"Jim Lux" wrote in message
...
Jerry wrote:
Hi Jim
It isnt clear to me that you read Howard Kowall's original post. He
intends to build his own antenna to communicate with low earth orbiting
satellites. I have information that will allow Howard to design and
build his own antenna that performs better than the design he chose
(Lindenblad).
Do you disagree that a DCA will perform better than a Lindenblad?
Nope.. haven't actually looked at it. BUT.. the thing I was pointing out
is looking at the overall system design, (for which design complexity and
tolerances are factors that need to be considered), it might not matter.
I have read many of your post and recognize that you are a smart guy
with lots of information about antennas. Thats why I wonder why you'd
write something as stupid as " And, for that matter CP is probably not
worth worrying about". You know that 3dB *is* normally something to try
to achieve while building an antenna.
Not if you've got plenty of link margin already, or if there's an easier
way to get the margin (e.g. rather than get 3dB more on the antenna,
shorten the feedline from 100 ft, etc.). Howard didn't say which 2m
satellite he's looking to listen to, or what kind of receiver he's using.
The reason you write that the circular
polarization is minimally significant seems to be that you are attempting
to minimize the value of the DCA. I wonder if you have any facts or
data, measured or calculated, to demonstrate that you know of any antenna
that performs better than a DCA for ground based reception from LEOs
Nope, that's not what I said. What I said was that sometimes, striving
for perfect axial ratio isn't worth it. A linear antenna will have 3dB
loss against a perfect CP, and that's a worst case. It might well be that
3dB is ok (for receiving WESAT on 137 MHz, for instance, where they have
pretty big EIRP, it wouldn't matter)
I agree with you that a Turnstile is a good antenna for LEO satellite
communication from Earth. But, I also claim that a DCA will perform
better than a Turnstile. Do you disagree?
No, I don't disagree or agree. Don't know how well a DCA does or doesn't
do.
Can you tell me more about why you wrote "For LEO satellites, you
don't
really want a hemispherical pattern anyway. You want something with more
gain at the horizon where the slant range is much greater (thousands of
km) than at zenith (where the range is hundreds of km). That is
precisely what I tried to address in the QST article. That is
precisely why the DCA performs better than all others. Besides, the
DCA is relatively easy to build , unlike the Quad helix.
My comment was general, on what sorts of patterns one might want for a
satellite antenna in a fixed position to communicate with LEO.
And, yes, your DCA is easy to build and probably non-critical in
dimensions and tolerances (have you checked this? either by modeling or
measurement?) But so is a turnstile or a turnstile with reflector or a
Lindenblad or even a quad helix, depending on how much variability you're
willing to tolerate
I will readily concede that building a quad helix for VHF is a mechanical
problem, compared to say, 1.5 GHz for GPS. It's going to be a physically
large structure (about the size of two gallon paintcans stacked), but if
you have a cookbook design (as in, buy X feet of aluminum rod or copper
wire, wind it around a plastic trash can, etc.)
I've built monofilar and quad helixes (and Lindenblads and turnstiles)
using copper foil tape on plastic buckets, rolled up paper, and all sorts
of things. Some work better than others, but mostly, it's mechanical
issues that are important. The "RF performance" is pretty much the same
for a given physical size. After all, for an "omni" sort of antenna close
to the ground, there's lots of other factors that probably have a bigger
effect. (which was where I started..)
Again, do you know of any antenna design that performs better than a
DCA for communication with LEOs from earth and doesnt require pointing?
How does one define "better"?
Is your article in QST posted somewhere? Got a NEC deck? (Can't get it
from ARRL because it's too new). Or, heck, rough dimensions and angles,
and I can build the NEC model. (googling KD6JDJ DCA doesn't turn up
anything useful.)
As you know, it's very challenging to get CP with good axial ratio in all
directions (sort of an extension of the hairy ball theorem). For that
matter, the axial ratio of the signal you're receiving may not be all that
hot.
Taking GPS as an example, the SVs have a spec that the axial ratio is no
worse than 1.2dB within 14.3 degrees of boresight for L1, and 3.2 dB for
L2.
I couldn't find any convenient data on ham satellite antennas. I think
AO51 uses some variant of a turnstile with separate ports for the two
transmitters, so one is LHCP the other RHCP. I did find a rough link
budget for AO51 (aka Echo) that shows path loss varying by about 8-9 dB
from zenith to horizon.
If we look at state of the art (at least in the 70s) for deep space
exploration, the Low Gain Antenna on Galileo (CP for 2.3GHz) had an axial
ratio of 2dB on boresight, and 11dB at 90 degrees off boresight.
See Bill Imbriale's book at
http://descanso.jpl.nasa.gov/Monogra...rce_external=0
(Volume 8)
for more details and lots and lots of measurements.
Jim
Hi Jim
I wont interlace my reply so that it might be easier for us to read.
You wrote that "it might not matter that a DCA performs better than a
Lindenblad" Well it does perform better for contact with a LEO satellite
and I'd expect that be enough to make the DCA worth considering. Maybe I am
missing something. Why would you *not* try making DCA for LEOs??
What (exactly is your point in writing about eliminating line loss when
the discussion is antenna sensitivity??? Why is it pertinent what LEO he
is interested in or what receiver he uses??? He asked about coax for an
antenna harness. I thought I was helping Howard when I pointed him toward
the DCA. Jim, if you dont need the DCA design concept there is no need for
you to consider it. But, please dont diminish the value of the DCA for
LEO use unless you have facts or data to show where I'm wrong about how well
the DCA performs.
Jim, are you writing that antenna sensitivity "doesnt matter" when
receiving NOAA weather satellite signals at 137-138 MHz? If so you are
completely wrong. Oh, you personally may have no interest in reception
from the NOAA satellites (APT) as low elevations. but, when recording
images of the Earth from NOAA satellites (APT) there is *no* antenna that
performs better than a DCA. And, that extra sensitivity using the DCA is
highly desired by most APT imagers.
Yes, I have made lots of measurements of the DCA. I made my own slotted
line so I could know the antenna's impedance. I have hundreds of actual
(measured) radiation patterns. I have plenty of EZNEC models of the DCA.
As I posted earlier, Patrik Tast posts lots of fundamental design
information on his Web Site
http://www.poes-weather.com/index.php .
Clearly, I am proud of the results I have realized with this DCA antenna
design project, so I'm always happy to share it with anyone interested.
Anyone interested in the DCA has always received answers to any/all
questions sent to me.
I define better performance as greater sensitivity to signals from LEO
satellites. But, the DCA has a much wider impedance match than a Quad
Helix.
I was pleasantly surprised that the axial ratio of the DCA radiation is
exceptionally good at most angles. EZNEC gives good prediction of AR at all
angles. Patrik Tast's Signal Plotter records the antenna sensitivity at 1
second intervals while the NOAA satellite is above the horizon. As I
remember, you acknowledged that I have developed a method of recording
actual radiation patterns using the program Patrik developed for me
(SignalPlotter).
I submit to you that, if you ever have need to develop a hemispheric
coverage antenna for CP signals, you could benefit from learning about the
DCA. It works.
Jerry KD6JDJ (who sincerely wants to know
facts about the flaws in the DCA design)