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.

[Robot Matrix]

If you're looking for shorter verticals, might look at antennas with top capacitive loading hats, especially for the lower bands, but also work across the HF band, especially in mobile situations.

Even inverted L antennas or T antennas utilize capacitive loading, which when used in conjunction with a loading coil at ground level seem to perform pretty well.

Google "cap antenna". At some point,  I'll use some form of that type of antenna for 160m for transmission in the near future (I let my ham license expire and have to retake the test, so I can get back on the air without hassle).

For now, I'm using an inverted L antenna for SW/LW reception with GREAT results... especially on 160-40m. Even up to 10m, I get great results with just a 9:1 unun, which I haven't made tuneable..  YET.