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Getting started with an omni [was: Opps!!!]



   So for now I iwll continue to work on my antennas and getting a 440 radio for 
   AO-27, HEY! PAYDAY IS FRIDAY!!!!!

   Also nobody has responded to the questions about the omni antennas I 
   mentioned either. Is this a sign that nobdy uses them or just waiting for 
   another chance to reply? The antennas in question are (1) W3KH Quadrifilar 
   Helix's from the article in August 1996 QST page 30, (2) Lindenblad antennas
   from the AMSAT web page? Any advice? Good,Bad,Ugly? or any other suggestions? 
   I also looked M2 eggbeaters but from being on a tight budget I would rather 
   spend 130.00+ dollars on part of a trackable beams system AFTER getting 
   something basic in the air first so as to start with.

The best thing to do at this point for AO-27 (or SO-35) would be to get an
'Arrow':

    http://members.aol.com/_ht_a/arrow146/arrowii.html#m2

That'll cost about half of what you're mentioned, since you won't need
a diplexer if you're using two rigs.  It's what most of the experienced
satellite operators are using for AO-27.  Just don't run more than about
10W into a hand-held antenna for RF exposure reasons.

I've tried to model a Quadrifilar Helix with the NEC2 antenna simulation
software and decided not to build that design.  I want an antenna that i
can understand and tune, and the NEC2 model i tried didn't seem very
stable numerically (and that might be a partly software issue).  I was
concerned that the dimensions for construction would be critical for
optimal performance, and at 70cm, a small errors do account for quite
a significant portion of a wavelength.

Lindenblad antenna sounds appealing to me, and is widely used in aviation.
It has a good pattern most of the time, and if you don't mind the trouble
to put one together, i'd think it to be worth considering.  Let us know
how it works out if you try it.

The 'Texas Potato Masher' is a rectangular version of an 'eggbeater',
which should have similar characteristics and be easier to build.  His
choice of using rectangular rather than square or cicular loops helps
with the impedance matching issues and vertical pattern control, so it
might even be better than an eggbeater.

Since you're using two rigs instead of a dual-bander, this may not be
applicable to you.  But i got about half-way to WAS with an FT-5100
mobile rig using a 2 meter quarterwave on my van tuned for decent SWR
in both bands and for low elevation passes, a gain omni.  The quarter-
wave such a simple antenna, but at 70cm (3rd harmonic), the pattern
happens to provide gain where one wants it for.  It doesn't work well
near the horizon (or for working repeaters) and the overhead fades are
annoying, but it about the cheapest way to get started if you're in a
quiet enough RF area and have a sensitive receiver on your dual-bander.  
Here's what the theoretical pattern for a quarter-wave ground-plane
looks like at 2 meters and 70 cm:

    http://www.qsl.net/kd6pag/nec/2mModGndP.html

But here's the biggest issue in terms of using an omni:  Is your area
quiet enough in the bands in question, and with respect the adjacent
commercial bands that may desense your receiver?  If you have a major
radio site nearby, it may not be the best choice.  You can make up for
low gain with a pre-amp, but it may make interference work.  I did well
with the mobile setup away from urbanized areas, but it was a really 
struggle at home.  Whether an omni works for you or not may not have 
that much to do with which omni you build but what your local conditions 
are.  But the best thing to do may be to get an 'Arrow'.

I'm looking for WV on satellite and hope to work you from California.

		       73's and good luck,

			    -- KD6PAG


Attachments:  Recent articles on Quadrafiler Helix, dual-band J-poles, and
	      the Texas Potato Masher

-------------------------------------------------------------------------------

From: TKVF99C@prodigy.com (MR STEPHEN G THOMPSON)
Date: Thu, 25 Feb 1999 21:53:49, -0500
To: amsat-bb@AMSAT.Org
Subject: [amsat-bb] The QHA antenna.
Sender: owner-AMSAT-BB@AMSAT.Org
Precedence: bulk
X-UIDL: ce53ed4cd9dd4790c3e2806cd6173bfd

What I need now is another project..yea right!  Like a
case of the Mung.  Actually I do enjoy homebrew projects but 
like a good book I find that I can really get caught up in
them.  Anyway, just about the time  K5OE Jerry's TPM antenna 
project showed up in the Journal I was closing out the
construction of an antenna someone had mentioned having
see on the web but I think they'd not yet built.  So having 
found the plans at the web address that was mentioned I
decided to build one for 2 meter Sat. operation.  It just so 
happens, although its a favorite receive antenna among WX
sat imaging enthusiasts, the web site included dimensions
for a 2 meter version that I'd hoped to build.  So no
scaling was involved, it had already been done.  After
having E-mailed a couple of questions like, which way is it 
polarized and does it work well on 2m for transmit?  The
answers were, Right Hand and I don't know, try it.  So I did 
and figured with the performance I found, some of you might 
be interested in a very simple antenna project that to me
ranks very hi in the performance vs time/effort/cost
ratio.  Oddly enough it turns out that the antenna also
works fairly well for local 2 meter FM operation and even
works the local 70cm repeaters, all with a safe VSWR range. 
But back to Sat. operation, this is what tickled me.  On
several occasions I've copied Mir's SSTV images with
absolute clarity till the bird hit the ground even
cleanly cutting some images right in half when LOS occured
in the middle of a picture transmission.  I'm located in
wooded North Carolina in a regular neigborhood full of the
familiar RF interferences that we all enjoy, but have had no 
problems of this type related to the antenna's performance. 
Anyway you might enjoy having one.  No overhead dead spots, 
no rotors.  Lord only knows what it would act like with a
preamp at the antenna...Wow!   Anyway check it out at:

http://www.personal.u-net.com/~pilotltd/qha.htm

73 de Steve
K5PK@amsat.org 

-------------------------------------------------------------------------------


>From AMSAT.Org!owner-AMSAT-BB Tue Mar 16 15:37:59 1999
From: John Mock <kd6pag@qsl.net>
To: "G. Keith O'Brien" <n4zq@ij.net>
cc: AMSAT-BB <amsat-bb@AMSAT.Org>
Subject: Re: [amsat-bb] Dual Band J Pole
Date: 16 Mar 1999 13:05:58 -0800
In-Reply-To: "G. Keith O'Brien"'s message of "Tue, 16 Mar 1999 08:04:45 -0500"
Sender: owner-AMSAT-BB@AMSAT.Org
Precedence: bulk
X-UIDL: 303e7542d677e1de124c5791c4d9ce3a

   Has anyone constructed a Dual Band (144/440) J Pole antenna, that they could
   supply the materials and dimensions used. Maybe you know of a Web page site,
   that has this information. Thanks for any help.. 73 N4Zq

Here's an extract from one from my WWW pages (with a copy of the linked
page).  This newsgroup is one of the few places where a dual-band J-pole
is worth pursuing.  Properly tuned, it won't hurt your rig to operate in
both bands, but the terrestrial performance at UHF is not very good.  If
you need instructions on tuning an antenna for dual band use, send me
e-mail.
			-- KD6PAG  (Networking Old-Timer, RF newbie)

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

	...

Make a 2 meter J-pole [http://www.qsl.net/kd6pag/usenet/J-pole.txt]
antenna from 300ohm twin-lead. [You will need to trim both the short and
long directions.  See also a recent QST article, which may be a better
source, and also gets into dual-band operation.] This project has appeared
in several forms, and others may be included later.  Note there is some
discussion as to which side the shield should be connected to.  I also
recall WA6TVM [now SK] suggested in the local net, the 'Antenna Forum',
using a sleeve balun with one of these.  While you can cut one of these 
to have a decent SWR on both 2m and 440, the pattern in vertical plane
probably does not make this a good general-purpose dual-band antenna, with
the pattern on the 440 side mostly at 45 degrees above the horizon
(e.g. better for satellites than repeaters).

	...

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

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Newsgroups: rec.radio.amateur.antenna,rec.radio.amateur.equipment,rec.radio.amat
eur.homebrew,rec.radio.amateur.misc
Subject: Re: Portable 2m Antenna for Mountaineering???
Message-ID: <1994Jan12.092451.1134@titan.ksc.nasa.gov>
From: Nguyent@snowmass.ksc.nasa.gov (Tom)
Date: 12 Jan 94 09:24:50 EST
References: <2gv7jp$3e0@netfs.dnd.ca>
Nntp-Posting-Host: 128.159.3.109
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Xref: agate rec.radio.amateur.antenna:1924 rec.radio.amateur.equipment:1729 
rec.radio.amateur.homebrew:1503 rec.radio.amateur.misc:52723

David,

Try a rolled up J-pole.  It's made of 300 Ohm Twin lead and tapped with a 
50 Ohm coax with BNC connector on opposite end.  It provides about 2.8dB
gain.  You can buy the ant from MFJ for  about $12 or build it yourself.  
The dimensions are below:
    -- for 146MHz center freq
                     |  
                     |
                     |
                     |
      this side	     |
           56"       |         
                     | | <---------------------------- trim  here for best SWR.
                     | |        14" on this side 
                     | |
	     coax====- -  1-3/4"   tap from bottom
                     | |
                     ---

I've experimented with lots of these  with good results (built about 30 or so 
for myself, friends, and local HAM classes).

Good luck and 73,
Tom
AD4NA

-------------------------------------------------------------------------------

From: K5OE@aol.com
Subject: [amsat-bb] Texas Potato Masher II
To: amsat-bb@AMSAT.Org
Date: Sun, 5 Sep 1999 19:02:43 EDT

For those out there who would like to build a 2 meter or 70 cm LEO antenna 
and don't need or want 3 kW EIRP, here's a couple of "new and improved" small 
satellite antennas for the do-it-yourselfers.

http://members.aol.com/k5oejerry/tpm2.htm

If you have heard me on any satellite in the last 3 months (including AO-10 
when it is close), these are what I am using (unless I'm mobile).  On 2 m I 
run about 30 Watts out into 50' of 9913F; sometimes much less on RS-13 and 
FO-29.  On 70 cm I have an ARR  preamp in the attic.

73,
Jerry, K5OE
http://members.aol.com/k5oe (home page)

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