OBSERVATIONS FROM NORWAY                                                           LA2QAA                                                        29th January 2008

AO-7 mode-A notes.

Do please remember that the propagation of radio frequencies does not suddenly stop at 30Mhz. Satellite signals *ARE* affected by the propagation, though sometimes in quite a different way than on the lower HF frequencies.

At VHF, scintillation and Faraday rotation are the main culprits. Presumably, one already knows about the idiosyncracies of the propagation at the higher end of the HF part of the spectrum  (mode-A downlink).

A *DEFINITE* advantage in mode-A operating is having the ability to change the polarisation though due to size this isn't as easy as it is at VHF. You can also try changing your polarisation at VHF ... I'm talking about the difference between horizontal and vertical ... for LEO's I wouldn't even bother with circular ... that is my personal opinion ... (mucho experience on the Royal Signals test range at Warminster, Wiltshire in the 1960's ... the principles remain the same in 2008).

Do  remember that the propagation at any given time is probably not necessarily the same at your location and at the station you're working's location. 200 miles (or more) in distance can show dramatically different propagation anomalies ... yes, honest !!.

Also ... remember that the signal is traversing the ionosphere TWICE !!.

When I recommended a full wavelength loop I did so because it is the simplest general purpose RX aerial for 10m. If you want a "dedicated" mode-A aerial, a 10m mini beam is the best option.

There's an Irish Ham's website with an excellent design but unfortunately, I can't remember the URL. A SPIDERQUAD design with a couple of bamboo poles is to be recommended. (use your browser to find a few designs).

As Mak SV1BSX commented, a compromise would be a small magnetic loop ... though in most cases, I wouldn't recommend compromise fort AO-7's mode-A downlink.

73 John.    <la2qaa@amsat.org>