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Re: Antenna Questions



on 12/24/00 12:12 PM, Gerald Payton N5GPC at n5gpc@earthlink.net wrote:

> I'm sorry to keep asking "newby" questions, but I am thoroughly confused
> about setting up my station. I don't think that I'll be interested in
> the digital birds since I'd have to "learn" them as-well-as "learn"
> about packet too.

Gerald, it can be daunting!  Let me try to help a little!

> 
> This brings me to what type of antennas to buy.  I'm using an IC-820H
> radio.  I have tried to listen to various birds with and without a
> preamp attached to a dual-band Comet GP-6 up about 25 feet.  Yes, I know
> that this is not what I want, but it is all I have right now & was just
> trying to "hear" the birds.

Yeah, a vertical like that will only really work OK right near the horizon.
The reason is that the radiation pattern is designed to be directed
horizontally to the ground and not into space.  Vertical antennas are
considered to be "omni-directional" but they are really only so in the
azimuthal plane around the antenna.  As soon as you start increasing your
elevation, you lose the omni-directional effects.

A 1/4 wave vertical has a radiation pattern somewhat akin to a donut.  The
donut hole looks upward with the rest of the donut orthogonal to the
antenna.  Now as you add gain to the antenna by making it a 5/8 or have wave
or colinear array, you essentially squash the donut so that it gets thinner,
but wider.  So you decrease the signal going into space and increase the
signal going along the ground.  That's why these antenna work great for
terrestrial uses, but not so much for space!

Good job on trying though!

> 
> I have had some success, but marginal.  It seems that I will need
> directional antennas and a rotor.  Here is where I get my "brain
> block."  It seems that I'd need cross-element beams to compensate for
> polarity, right or wrong?  Years ago when I got "the bug" it was AO-10 &
> 13 and I could not afford the equipment.  What is needed now?
> 

OK, you have several options.  If you want to work just the LEO birds, you
can *get by* with egg beater or turnstyle antennas.  These are
"omni-directional" with most of the radiation pattern pointing into space.
However, they don't really have any gain and unless you have a preamp and
some power to put into them on the uplink, you won't be happy.  They are
"simple" antennas, but not all that good, IMHO.

Now, the next step would be to add Yagis.  But what kind you ask?  Well, the
BEST is the circularly polarized ones.  The KLM 22 element on 2m and 40
element on 70cm are some of the best and most popular ones out there.  But
they are expensive and KLM is no longer.  They would be hard to get used.  M
Squared makes a CP antenna now as well.  These are good, but lack
polarization switching (nice, but not totally needed).  However, if you want
to save money, a standard yagi will work fine as well, but with diminished
performance.

A CP antenna is certainly best as that's what the satellite uses.  But
again, it's not necessary.  You'll just lose some gain in the overall link
equation, but a standard yagi is far better than an egg beater, I think.

There are also multiple lengths and sizes of the yagis for OSCAR work as
well.  Bigger is better for AO-10, but for AO-40 you'll be able to get along
with much smaller antennas.

Perhaps I didn't help, but it all depends on your real estate and how much
you can afford.  I have the two large KLM antennas.  I basically stole them
from a ham who just wanted to get rid of them (otherwise, I'd be very
intimidated to drop $700 or so into satellite antennas).  I built my own
AZ-El rotor setup from some old TV rotors.  They are completely manually
controlled.  So I went more of the low cost route to start with.

> I'm sorry, but I guess that I am trying to absorb too much too fast.

No, don't worry about it.  There is just a lot to absorb!

Don't worry about preamps yet.  I have worked most of my satellite QSOs w/o
preamps.  They do help, but start somewhere and work up.  You can always add
them later.

In terms of helix antennas, I don't think anyone builds them commercially at
UHF.  But I may be wrong.  I've never used one so I can't comment on how
they work.

73,

Jon
NA9D


-------------------------------------
Jon Ogden
NA9D (ex: KE9NA)

Member:  ARRL, AMSAT, DXCC, NRA

http://www.qsl.net/ke9na

"A life lived in fear is a life half lived."

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