How strong of a signal to reach a repeater from inside a house

[LeoG]

Here's another question I don't actually expect an answer to but figured I'd ask anyway.

I have a repeater about 17 miles away from my house and it's 10 miles from my sons house.  I was surprised that both of us can be inside our homes and talk somewhat reliably with 4.5 watt HTs.  My house has aluminum siding, his doesn't.

Anyone gander a guess at what the minimum signal might be to have this work?  dBm or µV would be nice to know.  I had one search produce a result of -70dbm which seems very strong, but I think they were talking about WiFi so in the GHz range.  What I'm trying to do is calibrate (somewhat) Radio Mobile to what it's showing me and what I'm actually receiving.  I know I'm not at the minimum because communication is fair to good, not sproratic or choppy.

Anyone want to gander an educated guess?

[WRKC935]

First thing to understand is what is fair to good signal level in the radio.

-95 dBm is whats required for an ERRS for in building public safety coverage.  That's a noise free solid signal.

-105 is the very beginning of noise in the receiver on a WIDE BAND FM signal

a typical radio will hit 12dB Sinad at somewhere around -112 to -115 dBm  that should be a full copy signal with noticeable static in the background.

 

So now we get to structure attenuation.

And that's going to depend on the structure, and the materials used. 

Assuming here that you have a standard stick build (wood 2X4) framing.  Drywall interior finish and vinyl or wood siding exterior.  The reason I know it's not stucco is that is applied to chicken wire which has MORE attenuation than the vinyl siding.  Metal siding is higher yet.  And the house is a bit older as the windows are probably not E-glass which has a very high attenuation level, comparable to sheet metal.

Stick build vinyl is going to have somewhere around 10dB of attenuation, stucco is around 20dB and metal is going to be 30dB plus.  I have walked building that were sheet metal exterior next to a transmit site (less than a mile) and had no usable (less than -105dBm signal level inside the building) with a -50dBm signal outside. 

And hearing the repeater isn't what really tells me the attenuation level.  It's the fact you can talk to the repeater.  The portable output signal is down at LEAST 12 dB from the repeater, assuming that you are running at 50 watts out, losses getting that down 4 to 6 dB in the cable and duplexer, and a modest gain antenna.  The portable has 4 watts, and no real gain from the antenna. 

It's all numbers.

power out - duplexer loss - cable loss + antenna gain = ERP

ERP - path loss - building attenuation +/- antenna gain = signal in the radio

All that should be calculated in dB.  Convert the power output from watt's to dBm and then run the numbers.  They will be right as long as the information is correct and will never lie. 

 

[LeoG]

Then I'm going to guess that I have a -105dBm to -100dBm signal in the house.  Full aluminum siding, normal stick built with drywall and old single pain windows with no E coating.  My son likely has a better signal inside his house since he's closer and has wood siding, no metal shield surrounding his house acting as a partial Faraday cage.

So if your guess of -30dB to get through my siding and walls then likely I have a -75 to -70dBm signal outside my house, which is pretty strong.

Let me throw that in the calculations and see if it makes any sense when I apply it to other situations.

[SteveShannon]

@LeoG @WRKC935’s analysis above is excellent. I’m go8ng to mark if somehow so I can find it again.

What’s your roofing material?  What are your doors made of?

You can have the walls wrapped entirely with aluminum siding, but given the regular glass window panes and possible door materials and very small wavelength of GMRS the siding might not matter as much as one might expect. 

[LeoG]

Asphalt and 3 wood doors and one metal.  I have another repeater that makes it into the house but I can't hit it from inside the house.