# Re: HF satellite Antenna

• Subject: Re: [amsat-bb] HF satellite Antenna
• From: "Robert Bruninga" <bruninga@xxxxxxxx>
• Date: Thu, 08 Apr 2004 15:37:55 -0400

```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

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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