[amsat-bb] Re: Icom 910H vs Kenwood TS2000

[Bruce Robertson]

On Mon, Nov 30, 2009 at 6:35 PM, Tom <k0tw at cox.net> wrote:
>
> Being a recent newbie to the LEOs it had never even occurred to me that too
> narrow a beam width could present a problem with staying pointed at those
> satellites. Thanks, Reid, for bringing that thought up in my mind. I wasn't
> taking it out of context. I was just wondering if it was an unforeseen
> problem (by me) that I needed to address. It's things like this that allow
> us to learn.
>
> It appears that a narrow beam width is not normally a problem and I thank
> John and others who pointed that out.
>

I'm going to elaborate on this discussion, for the benefit of
beginners who are considering building new stations with tracking
antennas. The narrower the beam, of course, the greater the gain when
pointing at the bird, both transmit and receive, and -- this is the
critical issue -- the lower the gain when you are pointed away from
the bird. Now with LEO satellites, some of which cross the sky in say
15 minutes, you need to have everything spot-on if your beamwidth is
very narrow: the station clock has to be accurate within a second, the
keps have to be up-to-date, etc. otherwise, your computer is telling
the rotors to point in the sky where the satellite is going to be in
five seconds, or will be in five. Especially with high passes, you can
be off by enough to not be able to hear the bird at all. So on
receive, long antennas, besides the additional expense and challenge
of mounting them, also add the challenge of getting your station
perfectly aligned, or you'll hear zippo.

On transmit, long antennas present another challenge: they
'concentrate' your signal so that it might well be excessively
powerful for the satellite in question. If my homebrew 7 - element
70cm yagi often needs to be down around 5w xmit on VO-52 to be in the
right range of effective power, how will I deal with things when I
have a 40-element beam? By all rights, I should put an attenuator
between the rig and the antenna so that I can get down under a watt!
It is my opinion, in fact, that a significant proportion of the
over-powered signals on our birds are from people in just this
situation: people using HEO antenna systems that simply can't provide
a small enough signal!

In fact, LEO satellites do not require these sorts of antenna systems
for reliable use. A beginner will be perfectly happy with, say, four
elements on 2m and 6-7 on 70cm (assuming the use of low-noise preamps,
which you are *crazy*to do without on long antennas, too). The beauty
of this system is that if a high wind knocks it slightly out of whack
in azimuth, it will not be the end of your satellite work: you'll just
have weaker signals, not silence. The other beauty of this system is
that it doesn't require an elevation rotor *at all*. Because the
elevation pattern of the antennas will more-or-less fill the sky if
you point the array up about 10-20 degrees (make it 10 if you have a
clear horizon). Now, suddenly, you've avoided all the hassle of
another rotor, you've made your array lighter and easier to work with,
and you have way less of a demand on your pointing system. Heck, if
you want to go ol' school, you can do the pointing yourself with a
twist of the dial.

These yagis do not need to be brilliantly built: mine were made with
welding rod and pine wood. They had very strange lobes off the side,
and all the rest, but they netted me lots of Q's and were very
reliable.

To be even more radical, I urge beginners to start with
omni-directional antennas and low-noise preamps. A wire dipole or a
vertical, both with almost no coax between them and the preamp, should
hear 'stuff' really well. Not Q-5, but a start. Then use this as a
baseline from which to compare the theoretical and real-world
improvement you get with your yagi array. If you aren't getting
improvement, then work out what's up.

This is not an argument against long arrays. I'm building some that I
bought from someone on this list around this time last year. I want to
do some exotic stuff like work Russia over the pole on AO-07 or hear
every last beep out of the newest cubesat. But I'm aware that in my
windy region these are going to be a bear to keep in place. So I'm
putting as much work into an omni array, too. I plan to transmit from
the latter when things get too QRO.

I guess in summary I'd say that in my opinion a big antenna array
isn't like a high-power computer, which works the same as a
lower-powered one, but has the umph when you need it; it is like
buying a high-powered plane as a new pilot: significantly more
challenging, and possibly leading to frustration.

73, Bruce
VE9QRP

>
>> I have used a Cushcraft 13B2 (13 elements) on 2m and a Cushcraft 719B (19
> elements) on 70cm on the
>> satellites with no problem.  Didn't experience either as being too narrow
> beamwidth for sat use.
>>
>> 73s John AA5JG
>>
>> I am amazed at how many people take things stated out of context on this
>> reflector. I never said that any of the arrays would not work due to
>> problem they were having hearing the satellites and it was stated that
>> if none of the other suggestions worked to consider that the arrays they
>> are using are pointed correctly in the correct direction of the
>> satellite. Pointing being off, the nulls could easily block the
>> satellite's reception. The larger the array the more this might be
>> evident. The larger the array, stacked arrays and even dish antennas,
>> pointing becomes a bit more critical.
>>
>> Reid, W4UPD
>
>
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