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Author Topic: Which mobile antenna is better for EmComm? Dual, or 2m 5/8?  (Read 12876 times)
W8JX
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Posts: 6445




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« Reply #15 on: March 21, 2013, 11:33:24 AM »

There was a study many years back with regards to antennas on police cars,
which tested various commercial offerings (which were stiffer than most ham
antennas) at speeds of 100mph, and came to the conclusion that there was little
or no advantage to the 5/8 wave whip, and it could actually be detrimental at
high speeds.

Some of the dual-band antennas that are about 5/8 wave on 2m are certainly
much stiffer than a plain whip, but my experience has been that the more joints
in the antenna the higher the chance that it develops an intermittent connection
somewhere... 

You seem to be shovelling it a bit thick here.  How about a link to that study--I think it's either a study by the makers of a thin whip antenna or is just simply a lot of bally-hoo by a group that wanted to cast doubt on a better antenna.  BTW, there are a lot of differently designed 5/8 wave 2 mtr/440 antennas out there. 

The only detriment to a stiffer antenna is that any sort of contact from an overhanging object would tend top snap it off.  The only advantage to a thin 1/4 wave 2 mtr. whip is its flexibility.

I agree. While a 1/4 wave can look cleaner, it is far more prone to "picket fence" signal  when getting to fringes when mobile and a 5/8 will do much better here and extend usable range on fringes. There is a phenomenon where VHF and UHF signals do not travel evenly in space a can create holes in coverage that you can see with moving a HT a foot or two at times. I longer antenna has a larger capture area that spans these holes better and why it picket fences far less than a 1/4 wave in practice.
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WB6BYU
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Posts: 13465




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« Reply #16 on: March 21, 2013, 12:12:57 PM »

Quote from: K1CJS
Quote from: WB6BYU
...the roof of the typical vehicle is nowhere near large enough to
provide the ground reflection that is the cause of that difference.  The end
result is that the 5/8 wave has a small advantage due to the point of maximum
radiation being higher in the air for the same mounting height.

You're kidding on this, right?  The 'average' ground plane requires only a 38 inch diameter circle.  Car roofs are always much larger than that--unless you drive a smart car!



No, I'm quite serious, and this leads to a lot of mistaken assumptions about
the behavior of a 5/8 wave antenna.

A 5/8 wave whip is basically a 1/2 wave radiator with the bottom raised
1/8 wavelength above the ground.  (There is a slight about of out-of-phase
radiation from the bottom 1/8 wave section, but that just contributes to
higher lobe formation.)  A 1/2 wave vertical dipole or a ground plane with
sloping radials (which is basically the same thing) in the same position as
the upper 1/2 wave section will give the same pattern.

While a dipole radiates from all along its length, radiation is maximum from
the center section where the current is highest.  For this analysis let's
assume a point source at the point of maximum current:  it isn't exact,
but the math is much easier to follow because we don't have to integrate
the current over the element length.

In the 5/8 wave whip the point of maximum current is 3/8 wave up from
the feedpoint, or 1/4 wave down from the top.  That's consistent with
the current distribution shown in any of the textbooks.  Let's look at
the radiation at an angle of 10 degrees above the horizon:  again, we
can repeat the analysis for any angle we want.

The gain of the 5/8 wave antenna is because the direct radiation from
the 1/2 wave element combines in phase with the ground reflection.
In this case, assuming a perfectly reflecting surface, we have one
ray going from the point source up at 10 degrees, and another going
down at 10 degrees, reflecting off the earth, and going back up at
10 degrees parallel to the first ray.  This is standard stuff, the same
way we analyze the lobes and nulls of a dipole over ground (except
that reflections are in phase for vertical polarization at low angles
and out of phase for horizontal polarization.)

We can then calculate the point where the ray reflects off the ground
using a bit of geometry as the elevation of the feedpoint divided by the
tangent of the angle.  For a feedpoint height of 0.375 wavelengths and
an angle of 10 degrees, tan(10 degrees) = 0.176, so 0.375/0.176 = 2.13
wavelengths.  At 146 MHz, that's over 14' from the base of the antenna.

The lower the angle of radiation you want, the further the reflection point
is from the base of the antenna.  If the ground plane doesn't extend out
to the reflecting point (and a bit past it, when we are considering the
radiation from the whole 1/2 wave element rather than a point source)
then the reflection doesn't fully reinforce the direct ray, and the 5/8 wave
element won't have the expected gain.



Quote
Quote

There was a study many years back with regards to antennas on police cars,
which tested various commercial offerings...


You seem to be shovelling it a bit thick here.  How about a link to that study...



I think this is it, but can't tell for sure, and my magazines are currently packed...

Test Data on 1/4- and 5/8-Wavelength Vertical Antennas for 2m-Mobile by
Reed Fisher W2CQH and Bill King W2LTJ, Ham Radio, May 1976 Page 46
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K1CJS
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Posts: 6055




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« Reply #17 on: March 21, 2013, 03:13:40 PM »

Thanks for the reply.  I'll check that article out. 73.
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K1CJS
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« Reply #18 on: March 23, 2013, 11:52:05 AM »

Since I can't find any sort of a connection to that article or its conclusions, and since I don't have/can't get that magazine, I have to discount that article as a fluke.  If it had any sort of truth, there would be additional notations and references to it, and from what I can see there are absolutely none.

I'll stick to my original thoughts and my experiences--5/8 wave antennas do a better job in most circumstances than 1/4 wave antennas do.
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M6GOM
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Posts: 990




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« Reply #19 on: March 24, 2013, 06:09:46 PM »


I'll stick to my original thoughts and my experiences--5/8 wave antennas do a better job in most circumstances than 1/4 wave antennas do.

...by an amount which you most likely won't be able to distinguish in normal use.

I really should do that Youtube video however its too damn cold at the moment to be outside swapping antennas about.
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W8JX
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Posts: 6445




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« Reply #20 on: March 24, 2013, 06:44:33 PM »

Well I can tell you a 1/4 vertical on UHF/440 is a very marginal antenna for a mobile or base.
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K1CJS
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Posts: 6055




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« Reply #21 on: March 25, 2013, 04:13:57 AM »

...by an amount which you most likely won't be able to distinguish in normal use.

If by normal use you mean yakking around town, then you could be right.  If however, you want to go further than that, you'll find that the 5/8 wave 'cuts through' ground floor noise and interference better than a 1/4 wave ever will. 

There is a repeater that is used as an 'area' repeater in Johnston, Rhode Island.  It's situated so that you can get into and hear it from anywhere in the state and beyond its borders.  When I used to travel the 495 beltway in Massachusetts, I could barely get into it with a 1/4 wave antenna--a lot of fade and picket fencing, but a 5/8 wave got into it full quieting.  Distance was between 30 to 60 miles. 
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KB7DZR
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Posts: 2




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« Reply #22 on: March 26, 2013, 08:14:32 AM »

For VHF the 1/4 works much better when your dealing with a station or repeater that is on a mountain top and your in a canyon. Tested with my APRS beacon many times and the 1/4 worked much better then the 5/8 when in the deep canyons. Also tested with voice.

Have a friend that is a radio tech for the forest service and they have found the 1/4 works much better for the mobile units for many reasons and one is they work better out of the canyons.

That said this area is deep canyons and high mountains with 2000 to 4000 foot differences.
Once you get into flatter areas where you need better range towards the horizon then the higher gain antennas are going to be the better option.

I mostly run the 1/4 wave for VHF and it works on UHF also but not the best of pattern. They handle the abuse of tree limbs and brush, cheap to replace and work far better in the canyons. May not be correct on paper in some points but I have found from real world use both in Ham, firefighting and commercial for it to work for me.

Scott KB7DZR
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M6GOM
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Posts: 990




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« Reply #23 on: March 26, 2013, 03:43:42 PM »

...by an amount which you most likely won't be able to distinguish in normal use.

If by normal use you mean yakking around town, then you could be right.  If however, you want to go further than that, you'll find that the 5/8 wave 'cuts through' ground floor noise and interference better than a 1/4 wave ever will. 


If you read my earlier posts you'll see I've done testing to a repeater 55 miles away.
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K1CJS
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Posts: 6055




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« Reply #24 on: March 28, 2013, 04:45:59 AM »

Over flat ground?  Any hills and valleys?  Fringe areas?  Beyond the reach of the 1/4 wave?  Just saying out 55 miles and both work the same doesn't mean one or the other is better--and that is all you said besides saying the repeater antenna was about 200 ft. higher.

Go out to a fringe area where the 1/4 wave won't get into the repeater reliably and you'll find the 5/8 WILL work better--it's fact.  73.
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M6GOM
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« Reply #25 on: March 28, 2013, 05:19:21 AM »

Over a range of hills.

http://goo.gl/maps/uKb3P

I'm in Driffield, the repeater is west just below and to the left of where it says Batley south of Leeds. As you can see there's a range of hills in the way to the west of Driffield.

The 5/8 may work better over flatter ground but the difference is a couple of dB, hardly noticeable. Although you may open the repeater and be able to hear that someone is there, the audio will be so intelligible to make the fact you can open the repeater quite pointless.
« Last Edit: March 28, 2013, 05:22:48 AM by M6GOM » Logged
WB6BYU
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Posts: 13465




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« Reply #26 on: March 28, 2013, 08:40:20 AM »

Things seem to be getting rather polarized here - and not just vertically.

In my experience switching antennas while driving through a marginal coverage
area, the difference between the 5/8 wave and 1/4 wave is about 1dB, but
to come up with that number I have to average over several days of switching
back and forth repeatedly because the change in signal strength as I go down
the road is much more than that.  If I stop to take a comparison either one might
be better, and if I roll forwards a couple feet it may change.  That's because the
area has strong multipath and the exact position of the antenna makes more
difference than the choice of whip length.

In another location I ran some tests and the 5/8 wave whip could hold the
repeater about 200' further down the freeway than the 1/4 wave could.  At
freeway speeds, that doesn't make any practical difference.  That was because
the freeway turned behind a hill, and both antennas dropped out quickly.

What we've seen on this thread is that, in some cases, the 5/8 wave made a
significant difference and sometimes it didn't.  I'd say that is a pretty accurate
reflection of the real world situation.  Some of the causes for the difference:

1) the actually antennas, mounts, cables, etc. used  in the comparison.  Not
all antennas are built the same, or in the same condition.  I have a couple
antennas that are worse than others of the same general type due to coax
losses, corrosion, poor adjustment, etc.  The 5/8 wave whips are particularly
prone to this:  a longer whip gives a better impedance match to 50 ohms but
has a higher angle of radiation.  Since more hams can measure SWR than are
equipped to do accurate gain measurements, some antennas get optimized
for the former.

2) The path conditions.  Some of us are more limited by geologic features than
by the curve of the earth.   Marginal signal areas may be due to multipath rather
than being over the horizon too far.  In the latter case the increased height above
ground of the radiating portion of the 5/8 wave may make more improvement
than the nominal gain, while it doesn't help in the former.  Repeaters placed on
5000' hills are more likely to be limited by other terrain features than those
where the a 200' water tower is the highest thing for miles around.  (I have no
problem hitting repeaters 100 miles away from my house using a HT on low
power, but we're on hills at opposite ends of a long valley with almost a direct
line-of-sight path.  Conditions are quite different while driving through hills,
on the flat part of the valley floor, or for repeaters that are only a few hundred
feet above average terrain.)

3) the height of the vehicle.  The 5/8 wave whip made more of a difference
on long flat paths when I drove a Honda Civic with a low roof, because the
taller whip helped get my signal up and over the other vehicles around me.
With my current van the difference is less, unless I get behind a large truck.
Parked on top of a hill it doesn't matter much.  Down in a dip it can make a
big difference.


So in the end it comes down to the fact that you just have to try it and see
which is better in your specific situation for the communication paths that
you want to use.  In my case, 10 watts to a quarter wave whip is adequate
to cover the county - except those remote canyons where higher power
and a longer antenna may not suffice, either.  In large, flat counties the
situation may be different.  The posters on this thread can provide anecdotes
and personal experience, but can't necessarily tell you which one will
work best in your situation.

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KW6LA
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Posts: 93




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« Reply #27 on: April 01, 2013, 05:13:19 PM »


Go out to a fringe area where the 1/4 wave won't get into the repeater reliably and you'll find the 5/8 WILL work better--it's fact.

This is what happens with my Larsen 5/8~ vs 1/4. all the time. Interesting that others find it not to be true.  Cheesy
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K1CJS
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Posts: 6055




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« Reply #28 on: April 02, 2013, 04:21:51 AM »

Go out to a fringe area where the 1/4 wave won't get into the repeater reliably and you'll find the 5/8 WILL work better--it's fact.

This is what happens with my Larsen 5/8~ vs 1/4. all the time. Interesting that others find it not to be true.  Cheesy

Thank you.  That is what I've been saying right along.
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KA4NMA
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Posts: 354




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« Reply #29 on: April 05, 2013, 08:47:20 PM »

The poster said he had a Larsen 2/70.  I thought it had gain on both 2m and 440?  For instance NMO2/70B 144 - 148 / 440 - 450 SPECIFICATIONS GAIN VHF: 3.8 dBi UHF: 5.2 dBi TYPE VHF: center loaded 1/2 wave UHF: collinear VSWR (see bandwidth) 2:1 BANDWIDTH @ 1.5 VHF: 2% / UHF: 3% BANDWIDTH @ 2.0 VHF: 4% / UHF: 6% COIL VHF: 146 / UHF: 445 MHz POWER RATING 100 Watts WHIP W2/70B, .100 open coil MAX HEIGHT 34 ¾

Which model of the 2/70 do you have?  I would go with a Larsen 2/70

Randy Ka4nma
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