Subject: Re: [amsat-bb] HF Satellite Experiment?
Bob - Calculate the resistance of 1000M of whatever wire you are using at the frequencies you are operating on. It will be low. The resistive component of the impedance will dominate the reactive component, so a simple transformer will do.
Turns out I just learned today that the tether is going to be about 2k ohms. That's 2 ohms per meter or Huge! For the first half wavelength at 7 MHz, this is 40 ohms or a nice dummy load...
Putting that into EZNEC changes the antenna from the expected long-wire cone with 14 dBi gain straight down to a nearly perfect Dipole pattern (off the sides) with a gain of - 64 dBi and a feed point impedance of 50000 +j36000. In other words, only the first 1/2 wave is really doing anything as far as pattern.
Again, these are only 1st cut assessments(we haven't really modeled the actual cubesat structure yet), but this high resistance of the tether is really going to need some looking at...
Bob, WB4APR
Cubesat HF experiment:
We have revised our HF satellite experiment proposal to be a CW telemetry beacon on 40, 20, 15 and 10 meter bands. The experiment will last only a few days, since the deployed 1000m tether will have so much drag, we will de-orbit in only a few days.
The idea is a keyed CW oscillator on 7.010 MHz with harmonics on 14.020, 21.030 and 28.040 MHz. These bands are all in the IARU Satelilte allocation. We will filter all harmonics above 28 MHz.
The challenge will be how to feed a 1000m long wire from only a 4"x4"x7" counterpoise (on all bands). transformer coupling?
The tether will be vertical and can act as a long wire antenna with a very narrow cone angle pointed straight down. Antenna gain will be more than
10
dB on all bands. This moving donut gain pattern sweeping across the earth should pose some interesting reception reports (If any of it gets through the ionosphere).
Launch Opportunity no earlier than March 2012. Putting us closer and
closer
to the Solar Max which would be the worst time for this experiment.
Other than a nice AMSAT experiment, if anyone can use this for valid ionosphereic science, come join us.
Bob, WB4APR
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-- KK6MC James Duffey Cedar Crest NM