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Re: HF satellite Antenna



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

Its wrapped around the cube 12 times in a slot...

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

Thats the problem.  Nitinol comes in 6 foot lengths.
Going to the MFR we can buy a 1800 foot spool
for $1800... Ha!   Nitinol is that fancy wire that has
excellent and programable memory.  Bend it to any
shape and heat it to 900 deg and then cool.  It
will hold that shape and remember it.  Then spool
it up, and when unspooled it will return to the
original shape.  In our case, its simply straight.

Another problem is that it is quite resistive... but
we can accept some loss. and end feeding it
is a high Z process anyway...

Bob

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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