How many MURZ farz in the woods?

[Lscott]

1 hour ago, SteveShannon said:

Because RF is emitted in all directions equally from an isentropic antenna, one way to compare the range of different power outputs is to think in terms of the volume of a sphere:

Actually the antennas respond to the "E" field. Look at the sensitivity rating of a radio's receiver. It's specified in microvolts.

To get a ruff idea the power density varies as the inverse square of the distance since the same power is spread out over a larger surface area of the sphere around the isotropic source.

So if the power density is:

PD=(watts)/(sphere's surface area)

Expressing sphere's surface area as a function of the radius:

sphere's surface area=Pi*radius^2

PD=(watts)/(Pi*radius^2)

And watts can be expressed as:

watts = (Volts^2)/(Resistance of free space), which is about 120*Pi, or 377 ohms.

So your power density becomes:

PD=[(Volts^2)/(377 ohms)]/[(Pi*radius^2)]

Or after a bit of rearrangement:

PD=[(volts/radius)^2/[(Pi*377 ohms)]

That will vary as the surface area of the sphere, or the inverse square of the distance. If the radius is in meters then you have the "E" field intensity in Volts/Meter.

PD=(E^2)/(Pi*377 ohms)

Note that antenna simulation software will specify the antenna's "E" field in volts per meter. Also be careful you want the "Far Field" number.

This also leads to another topic about "path loss" and how there is is a difference between VHF verses UHF for signal strength given antennas of equal gain. It turns out you need much more power at UHF to get the same signal strength you would see on VHF for the same power. Part of the gain you get on VHF is ruined due to the short stubby VHF antenna gains on HT's, reportedly around -5 to -6 db, which almost cancels out the reduction in path loss reduction. Just can't win.

[SteveShannon]

5 hours ago, marcspaz said:

 

The information above is technically accurate, but should we caveat that for the woods?  Since the forest isn't consistent in density, levels of foliage change, the amount of water in the trees vary, etc.  actual results may vary.

Yes!  Sorry, I thought I included the phrase “all other things being equal.“

I meant to anyway. 

[WRPG745]

The math is a great explanation of why more power doesn't always deliver what a user expects. It reminded me of a joke where the punch line includes "spherical cows in a vacuum" (iykyk)

[AdmiralCochrane]

13 hours ago, WRPG745 said:

The math is a great explanation of why more power doesn't always deliver what a user expects. It reminded me of a joke where the punch line includes "spherical cows in a vacuum" (iykyk)

The famous bovine derivative of the Schwarzchild metric

[WRYT601]

CB is far better IMO.  I have no trouble reaching my friend 18 miles away with 5-5 signals in my truck through heavily wooded and hilly terrain.  25 miles is about the limit of reliable comms for us with wooded hills between us, me mobile and him on his base station.  I have a homemade mag mount base-loaded whip 60 or 70" long on my truck and he has a sirio827 at the peak of his roof.   If you are willing to build or buy a decent CB antennas, a legal limit CB will beat a legal limit GMRS radio almost every time.  Even the 4' fiberglass CB antennas can reach a decent base station 10-15 miles out, but they would be my last choice.  Those silly little 2' mag mounts walmart used to sell only turn people off of CB.  Hand signals out-perform those things.

Considering that many people are interested in emergency comms, would you rather have a radio that draws 18A and might make it 10 miles with little chance of any enhanced propagation, or would you want something that can work 20 miles on a set of AA batteries in a pinch with a reasonable chance of reaching someone 1000 miles away 5 years out of 11?  The coax requirements are not as stringent on CB due to the lower loss, so if you need to run 50', your coax isn't a 35w heating element either.  

I've noticed little difference between VHF and UHF in the woods using HT's, but I think that is primarily because the handhelds don't have much of a ground plane at VHF resulting in a more spread out (in the vertical) radiation pattern.  Dipole for dipole, same ERP, I'd think VHF would win.  I had some good success with 5/8 NMO trunk mounts on VHF.  40w would get me about 20 miles to a ground plane at 20' quite reliably.

I monitor MURS 2 from a 5/8 wave GP at 15' up because I have a long range driveway alarm a mile down the road, but I never hear anyone on it.  Only the occasional Garmin GPS dog tracker (which I tracked down to a resort about 5 miles away).

As for FM on CB, IMO, it sucks.  Sorry to those who like FM, but if there are a lot of stations coming in on skip, forget it.  The human ear can pick out a single AM station among many far better than your FM detector can.  With FM, its the strongest that wins.  SSB provides, by far, the most distance per watt for congested band conditions running voice. 

[marcspaz]

8 minutes ago, WRYT601 said:

CB is far better IMO.  I have no trouble reaching my friend 18 miles away with 5-5 signals in my truck through heavily wooded and hilly terrain.  25 miles is about the limit of reliable comms for us with wooded hills between us,

 

This is my near exact experience.  I have said that CB is much better for peer-to-peer comms when compared to VHF and UHF service like MURS and FRS/GMRS, especially offroad.  The problem that drove (is driving) damn near every to GMRS is the fact that a vast majority of people have no idea how to properly install a CB (HF) anttenna on a vehicle and they buy trash antennas.  I lost count of people who asked me for help because I was talking to my friends over 15 to 25 miles and they were getting a mile or less.

I gave up trying to help people with it, too.  Most people I tried to help wouldn't take my advice, kept trying to half-a$$ it, and then get mad at me because it wouldn't work any better after the pretty much ignored what I said and did whatever they wanted.

[LeoG]

CB has no repeaters.

[marcspaz]

Just now, LeoG said:

CB has no repeaters.

 

This is a huge benefit for UHF, but if your mobile CB is setup right, it will outperform 99% of the GMRS repeaters that are on the air.  No question about it.

 

Where GMRS really shines is using repeaters to extend the range of low-power handheld radios with little antennas.  That won't happen on CB/HF.

[WRYT601]

32 minutes ago, marcspaz said:

 if your mobile CB is setup right, it will outperform 99% of the GMRS repeaters that are on the air.  No question about it

My experience as well.  These new plug-and-play GMRS repeater are kind of a joke IMO.  I have tried to reach several in my travels, and it seems you have to be right under them before they work.  The best those little notch-only duplexers can do is about -90dB isolation, and considering they have them tuned so they can be used on any repeater frequency, its more like 80dB if you are lucky.  Couple that with a transmitter that makes a lot of out of band white noise and cheaply designed receiver, you may as well forget it.  There is a ham who set one up on a 70' tower maybe 15 miles from me and I cannot reach it standing on my metal roof.  Online terrain maps show it to be line of sight.  However, I flip over to the 70cm repeater 25 miles away and my HT gets it done set to low power.  If someone wanted to homebrew a set of cavities and connect them to a pair of old motorola radios, they'd have something.

[WRUE951]

Just as in GMRS, talk distance differs in MURS..  I use both and have both dedicated UHF and VHF radios...  Conditions is what matters..   Sometimes UHF is better, sometimes VHF is.   I do like resorting to MURS while driving around Los Angeles where it is nearly imposible to use GMRS because its pretty bussy..  MURS is pretty much vacant and works much better than GMRS in those conditions  

[WRKC935]

Reality for "EMCOMM" is have everything.  At least if you are looking for emcomm abilities with others as well as your specific group. That means CB, MURS, HAM, GMRS and scanners for Public Safety and other government communications.  

Now if this is not really emcomm and instead just looking to communicate with your family during a situation that other means of communications has failed, that takes an approach like what you are seeing here.

Back to what I have said before on here.  Radio is first and foremost a tool.  

Consider drilling a hole.  If you have a 3/8 drill and a 1/2 paddle bit, you can drill a 1/2 hole in wood.  BUT, if you are like me, you have several 3/8, and 1/2 drills.   Some are corded, some are battery.  I have drill bits that range from wire gauge sizes to the biggest being 2 inch.  Then I have hole saws that are up to 5 inch for wood and metal.  Then I get into the SDS drills for concrete.  smallest SDS bit is 1/8, up to 1 inch for concrete.  After that, I have a coring drill that will spin up to a 12 inch coring bit for concrete.  

Then I have magnetic drill presses that attach themselves to metal plates and can cut with the drill bits and metal hole saws into metal.  

 

Point is that if I need a hole in something, I can make it.  You are limited to drilling a 1/2 hole in wood with what you have.  Right tool for the job.  Do you need every tool, every time, no.  And you can gather tools (radios) over time expanding your ability to listen and communicate.  Hardly anyone starts with nothing and immediately has everything, it's too expensive for many to go buy it all at once.  And putting all your eggs in one basket (all your radios services in one multiband radio) isn't a good plan either unless you have backups. Because a single radio failure shouldn't completely remove your ability to communicate.