Power required for max DX with a 1/4 wave antenna on VHF?

[prcguy]

I was using a common web based calculator which uses this formula, some as many others.

Maybe its just a coincidence that if you calculate free space loss at 1GHz and 10km then increase the frequency 4X to 4GHz but cut the distance by 4X leaving you with the same number of wavelengths in each calculation, you get the same free space loss.

 

[JustinWHT]

I was using a common web based calculator which uses this formula, some as many others.

I still don't see how injecting the number of wavelengths enters the picture.

While the Friss Equation is ultimately accurate describing the signal loss going into the far end receiver, it erroneously attributes this to free space loss.

A clue in my reasoning is where I said signal going into the far receiver.

 

[prcguy]

I still don't see how injecting the number of wavelengths enters the picture.

While the Friss Equation is ultimately accurate describing the signal loss going into the far end receiver, it erroneously attributes this to free space loss.

A clue in my reasoning is where I said signal going into the far receiver.

The formula doesn’t specifically address wavelengths but instead uses frequency. I’m just pointing out my observation that may explain why free space loss is higher as the frequency goes up and seems to correspond with the number of wavelengths. For example if you are 1,000 wavelengths away at some lower frequency like 100MHz the free space loss will be the same 1000 wavelengths away at 1GHz, but the distances will be shorter. On the other hand keeping the distance the same there will be more free space loss at 1GHz compared to 100MHz.

 

[JustinWHT]

The Friss Equation uses the wavelength of the signal

 

[JustinWHT]

The Friss Equation uses the wavelength of the signal

You're confusing two descriptions.

FSPL is determined by the inverse square law. It defines the loss between the transmit and receive point. Frequency (or wavelength) never enters the picture.

The Friss Equation describes the loss measured at the output of the receive antenna that has an aperture defined by the square area of the signal's wavelength - the shorter the wavelength, the smaller the capture area. The Friss Equation encapsulates the wavelength in it's formula.

 

[JustinWHT]

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