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Author Topic: High Gain VHF Mobile antenna needed.  (Read 3092 times)
5B4AFQ
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« on: December 21, 2005, 07:55:49 AM »

I work in a rough terrain mountain sourounted enviroment and i will like to install a high gain , hi-power mobile antenna on my jeep ( together with a 200w linear ) any sugestions on both items from the guys who have already been there done that ?

5B4AFQ
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K5LXP
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« Reply #1 on: December 21, 2005, 08:26:43 AM »

Gain will not help you with mountainous terrain, in fact a gain antenna could direct your energy in a direction it could do the least good.  My recommendation would be a quarter wave vertical on an adequate ground plane.  Works well in the mountains here.

Mark K5LXP
Albuquerque, NM
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KB1LKR
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« Reply #2 on: December 21, 2005, 10:18:53 AM »

If indeed it is mountainous, a 1/4 wave (~19"/48cm) w/ an NMO base (look at commercial antennas e.g. Larson, Antennex, PCTEL-A/S, etc.), mounted on the center of the roof of the vehicle would be best, but you say Jeep, does it have a metal roof, fabric or none. If not metal roof (for ground plane you might look at a 1/2 wave (not 5/8 wave) instead.

Also, if you're off road/in the open & always  have overhead clearance you could look elevating a 1/4 wave antenna w/ drooping radials on a mast -- get it up 6-10 feet/2-3 m more, that'll get you better range.

 
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VA7IC
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« Reply #3 on: December 21, 2005, 10:22:01 AM »

Having worked on Forestry radio systems for years I can tell you that you do not want a high gain antenna.  The antenna gets it's gain by flattening out the signal.  You need the signal getting up where the repeaters are.  We used low gain antennas on the repeaters and 1/4 waves on the vehicles for the best coverage into valleys.  Radio is line of sight, so a 200 Watt amplifier seems like overkill.  We ran 10 watts on our repeaters in the mountainous areas, and 20 Watts on the mobiles.  The only place power will really help is over a long unobstructed path.  You don't have that in the mountains.  The multipath you will probably create with so much power in mountainous terrain will probably make things worse, not better.
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5B4AFQ
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« Reply #4 on: December 21, 2005, 11:54:25 AM »

Exelent information , can you also explain in a bit more detail why hy gain is not advisable ? "flatering the signal " Huh does that have to do with angle of radiation in hi gain antennas ?
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AA4PB
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« Reply #5 on: December 21, 2005, 12:40:11 PM »

The only way to get gain is to take power from one direction and cencentrate it in another. With an omni-directional vertical the only place you can take it from is the vertical angle. So, a 1/4 vertical will have a very broad take off angle covering the horizon to high angles. A high gain antenna will concentrate the signal at low angles putting more energy near the horizon and less energy at the higher angles.
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AA4PB
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« Reply #6 on: December 21, 2005, 12:56:43 PM »

You might also want to do a little geometry on your location. It is often claimed that you need high angle radiation to get to a repeater located in a high location. If you work the geometry you find that for a vehicle antenna at 10 feet off the ground once you are a couple miles from the repeater the repeater will have to be very high in order for the take off angle to be more than a few degrees.

For example, if the repeater is 1 mile high and you are located 10 miles from the repeater then the take off angle is only 5.7 degrees (assuming your car is setting on level ground). If the repeater is 1 mile high then you would have to be located only 1 mile away to have a take off angle of 45 degrees. Of course when you are located close where the take off angle is high then you will normally have plenty of signal even though the high gain antenna concentrates most of the power at low angles.
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WB6BYU
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« Reply #7 on: December 21, 2005, 01:30:10 PM »

On the repeater that I use regularly, I find that around
50 to 75 watts is the maximum practical power.  Above
that, you may be able to hit the repeater when you can't
hear it, which is very annoying to other users.  Probably
a good general rule is not to run more power than the
repeater transmitter (NOT the repeater ERP) unless your
receiver is better than the one in the repeater.

I regularly drive through a valley in the hills where the
repeater coverage is very spotty.  I find a 5/8 wave whip
slightly better than a 1/4 wave whip in that situation,
to the point where, occasionally, I can be copied on the
5/8 wave whip but not on the 1/4 wave, but the day-to-day
differences in propagation are greater than the difference
between the two antennas.  And the difference is between,
for example, 40% copy vs. 70% copy - it depends on how
hard the other person is willing to work  to pull my
signal out.

If you want more gain, I'd recommend using a directional
antenna.  Many transmitter hunters mount a quad or yagi
on their vehicle, and this will give you more gain than
an omnidirectional antenna can.  Of course, you have
to rotate it, so it might be best to use it when stopped.
Also, the actual signal path may involve reflections
from one or more mountians, so simply pointing it in
the direction of the repeater may not give the best
signals.
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K0BG
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« Reply #8 on: December 21, 2005, 04:36:44 PM »

Read this: http://www.eham.net/articles/12596

Alan, KØBG
www.k0bg.com
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W8JI
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« Reply #9 on: December 21, 2005, 04:51:36 PM »

If you do some math, you'll see the argument about compressed pattern doesn't hold very much water. It's a popular argument, but very flawed.

The gain of a 5/8th wave antenna over perfect ground is:
0 degrees= 8.1 dBi
15 deg  =5.6dBi
32 deg =  -13.9 dBi

A 1/4 wl over perfect ground is:
0 deg =5.1 dBi
15 deg = 4.7dBi
32 deg = 2.7 dBi

They are equal in gain at about 20 degrees angle.

If the repeater antenna is at 5000 feet, you would have to be 13,500 feet from the repeater to be even between the 1/4 wave and the 5/8th wave, and as you moved further away the 5/8th wave would get stronger than the 1/4 wl antenna!!

You'd have to be about 7500 feet from the 5000 foot high repeater to have a 16.6 dB null, but at 7000 feet distance a few dozen milliwatts ERP would be a solid signal each way!!

The idea the 5/8th wave or any gain antenna practical on a mobile focus too much is very popular, but totally flawed unless transmitters are running only a few milliwatts.

The answer that a gain antenna might not do much is correct, but only for this reason. A 5/8th wave tall antenna only has gain over a 1/4 wave when it is installed over a nearly perfect flat groundplane that is several wavelengths in radius. The car roof is not that large, so the taller 5/8th wave antenna can actually have loss over a 1/4 wl antenna at useful angles.

Unless the repeater antenna is 15,000 feet higher than the mobile or unless power is in milliwatts the issue isn't the compressed pattern. The real issue is the groundplane is too small so the 5/8th wave never really has gain at the best angles, and has loss at other higher angles when compared to a 1/4 wl antenna.

So the end answer is the same, the reasons are just different. It isn't actually a pattern compression or null issue, it is a loss of gain issue because the roof is too small. The 5/8th wave places the null so high that to be in the null you would be so close the null depth wouldn't matter with reasonable power of a few watts. The darned car roof is just too small to be a good groundplane.

73 Tom
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KT8K
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« Reply #10 on: December 22, 2005, 06:30:46 AM »

I can't verify this with any scientific evidence, but my impression has been that, in a mountainous or big city (tall buildings) environment, a lot of VHF propagation beyond line of sight is by reflections from the higher mountain tops/buildings.  A gain antenna decreases the high angle radiation that would be reflected from these places, so it actually does worse than a simple "no-gain" quarter wave with a _good_ ground plane like the metal roof of a vehicle.  I have seen jeeps set up with a mast on the back bumper, extending above the roof line, with a quarter wave ground plane antenna at the top, too.  

To illustrate my point about the importance of reflections from high objects, when participating in a VHF fox hunt on 2 meters, I and several other competitors were led astray for miles by a water tower that was reflecting the fox's signal -- we were receiving the signal much better from the water tower than we were directly from the "fox", even though the fox was closer to us than the water tower.  Years later, as the fox, I aimed a small beam at a water tower a few miles away to confuse the "hounds", and it worked quite well.  

The quarter wave "no gain" vertical is the best mobile antenna unless you live in the plains away from high hills or buildings.
Gud rx & 73 de kt8k - Tim
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K4III
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« Reply #11 on: December 22, 2005, 09:35:33 AM »

A 1/4 wave antenna on VHF is even superior to a higher gain VHF antenna here in heavily forested flat wooded areas. I set up a uhf system and was shocked at how badly the rf signal was "eaten alive" by the brush. At 50 feet transmitting 20 watts on 440, it was unreadable 3 miles away with a mobile due to the terrain! At 500 ft, a 450 repeater's coverage is about 25 miles max whereby a 150 repeater's coverage at 250ft is 40 miles!

I decided to experiment with vhf & uhf antennas on my vehicle. It seems the RF needs to get "UP into the SKY" to be picked up by the repeaters. Using a high gain antenna focused too much of the VHF signal into the trees and woods along side the roadways. But the 1/4 wave seems to work a lot better. That is why I've set up these antennas on North FL Forestry vehicles using vhf in our area. For UHF, using a 1/4 VHF 2m antenna tops any high gain uhf antenna and it being 3/4 wavelength on UHF, you will see that it has 3db gains at 15degrees from the vertical. Using a VHF 1/4 wave on 440 is usually used for satellite operation due to the high angle of radiation, but getting that steep angle gets it above the tree line when traversing roadways in heavily wooded areas. I believe a direct line of sight is impossible, but rather, the repeater uses its preamp to amplify the weak signal found above the treeline on UHF in the area.

If you can get above the 120ft pines treeline, you are as good as gold!

I remember in S Florida, a 40ft UHF repeater could cover 60 miles! What a change in terrain & fauna and the effect it has on rf!


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K4III
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« Reply #12 on: December 22, 2005, 09:38:24 AM »

A 1/4 wave antenna on VHF is even superior to a higher gain VHF antenna here in heavily forested flat wooded areas like in N FL. I set up a uhf system and was shocked at how badly the rf signal was "eaten alive" by the brush. At 50 feet transmitting 20 watts on 440, it was unreadable 3 miles away with a mobile due to the woods! At 500 ft, the 450 repeater's coverage is about 25 miles max whereby a 150 repeater's coverage at 250ft is 40 miles!

I decided to experiment with vhf & uhf antennas on my vehicle. It seems the RF needs to get "UP into the SKY" to be picked up by the repeaters. Using a high gain antenna focused too much of the VHF signal into the trees and woods along side the roadways. But the 1/4 wave seems to work a lot better. That is why I've set up these antennas on North FL Forestry vehicles using vhf in our area. For UHF, using a 1/4 VHF 2m antenna tops any high gain uhf antenna and it being 3/4 wavelength on UHF, you will see that it has 3db gains at 15degrees from the vertical. Using a VHF 1/4 wave on 440 is usually used for satellite operation due to the high angle of radiation, but getting that steep angle gets it above the tree line when traversing roadways in heavily wooded areas. I believe a direct line of sight is impossible, but rather, the repeater uses its preamp to amplify the weak signal found above the treeline on UHF in the area.

If you can get above the 120ft pines treeline, you are as good as gold!

I remember in S Florida, a 40ft UHF repeater could cover 60 miles! What a change forests & trees have on the effects of rf!


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W8JI
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« Reply #13 on: December 22, 2005, 05:11:37 PM »

It's all simple geometry.

A 500 foot tall building ten miles away is, for all practical purposes, at zero degrees elevation. There is no "aiming up" to get to it, it is 1/2 degree above the horizon.

While knife edging is certainly possible and common, the real reason these  magical gain antennas don't show gain and sometimes show loss is they aren't really gain antennas at any angle.

I compared a ~22 foot long 5.2 dB gain Super Stationmaster to a ~14 foot long 9 dB gain hambone superdeluxe yellow package wrapper special, and the StationMaster had about 5dB more gain along the horizon.

73 Tom  
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W7FRS
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« Reply #14 on: December 22, 2005, 05:26:53 PM »

Hey guys; what about 2 1/4 wave verticals spaced x feet apart and fed x degrees out of phase?  Then one could choose what works best.  Please forgive me if this is just a brain fart.  I live in the mountains. When I am going some where I am generally going in one direction.

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