better antenna? 5/8 over 5/8 vs 1/2 wave?

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jassing

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I'm looking at a couple of antenna options for my truck. Due to various equipment, I have some limitations - but I'm looking for a "best 1st try" option in getting an antenna.

So here's my options:

1) 1/2 wave base loaded antenna mounted on truck roof.
or
2) 5/8 over 5/8 ("no ground plane" type) mounted on back of cab; with the antenna/whip right near the roof line.

This is on 460mhz and 50 watt radio.
Thanks for any advice you can offer.
-josh
 

kayn1n32008

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I'm looking at a couple of antenna options for my truck. Due to various equipment, I have some limitations - but I'm looking for a "best 1st try" option in getting an antenna.

So here's my options:

1) 1/2 wave base loaded antenna mounted on truck roof.
or
2) 5/8 over 5/8 ("no ground plane" type) mounted on back of cab; with the antenna/whip right near the roof line.

This is on 460mhz and 50 watt radio.
Thanks for any advice you can offer.
-josh
Go with the 5/8 over 5/8 and mount it on the roof of the truck. as you would with the 1/2wave. no point in going to a better antenna and mounting it in a less than ideal place.

I am using a Larsen NMO2/70 (5/8 over 5/8 on Uhf) and on a few of the ham Uhf repeaters here in Alberta I am routinely getting better than 80 km radius of coverage.
 
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jassing

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Go with the 5/8 over 5/8 and mount it on the roof of the truck. as you would with the 1/2wave. no point in going to a better antenna and mounting it in a less than ideal place.
I cannot do that. which is why I am asking if the better antenna in a less optimal place will still be better than a lesser antenna in an optimal place.

1/2 wave = 3 db gain optimal location
5/8 over 5/8 = 5 db gain, in sub-optimal location, but no ground plane required and antenna would still be over roof line.
 

prcguy

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When I change from a UHF 1/4 wave whip to a Larsen, A/S or other stacked colinear antenna on the same mount the improvement is noticeable but not drastic. Slightly fuzzy areas are a little more full quieting and really fuzzy areas are less fuzzy.

If you start out with a 1/2 wave which may be slightly better than my 1/4 wave but then place your gain antenna in a less desirable location the results may not be as good. You might notice some improvement but it will never be a life changing event.

BTW, a 1/2 wave antenna is more like 0dBd gain ad a dual stacked 5/8 over 5/8 colinear is a little less than 3dBd gain.
prcguy
 

N4KVE

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I used to use a 5/8 gain antenna with 5.5 db on the roof with a magnetic mount, so no ground plane. When I switched cars, I used that same antenna with a trunk lip mount, & now have a ground plane. I have noticed no difference, even using repeaters 60 miles away. GARY N4KVE
 

jassing

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BTW, a 1/2 wave antenna is more like 0dBd gain ad a dual stacked 5/8 over 5/8 colinear is a little less than 3dBd gain.
prcguy
I was going off marking claims by laird ... so hopefully whatever their gain calculations are; they apply them on all their antennas.

So it sounds like it might be a wash -- both will perform close to the same?

The 5/8 over 5/8 is specifically a "no ground plane" antenna; vs just being installed w/o one; and the 1/2 wave is a base loaded whip.
Item # B4505CN, Mobile Load Coil Antennas on Laird Technologies cab mounted, antenna at roof line
or
http://goo.gl/jjQ7o on the roof....

(Hmm. not sure why I thought the 2nd was a 1/2 wave -- both 5/8)

It's the height 1' vs 3' that makes the location change.

if they both will perform similarly, I'll do the roof mount... if one has a clear edge, i'll go that one... Just trying to cut down on cutting holes in the metal for experiments.. don't want to make swiss cheese out of the body...

Thanks
-josh
 

jassing

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Good read...

Marc's Technical Pages: Antenna Gain Explained
In part, it says:

The higher the gain of an antenna the smaller the effective angle of use. This is the part people forget i.e. that they have robbed power from other directions and superimposed it on the radiation in the intended direction.
Reading it; this makes sense; and then when coupled with the formula for determining gain (logarithm based); it seems that the 5/8 whip roof mounted will be the better choice...
 

zz0468

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I prefer just a 1/4 wave spike on UHF. It's small, unobtrusive, and when working with a decently performing repeater, it works just fine. On an FM channel, the 3 or 5 or 6 db gain (or whatever) is almost completely insignificant, especially in a mobile environment.

If the repeater is at a substantially higher elevation, and close in, a gain antenna can actually deliver less signal to the repeater than something with a broader vertical pattern.
 

jassing

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Thanks zz -- while most of this will be vehicle to vehicle Since a 1/4 wave would fall in the size I can mount on the roof; maybe I'll go that route, then go up if I need to.
-j
 

zz0468

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Thanks zz -- while most of this will be vehicle to vehicle Since a 1/4 wave would fall in the size I can mount on the roof; maybe I'll go that route, then go up if I need to.
-j
You'll probably find, like I have, that a 1/4 wave antenna on the roof will outperform a gain antenna mounted elsewhere.
 

jassing

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You'll probably find, like I have, that a 1/4 wave antenna on the roof will outperform a gain antenna mounted elsewhere.
And the nice thing about 1/4 wave is -- well; it's cheap & small! and I can always bump it up if I think I need to.

I know it's different, but "way back when" "bigger = better" for cb.. I guess I still have that in my head.

Thanks - 1/4 wave it will be ...

-j
 

62Truck

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Just a thought being that you can't mount it on the roof. What about a fender mount?
 
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kb0nly

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I was just going to suggest that... Fender mount it opposite of your FM broadcast antenna then go with the 5/8w antenna.
 

zz0468

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You can get extra gain, but probably not the way you like. If you space the antennas correctly, and phase them correctly, you'll get a figure-8 pattern, with deep nulls in one set of directions, and modest gain in another set of directions - not real useful in a mobile environment.

If you randomly space and phase the antennas, you'll get random results, maybe a bit of gain in one or two directions, and mostly loss in all the other directions.
 

RADIO_ROOKIE

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Thank you for making clear for me this things! B.T.W. I know in CB you must position your antenna in the middle of the roof to maximize antenna efficiency . Does it apply to UHF as well? Maybe because the wave length is not so big as in CB, the antenna does not need to be in the middle of the roof and it works fine with much smaller ground plane.
What about magnetic mounts..? I heard they are suitable in CB but in UHF the magnetic base behave like a capacitor . And the whip itself needs to be as close as possible to the ground plane to have a good SWR.
 

LtDoc

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A magnet mount antenna works the same no matter what band it's for. It works because of capacitive coupling with the metal it sits on. If that metal isn't 'large' enough to provide a decent "groundplane" or counterpoise (the other half of the antenna) then you can have problems. At VHF/UHF the standard sized vehicle's roof should provide more than enough for that 'other half' of the antenna.
For HF bands, the roof isn't all that makes up that "groundplane", what the vehicle is sitting over also contributes to it. How? The same way that mag-mount works, through capacitive coupling. The vehicle is one plate of that capacitor, the earth is the other plate. Ever noticed how things seem to improve when driving over different things, bridges for instance? More metal in that bridge than there is in the average street/road or plane dirt. That means a better coupling.
- 'Doc
 

zz0468

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...B.T.W. I know in CB you must position your antenna in the middle of the roof to maximize antenna efficiency . Does it apply to UHF as well? Maybe because the wave length is not so big as in CB, the antenna does not need to be in the middle of the roof and it works fine with much smaller ground plane.
If you mount an antenna near the edge of the roof, the pattern will be skewed a bit off omni-directional. How much would depend on a number of factors. The center of the roof gives the best omni-directional pattern.

There are other considerations in installing a mobile antenna, however, and depending on the circumstances, could be more important that a semi-perfect omni pattern.
 

LtDoc

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Will it matter if there are other antennas on that roof? Yes, to some extent, but there are a lot of "if's" in that. It's very seldom that an antenna is truly 'omnidirectional'. There can/will be some variations in symmetry because of various things (other objects on the same roof or near by), and the shape of the roof for one thing. In most cases, you will never know that the antenna's radiation pattern is slightly 'skewed', and just isn't worth worrying about (too much). And then you get to determine what's 'practical' for your particular circumstances. If you just have to have a dozen or so antennas on your roof for whatever reason, then expect some variations in radiation patterns. After you figure out what those particular deviations are, compensate for them (point the vehicle in a different direction?). Large deviations are easier to figure and compensate for. Smaller ones are almost impossible to determine by the average person and his equipment, so why worry about them? (It isn't easy to find a place to drive in circles to check radiation patterns, and people seeing you do it will think you are crazy, you know? Oh well, they probably think you're crazy anyway with all those antennas. :))
- 'Doc
 
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