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Author Topic: Multiple antennas on one roof? Spacing? Performance? 2 Meters CB etc.  (Read 6849 times)
W4JST
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« on: April 15, 2012, 01:26:26 PM »

How do multiple antennas on the same roof effect performance?

The vehicle is a Sprinter van with a large roof.

I currently have a hole drilled in the center of the roof with a permanent 3/8" 24 mount for my CB antenna
and another of the same kind of mount near the back of the roof (in the center side to side, but near the back of the van so I can easily change the antenna).

I have been using the mount near the rear of the van for my 2 meter 5/8 whip.

Primarily I want the best performance for 2 meter simplex.
Should I put the two meter antenna in the center of the roof?

I wouldn't mind another mount for an HF antenna but am not sure if having other antennas too close to my 2 meter antenna would effect it in a bad way or not.
Again, my primary concern is the best possible 2 meter simplex range.
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KD4LLA
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« Reply #1 on: April 15, 2012, 04:39:09 PM »

On a car, everything is a compromise...  Since you intend to use "simplex" for both bands (CB/ 2m), why even let it be a consideration?

A quarter-wave whip in the center of the roof will out perform any 5/8 wave mounted anywhere else on a vehicle.  Ever seen a police vehicle with a 5/8 wave antenna on it?

In general a quarter wave CB antenna  is already such a compromise your communication range is already restricted to 3 to 5 miles under most conditions.  I have a 19.5" whip and a 24" CB antenna on my Dodge Caravan, each mounted in opposite rear roof corners.  They work exactly as planned.  I can hit the local  repeaters, work simplex on 2m and talk to the truck drivers as I travel the Interstate.

Mike
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KCJ9091
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« Reply #2 on: April 15, 2012, 05:38:12 PM »

A quarter-wave whip in the center of the roof will out perform any 5/8 wave mounted anywhere else on a vehicle. 

Maybe yes, may be no.  There are a lot of variables involved to make such a blanket statement.  VHF high band in the city where you are close to antenna mounted on a tall tower 1/4 wave would work better with the higher take off angle.  However for out in the rural areas where you are farther away from the tower, are talking simplex to someone on their home station who does not have a lot of antenna height, or someone in another vehicle the lower take off angle of the higher gain 5/8 antenna would out perform the 1/4 mounted in the same spot.


Ever seen a police vehicle with a 5/8 wave antenna on it?

Quite a few actually.  Most every patrol car I have driven has had a 5/8ths vhf antenna on it.  Moonlighting for the radio shop I installed several over the years.  Again small rural departments with a large area to cover.

In general a quarter wave CB antenna  is already such a compromise your communication range is already restricted to 3 to 5 miles under most conditions. 
Mike

Say what?  The low power limits and conservative factory tune are more a limit to your range on a legal CB than the antenna.
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W4JST
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« Reply #3 on: April 15, 2012, 05:48:40 PM »

I appreciate the responses. I am not looking for any advice regarding the CB antenna etc.

I would like to know if anyone call tell me how performance will vary, if any, from having my antenna in the center of my large roof versus near the back.

I want the greatest two meter simplex range.


I would also like to know how mounting antennas near each other will effect this performance.

I may be able to move my two meter antenna to the center of my roof, if it will help two meter FM simplex performance, but if I add an HF antenna, my distance between antennas will be decreasing.
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KD0PBO
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« Reply #4 on: April 15, 2012, 06:41:05 PM »

I appreciate the responses. I am not looking for any advice regarding the CB antenna etc.

I would like to know if anyone call tell me how performance will vary, if any, from having my antenna in the center of my large roof versus near the back.


QST actually had an article about this in the Febuary edition if memory serves me right. Basically placing the antenna anywhere off center will effect the radiation patern. If you mounted the antenna in the center but moved it back on the vehincle's roof you will have better recieve and transmit off the front of the vehicle, essentially becoming mildly directional. Vise versa if you moved it up to the front center of the roof. QST did the testing with a 1/4 2m antenna but I would imagine that the same can be applied to a 5/8 wave.

So for BEST 2m simplex performance, dead center would theoretically be best. I can't speak for simplex but with a 5/8 mounted off-center for repeater use, its hard to notice a difference between a center mounted antenna.

Hope that helps!
KD0PBO
Miles D.
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W4JST
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« Reply #5 on: April 15, 2012, 07:45:54 PM »

Thank you for the information.

What about proximity to other antennas?

I know at the hamfest there's always several vehicles with the roof, and rest of the vehicle, covered in antennas.

If I place another mount and a HF antenna on my roof or another two meter or 440 MHz or whatever, is that going to negatively effect my 2 meter simplex performance?
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KCJ9091
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« Reply #6 on: April 16, 2012, 06:40:58 AM »

On the roof of my truck I have a 5/8 vhf 18" away from a 2m/70cm dual band 18" away from a 33cm antenna.  There may be a little skewing of the pattern but there has been not ill effects to the radios.  I do try to keep to the lowest power level to be safe but have run the radios at full power with no problems.
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NT6U
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« Reply #7 on: April 16, 2012, 12:52:23 PM »

   I have ran a 5/8" 2 meter antenna along with a C.B. mobile- 10 meter- antenna on the roof of several vehicles since 1992, never a problem.  I kept the 2 meter in front of the C.B.,center line of the roof on the cab of an F-250 P/U just mainly for appearance of the shorter antenna first.
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W4JST
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« Reply #8 on: April 16, 2012, 05:58:39 PM »

Is there any certain spacing distance I should keep in mind? Probably some factor of a wavelength as it varies from frequency to frequency or what?

I may be drilling another hole for my HF antenna on the roof but like I said maximum two meter FM simplex range is my first priority right now.

Thanks everyone.
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KD0PBO
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« Reply #9 on: April 16, 2012, 08:03:55 PM »

Thank you for the information.

What about proximity to other antennas?


My rule of thumb...The farther apart the better!  Grin I think keeping antennas at least a 1/4w length apart should keep effects down to a minimum. With HF however, a 1/4w length can become a pretty hefty distance.

Hope that helps (again)
KD0PBO
Miles D.
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KD8RFT
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« Reply #10 on: April 26, 2012, 10:15:18 AM »

I appreciate the responses. I am not looking for any advice regarding the CB antenna etc.

I would like to know if anyone call tell me how performance will vary, if any, from having my antenna in the center of my large roof versus near the back.


QST actually had an article about this in the Febuary edition if memory serves me right. Basically placing the antenna anywhere off center will effect the radiation patern. If you mounted the antenna in the center but moved it back on the vehincle's roof you will have better recieve and transmit off the front of the vehicle, essentially becoming mildly directional. Vise versa if you moved it up to the front center of the roof. QST did the testing with a 1/4 2m antenna but I would imagine that the same can be applied to a 5/8 wave.

So for BEST 2m simplex performance, dead center would theoretically be best. I can't speak for simplex but with a 5/8 mounted off-center for repeater use, its hard to notice a difference between a center mounted antenna.

Hope that helps!
KD0PBO
Miles D.


I spent a lot of time looking at that article and those radiation patterns that were modeled. The repeatable conclusion from looking at the plots, was that you would get less reduction in performance with more sheetmetal in the direction of interest. The center of the roof is the best place to mount the antenna. If you move the antenna towards the rear, there is less shetmetal behind you and more in front. The pattern will be better in front than in rear. This is repeatable in any direction. The front pattern does not increase beyond ideal, the rear decreases.

If you look at the pattern from placing the antenna in the center of a normal rectangular roof, you will see similiar pattern front/back/left/right but notice the lobes in the corners, because there is more sheetmetal in the direction of the corners (roof is not a circle, length to corner is hypotenuse of right triangle).

Also, if you look at the plots, you do not add gain in any biased direction, you only lose gain in less surface sheetmetal direction from ideal "antenna in the center" plot.

At least, this is how I perceived the data/plots from the article.
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K1CJS
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« Reply #11 on: May 02, 2012, 05:51:46 AM »

A center mounted antenna would give the most optimal performance, but as long as you keep at least 19 to 20 inches away from the edges of the vehicle roof, your two meter signal pattern shouldn't be too bad for good performance.
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KI4SDY
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« Reply #12 on: May 02, 2012, 05:50:46 PM »

If your main aim is to use simplex and no repeater, I would mount the vhf antenna on the rear and use the directional properties of the result to aim your signal to the target. That will provide the best reception and transmission under the circumstances. It should still work fine on the local repeaters Wink
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N6AJR
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« Reply #13 on: May 08, 2012, 01:04:04 PM »

I have , on the roof of my 94 ford 1/2 ton extended cab pickup, a 2m/440 antenna , a 220 antenna a 900 mhz antenna and a 1.2 g antenna, and a 2m 440/ antenna, and a straight 2m antenna ( large one for simplex) and on the other ( left side) bed rail I have a DK-3 and  AYAS 120.  I have 5 multiband  radios in the truck  including  HF and I use any and all of them, and don't see any problems, except some times the 220 mhz rig gets into the FM broadcast radio.  I use a greenlee punch to do the roof and a step drill for the bed rails.  They work for me and I have no idea what the  radiated patterns are, it is what it is.
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K5LXP
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« Reply #14 on: May 08, 2012, 03:10:08 PM »

They work for me and I have no idea what the  radiated patterns are, it is what it is.

I couldn't agree more.

I have had six antennas on the roof of my pickup for at least 15 years now.  My only concern was that they weren't close enough to touch each other when whipped around in the wind, which would likely cause some damage to something if that happened while keyed.  I could give a whit about a few dB of pattern distortion in one direction or another due to some coupling or asymmetric groundplane.

In a mobile environment if the antenna is there and working, odds are it will work to line of sight in every direction.  If a few dB here or there is going to matter all that much, you're asking a lot no matter how "perfect" you make the installation.


Mark K5LXP
Albuquerque, NM
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