Clarification regarding FCC regs on output power

[Riktar]

50 watts is the max output on the channels listed for GMRS simplex and repeater channels.

I get that.

What I am wondering is if that 50 watts is specific to what is coming out the back of the radio itself or is that the maximum that can be applied from the first connection of the coax/antenna cable?

IE: If a repeater is setup using a duplexer, there is wattage lost going from the output side of the radio to the first coax connection point. Is it a general fact that duplexers soak up half of the wattage from the radio?

So even with a full 50 watt radio/transmitter, if a duplexer is used, only 25 watts are going to be available at the first connection of the coax going to the antenna yes?

 

[Guest]

That’s a good question actually. I’d bet that since you should be using gmrs cert radios for that repeater and those radios would be topped at 50w then it’s the 50watts at the radio into the duplexer.    I do know that a good duplexer does not take 1/2 your  wattage.  A good duplexer is more like 1/4 loss.  But even 25w into good coax and a good antenna won’t matter at all.  A 20w radio with good coax and antenna will easily do 200miles.  Line of sight is much more important then watts.   Most people don’t need 50watts.  Location (height)  antenna, coax, line of sight  is key. 

[marcspaz]

It's possible to build a radio with an internal duplexer.  If it meets the certification requirements, you could have the full 50w at the output.  However, I an not aware of any such radio existing at the moment (built-in duplexer and 50w).  That means you are going to be restricted to the 50w out of the radio and the ~25w out of the duplexer.

[nokones]

Accordingly to my recently calibrated Bird 43 and my new 200-500 MHz 100W 100D slug because the old slug did not pass muster on the calibration, my Vertex Standard EVX-R70 40 watt rated repeater is putting out 42 watts into the EMR Corp Duplexer and spitting out a tad shy of 31 watts from the duplexer into a Laird FG4605 omni 5 dB antenna with the tip at approx 30 feet.

 

[marcspaz]

3 minutes ago, nokones said:

EMR Corp Duplexer

 

Those are advertised as a 1dB loss... pretty good.  Mine are closer to 3dB... but they were a gift and I am thrilled to have them. 

[Guest]

On 8/18/2024 at 12:04 PM, Riktar said:

50 watts is the max output on the channels listed for GMRS simplex and repeater channels.

I get that.

What I am wondering is if that 50 watts is specific to what is coming out the back of the radio itself or is that the maximum that can be applied from the first connection of the coax/antenna cable?

IE: If a repeater is setup using a duplexer, there is wattage lost going from the output side of the radio to the first coax connection point. Is it a general fact that duplexers soak up half of the wattage from the radio?

So even with a full 50 watt radio/transmitter, if a duplexer is used, only 25 watts are going to be available at the first connection of the coax going to the antenna yes?

 

The FCC rule uses the term "transmitter output power" which is also known as "carrier power" in other subparts of 47 CFR including Part 95. That definition is "the transmitter output into the antenna or impedance matching circuit"... aka "the port on the back of the radio or transmitter". That means your 50w is to be measured BEFORE any filtering, feed line, or antenna. 

So if you have a 50w repeater with 1.5dB loss into the duplexer and 0.3dB loss in a small feed line - you've already lost a good 40% of your power - however the FCC does not designate an ERP limit which means you can enjoy the largest gain antenna you can afford/install.

However - remember ERP doesn't come out of thin air - higher gain antennas take the radiation pattern of the antenna and "squish" it - almost like thinking of your radiation pattern being a marshmallow on a skewer... now stack another few marshmallows for each dipole section and give them a squish - your power is just going more towards the horizon than "everywhere". The small benefit to this is that most of your users are going to be at the horizon so pushing the radiation pattern at 0 degree or even -1, -2, -3 degrees down if needing to cover a valley with a tall site makes the antenna a very valuable design consideration to make the most of the RF you'll get out of your system.

 

[JLeikhim]

On 8/19/2024 at 3:22 PM, NavyBOFH said:

The FCC rule uses the term "transmitter output power" which is also known as "carrier power" in other subparts of 47 CFR including Part 95. That definition is "the transmitter output into the antenna or impedance matching circuit"... aka "the port on the back of the radio or transmitter". That means your 50w is to be measured BEFORE any filtering, feed line, or antenna. 

So if you have a 50w repeater with 1.5dB loss into the duplexer and 0.3dB loss in a small feed line - you've already lost a good 40% of your power - however the FCC does not designate an ERP limit which means you can enjoy the largest gain antenna you can afford/install.

However - remember ERP doesn't come out of thin air - higher gain antennas take the radiation pattern of the antenna and "squish" it - almost like thinking of your radiation pattern being a marshmallow on a skewer... now stack another few marshmallows for each dipole section and give them a squish - your power is just going more towards the horizon than "everywhere". The small benefit to this is that most of your users are going to be at the horizon so pushing the radiation pattern at 0 degree or even -1, -2, -3 degrees down if needing to cover a valley with a tall site makes the antenna a very valuable design consideration to make the most of the RF you'll get out of your system.

 

"the transmitter output into the antenna or impedance matching circuit"

Easy enough. I have one of these Z-Match babies right after my duplexer.

 

 

[JLeikhim]

What the rules actually read:  

§ 95.1767 GMRS transmitting power limits.

This section contains transmitting power limits for GMRS stations. The maximum transmitting power depends on which channels are being used and the type of station.

(a) 462/467 MHz main channels. The limits in this paragraph apply to stations transmitting on any of the 462 MHz main channels or any of the 467 MHz main channels. Each GMRS transmitter type must be capable of operating within the allowable power range. GMRS licensees are responsible for ensuring that their GMRS stations operate in compliance with these limits.

(1) The transmitter output power of mobile, repeater and base stations must not exceed 50 Watts.

(2) The transmitter output power of fixed stations must not exceed 15 Watts.

(b) 462 MHz interstitial channels. The effective radiated power (ERP) of mobile, hand-held portable and base stations transmitting on the 462 MHz interstitial channels must not exceed 5 Watts.

(c) 467 MHz interstitial channels. The effective radiated power (ERP) of hand-held portable units transmitting on the 467 MHz interstitial channels must not exceed 0.5 Watt. Each GMRS transmitter type capable of transmitting on these channels must be designed such that the ERP does not exceed 0.5 Watt.