Effective Radiated Power (ERP) Calculator

[LeoG]

I found this calculator searching around the internet.  The only thing is minimum baseline is LMR-400 cable.  I was hoping it would go down to RG58 so you could make comparisons.

https://www.antennas.ca/calc_ERP.htm

 

My little system with 6.4 watts out, 50' LMR400 to an antenna with 7.2dBi of gain radiates and ERP of 24.5 watts.

[Socalgmrs]

Nice my base station ays 289 and my trucks is 115. 

[LeoG]

That sound pretty nice.  So 50w transmitter, 50' LMR400 antenna with a gain of 9?

[WSAM454]

Well, not wanting (or able to) get into the actual math, LMR 400 has an approximate loss of 3dB at 450 MHz, and RG8X has an approximate loss of 9 dB.   That is a 6 dB difference, which translates (roughly!) to 1/4 the power level.   So an ERP of 40W using LMR 400, according to the chart,  will  have an approximate ERP of 10 W using RG-8X.

[LeoG]

Right now what was sent to me is RG8/U with the kit I got and I'm waiting on my LMR400 before I actually put the antenna in the air on a semi permanent basis.  I have two spots it might go 2 miles from each other.  Want to find which one will work best for me.  I guess the antenna gain is almost eliminated by the RG8 coax run.

 

[SteveShannon]

28 minutes ago, WSAM454 said:

Well, not wanting (or able to) get into the actual math, LMR 400 has an approximate loss of 3dB at 450 MHz, and RG8X has an approximate loss of 9 dB.   That is a 6 dB difference, which translates (roughly!) to 1/4 the power level.   So an ERP of 40W using LMR 400, according to the chart,  will  have an approximate ERP of 10 W using RG-8X.

That 6 db difference is true for 100 feet of LMR400 compared to 100 feet of RG8x, but for any other distances the losses must be scaled up or down according to the distance. For instance, at 200 feet, the losses through LMR400 will be about 6 db (3/4 of the power is attenuated), but the losses through RG8x will amount to 18 db, meaning only 1/64 of the RF power reaches the antenna; 63/64 is attenuated. Putting that into ERP values with a 6 db antenna and 50 watt output radio, 12.5 watts makes it through the LMR 400 to the feedpoint. Then the antenna adds 6 db gain so the ERP is 50 watts.  For the RG8x, 50/64 watt makes it to the antenna (0.78125 watts) and the ERP is 3.125 watts.

[LeoG]

Maybe this would help

https://www.noisemeters.com/apps/db-calculator/

[WSAM454]

Thanks for the clarification, Steve.

For followers of this thread, I have found this site after reading your posting, and since it offers more coax choices, it may be more useful than the OP's.

Scroll down near the bottom and there is a set of calculators:

 

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

 

 

 

[LeoG]

That's a nice calculator.  You have to do it in 2 steps to figure it out.  Since my actual cable isn't listed, RG58/U, I used the Belden 8240.

Using that cable gave me an effective output of 11.8 watts vs 24.5 with the LMR.  Quite the difference.

 

Thanks for the calculator.

[WRKC935]

The physical math for this stuff isn't as hard as you might think.

Calculating ERP the 'easy' way.

First thing is to convert your RF power in watts to dBm.  Lots of phone apps that will do that for you.

Then you subtract the cable loss in dB from the dBm number from the above conversion.

Then ADD the antenna gain in dB to that number. 

Take the resulting number and convert it back to watts from dBm.

That's all there is to it. 

 

Now there is a second part to it if you are so inclined to do it. 

Mind you this is a comparison number and not a 'true' ERP number. 

But for every doubling in antenna height, you 'gain' an additional 6dB.

Now of course, cable losses increase with this, but it's pretty much a standard calculation used by RF engineer types.  So that can be applied too if you are trying to figure out how much better having an antenna further up than you already have it.

 

[SvenMarbles]

I use this one. https://kv5r.com/ham-radio/coax-loss-calculator/
 

This one will take into account, all varieties of coax at different lengths, actual frequencies (467, not 450), input wattage, your SWR,  and antenna gain. 
 

Don’t confuse DBI and DBD. Deduct 2.15 from your antenna’s dbi rating for the dbd. 

[jwilkers]

I found this calculator searching around the internet.  The only thing is minimum baseline is LMR-400 cable.  I was hoping it would go down to RG58 so you could make comparisons.
https://www.antennas.ca/calc_ERP.htm
 
My little system with 6.4 watts out, 50' LMR400 to an antenna with 7.2dBi of gain radiates and ERP of 24.5 watts.

Gain does NOT increase your ERP!

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[LeoG]

Gain does not increase power would be correct.  Gain does increase where the power goes.  Zero gain is an antenna that radiates equally in a 360º sphere.  Most antennas don't do that.  An "Omni directional" antenna is really a lie, but in a good way.  It still radiates in a horizontal circle (approx) but the beam of energy radiating from the antenna is cut off on the top and bottom so that power is all included in a beam that is 20 or 30 or whatever degrees in the vertical thereby increasing the effective power in that area.  You aren't wasting it going straight up or down.  It is being put to better use in the horizontal plane that it will be received in "effectively" giving you the result of more radiated watts.

Gain doesn't multiply your radiated power, it directs it to a more useful area.

 

Better?

[SteveShannon]

18 minutes ago, jwilkers said:

Gain does NOT increase your ERP!

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That’s completely wrong. To calculate ERP you take the RF power input to the antenna and apply the gain. 

[dosw]

22 minutes ago, jwilkers said:

Gain does NOT increase your ERP!

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Interesting suggestion but it defies the explanations provided in amateur radio study guides (here's one: https://www.kb6nu.com/extra-class-question-of-the-day-effective-radiated-power/ ), and in Wikipedia (https://en.wikipedia.org/wiki/Effective_radiated_power ).

Putting "not" in caps and adding an exclamation point doesn't make an incorrect statement true. But I could be persuaded that all the reputable sources of information are incorrect if a persuasive argument were made with supporting proof.

[LeoG]

I am assuming he means you aren't getting more actual "watts" from gain which is true.  That's why it's called "Effective" Radiated Power and not True Radiated Power.

[SteveShannon]

Here’s what the FCC says:

[LeoG]

The FCC must be wrong

[SvenMarbles]

Here’s a simple way to explain antenna gain to children. 
 

take a garden hose and turn it on upright in your fist. The water will gush out as you’d expect it to. That’s a zero gain scenario. Now put your thumb on it to spray it further. That’s antenna gain . There’s no net increase in water volume, you’ve just concentrated it in a direction, and in doing so it’s pretty effective..

ERP is a metric to assume how, that now more concentrated and directed water, is behaving supposing that you hypothetically accomplished that same “splash out” by increasing your net power instead of concentrating it…
 

 

[BoxCar]

2 minutes ago, SvenMarbles said:

Here’s a simple way to explain antenna gain to children. 
 

take a garden hose and turn it on upright in your fist. The water will gush out as you’d expect it to. That’s a zero gain scenario. Now put your thumb on it to spray it further. That’s antenna gain . There’s no net increase in water volume, you’ve just concentrated it in a direction, and in doing so it’s pretty effective..

ERP is a metric to assume how, that now more concentrated and directed water, is behaving supposing that you hypothetically accomplished that same “splash out” by increasing your net power instead of concentrating it…
 

 

Another way -

Your antenna radiates your signal in a pattern that looks like a beach ball. If you push the ball on the top and bottom, the sides squirt out. The amount you squeeze is the antenna gain.

[jwilkers]

That’s completely wrong. To calculate ERP you take the RF power input to the antenna and apply the gain. 

Negative. See the first reply to my comment. His explanation is spot on.

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[jwilkers]

I am assuming he means you aren't getting more actual "watts" from gain which is true.  That's why it's called "Effective" Radiated Power and not True Radiated Power.

There are no more watts emitting from the antenna. A 25 watt signal is still.25 watts, regardless of antenna design. Gain just indicates how that power is used.

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[jwilkers]

Here’s what the FCC says:

Broken link

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[jwilkers]

The FCC must be wrong

The FCC is always right. Mostly.....

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[SteveShannon]

4 minutes ago, jwilkers said:

Broken link

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