[amsat-bb] Lindenblad or DCA??? (neither?)

[Robert Bruninga]

Neither are going to hear much on the horizon for typical weak amateur
satellites where they spend most of their time.  These antennas have gain
that  is at best 3 dBi

It is very important to understand that all Low Earth Orbit sateliltes
spend more than 70% of their in-view time below about 22 degrees.  And
satellites that low are 6 to 10dB farther away than when they are high up
above 45 degreees.  See the scale drawing of an orbit on the right side of
this page:
http://aprs.org/LEO-tracking.html

The DCA and Lindenblad and Eggbeaters and all other such omni antennas are
designed to give good performance over all elevation angles for
sateliltes.  But to do this, they are equally poor in all directions.
This is wasteful since you need less gain at high elevations and much more
gain at lower elevations for LEOS (98% of all amateur sateliltes?).

So you have two choices.  Use an omni and only get high passes (less than
30% of all time-in-view), and hear only strong satelites down on the
horizon...

Or, use a terrestrial style antenna with gain on the horizon... but by
definition, these will have a null higher up, and for a typical mobile
whip, that null might be at 15 or 20 degrees which is right where
satellites spend their most time.

Best option is to switch back and forth... but then if you are going to be
sitting there, y ou may as well control a beam...

(I still have not found such an ideal antenna (though lots of claims are
made)...

On the other hand, if I am going to simply want OMNI coverage for
satellites above about 30 degree, then I prefer the simple 19.25"
quarterwave whip over a big ground plane.  Because this gives 5 dBi on 2
meters and about 7 dBi on UHF and is nothing more than a 19.25" piece of
wire sticking up from a large ground plane.  Much easier than all those
other omnis that provide less gain everywhere.

Though I have never done rigorous studies to prove its performance.  But
what could be simpler to test.  If I had any of those complex omni's., I'd
love to do such a comparison some day... after I retire...

Bob



-----Original Message-----
From: AMSAT-BB <amsat-bb-bounces at amsat.org> On Behalf Of Hans (BX2ABT)
Sent: Monday, April 02, 2018 10:16 AM
To: amsat-bb at amsat.org
Subject: [amsat-bb] Lindenblad or DCA???

Hello Joe,

Thanks for the reply, appreciate it.

Yes, the phasing harness is different, but my guess was that this
shouldn't make a lot of difference as long as you get the 90 degree phase
and the impedance right. If you look at the radiation patterns for both
then they are nearly identical, hence my question if they are not one and
the same thing.

It came up here because I have two DCAs right now, one for 2m, one for
70cm, both RHCP, no pre-amps. I do hear some birds, but signals are very
weak. The 2m one works fine when receiving the NOAA wx sats, so I was
wondering if the 30 degree angle of the Lindenblad would be better for
amateur sats. I lack the knowledge to understand the difference between,
what you say, the geosynchronous orbit of NOAA sats and the LEO orbits of
our sats. NOAA wx sats are also LEO sats, not? I'll see if Google turns up
something there.

It's easy for me to change the tilt of my DCAs to 30 degrees, so I'll try
that this week. Should be a nice experiment.

73 de Hans
BX2ABT


> Hans,
>
> Here are links with antenna patterns for the DCA:
>
> https://drive.google.com/file/d/0B0Dkk87YYl-xX2gzLXk5MHZKVU0/view
>
> and the EZ-Lindenblad:
>
> http://www.amsat.org/wordpress/wp-content/uploads/2015/08/An-EZ-Linden
> blad-Antenna-for-2-Meters2.pdf
>
> The difference is in the lengths of the "RG-59 (75ohm) phasing harness"
> and dipoles to achieve a 50 ohm impedance at the required frequency.
>
> The angle of the crossbooms affects the directivity of the antenna, in
> the examples given, 60 degrees for a NOAA satellite in a
> geosynchronous orbit and 30 degrees for a LEO orbit.
>
> The only differences I see are the 50 ohm load feedpoint and the
> Lindenblad dipoles are not exactly in the same plane as the DCA.
>
> -73, Joe Spier, K6WAO
> President, AMSAT
_______________________________________________
Sent via AMSAT-BB at amsat.org. AMSAT-NA makes this open forum available to
all interested persons worldwide without requiring membership. Opinions
expressed are solely those of the author, and do not reflect the official
views of AMSAT-NA.
Not an AMSAT-NA member? Join now to support the amateur satellite program!
Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb