or should I just stay out of it?
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K3IMW
Member

Posts: 80




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« Reply #10 on: February 08, 2002, 01:22:23 AM »

Interesting idea .. my first thought was that I don't know where the other
vehicle is, so directional
solutions don't work - but of course, you always have a pretty good idea
where you were last in
contact, so this could help (as could a Yagi or whatever).

We have a desert trip coming up in just over a week so I plan to try a few
things out, and
hopefully report back.

--ian k3imw/6


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