4nec2 hidden variables.
 

On Tue, 16 Aug 2011 11:21:20 +0200, Helmut Wabnig [email protected] ---
-.dotat wrote:

Thank you for the links, appreciate your help.

Y'er welcome. However, I still don't have a clue what you're trying
to accomplish. If you want super front to back ratio for a direction
finding antenna, you need a wall, not a phased single wire reflector
as in a yagi. Something like a dish or a panel reflector will work,
but is probably not practical. As you discovered, the position of the
reflector is very critical for maintaining the best F/B ratio. It's
also very sensitive to frequency. You might be able to make it work
with a flimsy tape measure, but it probably won't be very stable in
the field.

Permit me to offer another idea. Instead of direction finding by the
peak signal, do it with a null. Two identical dipoles, a T connector,
and some coax, are all that are required. Space them about 1/4
wavelength apart. Insert a 3/4 wave coax phasing section between the
two dipoles. Signals from the front antenna will cancel when they
reach the 2nd antenna yielding a cardioid pattern. It will be just as
critical and sensitive to frequency and position as your yagi, but
methinks will work better. I've obtained 50dB deep nulls with this
method. The good news is that you really only need one adjustment on
the antenna, which is the relative distance between dipoles, which is
easily accomplished by sliding the antenna on the boom. The
adjustment range is limited, so an assortment of coax phasing lengths
will also be needed. Adjusting the antenna for best null at a
specific frequency, prior to transmitter hunting, is a necessary
complication.

I also use this method on my home base station to null out a rather
strong repeater, so I can talk to a distant repeater on the same
frequency. I've also built one using 4 identical rubber ducky
antennas for 2meters. It sorta worked but I could only get a 10dB
deep null.

Both EZNEC and 4NEC have HF examples of this type of antenna.
cardioid.ez and \models\HFvertical\cardioid.nec

I fear that in the end I will have to cut and tweak using the
antenna analyzer, and with the time perhaps find
"tape measure correction factors".

More bad news. You really do have to TEST your creations to see if
the simulations and models are correct. I tend to make far too many
simplifying assumptions in my models, which show up as errors in the
actual construction.

Want to keep the design as simple as possible,

Simple is good. Crude is not. Recognizing the difference is
difficult.

and hope it works without a balun or additional tricks.
Am not yet sure if it will work that way, or not.

It won't work well without a balun. The coax will radiate, giving
unpredictable directional indications. You can minimize the effects
with ferrite beads, a sleeve balun, or with just a coax loop.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

4nec2 hidden variables.
 

On Tue, 16 Aug 2011 11:14:24 +0200, Helmut Wabnig [email protected] ---
-.dotat wrote:

The SteppIr does vary the elements, if I understand it correctly?
For practical reasons they do not change element positions on
the boom but sufficiently adapt to each frequency band.

Perhaps reading the Steppir patent might be illuminating:
http://www.google.com/patents?id=rw6xAAAAEBAJ
Note that the elements do not change position, only their length.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

4nec2 hidden variables.
 

On 8/16/2011 9:25 AM, Jeff Liebermann wrote:
On Tue, 16 Aug 2011 11:21:20 +0200, Helmut [email protected] ---
-.dotat wrote:

Thank you for the links, appreciate your help.

Y'er welcome. However, I still don't have a clue what you're trying
to accomplish. If you want super front to back ratio for a direction
finding antenna, you need a wall, not a phased single wire reflector
as in a yagi. Something like a dish or a panel reflector will work,
but is probably not practical. As you discovered, the position of the
reflector is very critical for maintaining the best F/B ratio. It's
also very sensitive to frequency. You might be able to make it work
with a flimsy tape measure, but it probably won't be very stable in
the field.

Permit me to offer another idea. Instead of direction finding by the
peak signal, do it with a null.

Exactly.. you can get very sharp nulls pretty easily. For simple
antennas, there's usually an ambiguity, but you can resolve that by
other means.

4nec2 hidden variables.
 

On Tue, 16 Aug 2011 11:14:24 +0200, Helmut Wabnig [email protected] ---
-.dotat wrote:

"Signals from everywhere and from all directions."

Hi Helmut,

The signal is too STRONG. I'm surprised you have not recognized this
common fox hunting problem.

There needs to be an attenuator between the antenna and the receiver.

73's
Richard Clark, KB7QHC

4nec2 hidden variables.
 

On Tue, 16 Aug 2011 09:51:53 -0700, Jim Lux
wrote:

On 8/16/2011 9:25 AM, Jeff Liebermann wrote:
Permit me to offer another idea. Instead of direction finding by the
peak signal, do it with a null.

Exactly.. you can get very sharp nulls pretty easily. For simple
antennas, there's usually an ambiguity, but you can resolve that by
other means.

With the cardioid pattern, there's no ambiguity. With a deep null,
and a non-reflective environment, it's easy to use. However,
reflections off buildings and hills are a problem. The antenna is
roughly an omnidirectional antenna, and reflections will appear to
fill in the deep null. Not much can be done about that except to move
and try a different location. My favored method is by using a map.
Find a position, determine a bearing, and draw a line of position on
the map. Then, find a different location, and do it again. Repeat as
often as practical generating as many LOP's as possible. Some of the
LOP's will be totally insane, but a majority should cross at one point
on the map. (This has been automated in software, but a map is good
enough).

In 1976, I helped design the AN/SRD-21 homer DF, which DOES have a 180
degree ambiguity.
http://802.11junk.com/jeffl/AN-SRD-21/
If the homing receiver works in the same manner as described in the
above manual (PIN antenna switch, synchronous demodulator, charge
pump, and zero center meter), resolving the ambiguity is easy. Just
rotate the homing antenna to the right. If the zero center meter
indicates that you should turn back to the left, then you have the
correct direction. If it indicates that you should continue to turn
to the right, then you have the wrong direction.

This has nothing to do with the topic at hand, but I thought it might
be of interest. Some of my ancient comments on Doppler direction
finding:
http://www.qsl.net/n9zia/doppler_notes1.txt
http://www.qsl.net/n9zia/doppler_notes2.txt

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

4nec2 hidden variables.
 

In article ,
Jeff Liebermann wrote:

With the cardioid pattern, there's no ambiguity. With a deep null,
and a non-reflective environment, it's easy to use. However,
reflections off buildings and hills are a problem. The antenna is
roughly an omnidirectional antenna, and reflections will appear to
fill in the deep null. Not much can be done about that except to move
and try a different location. My favored method is by using a map.
Find a position, determine a bearing, and draw a line of position on
the map. Then, find a different location, and do it again. Repeat as
often as practical generating as many LOP's as possible.

I have seen recommendations that one do this by proceeding in a
straight line, taking bearings at positions of around 1/4 wavelength
or so. The effects of multipath will tend to cause the measured
bearing to wobble back and forth across an arc (as you go down the
line) and the true bearing will tend to be close to the center of that
arc.

I haven't tried this approach myself... but it might combine well with
your "take readings from lots of different locations" approach, by
helping cancel out some of the effects of multipath.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

4nec2 hidden variables.
 

On 8/16/2011 3:16 PM, Dave Platt wrote:
In ,
Jeff wrote:

With the cardioid pattern, there's no ambiguity. With a deep null,
and a non-reflective environment, it's easy to use. However,
reflections off buildings and hills are a problem. The antenna is
roughly an omnidirectional antenna, and reflections will appear to
fill in the deep null. Not much can be done about that except to move
and try a different location. My favored method is by using a map.
Find a position, determine a bearing, and draw a line of position on
the map. Then, find a different location, and do it again. Repeat as
often as practical generating as many LOP's as possible.

I have seen recommendations that one do this by proceeding in a
straight line, taking bearings at positions of around 1/4 wavelength
or so. The effects of multipath will tend to cause the measured
bearing to wobble back and forth across an arc (as you go down the
line) and the true bearing will tend to be close to the center of that
arc.

I haven't tried this approach myself... but it might combine well with
your "take readings from lots of different locations" approach, by
helping cancel out some of the effects of multipath.


There's a photograph of this technique on a field with little flags at
different distances all oscillating around a single line illustrating it
in some ARRL publication (probably an antenna compendium, but maybe the
handbook or antenna book).

4nec2 hidden variables.
 

On Tue, 16 Aug 2011 17:13:15 -0700, Jim Lux
wrote:

On 8/16/2011 3:16 PM, Dave Platt wrote:
In ,
Jeff wrote:

With the cardioid pattern, there's no ambiguity. With a deep null,
and a non-reflective environment, it's easy to use. However,
reflections off buildings and hills are a problem. The antenna is
roughly an omnidirectional antenna, and reflections will appear to
fill in the deep null. Not much can be done about that except to move
and try a different location. My favored method is by using a map.
Find a position, determine a bearing, and draw a line of position on
the map. Then, find a different location, and do it again. Repeat as
often as practical generating as many LOP's as possible.

I have seen recommendations that one do this by proceeding in a
straight line, taking bearings at positions of around 1/4 wavelength
or so. The effects of multipath will tend to cause the measured
bearing to wobble back and forth across an arc (as you go down the
line) and the true bearing will tend to be close to the center of that
arc.

I haven't tried this approach myself... but it might combine well with
your "take readings from lots of different locations" approach, by
helping cancel out some of the effects of multipath.

There's a photograph of this technique on a field with little flags at
different distances all oscillating around a single line illustrating it
in some ARRL publication (probably an antenna compendium, but maybe the
handbook or antenna book).

I think the photo was on interferometry for direction finding, where
they were trying to plot the wavefront pattern. I recall the photo,
but I couldn't find it in the ARRL Antenna Handbook (19th edition).
However, the handbook does have a full chapter on direction finding
(Ch 14) which includes the cardioid antenna.

The purpose of me using multiple bearings is simply to eliminate the
effects of reflections. It's not totally foolproof, but better than
chasing a single bearing.

There are better ways, but they're usually not very portable. A
rotating antenna direction finder can easily distinguish between the
incident signal from a reflection. Such direction finders display
something like the antenna pattern on a polar display. The incident
signal is very steady and does not move in azimuth. Reflections
jitter dramatically in both amplitude and azimuth. They also tend to
appear and disappear rapidly and erratically.

Rotating antenna DF antennas:
http://www.rockwellcollins.com/sitecore/content/Data/Products/EW_and_Intelligence/SIGINT/ANT-1040A_Airborne_Spinning_DF_Antenna.aspx
http://www.rockwellcollins.com/sitecore/content/Data/Products/EW_and_Intelligence/SIGINT/ANT-1040_Spinning_DF_Antenna.aspx
20 in diameter, 25 in high, 50 lbs.

Another method that I've used is to generate a narrow "beam" using two
identical yagi antennas that share a common reflector. The antennas
are oriented about 20 degrees apart with the reflector at the vertex.
A near perfect audible square wave drives two identical PIN diode
switches on the driven elements. While the antenna may have a -3dB
beamwidth of perhaps 10 degrees, the LOP of equal antenna signal
levels is extremely sharp, and often less than 0.5 degrees. A
synchronous demodulator, charge pump, differential amp, and zero
center meter complete the systems. You can also do it by ear by
listening to the null at the switching frequency. If the antennas and
PIN diode switches are perfectly symmetical, all boresight error
cancel. I originally contrived the system by reading about the Lorenz
beam bombing system used by the German's during WWII, but later
discovered that others had anticipated the idea.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

direction finding
 

On 8/16/2011 9:35 PM, Jeff Liebermann wrote:
On Tue, 16 Aug 2011 17:13:15 -0700, Jim
wrote:

I think the photo was on interferometry for direction finding, where
they were trying to plot the wavefront pattern. I recall the photo,
but I couldn't find it in the ARRL Antenna Handbook (19th edition).
However, the handbook does have a full chapter on direction finding
(Ch 14) which includes the cardioid antenna.


Could be..



The purpose of me using multiple bearings is simply to eliminate the
effects of reflections. It's not totally foolproof, but better than
chasing a single bearing.

There are better ways, but they're usually not very portable. A
rotating antenna direction finder can easily distinguish between the
incident signal from a reflection. Such direction finders display
something like the antenna pattern on a polar display. The incident
signal is very steady and does not move in azimuth. Reflections
jitter dramatically in both amplitude and azimuth. They also tend to
appear and disappear rapidly and erratically.

snip

Another method that I've used is to generate a narrow "beam" using two
identical yagi antennas that share a common reflector. snip



A popular scheme from WJ 30 years ago (at the advent of digital signal
processing) used 3 antennas in a triangle, and basically did
interferometry. Sampling the data and using an FFT lets you do a wide
band at one time (which is handy if you want to DF frequency hopping
radios, which became very popular in the 80s)

direction finding
 

On 8/17/2011 11:02 AM, Jim Lux wrote:

A popular scheme from WJ 30 years ago (at the advent of digital signal
processing) used 3 antennas in a triangle, and basically did
interferometry. Sampling the data and using an FFT lets you do a wide
band at one time (which is handy if you want to DF frequency hopping
radios, which became very popular in the 80s)

As far as VHF, I know a couple here in town I've seen using arrays
for direction finding. But they used four verticals in a square.
It worked quite well. We had one guy that kept jamming one of the
repeaters, and the user of said array was easily capable of tracking
him down to a shopping center parking lot where the guy was sitting
in his car. I suspect the expression on the jammers face when the
tracker tapped on his window was priceless. :)
The device in the car used an array of LED's in a cross configuration
if I remember right.
He had the antenna array on the back of his truck. I don't recall the
element spacing.