GMRS Antenna question.

[WRVD377]

Since I have little to no knowledge of what is best for a repeater. I initially used what I could find that was "gmrs tuned". Optek UH-2401.

Now after many forums search's I find that maybe a Dipole might be better.

If a person tried to look for the best universal direction output, best gain, and working on antenna altitude, but may not be able to fix that much.

Are my assumptions from my reading even close to reasonable?

If I had the money,

ASP705K might be the best answer.

DB420-B May be next best.

DB408-B Then this.

DB404-B Then this.

Compared to the radiated energy of a Optek UH-2401 like antennas.

Am I missing ground plane / grounding complications?  Other things I am too stupid to know?  Keep in mind altitude is not an option more that 20'9" change to 33'2" mast. Hills North and South.

My assumption is DB404-B in elliptical layout should be better than Optek UH-2401 like antennas.  Is this reasonable  understanding?

Thank you,

Royce

 

 

 

[SteveShannon]

2 minutes ago, WRTZ750 said:

Can someone direct me to where I can learn about what it means to have an antenna with "gain"?  In the context of a repeater antenna, does a higher gain omnidirectional antenna have a larger "dead spot" area near the tower then?  How does one calculate what this dead spot size/area is?  Or does it really apply when doing shorter tower hights of 100 feet or less?

Theoretically radio waves would be radiated from a single point at exactly the same strength in all directions, forming a perfect sphere. That’s unity gain, or no real gain.
However, in real life that sphere may take on other shapes, with more strength in a specific direction, or flattened at the top or bottom. But there’s just as much RF energy being emitted, so just like a water balloon, if you flatten it, the circumference becomes larger. That’s said to be gain in that direction. 
When there is gain in a direction, other directions suffer. Those may be directions that don’t matter. For instance, directly above or below the tower. 
The size and shape can be simulated using antenna simulation software or it may be estimated using past experience or it may be measured using a field strength meter. 
Most commercial antennas have some kind of published information. 

[gortex2]

a DB404 will work fine. I run a 404 on 3 of my repeaters. 2 are on tower sites 1000' higher than the surrounding area. 1 is on a 60' tower. I also run a DB408 as a receive antenna at one tower site with a DB404 for the transmit thru a combiner. The 408 and 420 are higher gain so if your way up may over shadow the area you are trying to cover. In the end all three are commercial made antennas used by Public Safety and LMR all over the US and are built to last. 

[WRVD377]

Thank you

[WRVD377]

On a like question, You know how many antennas give you a radiated emissions pattern. E-plane some call it.

Is there an easy way to determine how to mount it to point the best pattern the direction you want.  Meaning an Optek, if I read it right likely am not, appears to have a pattern that is best 180 degrees from each other, but from what mount point or mark on the hardware.

My old Microwave days was easy, it was two hands, 1 vertical and 1 horizontal, and an ohm meter till you had them aligned. Spread your finger out both planes and hit them both from both sides 1 mile apart. Round dishish shape antenna, easy to know which way radiation was going.

[Lscott]

Just so people have an idea what these antennas look like and the performance I attached a datasheet for the DB408. For repeaters there is one spec that often gets overlooked, that's the "down tilt" angle if it has one. That's important at high gains and you need close in coverage to the site.

https://www.kpperformance.com/Antenna-Downtilt-A-Practical-Overview.html

https://www.telecomhall.net/t/what-is-antenna-electrical-and-mechanical-tilt-and-how-to-use-it/6388

DB408-B Product specifications.pdf

[VETCOMMS]

Can someone direct me to where I can learn about what it means to have an antenna with "gain"?  In the context of a repeater antenna, does a higher gain omnidirectional antenna have a larger "dead spot" area near the tower then?  How does one calculate what this dead spot size/area is?  Or does it really apply when doing shorter tower hights of 100 feet or less?

[VETCOMMS]

Thanks for that explanation Sshannon.

[SteveShannon]

2 minutes ago, WRTZ750 said:

Thanks for that explanation Sshannon.

You bet.  Did it answer your questions?

[VETCOMMS]

Absolutely, I think I've seen the radio wave disbursement pattern with antennas before.  Not sure what the actual term is for that with colors and blobs going out in different directions from the antenna.  What I don't know is if on a 50 to 100 foot tall tower there is much of a dead spot area to be concerned about or if that's more for the big boys that have antennas way up high.

Along the same lines, I think mygmrs.com will email me to tell me to mark if my question was answered but I don't know how to do this.  Are you familiar with how to say, "yes, it helped me"?

 

[SteveShannon]

Somethings to keep in mind is that height above ground of an antenna has a huge effect on gain.

Although gain patterns look pretty regular; gain is not.  It can look like tendrils emanating in all kinds of directions.  Here's an idealized picture from https://www.netxl.com/blog/networking/antenna-gain/:

 

But here is a pattern generated from EZNec, an antenna simulation tool:

[SteveShannon]

4 minutes ago, WRTZ750 said:

Absolutely, I think I've seen the radio wave disbursement pattern with antennas before.  Not sure what the actual term is for that with colors and blobs going out in different directions from the antenna.  What I don't know is if on a 50 to 100 foot tall tower there is much of a dead spot area to be concerned about or if that's more for the big boys that have antennas way up high.

Along the same lines, I think mygmrs.com will email me to tell me to mark if my question was answered but I don't know how to do this.  Are you familiar with how to say, "yes, it helped me"?

 

You can vote an answer up or down along the left hand side, but that's the only way I know.  I'm just glad it helped.

Yes, you can have areas near tall towers with high gain antennas that are dead-zones.  Some antennas even have an RF downward tilt to minimize that area. One way to do that is with slightly out of phase antennas in a colinear array.  They're pretty fascinating.  The harness running to the antennas has very slightly different lengths of feedline to create a delay to some of the antennas in the array, causing a downward shift in the radiated pattern. Fortunately, the manufacturer does all the math.

Steve

[VETCOMMS]

So in the top picture, the higher the gain, the more focused or narrower the beam of RF that's emitted, correct?

And using the lower image, that particular antenna radiates more RF to the right because that's the larger cone coming out in that direction.  Am I understanding this stuff correctly?  I'm not a HAM and don't have lots of radio transmission experience but it's always interested me....

[SteveShannon]

4 minutes ago, WRTZ750 said:

So in the top picture, the higher the gain, the more focused or narrower the beam of RF that's emitted, correct?

And using the lower image, that particular antenna radiates more RF to the right because that's the larger cone coming out in that direction.  Am I understanding this stuff correctly?  I'm not a HAM and don't have lots of radio transmission experience but it's always interested me....

Yes; you're understanding it correctly.  For most purposes you probably wouldn't want something like that (and those are pretty idealized images anyway).

It's just like a light bulb.  If you just put a bulb on top of a pole with no reflector and no focusing, it's a small speck of light, but if you add a reflector you get brighter light (gain) in one direction at the cost of less light (or even no light) in other directions. That's what a parabolic antenna might do, but that's the kind of antenna that's only useful for certain applications, such as fixed point to fixed point communications.  Most of us want something with a lot less directionality. Many repeater antennas or mobile antennas radiate a pattern that's like a flattened ball, wide outward horizontally, but with less gain vertically (both up and down).

[KAF6045]

4 hours ago, WRTZ750 said:

So in the top picture, the higher the gain, the more focused or narrower the beam of RF that's emitted, correct?

And using the lower image, that particular antenna radiates more RF to the right because that's the larger cone coming out in that direction.  Am I understanding this stuff correctly?  I'm not a HAM and don't have lots of radio transmission experience but it's always interested me....

Simple half-wave dipole, 20 meters (~66 feet) above ground. Max gain 5.57dBi.

Same antenna 100 meters (~330 feet) up. Max gain 7.36dBi

 

FYI: EZNec Pro/2+ v7.0 is now a free download (the author has stopped providing updates). https://www.eznec.com/

 

[WRVD377]

Nice to have many of you folks to answer. When I asked, it was mostly related to my fish bowl issue. Is dipole from a hole or typical single pole "omni" ground plane 5', 10', 16' better do to the way it sends out the signal. I am not sure, but overall if you don't have altitude, seams range is better with single pole omni, again I don't know.

[KAF6045]

On 1/16/2023 at 11:09 PM, WRVD377 said:

Nice to have many of you folks to answer. When I asked, it was mostly related to my fish bowl issue. Is dipole from a hole or typical single pole "omni" ground plane 5', 10', 16' better do to the way it sends out the signal. I am not sure, but overall if you don't have altitude, seams range is better with single pole omni, again I don't know.

Same dipole at 3m (~11 feet) (FYI: I'm using 465MHz as that is between the 462MHz output and 467MHz input frequencies)

And at 1.5m

At 23deg you are down 3dB -- or half the ERP seen at 5deg.

[KAF6045]

On 1/15/2023 at 5:50 PM, WRTZ750 said:

So in the top picture, the higher the gain, the more focused or narrower the beam of RF that's emitted, correct?

And using the lower image, that particular antenna radiates more RF to the right because that's the larger cone coming out in that direction.  Am I understanding this stuff correctly?  I'm not a HAM and don't have lots of radio transmission experience but it's always interested me....

Note that those images are AZIMUTH images as specific elevation angles. My plots are elevation images. For a dipole, azimuth plots are boring.

Perfect circle...

[WRVD377]

 

I think I see some of my confusion. One vendor shows elevation pattern the other shows Azimuth.  So thanks for the writing everyone has done such that I finally caught that.

GP-6NC: Is my red line correct for how they word where the antenna is?

DB404: Did I assume correctly on antenna placement for two types of deployment methods?

If I got those correct, then does the length of either type antenna change much of anything. GP-6NC is 10', DB404 is 5'.

[KAF6045]

Note that the image references "FREE SPACE" -- which is essentially a non-existent condition in real life (even a probe moving between Jupiter and Saturn will have the radio and power source acting as some form of reflector/ground).

Compare:

Free Space half wave dipole (note the 2.15dBi figure -- that's where the conversion from dB Isotropic to dB dipole [dBd] comes from)

"Perfect Ground", antenna 100 meters up

And finally, "Real Ground" using high-accuracy model (0.005 S/m conductivity, 13 dielectric constant), same 100 meters up

[axorlov]

Would be interesting to see elevation chart of half-wave (yes, half-wave) over real ground at 1.5m, like when it's installed on jeep tailgate or on small car with plastic roof, or being clipped to backpack on a shoulder height.

[KAF6045]

On 1/22/2023 at 2:09 PM, axorlov said:

Would be interesting to see elevation chart of half-wave (yes, half-wave) over real ground at 1.5m, like when it's installed on jeep tailgate or on small car with plastic roof, or being clipped to backpack on a shoulder height.

Second image of my Tuesday 11:37 post -- though I'd really need to model the metal parts of the vehicle to be most accurate.

[axorlov]

1 hour ago, KAF6045 said:

Second image of my Tuesday 11:37 post -- though I'd really need to model the metal parts of the vehicle to be most accurate.

Jesus Chrysler! It was here all along. Thanks! I need better glass-wiping tissue.

And no, you don't need to model metal parts, I suspect they will not change the picture in a single tiny bit.

[KAF6045]

15 hours ago, axorlov said:

Jesus Chrysler! It was here all along. Thanks! I need better glass-wiping tissue.

And no, you don't need to model metal parts, I suspect they will not change the picture in a single tiny bit.

They are likely to (for a tailgate location) to slightly tilt the elevation pattern, and may turn the azimuth pattern from a circle to an ellipse. All the metal on one side will make it seem as if the ground is tilted.

[Lscott]

16 hours ago, axorlov said:

Jesus Chrysler! It was here all along. Thanks! I need better glass-wiping tissue.

And no, you don't need to model metal parts, I suspect they will not change the picture in a single tiny bit.

You may need to do a model if the metal structures are a 1/4 wavelength or larger in size in the immediate area of the antenna. Things like roofs, hoods and so on can affect the radiation pattern. Those structures are approximated using a grid of wires in the model.

This is an old model I did looking at the radiation pattern for a 2 meter horizontal loop antenna using a magnet mount on the roof of my old Jeep. I wanted to see if the 18 inch mast they sell with the magnet mount was sufficiently high for good performance. Doesn't look very good. Both photos have the same exact 3D orientation. 

As you can see there are two main areas of high signal radiation. One is mostly directed skywards, useless. The second in in a horizontal plane, which is where you want it. Because the roof is a rectangle the horizontal part is not symmetrical, more signal in the direction of more metal in the roof.

[nokones]

I'm using a Laird FG4605 5 dB omni 460-470 MHz antenna.