[TowerTalk] Choke on feed point of dipole

[Jim Lux]

If you're spending millions of dollars on a system that depends on HF propagation (or lack thereof) or trying to understand the physics of the ionosphere, then the tools are pretty handy.

Practical example: I'm the project manager of the SunRISE mission (https://www.jpl.nasa.gov/missions/sun-radio-interferometer-space-experiment/) which is basically flying six receivers for 100kHz to 23 MHz just above GEO synchronous in a cluster about 10-20 km across.  It will image the RF emissions from the Sun (Type II and Type III radio bursts). 

However, we'd like to calibrate it after launch with a known source with high SNR - so the question comes up - if I put a 100W transmitter on the surface of the Earth, will it still be a "point source" after passing through the ionosphere, and how much will the direction be perturbed by the ionosphere.  Are there particular times of day or particular places in the world that work better? (Our best data on the galactic background at low frequencies comes from measurements made by Grote Reber in Tasmania with a huge array of wire antennas - it's a particularly good place, apparently, relative to the magnetic poles, terrestrial interference, etc. - who'd a thunk it?) 

We are also concerned about interference from broadcast stations (lots of which are in the 100kHz to 23 MHz band, and some have enormous EIRPs in the hundreds of MW range - 500 kW into a 24dBi antenna is impressive) - Some will make it through the ionosphere - yeah, we talk about the ionosphere being like an RF mirror, but it's not a great mirror and it's kind of lumpy.

So tools like PHARLAP are useful.



On Thu, 15 Jan 2026 12:25:40 -0600, kq2m at kq2m.com wrote:

> It's sort of like ionospheric propagation - you can use VOACAP and it's
> ilk, which make some simplifying assumptions (Chapman layers, etc.) or
> you can just use PHARLAP (which is a full on 3D raytracing).  

Or you can operate a lot and learn how propagation actually works in
real-time by observing the constant changes in path, wave-angle, signal
strength, etc. and have fun working people too!

And then, when you become a propagation master, you can start to predict
what will happen next just by listening to subtle changes in the band
noise. That's way beyond VOACAP or any propagation program that exists.

73

Bob, KQ2M