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