LF/MF Short Vertical Antenna calculator

[tybee]

Came across this calculator, you have to enter four variables: https://people.physics.anu.edu.au/~dxt103/calculators/Rrad.php

1. Enter.antenna height in metres: 3
2. Enter frequency in MHz: 1650
3. Enter transmitter power output in watts: .01.

But I get stumped when it ask me to
Enter antenna impedance (dominated by ground loss plus inductor loss) in ohms:
It defaults to 40, but what should I be entering there?

https://people.physics.anu.edu.au/~dxt103/calculators/Rrad.php

[RobRich]

A quarter-wave vertical over a lossless ground plane would be around 36 ohms.

A lossless ground plane is highly unlikely whether using on-ground ground radials or elevated ground radials. The additional ground loss adds to the 36 ohms value, though again, if assuming a quarter-wave vertical radiator.

The "ideal" 36-ohms number can change considerably for added loading coils to shorten the vertical radiator, capacitive coupling, and various other potential system losses.

I suspect the 40-ohm default value is assuming more like a 20-ohm radiator with an additional 20-ohms of ground loss. You can model the antenna to get a better idea, but for simplicity's sake, the 40-ohm value is a generic enough ballpark starting point for a basic short vertical antenna design IMO.

Vertical radiator length, radial lengths, loading, capacity hat sizing (if present), feedpoint matching, etc. can be be tweaked once deployed to change system resistance if needed.

[tybee]

Just of note, the default of the impedance is set for 40,.. but note also the default for the ant length is set at 10 meters, the frequency  default is 0.475 and the transmitter power is set default of 5 watts, so I'm guessing that the impedance default is not what it should be for 0art 15.

Top be honest, although I think the calculator is cool, I wouldnt know how to interpret its results anyway.