Antennas in the real world.

[Kris Amy]

Hi, I have a 16 slot and a 32 slot waveguide, best option was to get the
holes laser cut to make sure that they were right.

Kris
On Sat, Dec 21, 2002 at 12:29:58AM +1100, Lyle Williams wrote:
> 
> 
> Feeding of the four helicals was stolen from the ARRL Antenna Handbook.  The
> basic idea was to drop the feedpoint impedence of a single helix from 140
> ohms to closer to ~50 ohms.  All helixes were aligned identically on a
> square aluminium reflector.  Equal lengths of wire linked the helixes to a
> central point behind the reflector, where a N-connector led to a short bit
> of LMR-195 and a RP SMA.
> 
> On the humerous side, having built 8 helixes in a batch, I found that I had
> somehow made one #$%^ left hand circular polarised.  I have no idea how I
> wound one backwards without noticing.  Anyway, that one went into the bin,
> and I made a ninth one RHCP.
> 
> Performance certainly wasn't worth the construction effort.  The length and
> weight of the helicals makes them hard to mount.  I have an unguyed 7m mast
> with a vertical antenna for 144/430MHz on the top.  Mounting the 180 degree
> slotted waveguide under the existing antenna would be really simple, and
> create negligible torque to topple/bend the mast.  Also, it probably will be
> easier to get mast space around town if you don't need to be on the top of a
> mast.
> 
> 
> 
> Lyle Williams
> VK1XLW
> (Still hoping an angel with a computer controlled milling machine would
> appear out of the blue...  :^)
> 
> 
> 
> 
> 
> 
> ----- Original Message -----
> From: "Jason Hecker" <jhecker at www.wireless.org.au>
> To: "Lyle Williams" <lyle.williams at bigpond.com>
> Cc: <wireless at lists.samba.org>
> Sent: Friday, December 20, 2002 11:25 PM
> Subject: Re: Antennas in the real world.
> 
> 
> > Ahhh,
> >
> > You went crazy whacked four together.  How did you go linking them
> > together?  What did you use as a splitter?  Did you use matched cables
> > lengths from the splitter?
> >
> > I have seen the data speaking of optimistic results with helicals with
> > Kraus's formulae bing 3-4dB too high.  ;(
> >
> > j
> >
> > On Fri, 20 Dec 2002, Lyle Williams wrote:
> >
> > > Well,  I ran around with two HUGE quad helicals experimenting with
> antenna performance.  On paper, the 4x28 turn monster was ~27dBi and the
> 4x17 turn ~24dBi.
> > >
> > > Paper is an exceptionally good propagation medium - I recommend it
> wholeheartedly.  Unfortunately my radio waves had to get through air, rather
> than paper.  Real world gain on the helicals was nothing like the paper
> gain.  The original gain formulas published for helicals were too
> optomistic, and more modern formulas discount the gain by about 5dB.  Less
> another 3dB if working circular polarisation into linear polarisation.
> > >
> > > Implementaion losses probably crept in here too, but I did spend quite a
> bit of time trying to get the helicals right.  Test paths from Gungahlin to
> Black Mt and Gungahlin to Mt Ainslie came up OK.
> > >
> > > I built a pair of 123x123mm biquads with 30mm skirts, as described at
> http://www.trevormarshall.com/biquad.htm .  These perform very well, and
> given the ease of construction, seem like an excellent choice for starting
> out.  The biquads worked over the paths mentioned above too.  They also
> occupy about 1/250th the volume of the helical arrays.
> > >
> > > I am taken by the slotted waveguides described at
> http://www.trevormarshall.com/waveguides.htm . By flattening the radiation
> pattern to a narrow area close to the horizon, these antennas give
> reasonable gain over 180 or 360 degrees (depending upon the design)  WA
> FreeNet seems to be getting excellent results (25km paths between omnis!)
> out of similar designs.  This morning I picked up the parts to make four 16
> slot (or 16+16 slot) waveguides.  Damage was around $150.  The major
> construction challenge seems to be cutting rectangular holes in the
> aluminium.  Has anyone got any tips for doing this?  Alternately, a
> waveguide design that allows for slots with curved ends would be nice
> (easier to cut with a router)
> > >
> > > All tips and pointers gladly accepted!
> > >
> > > Lyle Williams
> > > VK1XLW
> > >
> > >
> > >
> > >
> >
> > --
> > Cheers,
> > jASON
> > ---------------------------------------------
> > --== http://www.wireless.org.au/~jhecker ==--
> > ---------------------------------------------
> >

-- 
Kris Amy

System Administrator,
UCB Australia

+---------------------------+
| Email: kris at amy.id.au     |
| Phone: +61 411 202 258    |
| Msn: linorz2k at hotmail.com |
| ICQ: 38459156             |
| GPG/PGP: B42267528B29886E |
+---------------------------+
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 189 bytes
Desc: not available
Url : http://lists.samba.org/archive/wireless/attachments/20021221/b5a03e9a/attachment.bin