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Re: HF satellite Antenna



The impedance would lower, and it would become more dependant on that 
ground plane reflection to complete the antenna as you move away from a 
voltage node.  The full half wave would radiate much like a dipole with the 
proper matching circuit.

What if you created a fractal antenna that used four of the sides as a 
folded radiator that summed up to a resonant antenna?  It probably would be 
too close to the metal within, or the sides themselves, but it is "out of 
the box" thinking.

I still didn't catch how you plan to deploy the antenna.

Where would I find some of the wire, say 16.98 feet or so, so I could do 
some testing?

Mike kf4fdj

At 03:37 PM 4/8/2004, Robert Bruninga wrote:
>Well, I found a few minutes and modeled it on EZNEC and
>it looks like the impedance is a simple 225 - j35 ohms for
>the half wave end fed.  Now I just have to see what happens
>as we shorten it..
>
> >>> "Robert Bruninga" <bruninga@usna.edu> 4/8/04 2:32:18 PM >>>
>Thanks, and  Yes to all below.
>I assume then we add capactive reactance to tune out
>the inductance of the tapped coil.  Then we can also
>tune out any off resonance.  Which then gets back to the
>original question.  What is the optimum length while trading
>off length, versus efficiency, versus the penalty of length
>as having increased drag and thus shortening the
>life of the satellite...  I need to find some time to play with
>EZNEC and just see... but thought some others might
>want to try their hand...    de WB4APR, Bob
>
> >>> Mike Gilchrist <32one@earthlink.net> 4/8/04 1:54:49 PM >>>
>At 08:28 AM 4/8/2004, Robert Bruninga wrote:
> >Antenna designers note:
> >
> >Our next amateur satellite will be a 5" cubesat but it needs
> >a piano wire HF antenna for 29 MHz.  A dipole is impossible with
> >our planned deployment design due to risk of entanglement.
> >
> >Thus a monopole.  But with only the counterpoise of the
> >5" cube, a 1/4 wave monopole wont work.  We are thinking
> >of a full size ½ wave radiator end-fed.  The antenna will be
> >a .02" dia length of rigid nitinol wire.  If someone wants to
> >volunteer a matching circuit and optimum length, we'd be
> >  happy for your input.
>
>Hi Bob:
>
>1/2-wave vertical antennas "ground independent." If you feed 1/2-wavelength
>of wire at one end, the feed point impedance is extremely high. This is
>called a "voltage fed," rather than a current-fed antenna. The feed point,
>at a voltage maximum, has high impedance, and thus must use a critical
>matching network to be fed with conventional coaxial cable.  The matching
>network could be a transformer with a 2000 Ohm secondary and a tap at the
>50 Ohm point. In any case, since the natural impedance of the end-fed
>half-wave is so high, it is not influenced by the presence or absence of
>ground, hence the "ground independence."  This would also lend itself well
>to tweaking the tap point to find an ideal point, while engineering the
>antenna.  Once it is on the "cubesat," there will be little change in the
>characteristics.
>
>How will you deploy the antenna?  Off of a spool or something?  The reason
>I ask, is that you could take the first 1/2 wave element and add extra
>elements ala collinear fashion to get more gain.  It would be helpful to
>know how much "storage" and weight restraints you have on the antenna
>deployment.
>
>73, Mike kf4fdj
>
>
>
>          Mike Gilchrist
>          Disruptive Technologist
>          DayStar Communications
>          Black holes were created when God divided by zero.
>
>+  18215 Paulson Drive
>+  Port Charlotte, FL   33954
>+  m.gilchrist@daystar.net
>(  direct:      941-206-7810
>4  FAX:         941-629-4452
>J  http://www.daystar.net
>
>
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>
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