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Re: HF satellite Antenna



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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