Why More Power Isn't Your Best Option - My Opinion

[marcspaz]

 

Hey folks.  I have been helping someone here with questions about getting more power out of their radio and into the antenna.  I thought the discussion could help many new people, so I am sharing some of that conversation here.

This is a really long post.  Sorry about that, but I think it's worth it.  If you have any questions, just ask.  I am sure myself or one of the other experienced operators or engineers can help.

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I think, to help make sense of power vs performance in GMRS, I'll explain how the signal strength is impacted by output wattage.

UHF (where GMRS lives) is a Line of Sight service.  This means exactly what it sounds like.  If it's in the RF line of sight, you can talk to it.  When we talk to radios on the surface of the earth, you can only talk to the visual horizon, plus about 15% (aka the Radio Horizon).  The higher both antennas get, the further the horizon and the further you can talk.  From the roof of a 15 story building, I can use 0.5w from my handheld to talk on a repeater over 50 miles away.  I talk to satellites in orbit, 240+ miles above Earth, with only 0.25 watts because there is nothing but empty space between us.

There is some very minor benefit to more power.  One of the reasons why the GMRS radio waves can be heard past the visual horizon on earth is because some of the signal gets bent / refracted, as well as scattered in all directions while it travels, creating the referenced RF horizon.  Some of this signal scatters back to earth while the rest travels out into space.  It's a very small percentage and it varies depending on the atmospheric conditions.  That said, if you have 100w out and only 1% of your signal gets scattered back to earth (example only), someone can hear that 1 watt and you may be able to talk if the other person's signal scatters about the same, coming back to you.  Scatter is extremely unreliable, however.

I'll explain why getting the antenna as high as possible is for the best, compared to adding more power.  In the radio world we use a quality scale to define how well your signal is being heard.  This is called the RST System.  RST stands for Readability (how intelligible your words or information is), Signal Strength (how strong your signal is heard by the receiving station), and Tone (mostly used in Morse Code communications). 

For the voice side of things, we typically only use the RS portion.  R is on a scale of 1 to 5, with 1 being aware that someone is transmitting voice, but can't make out anything. An R5 means, regardless of signal strength, your words are 100% intelligible.  For the S, we use an S-meter and provide the total number of S-units read on the meter.  This indicates how strong your signal is.

This is a picture of an analog S-meter scale.

 

The letter S on the left represents one (1) S-unit.  The largest S-unit reading on s-meters is the 9 or S-9.  I'll explain the +10, +30, +60 in a moment,  So, just referencing the RST system and the meter above, the best signal report would be a 5/9 (pronounced five nine) or 59.  This means your voice is completely understandable and your signal is strong enough that the needle moves to 9 s-units, the maximum value on the number scale (normally there is zero static or noise heard at this level). 

The commonly accepted minimum signal report that is considered "usable" is a 2/1, 2 for voice quality and 1 S-unit.  However, that is absolute worst case, if it was an emergency and you can repeat information over and over to get the communications out.  There is a tremendous amount of noise and your voice is barely being understood when it is heard. 

Conversational minimal signal reports are going to be about a 3/2, but that is still aggravating and most people give up quickly due to the static and missing every other word or every third word.  It's mostly "call me on the phone" or "almost there" kind of stuff. 

And then there is what most people tolerate for chit-chat, and that is a 4/3.  Meaning, I can hear everything you say.  There is much less static, and the signal is strong enough that you have established reliable communications.

So, to understand what an S-unit is and how it impacts the quality of communication, I have to explain what the meter is actually measuring.

One s-unit is equal to 0.2 microvolts detected at the receiver antenna input.  Regardless of how much power the transmitter is and regardless of how far that station is, if 0.2 microvolts makes it to your radio, that radio is hearing 1 s-unit of signal.

Lets say you have 10w going into the transmit antenna and the receiving station is fairly far from your radio, only receiving 1 s-unit. 

10w

We really want to get to 3 s-units for reliable communications.  Well, in order for the received voltage to climb 1 s-unit, you need to multiply your power by 4.  That means to move 1 s-unit on the receiver, you need to increase your transmit power to 40 watts. 

40w

However, we already know that 2 s-units isn't really usable.  To get to 3 s-units, you need to multiply your power 4 times, again.  That's 160w into the antenna to go from "I know you're there" to "I can actually talk to you" (assuming your audio is good).

160w

Lets assume you wanted to try to get the s-meter to swing to 9 s-units.  Well, that will take 655,360 watts.

655,360w

Now, the + scale is even more fun  You need to double your power for every 3dB increase in measured power.  You need go up 4 times the power to increase by 6dB.  Those + readings are for strength in dB over the s-9 signal strength. 

Since in our specific example we know you need 655,360w to go from s-1 to s-9, to get to +30dB over s-9 you need to increase your transmit power to 655,360,000w.  That's 655 million.  Not a type-o.

655,360,000w

So now, lets go back to the original limit of GMRS being a Line of Sight service.  That 160w you needed to get to 3 s-units, basically netted you zero distance compared to the 10w, because the horizon didn't change.  All you did was make it so that those who could hear you, now understand what you are saying... maybe. 

To make things even more bleak, if you are communicating due to scatter and 1% or less of your signal is being heard, the amount your transmit power would need to increase is unfathomable.  And did I mention that all that transmit power did absolutely nothing for your receive capability? So, you can see why it's not practical to chase power output.  If you get your antenna 30 or 40 feet higher, you would actually improve your communications range and quality of reception much more than if you can increased your power from 10w to legal limit.

 

[OffRoaderX]

Great post!
Is not more power also helpful in penetrating objects like walls, and bouncing the signal through canyons, etc, for example when off-roading?

 

[BoxCar]

Yes, more power does mean better penetration, but again it's another relative thingy. It depends on what you are trying to penetrate and the density of the obstruction. As far as bouncing (or reflecting) the signal off rocks like mountain sides or canyon walls, it's again the material used as the reflector. So more power may boost the signal from a poor, but marginal surface, at the levels we're using it's negligible.

[marcspaz]

10 hours ago, OffRoaderX said:

Great post!
Is not more power also helpful in penetrating objects like walls, and bouncing the signal through canyons, etc, for example when off-roading?

 

 

Great question!

If you have walls of a canyon that are high in iron and other conductive material, there can be some tunneling or directional impact, but its rare for it to be in high enough concentration to work.  Most natural barriers, such as the trees and hills/mountains absorb UHF signals... however, UHF penetrates man-made structures, like buildings, much better.  More power would be more beneficial in urban areas than in other types of areas. 

More power is going to be a benefit in rough terrain for other reasons, though.  RF shadowing is like shadows from the Sun.  There is never complete darkness behind a rock or mountain.  Same with radio waves.  Refraction and scatter places the radio waves behind stuff... the sharper the angle, the less scatter.  So, the more power you have, the less you are going to be impacted by shadowing, but only to a limited degree.

HF signals will bounce off of stuff pretty well, and goes through trees and foliage with no issues.

[WRUU653]

 ...but this goes to eleven.   In all seriousness though Marc thanks for this post, good stuff sir. 

[marcspaz]

11 hours ago, WRUU653 said:

 ...but this goes to eleven.   In all seriousness though Marc thanks for this post, good stuff sir. 

 

Thank you! My pleasure, truly. 

 

I'm no expert, but I try to be as accurate as possible and deliver it in a way that everyone can understand, regardless of the reader's experience level. My goal is to help as many people as possible, because if it wasn't for the community, I would have been lost.

[Sshannon]

Very nicely explained.  I understood the relationship between decibels and S units, but this helped me better understand the “I hear you five by nine!” reporting. Well done!

[WRKC935]

As a rough rule of thumb it is often said that doubling the height of an antenna will give a 6 dB increase in gain. Although this will depend upon the actual situation and a host of caveats, etc, studies have shown that it is generally not too far from the truth.

This kind of goes hand in hand with what Marc is saying here.  But of course there is a point of diminished return on investment.

To go from 10 to 20 feet or 20 to 40 feet would be a good investment in getting your signal out there, but if you are at 300 feet, the cost alone to double that is going to be too high for the perceived gain you get from doing it.

There are other factors in play here of course such as topography of the land around you, so consider this to be pretty close to correct over flat ground, but in hills and valleys your mileage will vary.

But, as mentioned before by both Marc and myself, height is king when it comes to antenna's.  And it will have a more dramatic effect on your signal than doubling your power. 

 

[Borage257]

You can play around with the Radio Mobile Online website to see the local affect of increasing antenna height, antenna gain and radio watts.

 

[WRUW886]

Awesome post.

[marcspaz]

3 hours ago, WRKC935 said:

it is often said that doubling the height of an antenna will give a 6 dB increase in gain.

 

I have heard this as well.  It might be fun to actually test this.  I'll have to see, but I may hook up a spectrum analyzer to an antenna and then use a telescoping mast to see what the dB reading are on the analyzer side.  It may be a bit more than a week, as I am traveling through Monday and have to work, but I am penciling that in.

[KAF6045]

3 hours ago, marcspaz said:

I have heard this as well.  It might be fun to actually test this.  I'll have to see, but I may hook up a spectrum analyzer to an antenna and then use a telescoping mast to see what the dB reading are on the analyzer side.  It may be a bit more than a week, as I am traveling through Monday and have to work, but I am penciling that in.

I'd suspect it is more a distance aspect than local. People near the original horizon cut-off with a low antenna are now well within the new horizon distance and likely also have less environmental losses vs a signal skimming over the ground.

Some may be ground effect, but again, that is in the far-field radiation pattern, not near-field. I've not used the EZNEC Ground Wave Distance option before but...

Half-wave dipole, vertical, /bottom/ at 3m (~10 ft), with distance 5km (ignore the window title -- I haven't found where to change the description):

And at 10 meters (~33 ft)

10dB less loss at 5km. Turning off ground wave distance and plotting elevation... 3m elevation:

Interesting, over high-accuracy ground type (medium 0.005 S/m Conductivity, 13 Dielectric Constant), the half-wave shows 6.6dBi at a 3deg elevation. Now at 10m:

At 10m it shows 7.5dBi at a 1 deg elevation angle (at 3 deg angle it is only 5.6 dBi). Going back to the 3m elevation, at 1 deg take-off angle, it is only 1.85 dBi. So 3m elevation is 5.66 dB below the 10m height at the angle of max dBi at 10m.

I also ran with Free-Space to confirm dipole pattern and dBi max:

As expected, 2.15 dBi.

 

If you (generic readers) don't already have EZNEC, GET IT. The creator has retired and stopped making updates, but has made EZNEC Pro+ v7 a free download (it used to be $$$) https://www.eznec.com/

[axorlov]

Great post by Marc. But consider some practical applications. When you are in the forest (as opposed to an open range) the more power is always the answer. And we had our discussion about city, I don't want to delve into this now.

[marcspaz]

53 minutes ago, axorlov said:

Great post by Marc. But consider some practical applications. When you are in the forest (as opposed to an open range) the more power is always the answer. And we had our discussion about city, I don't want to delve into this now.

 

My post really focuses on base stations. May e I should mention it specifically, but I assume telling people to put their antenna 30 or 40 feet higher was an indication. If the antenna is higher than the trees, then you don't have to worry about them. LoL

[WRKC935]

11 hours ago, marcspaz said:

 

My post really focuses on base stations. May e I should mention it specifically, but I assume telling people to put their antenna 30 or 40 feet higher was an indication. If the antenna is higher than the trees, then you don't have to worry about them. LoL

Yeah, it's sort of hard to run a 20 foot mast on a vehicle ANYWHERE.  Trees, bridges, power and telephone lines that run across the road would all create problems.  I have seen some masts that could be deployed at a camp site, but not moving down the road or through the woods.

 

And this perceived gain would also apply more to suburban and rural locations more than urban or city locations.  Reason being is the buildings and the attenuation they cause.  A residential structure typically will be a 10dB attenuation of the signal both directions (TX and RX).  Commercial structures depending on the building materials typically range from 20 to 30 plus dB of attenuation.  The worst being metal and E-glass materials for blocking RF.  And those numbers are from inside the building.  If you are on the far side of it, and talking through the whole structure, it can be twice that. 

Height increase is not as effective in city and urban situations for this reason.  There are a couple reasons for this, but that's a whole page of explanation I am not gonna get into at 7 AM

 

[marcspaz]

@axorlov / @WRKC935  I'm kind of confused.  Are you saying that antenna height doesn't help with propagation as much as additional power in cities because of buildings.  Or are you saying height doesn't help when you are actually in a building?  Or neither of those?  I think I am missing something?

[KAF6045]

Height helps -- at the repeater site. The (grandfathered business, city owned) Grand Rapids MI GMRS repeater is on a 290 ft tower, and has a gain antenna providing 200W ERP. It has a range of 15-30 miles depending on intervening mountains.

[WRKC935]

10 hours ago, marcspaz said:

@axorlov / @WRKC935  I'm kind of confused.  Are you saying that antenna height doesn't help with propagation as much as additional power in cities because of buildings.  Or are you saying height doesn't help when you are actually in a building?  Or neither of those?  I think I am missing something?

Height helps always.  However, realized gain due to height in dense urban area's is lower than it would be in a rural environment.

 

[marcspaz]

32 minutes ago, WRKC935 said:

Height helps always.  However, realized gain due to height in dense urban area's is lower than it would be in a rural environment.

 

 

100% agree.