What should the Watts be reading at the end of a 65 foot cable run?

[CentralFloridaGMRS]

50 watts out. Checked the reading at the end of the 65 foot cable run. At about 20 feet it reads 48 watts out before the Bulkhead Lightning arrester. At the end, it's reading about half. at around 25 watts. Is this normal? See the order below for my equipment 

UHF Female SO-239 to UHF Female SO-239 Bulkhead Lightning Arrester (Not Compatible with Helium)	arrester-uhf-ff	1	$23.95
Times Microwave LMR-400 Cable build Length 46 FT

[Lscott]

If it’s real LMR-400 coax you should read about 67% of the input power at the end of the 65 foot run. That’s at 450MHz from a data table I have. GMRS is a bit higher frequency so the loss is a tiny bit higher from that factor. Crappy feed throughs and other poor coax end connectors can reduce the power more. 

[SteveShannon]

1 hour ago, WRPG818 said:

50 watts out. Checked the reading at the end of the 65 foot cable run. At about 20 feet it reads 48 watts out before the Bulkhead Lightning arrester. At the end, it's reading about half. at around 25 watts. Is this normal? See the order below for my equipment 

UHF Female SO-239 to UHF Female SO-239 Bulkhead Lightning Arrester (Not Compatible with Helium)	arrester-uhf-ff	1	$23.95
Times Microwave LMR-400 Cable build Length 46 FT

@Lscott is right. I would suggest you hook the wattmeter between every section to see where the loss is. 

[MarkInTampa]

Just guessing on the SWR

But even with perfect SWR or dummy load, not much different

 

 

I'd guess the  Bulkhead Lightning arrester might have a bit of loss. Don't know.

FYI: https://kv5r.com/ham-radio/coax-loss-calculator/

[CentralFloridaGMRS]

OK. Now it's reading higher around 33 watts to 35.  I might have had the power levels to medium on my radio. Someone said I should be getting 50 watts at the Antenna. Thought that was wrong

 

[SteveShannon]

4 hours ago, WRPG818 said:

OK. Now it's reading higher around 33 watts to 35.  I might have had the power levels to medium on my radio. Someone said I should be getting 50 watts at the Antenna. Thought that was wrong

 

Pumping 50 watts in and getting 33 to 35 watts out at the far end of a 65 foot run of LMR400 is completely reasonable at 450 MHz. The higher the frequency, the greater the attenuation. 
Don’t get this confused with SWR. It’s not the same thing. Measuring SWR at the two endpoints of a lossy cable will result in different measurements of SWR. At the radio end an SWR meter sees the full strength of the forward power but only measures the reflected power after it has been attenuated in both directions, making the antenna appear to have a lower (possibly much lower) SWR than it really is.

At the antenna end of a lossy coax cable an SWR meter measures the actual forward RF delivered to the antenna after losses in the cable.  The SWR meter also measures the actual reflected power from the antenna, yielding a more accurate measurement of SWR.

 

[Lscott]

1 hour ago, WRPG818 said:

OK. Now it's reading higher around 33 watts to 35.  I might have had the power levels to medium on my radio. Someone said I should be getting 50 watts at the Antenna. Thought that was wrong

 

That’s almost exactly what I expected for a 65 foot run of LMR-400 based on the cable loss factor. That’s a 1.76db loss.

[CentralFloridaGMRS]

Thanks all for the response. It's all good now.