You could cut up a military surplus RF blanket :-) For those who don't
know, you cover the radome of an aircraft with this when the radar is
transmitting, so you don't cook anyone nearby. I never knew how
effective it was, but I didn't walk in front of the aircraft, just to
be safe.

Joe Gray
W5JG

On Wed, Apr 4, 2012 at 6:53 AM, Michael Baker mpb45@clanbaker.org wrote:

You can get inexpensive conductive foam from Amazon.

On 4/4/2012 5:53 AM, Michael Baker wrote:

So it can be done: try to emulate the Zephyr GPS antenna with the RF
absorber.

On Wed, Apr 4, 2012 at 3:56 PM, Robert Berg bob.b@pobox.com wrote:

On 4/4/12 6:56 AM, Robert Berg wrote:

Not all conductive foam works as a decent RF absorber.  If the
conductivity isn't well matched to 377 ohms, then the RF reflects right
off of it.  The black foam that ICs used to come in is a good example.

The sheet RF absorber (as opposed to the pyramidal kind) typically has
multiple layers of conductive sheet separated by a fairly lossless foam,
with the conductivities and spacing of sheets chosen to optimize the
absorption for a particular frequency range and angle of incidence. As
with any RF load, the important thing is the "match".

Pyramidal absorbers (like you see in an anechoic chamber) make use of
"cones" (so there's not a real sharp "transition" in impedance), and for
higher frequencies, the reflections head down deeper into the valleys
between the peaks.  (at least for angles of incidence close to normal).

All of the absorbers have very different properties at grazing angles
than they do at normal incidence.

And, what you might be seeing is actually a magnetic absorber to
suppress creeping waves along the surface.  It's a ferrite loaded
elastomer. We use a lot of it at work, for instance, around the outside
of a corrugated horn to suppress back/side lobes.

There's a new "choke ring" style antenna (patented, of course, and they
deserve it) which uses spikes instead of solid rings. And, they wrap the
choke over a hemispherical surface as opposed to on a plane.

Much tougher to design and fabricate  (no buying sets of cake pans any
more), but if you want to differentiate yourself from the horde of
Ashtech/D&M style chokes, you need something.

At JPL, we also use what's called a "helibowl" for ground testing.  It's
a quad helix or other element inside a bowl.  Doesn't have much pattern
close to the horizon.  I suspect you can google and find more details,
or if people are interested, I can ask around about design information.

If foliage does such a number on GPS signals, just fill a big garbage bag
with yard debris and set the antenna in the middle of that.

On 04/04/2012 05:53 AM, Michael Baker wrote:

--
Chuck Forsberg WA7KGX N2469R    caf@omen.com  www.omen.com
Developer of Industrial ZMODEM(Tm) for Embedded Applications
Omen Technology Inc      "The High Reliability Software"
10255 NW Old Cornelius Pass Portland OR 97231  503-614-0430

On 4/4/12 5:53 AM, Michael Baker wrote:

This material may or may not be what you want
http://www.chomerics.com/products/CHO-MUTE.htm

Parker-Chomerics does do samples  (probably a 10-15cm square piece
that is mailable)

http://www.lairdtech.com/ is another supplier

http://www.masttechnologies.com/magnetic-vs-dielectric-absorber-selection/

http://www.emc-products.dk/

http://www.arc-tech.com/

http://www.aemi-inc.com/  - pyramidal absorber
http://www.ets-lindgren.com/Absorbers

If you get the blue pyramidal absorber, get the kind that has the
waterproof coating.. not only is it more rugged, it doesn't shed as much
of that horrible blue/black conductive dust.  (we had a roof leak in one
of our indoor ranges at JPL a few years ago, and wound up having to buy
all new absorber.. turns out that the fire-proofing and conductive salts
wash out..)

Even if you have to buy the stuff, it's not all that expensive, usually.
Last time (5 years ago) that I bought pyramidal absorber it was
something like $20/2x2 foot panel.  The elastomer isn't a whole lot more
expensive: it's made in huge quantities and the raw materials are cheap.
The dominant cost is the packaging/shipping & handling.

the ferrite loaded elastomer is quite dense.  It reminds me of the lead
apron they put on you when you get dental x-rays.  It's thin and floppy,
but surprisingly heavy.

Once you get a handle on the names and kinds of stuff you might want,
you can probably find a way to get it.  You might find it at somewhere
like Newark or Allied.

On 4/4/12 7:35 AM, Chuck Forsberg WA7KGX N2469R wrote:

wet yard debris..

Maybe get some a wide tupperware dish and fill it with water and just
a little salt.  It may not be as portable but it would turn RF into
heat well enough.  The trick is to add sale until the water will just
conduct RF so you need an RF signal generator and a pair of metal
probes and a way to measure RF current

In all cases if you build this using any material, water, foam,
conductive paint or whatever you will need to have some way to measure
its effectiveness.    How to measure GPS multi path resistance?

Better and faster way to get good performance is to buy a 10 foot
section of 3/4" pipe and a pipe flange.  Place your GPS antenna on the
flange and then attach the pipe to the tallest thing around.  Then
there will be no reflectors that are above the antenna's horizon and
no multi path.

Surveyors need the special antenna because they work close to the
ground and can't choose a good antenna location.  You are not a
surveyor and can simply move the antenna.

On Wed, Apr 4, 2012 at 5:53 AM, Michael Baker mpb45@clanbaker.org wrote:

That should work, but if you live in thunderstorm country, making your antenna the tallest thing around can come back to bite you.

On Wed, Apr 4, 2012 at 10:31 AM, beale beale@bealecorner.com wrote:

people say this all the time but many ham radio operators have antenna
masts that are very tall and actually survive storms.  All TV and
radio stations do too.    And then I owned a sail boat for years along
with thousands of others.  and it does not take much of a mast for
you to be the tallest thing on the water.    The trick to being safe
is to make sure the antenna has a VERY low impedance path to a good
ground.  On my boat I had a 60+ foot aluminum mast that was over a
6,000 pound cast lead keel and the two where electrically connected.
So one end of the system had a good saltwater ground.  It is hard to
do that well at home but you can come close.

People will argue that if you ground the pole it then will become a
"lightening magnet"  That thinking is 180 degrees backward.  A pole
becomes a lightening magnet if it is allowed to charge above ground
potential.  So for most of us who don't live in Florida and #10 wire
clamped to the mast and run off to a ground rod is good enough.  It is
also required by most building codes.  Even outdoor TV antenna have
the same code requirement.

One more argument:  Let's agree the a tall grounded pole is a
lightening rod.  then if the house is going to be struck would you
prefer to have the rod or not?  Without it, the lighting will find
some other patch like a furnace exhaust vent or whatever

So I guess I'd say that a GPS on a mast is not worse than any other antenna.

--

Chris Albertson
Redondo Beach, California

Hi Mike:

I used military surplus radar absorbing material to stop reflections on a satellite antenna by applying it to the rain
gutter, see:
http://www.prc68.com/I/Images/SB_angw.jpg

A sheet of this stuff about 4 x 6 feet weighed maybe 40 pounds.  An easy way to confirm it works is to use it to block a
receiver antenna and see how far the signal goes down.

Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/Brooke4Congress.html

Michael Baker wrote:

Hi Brooke,
RAM mat is a good idea, but I'm not sure that signal blocking is a good test, it could just be reflecting. A better test is to put a small piece in a microwave along with a cup of water (as dummy load) and see how hot it gets. Best done at low power and short time when household manager is not around. The smell if burning rubber in the microwave may not go down well.
 
Robert G8RPI.

From: Brooke Clarke brooke@pacific.net
To: Discussion of precise time and frequency measurement time-nuts@febo.com
Sent: Wednesday, 4 April 2012, 21:28
Subject: Re: [time-nuts] Improving performance of a GPS antenna...?

Hi Mike:

I used military surplus radar absorbing material to stop reflections on a satellite antenna by applying it to the rain
gutter, see:
http://www.prc68.com/I/Images/SB_angw.jpg

A sheet of this stuff about 4 x 6 feet weighed maybe 40 pounds.  An easy way to confirm it works is to use it to block a
receiver antenna and see how far the signal goes down.

Have Fun,

Brooke Clarke
http://www.PRC68.com
http://www.end2partygovernment.com/Brooke4Congress.html

Michael Baker wrote:

time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

Jim,

There are actually two new designs one coming from Novatel where the
"rings" are put in a pyramide/cone. The "rings" are also shopped of into
small ring segments.

 http://webone.novatel.ca/assets/Documents/Papers/GNSS-750.pdf

The other, coming as a response to the Novatel antenna, is from Topcon
which uses spikes.

http://www.topconpositioning.com/products/gps/antennas/pn-a5
http://www.topconpositioning.com/sites/default/files/PN-A5_white_paper.pdf

I am interested in more information. Will look for whats later in the
thread. I have a situation where there is interference coming from a
specific direction (close to the horizon). Is it feasible to
block/attenuate/absorb L-band signals from one direction?

--

Björn

How do you know it is coming from a specific direction? There is a
terrestrial L-band transmitter? A radio link? Try to put a screen
n*lambda+lambda/2 from the GPS antenna to create a null for the offending
L-band frequency...

On Thu, Apr 5, 2012 at 2:49 PM, Chuck Forsberg WA7KGX N2469R
caf@omen.comwrote: