Radio Watts for both receiving and transmitting

[Guest RKitchens89]

I couldn't be more green with GMRS so I'm sorry in advance if this is a "DUH" type question.

I am looking to use GMRS for off-roading. I am thinking of getting a 50W mobile unit mounted in my camper for a "Base" unit and my friends and I all carrying 5W handheld units in our vehicles. This way we could talk amongst ourselves with our handhelds in close proximity to each other but the base unit’s 50W would be the workhorse for long transmissions between base and our vehicles.

In my head I am envisioning that on either Transmitting or Receiving the 50W radio will have this large reaching circle with the camper in the middle and at the same time the vehicles will have a smaller 5W circle for both Receiving and Transmitting. If the two circles overlap, my messages would be equally as clear no matter if the message is being transmitted from Base or one of the vehicles. Is this how it works?

 

I have attached a simple picture I made to hopefully clarify what I'm thinking. Thank you in advance!

 

https://www.dropbox.com/s/pk1f7jem65qrw32/GMRS.jpg?dl=0

[WRQC527]

That makes sense, up to a point. The reason being that your base station with 50 watts can transmit farther than a handheld. Meaning that your handheld may be able to hear the base, but the base might not be able to hear your handheld. The trick would be to not travel farther from the base than your handheld can transmit. You can also transmit to someone closer to the base and they can relay for you, but that's assuming there is someone between you and the base. There's lots of variables to this, like how high the camper is, the terrain, etc. The way your circles are configured in your diagram would mean that your handhelds would not be able to reach the base. Just because the circles overlap a little doesn't help you. The base needs to be in the circle your handheld covers. Hope that makes sense.

[axorlov]

If this map is a real map and if I read it correctly, your base camp is at the same altitude as vehicles and not blocked by terrain. This condition is close to ideal, you may not even need 50W to have clear comms. But 50W will not hurt. The mobile mounted in the camper should also have an advantage of better antenna and better receiver than on HTs, so it also should hear better than HT. Other areas might be blocked, like Gibbon Picnic Area in the north, because it's in the dip behind the hill.

[gortex2]

If your all using handhelds 4 watt portables a 50 watt base wont receive them any better than a 10 watt base radio. Just something to think about. 

[marcspaz]

 

In that map, the two radios would need to be inside both respective circles.

 

If you are going to have either a permanent base station or a portable/temporary base station at a camp, you would be much better served with a 5w-10w mobile used as a base station, and a portable mast with a high-gain antenna on it.  This will increase your range due to the base antenna being higher and the gain from the antenna helps with transmit power and receive sensitivity.

 

Below are a couple of my portable antenna's for example.

 

 

 

[BoxCar]

19 minutes ago, gortex2 said:

If your all using handhelds 4 watt portables a 50 watt base wont receive them any better than a 10 watt base radio. Just something to think about. 

+1 on this. Watts is output power and has no bearing (mostly) on receiving ability. The key for both is antenna height and the higher the better. Second to antenna height is the antenna itself. Antennas have a gain measurement which translates to the amount the signal at the antenna is amplified by the antenna design both outgoing and incoming. There are two scales commonly used, the dBi scale and the dBd scale. The dbi scale is how the antenna output compares to an imaginary antenna while the dBd scale is measured against real conditions. The dBi scale can be converted to dBd by subtracting 2.1 from the dBi figure.

[KAF6045]

4 hours ago, BoxCar said:

There are two scales commonly used, the dBi scale and the dBd scale. The dbi scale is how the antenna output compares to an imaginary antenna while the dBd scale is measured against real conditions. The dBi scale can be converted to dBd by subtracting 2.1 from the dBi figure.

For purposes of visualization: dBi (isotropic) is an imaginary point source radiating in an even sphere. dBd (dipole) is a, well, half-wave dipole radiating in a "doughnut" pattern.

Also to be considered is that often the antenna gain rating may be based upon "free space", not real ground conditions. Compare:

Half-wave dipole in free space...

Same antenna over real ground conditions.

Elevation view -- while a dipole in free space is 2.15dBi, note that this example actual peaks out at 7.51dBi (this antenna is modeled at 330 ft [100m] above ground)