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Author Topic: mirco directional antenna how to ?  (Read 8306 times)

KE5BKD

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mirco directional antenna how to ?
« on: May 08, 2006, 05:20:34 PM »

i would appreciate any help in this. i would like to build (buy?) a micro directional antenna for fox hunting. i would prefer a yagi but will build, buy, use anything as long as it is compact. our local pd has a problem with people tx-ing on their freq and most of the people are smart enough to move around. the one thing i really need is something small enough not to be noticed as most of the time these people are found in residencial areas or crowds and disappear when anyone with an antenna is spotted. thanks for any help you may offer.

                           73,
                           ke5bkd
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WB6BYU

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mirco directional antenna how to ?
« Reply #1 on: May 11, 2006, 10:55:39 AM »

There are a number of approaches for this.  The first
problem is that, as the antenna size decreases, so does
the received signal pickup.  If you are looking in a close
area where the signal is fairly strong, this probably
isn't a problem.  But it is hard to build a small antenna
that will be sensitive enough to use at a long distance.

Some combination of the following may work for you:

SMALL ANTENNAS

For a yagi, you can shrink the elements 50% or so and
still have it usable, but you can't shrink the boom much.
The shortened elements tend to have very narrow bandwidth,
however, so need careful tuning to maintain a pattern.
A 2-element yagi can be made very short (4" to 6" in
the 150 MHz range) but the tuning is even sharper.  You'll
find some good information on shortened yagis here:

http://www.cebik.com/fdim/fdim1.html

Where signals are strong a loop antenna would be a good
choice, such as this:

http://www.arrowantennas.com/fhl.html

The pattern will be bidirectional, so you will have to
triangulate to locate the transmitter.  I'm sure you
could build your own - I've seen a couple VHF loop antenna
designs on the internet, and the basic design is simple.


HARMONIC SNIFFFING

Most transmitters also radiate some signal on harmonics,
even if it is down 60dB or more from the fundamental.
At close range it may be easier to sniff the harmonics
because antennas are smaller and can have a much sharper
pattern.  For example, I use 730 MHz beams for hunting
both 2m (5th harmonic) and 121.5 MHz (6th harmonic).
A 6-element yagi is a bit over a foot long and 8" wide
and has a very sharp pattern compared to a 3-element
yagi for the fundamental.  I can only hear some of my
2m transmitters when I'm within 100 feet, but it works
fine for sniffing them out of the bushes.  The 2nd and
4th harmonics of 450 MHz fall near the cell phone bands
around 900 and 1800 MHz, and you may be able to find
commercial beams designed for those frequencies.  I've
had good luck scaling these designs for harmonic sniffing:

http://www.clarc.org/Articles/uhf.htm

or you can use W4RNL's quad calculator formulas here:

http://www.cebik.com/quad/2mq.html

Of course, this requires that you have a receiver that
covers the harmonic frequency.


TDOA (Time Difference of Arrival) DF SETS

These units require a relatively simple antenna and can
give good direcitonal resolution.  They can also get
horribly confused with reflections, so use them with care.
Basically they consist of a pair of dipole elements mounted
perhaps a quarter wavelength apart with a switching
system that connects them alternately to the receiver.
The output of the receiver goes to a phase detector that
indicates whether to turn the antenna left or right.
There are a number of constructional projects on the web -
here is one example:

http://home.att.net/~jleggio/projects/rdf/tdoa1.htm

You can use shortened antennas with these as long as you
carefully match them.


CAMOUFLAGE

What would someone reasonably be walking around carrying
that could hide an antenna?  A 2m quad can fit in a box
on top of a car - perhaps a large pizza sign?  Political
or protest signs can have yagi elements inserted into the
corregated plastic that is normally used these days.  A
tennis racquet could hold a loop or a small yagi woven
into the strings.  Especially for harmonic sniffing the
item doesn't have to be large:  a plastic briefcase or
binder will hold a yagi, a dummy telephoto lens could
hide a quad.  Anything rectangular could enclose a corner
reflector antenna built into one side.  You just have
to think of some object that doesn't look like an antenna.

I actually built a TDOA set once for 2m with a pair of
whips mounted on the sides of a set of headphones.  The
whips had balls mounted on the ends so they looked like
"dealy-bobbers" (though I must admit that I was a bit
too old to look like I would wear such a thing.)  I had
the TDOA circuitry connected so the receiver audio would
appear in my left ear if I had to turn the antenna towards
the left, etc.  Two problems:  first, reflections are a
problem with TDOA units.  Second, trying to walk down a
sidewalk while taking a bearing on a station to the side
caused me not to pay attention to where I was going.

AA4PB

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mirco directional antenna how to ?
« Reply #2 on: June 30, 2006, 09:59:27 AM »

One method is to use three people with three DF sets located away from the area where they won't be spotted. Each one takes a bearing and someone plots them on a map. Where they intersect is the approximate location of the transmitter. This permits you to get a position with even a fairly short transmission.
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Bob  AA4PB
Garrisonville, VA

WB6BYU

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mirco directional antenna how to ?
« Reply #3 on: June 30, 2006, 01:46:09 PM »

In my experience using hand-held equipment (3-element yagi)
and plotting bearings on a map, the best I can expect is
an accuracy of about 1 in 10.  Using a 6-element 732 MHz
yagi on the 5th harmonic gives a sharper bearing when the
signal is strong enough, but still the process of triangulating
the bearing onto a map of appropriate scale is not trivial.  
If you are 100 yards away, this can work where
the people ar 50 feet apart, but in a crowd you probably
won't be able to get close enough to identify an individual
before they are elsewhere.

This is using a compass with a reversed bezel mounted
on the yagi boom (taking care to keep it away from steel
objects).  One of the limiting factors is knowing your OWN
position closely enough to plot an accurate bearing from it.
And of course this ignores any reflections or shielding
from buildings, etc.:  can you get the antenna up above
a crown of people enough to get a good bearing without
attracting attention?

AA4PB

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mirco directional antenna how to ?
« Reply #4 on: June 30, 2006, 06:26:48 PM »

Of course you won't be able to triangulate accurately enough to pinpoint which person is carrying the transmitter. It can get you close enough to to identify the area in which the transmitter is located so you can go in with smaller, short range equipment.

As far as identifying your location, you use a GPS receiver. In fact it is possible to use automatic DF equipment and automatically transmit the positions and bearing lines to a computer mapping system and have it all display instantaneously. This can be real helpful when the offender is only making short transmissions.

One way to covertly DF short range is to use a body worn antenna where you use the body to shield the antenna in one direction. Use a special receiver that provides signal strength via a varying tone in an earpiece. Turn your body to determine the direction of maximum signal strength. Nobody even knows you are using a DF receiver.

If the offender is using an HT, all you have to do is get close enough to see him use it.
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Bob  AA4PB
Garrisonville, VA

AA4PB

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mirco directional antenna how to ?
« Reply #5 on: June 30, 2006, 07:14:00 PM »

There are some very good commercial automatic DF receivers available that use either three or four monopole whips in a pattern spaced 1/4 wavelength apart. Some of the newest stuff uses DSP to do a pretty good job of sorting out the multipath. Some of the PDs, the FAA, and the FCC use this type of equipment. The down side is that a system costs between $10K and $20K. The PD may be able to find some funding or perhaps borrow a system from one of the larger PDs. It makes it a lot easier that walking around with a 3ele yagi and an HT.
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Bob  AA4PB
Garrisonville, VA

K8MHZ

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mirco directional antenna how to ?
« Reply #6 on: March 09, 2007, 06:46:43 AM »

"our local pd has a problem with people tx-ing on their freq"

The first thing they need to do is to either add or change the tone required to open up their repeater or other radios.

If the prob goes away chances are you are dealing with someone that somehow snagged a cop radio.  Special equipment is needed to set the tones.  If it comes back in a short time you can be pretty sure you have someone with a ham rig that has been opened up.  Any info you can get is good info, plus changing the tones may eliminate the problem.

If this continues to be an issue have the PD's radio tech program the input to the repeater (if that is what is being jammed) into all the radios in the cruisers and have them check the input freq when the jamming occurs.  A rough location can be figured out very quickly that way.

Now, having done a fair amount of foxhunting let me tell you this; you will get a full scale reading on a Yagi from any direction if you get close enough to the jammer for them to see you.  A regular size Yagi on 2 meters can be used to get a heading from a distance.  The gain of the Yagi will peg your meter even on the backside when you get about a half mile away.  Learn to read your heading from a distance and TRUST it.  Learn where the lobes are and where the nulls are.  Notice those words are both plural?  Yagis used close to the ground usually have at least three lobes and nulls. Then, using a Yagi made for 440 sniff the third harmonic.  You can get within about 75 feet before you start to saturate both sides of the antenna on the thirds.  If you are really clever you can build an offset attenuator with a frequency mixer and get within 5 feet before either antenna saturation or front end overloading occurs.
 
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