Mobile vs. Base antennas powers

[nyc787]

I’ll be using my mobile radio as base radio but I’m trying to understand main differences in performance from mobile antennas vs. base antennas. How is it possible for a 40” mobile antenna to have 6db when 9 foot base antennas can also be 6db? What would make the base antenna more powerful then?

[kidphc]

This is over simplifying things a bit. A base antenna usually can be put up higher. Not that it is more powerful. Db values and wattage are misleading also.

 

UHF is pretty much line of sight. So being higher up with less obstructions makes for futher contacts, all else being equal.

 

Higher DBi values are not always better then lower DBi values. Note the "i", indicates compared to a reference antenna. Although, a lot of manufacturers never state what reference antenna.

 

For instance off roading a 3 DBi antenna can be more beneficial then a 6 DBi antenna. The radiation pattern of the 3DBi antenna is going to be more sphere like and easier to make contact with vehicles below you on a berm. Where on flat lands the flatter radiation pattern of a 6DBi antenna is going to reach out further.

 

Mind you DB and DBi are just the gain values of a given antenna and irrespective of it's wave length. Although, it may seam related all the time.

[BoxCar]

Another way to think of gain is the difference you get when spraying water from a hose. A 3 dBi antenna covers the area with a fine mist while a 6 dBi antenna covers the same area with a heavy spray. You'll get more penetration or the ability to go through trees and buildings with the higher gain antenna but the total amount of area covered remains pretty much the same. With all radios above roughly 100 MHz the higher the antenna is above the surrounding terrain the more coverage area as the distance to the horizon increases. The visible horizon changes with height. you can see about 3 miles when standing on flat ground, but then from your rooftop you can see 10 to 12. Go another 20 feet (or 6.1 meters) above the roof and you have close to a 30 km coverage area,

[n4gix]

@BoxCar that is way too optimistic...

 

At 20' above ground level (AGL) the distance to the "radio horizon" (RH) is 10 km (6 miles).

 

At 40' AGL, RH is only 14 km (9 miles).

 

At 100' AGL, RH is 23 km (14 miles)

 

At 500' AGL, RH is 51 km (32 miles).

 

As you can see, RH is not a linear value at all...

 

See: https://www.qsl.net/w4sat/horizon.htm

 

https://puu.sh/E77Md.png

[berkinet]

Another way to think of gain is the difference you get when spraying water from a hose. A 3 dBi antenna covers the area with a fine mist while a 6 dBi antenna covers the same area with a heavy spray. You'll get more penetration or the ability to go through trees and buildings with the higher gain antenna but the total amount of area covered remains pretty much the same. ...

Close. To use your analogy, and limit it to omni-directional antennas, a low gain antenna, say a 0db 1/4 wave, is like the sprayer in the middle of your yard the kids run through. Water sprays a short distance in every direction, including up, within a 180° hemisphere. A gain antenna is more like a quality

lawn sprinkler head. The water only goes out sideways, not up. So, the same amount of water covers a larger circle. (BTW, unlike radio waves, water that goes up will, at least according to Isaac Newton, come back down. So, the analogy is imperfect. You have to imagine that the water going up from the sprayer evaporates before it can drop back to the lawn.)

 

Now, to extend the analogy, imagine you ditch the sprinkler and sprayer and get a nozzle. Now you have a strong stream of water in one direction only. So, even though only one person will get wet, they will be drenched. And, that is a directional antenna.

 

Pure and simple, the power out put from the radio is constant. All the antenna design does is focus the direction, including up or down, that the radio energy is radiated. Less energy radiated towards the sky means more radiated sideways... hopefully to another antenna.

[BoxCar]

Actually the analogy works for any antenna as the spray represents field strength and it works for both omni and directional.

[n4gix]

Every 3dBi gain will increase the effective radiated power (ERP) by a factor of 2. Fifty watts into a 3dBi antenna has an ERP of nearly* one-hundred watts.

 

* nearly because of inevitable cable losses.

[berkinet]

Every 3dBi gain will increase the effective radiated power (ERP) by a factor of 2. Fifty watts into a 3dBi antenna has an ERP of nearly* one-hundred watts.* nearly because of inevitable cable losses.

Very true, though with the caveat that the “gain” is limited to specific radiation pattern.

[nyc787]

I had chance to go on my 6 floor roof to see where I can mount the antenna. One of the issues is the nearby TV dishes from neighbors. Will I be causing disruptions to them with my antenna?

Primary goal is for antenna to hit 6-10 miles over.

 

Planning to use LMR400 need less than 20 feet to reach my window. Will it flex enough over this wide ridge?

2 spots where I can mount antenna. Yellow arrow shows old metal mount where I can attach to without having to drill anything new.

What would be the max allowed antenna height on 6 floor roof?

Will a ground to a wall be sufficient lightening protection or do I must install a lightening arrestor?

 

photo upload website

[berkinet]

Will I be causing disruptions to them with my antenna?

Planning to use LMR400 need less than 20 feet to reach my window. Will it flex enough over this wide ridge?

2 spots where I can mount antenna. Yellow arrow shows old metal mount where I can attach to without having to drill anything new.

What would be the max allowed antenna height on 6 floor roof?

Will a ground to a wall be sufficient lightening protection or do I must install a lightening arrestor?

Well, lots of (good) questions...

• Normally you will not cause interference to your neighbours unless their equipment is poorly maintained, for example, with corroded connections.
• The LMR400 should be fine. You can always run it at an angle across the ridge to increase the run length. However, given the relatively short distance, if the curve over the ridge is too tight for the LMR400, you can easily switch to a more flexible cable. Loss will not be a factor on a 20" run
• By all means use the existing mount, if it is strong enough.
• The max height will depend on local zoning and your landlord or home owner's association. Make sure you get a base station antenna (I.e. with a ground plane) and then mount it on the highest mast you can - as limited by the local rules and your ability to properly support/guy the mast. Finding good anchor points for the guy wires for the mast may be the hardest part. BTW, I'd just do one set of guy wires and given your roof layout, I'd do three wires at 135º, 90º, 135º. Or, with a short mast, you might get away with just attaching it to two points on the existing mount (high and low).
• As for lightning protection. I'd ask a local radio shop or ham group for advice. But, I strongly suspect the wall will not work. However, if you live in an area prone to lightening, you are likely to find a solid ground up there somewhere.

Good luck and be careful up there.

[Logan5]

Not sure I would feel comfortable mounting to that existing bracket and raising more than a few feet. I would get a masonry bit and drill your own in the more ideal location. Then you can easily go up 10 feet. also keep in mind, off set dish antennas beam is more up than out. even on that old bracket you will not be in line of sight.

[nyc787]

I can also get a schedule 80 PVC, clamp the antenna to PVC and stand the PVC next to the bracket and tether it, that way, the bracket won’t have to hold all the weight 10 feet in the air. I’m going to search around how people ground their antennas properly on roofs here.

[Radioguy7268]

If you're just trying to hit a repeater, then you really don't need any more height than what you have. When you say that you want a "base antenna"  - what do you intend to use it for? 

 

If you're in NYC, there's already quite a few GMRS repeaters already on the air with excellent coverage. If you're trying to put up your own repeater, I'd ask "Why?"  

[nyc787]

If you're just trying to hit a repeater, then you really don't need any more height than what you have. When you say that you want a "base antenna"  - what do you intend to use it for? 

 

If you're in NYC, there's already quite a few GMRS repeaters already on the air with excellent coverage. If you're trying to put up your own repeater, I'd ask "Why?"

 

I intend to use my setup for simplex comms 10 miles out and also hit repeaters around me, but in case I won’t be able to use repeaters, I want the antenna to be strong enough to reach 10+ miles out without problems. I have no interest in setting up my own repeater.

I say base antenna because I don’t think a 20” mobile antenna is going to work in my application.

[nyc787]

My superintendent was busy and wasn’t around to go with me on the roof to show me what would be proper way to ground it. I don’t see any grounding sources on that roof. Correct grounding is my only setback to the project. Once I figure that out, I’ll get a Laird 5db FG4505W.

[Radioguy7268]

Those Dish TV antennas should have grounding on them (assuming they were professionally installed by a certified DISH installer). Check them out to see where they were grounded.

 

Make sure you use a Polyphaser or similar surge protector down where your coaxial cable enters the building. You'll need to ground that also.

 

The Laird FG series is a decent antenna for the money.

[nyc787]

I'm looking at polyphasers but there are a million models, which one is correct?   Can the polyphaser be attached to antenna first and coax to polyphaser?

 

https://www.polyphaser.com/search?Category=RF+Surge+Protectors&Rfpssp99app=Amateur+Radio&sort=y&view_type=grid

[Jones]

https://www.polyphaser.com/type-n-surge-protector-1ghz-blocking-cap-gas-tube-is-b50hn-c2

 

Put it as low and close to the ground as you can get it, or at very least, right outside the coax entrance to your shack, NOT on the antenna itself, and NOT inside the house.  I do not know if you can get the coax down to a close-enough-to-the-ground level for this to do much good in your case, unless you live in the basement, or on the first floor.  It needs to be grounded to the building's ground system to do it right.

 

Best idea in an old high rise... leave a coax connector inline just outside of a window, where you can reach outside, and disconnect it from the coax going to your radio equipment during a thunderstorm. ...or buy good insurance.

 

The correct way to ground something like this would be to have a copper strap running from the ground, (which would be bonded to the building's electrical system ground) all the way up to the roof,and have all antennas, air conditioners, and anything else electrical up there tied to that main strap.  (Your building MAY already have this) Still, that's only good for the DC component of lightning.  Lightning has a lot of RF components as well, and let's face it, ANY LONG WIRE tied to ground on one end, is not ground at the opposite end. It is an antenna.  The longer the wire, the lower, and more frequencies it will resonate on, and the more damage can occur due to lightning.

 

Also, if you DO have such a ground system on your building, (I'm betting old buildings don't, but if you do...) then ground the base of your antenna to that system strap with a #10 or larger stranded wire. You should also ground your polyphaser to that same strap. Do not loop-through the ground wire from the antenna to the arrester to ground, Run separate wires from each to a common ground point if you can, or better if possible to BOLT the polyphaser directly to the ground strap.  If the main ground is on the opposite side of the building, you may be out of luck.

 

Still best idea: during a thunderstorm, unplug the coax, and let it dangle outside the building.