Antennas in the real world.

[Lyle Williams]

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 ==--
> ---------------------------------------------
>