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Vertical vs Dipole Array (splitter vs harness)

mvrx

Member
Joined
Mar 29, 2012
Messages
23
Hi,

I'm planning some changes to a site where I have in use an UHF DMR repeater (440MHz).
The main problem is the elevation difference between antenna position (6738 ft) and the road it needs to cover (2896 ft).
The Line-of-sight looks like this


What can be covered by an omni antenna looks like this :

Legend :
blue circle = antenna
green area = possible coverage
red circles/line = what needs to be covered (road / cities)
waves area = no coverage (high relief near site)

What I need to ask you guys is what would you use to achieve the best performance?

1. An omni or a dipole array?
2. If dipoles, should I get a harness or a splitter/combiner?
due to the mast size I can only mount 2 dipoles

I have available the following antennas, ready to install :
OMNI
DIPOLE

3. Can this product be used with the dipoles above?

Other details about the repeater :
RF Power : 20W
Coax : 25 ft / Andrew LDF4-50A

Thanks!
 

mmckenna

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Joined
Jul 27, 2005
Messages
13,801
Location
SNCZCA01DS0
I've had similar installation challenges like that. 3000 foot high UHF site trying to cover deep valleys and a roadway that was down at sea level. The original installer had put in a vertical antenna, and coverage was lousy. Variations of 0dB, 3dB and 6dB antennas were tried, none of them really worked well enough for the end users.
When I took over the site, I replaced the antenna with a Telewave ANT-450 UHF folded dipole. Setting the spacing from the mast to give it a more cardioid pattern helped with the coverage quite a bit. I went as far as to add a second mast with some brackets to angle the antenna down a bit to improve coverage below. Since this was on a high ridge with a near vertical drop down 3K feet, the combination of the radiation pattern and mechanical downtilt did the trick.

It's a single element antenna, so no phasing harness needed.

I think if you want to run two dipoles, then you need some way of evenly splitting the power between them and keeping them in phase. Not sure what the best approach is. Where we've used stacked dipoles, we always ordered them that way from the manufacturer with the harness.
 

mvrx

Member
Joined
Mar 29, 2012
Messages
23
I think if you want to run two dipoles, then you need some way of evenly splitting the power between them and keeping them in phase. Not sure what the best approach is. Where we've used stacked dipoles, we always ordered them that way from the manufacturer with the harness.
Thanks for the reply. Unfortunately I got the dipoles (10 of them) but no harness was provided. I have these but not sure if I can use 2 dipoles with one of the divider :

 

mmckenna

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Joined
Jul 27, 2005
Messages
13,801
Location
SNCZCA01DS0
Not sure about those power dividers, but you also need to address the impedance mismatch of combining two 50Ω antennas on to one feed line.
 

prcguy

Member
Joined
Jun 30, 2006
Messages
9,995
Location
So Cal - Richardson, TX - Tewksbury, MA
In my opinion I would look for antennas with more gain and some electrical down tilt to make the best use of the gain. I have used a version of the Telewave ANT450F2 that you show but with 10dBd of gain and a lot of electrical down tilt on a 5,700ft mountain. I contacted the factory with my site specifications and they recommended the amount of down tilt for a 5,700ft site and it worked very well to hand held radios out to 75mi in most directions and also up close at the base of the mountain.

Here is a picture of a recent install I did at 5,000ft which covers about 180 degrees from the antenna up and down the mountain range. The base of this mountain is about 2,000ft in altitude and 20mi away its down to about 500ft altitude. This might be similar to your situation. The most distant dark mountains sticking out of fog is an island 60mi away and the dark hill just before it is about 35 miles away. This UHF system has great coverage to hand held radios inside a car just about anywhere within 40 miles around and to mobiles well past 100 miles in some directions.

Lukens.JPG

The antennas at this site are a 9dBd gain receive antenna with about 120 degrees of coverage and a 6dBd gain omni directional transmit antenna, both with a good amount of electrical down tilt to focus the antenna beam where it will do the most good. I don't remember the exact amount of down tilt but its more than 4 degrees and less than 8 degrees. The receive antenna might be the same that mmckenna recommended, a Telewave 4 bay dipole array model ANT450D6-9.

Many antennas can be ordered with electrical down tilt or for your specific site and coverage. For your specific site it looks like you have about 120 degrees of coverage and possibly less. In this case you can sometimes use mechanical down tilt with a high or medium gain antenna and achieve great results. For example a 6dBd gain antenna with no electrical down tilt but you mechanically tilt the antenna over maybe 10 degrees toward the town. This will concentrate the main lobe down the hill towards the town but at +/- 90 degrees from the direction of tilt there will be no down tilt, so if the road shown in your picture heading straight up from the radio site is at a higher altitude this should work out well.
 
Last edited:

ko6jw_2

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Premium Subscriber
Joined
May 18, 2008
Messages
1,065
Location
Santa Ynez, CA
If you buy from Telewave, they will make the harness to your specs. You can choose the directional pattern based on several factors. When we ordered we could also specify the down tilt for the pattern. They do offer discounts to hams. It took a while to get the dipoles, but we are very happy with the performance .
 

mvrx

Member
Joined
Mar 29, 2012
Messages
23
For your specific site it looks like you have about 120 degrees of coverage and possibly less. In this case you can sometimes use mechanical down tilt with a high or medium gain antenna and achieve great results.
Right now I have a 2dBd, 3/4 Wave Coax J-Pole with a Beamwidth at -3 dB of 60 degrees which does the job well but it's a cheap stick that will not last a winter. The Telewave antenna has already been in service for 2 years but got replaced by an Amphenol 4-bay array.
The bigger the gain, the smaller the beamwidth so the mechanical tilt makes sense on verticals.

Thank you!

If you buy from Telewave, they will make the harness to your specs. You can choose the directional pattern based on several factors. When we ordered we could also specify the down tilt for the pattern. They do offer discounts to hams. It took a while to get the dipoles, but we are very happy with the performance .
Thanks! The only thing I might order is the harness for the electrical down tilt advantage. Mechanical tilt can be adjusted on site if needed.

So, based on your input, I should go with the dipoles, 45 degrees offset.

Thank you all!
 
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