Here is the text without the images, please download PDF from
http://radiation.weblife.at/main

###################TEXT########################### ########


What you always wanted to know about Fox Hunt Antennas,
especially the "Tape Measure Antenna".


There are several webpages describing how to calculate the basic
design parameters of three- element Yagi antennas and practical
examples of how to make them out of various materials.
The classic design page is by Joe Leggio, WB2HOL on the webpage
http://pages.videotron.com/ve2jmk/tape_bm.htm Please read, I do not
repeat the contents.

The design is made of PVC tubes and tape measure pieces as radiating
elements. Note the taper of the elements. It seems that the second
element from the left, the radiating dipole, is tapered more than an
estimated linear decrease. Further note, that the center element is
positioned more to the left. We will soon discuss what that tells us.


(Image from the Leggio page)


The Leggio design is based on non-conductive boom material. Note that
the front part of the antenna is quite long as seen from the center
dipole. The distance will only be correct when we use an insulator as
boom carrier. On the contrary, the left half is shorter, and there is
the cable running along the center from the radiator to the leftmost
element, the reflector. The non-conducting boom design is defunct now,
on that part of the antenna. Therefore the calculated element
distances will have to be changed. There is no need for a galvanic
connection at the center of the reflector element, because the voltage
in the center point is zero anyhow and no current will flow between
reflector and boom. So we have a boom design of half a conductive and
half non-conductive material, effectively. All electromagnetic waves
shorten their wavelengths when entering a medium, and waves travelling
along a conductor have a shorter wavelength than in free space.

To aid our calculations we may use the software YagiCad4.1 from here
http://www.ipass.net/teara/kq3.html and a ready made recipe from
there http://home.att.net/~jleggio/projects/rdf/tape1.zip
That little gem can be learned and mastered within 15 minutes, while
also very fine, good and marvellous pieces of software like
http://home.ict.nl/~arivoors/ will take you five weeks to master. For
the design of our fox-hunt 3 element yagis, the YagiCad is sufficient.
As a homework, go and enter the original WB2HOL data into YagiCad.
Surprised?

Well, let me skip and leap forward. I do not intend to write a YagiCad
turorial here. To proceed with the story, and come to the more
interesting part, let me tell you how my copy of that antenna worked:
it did not. It was clumsy and heavy because good PVC is not available
here, and I of course scaled the design from the 146,565 mentioned by
Joe Leggio up to 150 MHz by percent-wise shrinking every part. Time to
look around.

The Cebik pages, W4RNL.

May those pages last forever: http://www.cebik.com .
For the topic chosen we must read http://www.cebik.com/ao/ao1.html .

Let us jump into medias res:
(Image from Cebik page)

Foxhunting requires good front-to-back ratio, and not maximum gain. In
most cases the frequency used will be given and will not change, so we
may design an antenna with our desired frequency as center frequency,
using YagiCad. By varying the overall length of the antenna we find
out that length influences gain. Positioning the centre element, the
active dipole, we notice that the position influences the
forward-to-back ratio. Because everything influences everything else
on an antenna, the observations we make will not be as clear from the
beginning , but after a while of experimenting we get the feeling. Let
YagiCad do the optimising. Compare the results with the recipes given
by Cebik. Note, that both sources assume a non-conducting boom, and
the obtained numbers refer to isolated elements
in air.

What happens when we use an antenna a few megahertz above or below the
nominal design frequency? Gain will not change much. Impedance will
change somewhat, but we are less concerned about when doing fox hunts.
What changes most, is the F/B ratio, actually it changes
catastrophically if only a few hundred kilohertz off center. That
quickly renders an antenna useless, and especially beginners then
claim to hear signals "from all directions". Impossible to distinguish
between direct signal and reflections and the reception from 180°
backwards. And do you really expect your home made antenna to be as
good as the data from your calculations? Do you expect the wire
skeleton to resonate precisely at your target frequency? Remember,
even the coax cable to the rear has an influence, and what if your
boom is constructed of metal?

Optimizing F/B ratio on real antennas.

You guess, just make the center element, the driven dipole, moveable.
The direct advancement is that we can equal out manufacturing
tolerances and practically unknown parameters of small influence. But
the main achievement is that we can precisely adjust the front to back
ratio for our desired frequency. We will never be able to reach the 57
dB which theory promises, but we get the best out of our real existing
home made marvel. Then we may be able to do a nice observation.

A tape measure antenna with metal boom, working at the correct design
frequency, and adjusted to best F/B ratio at 150 MHz.
Compare the setup with the Leggio and with the Cebik images.
To the left (out of sight) is a noise generator, transmitter PA and a
transmitting log-periodic antenna. The analyzer displays what the
receiving antenna delivers.

What does this tell us? Obviously the
Leggio Antenna is detuned from its
center design frequency. It may work
or not, if you copy the design for your
purpose. Make the center T on the PVC
boom moveable, success guaranteed!


Final remarks.

'Nuff said, just a few additional pictures.

The slider, top view. Side view,
elements mounted.


Do we need an attenuator? Absolutely!
In some cases an offset attenuator may be recommended, especially if
you know the precise frequency of the target, the "fox", but sometimes
we are better off with direct frequency equipment, especially when
searching for radio interference sources.

Element mount (director)


To the right and below: Complete fox hunt equipment, antenna,
attenuator
and receiver.

By Helmut Wabnig, OE8UWW, Sept. 2007.

while this may be good about yagi antennas, that is only one type of antenna
used in fox hunts. they are usually good for long distance hunting, but
tend to be too broad for close in work. loop/sense antenna combinations
with a deep cardiod pattern null are another type that is very useful, even
plain loops with good side nulls are helpful. then there are also the
various forms of 'doppler' antennas. beside the classic 4 vertical doppler
design that is usually limited to car top applications there is a simple 2
vertical design that is good for use on foot. and example of that is at:
http://www.k1ttt.net/technote/doppler.html


"Helmut Wabnig" .... .-- .- -... -. .. --. @ .- --- -. dot .- - wrote in
message ...
Here is the text without the images, please download PDF from
http://radiation.weblife.at/main

###################TEXT########################### ########


What you always wanted to know about Fox Hunt Antennas,
especially the "Tape Measure Antenna".


There are several webpages describing how to calculate the basic
design parameters of three- element Yagi antennas and practical
examples of how to make them out of various materials.
The classic design page is by Joe Leggio, WB2HOL on the webpage
http://pages.videotron.com/ve2jmk/tape_bm.htm Please read, I do not
repeat the contents.

The design is made of PVC tubes and tape measure pieces as radiating
elements. Note the taper of the elements. It seems that the second
element from the left, the radiating dipole, is tapered more than an
estimated linear decrease. Further note, that the center element is
positioned more to the left. We will soon discuss what that tells us.


(Image from the Leggio page)


The Leggio design is based on non-conductive boom material. Note that
the front part of the antenna is quite long as seen from the center
dipole. The distance will only be correct when we use an insulator as
boom carrier. On the contrary, the left half is shorter, and there is
the cable running along the center from the radiator to the leftmost
element, the reflector. The non-conducting boom design is defunct now,
on that part of the antenna. Therefore the calculated element
distances will have to be changed. There is no need for a galvanic
connection at the center of the reflector element, because the voltage
in the center point is zero anyhow and no current will flow between
reflector and boom. So we have a boom design of half a conductive and
half non-conductive material, effectively. All electromagnetic waves
shorten their wavelengths when entering a medium, and waves travelling
along a conductor have a shorter wavelength than in free space.

To aid our calculations we may use the software YagiCad4.1 from here
http://www.ipass.net/teara/kq3.html and a ready made recipe from
there http://home.att.net/~jleggio/projects/rdf/tape1.zip
That little gem can be learned and mastered within 15 minutes, while
also very fine, good and marvellous pieces of software like
http://home.ict.nl/~arivoors/ will take you five weeks to master. For
the design of our fox-hunt 3 element yagis, the YagiCad is sufficient.
As a homework, go and enter the original WB2HOL data into YagiCad.
Surprised?

Well, let me skip and leap forward. I do not intend to write a YagiCad
turorial here. To proceed with the story, and come to the more
interesting part, let me tell you how my copy of that antenna worked:
it did not. It was clumsy and heavy because good PVC is not available
here, and I of course scaled the design from the 146,565 mentioned by
Joe Leggio up to 150 MHz by percent-wise shrinking every part. Time to
look around.

The Cebik pages, W4RNL.

May those pages last forever: http://www.cebik.com .
For the topic chosen we must read http://www.cebik.com/ao/ao1.html .

Let us jump into medias res:
(Image from Cebik page)

Foxhunting requires good front-to-back ratio, and not maximum gain. In
most cases the frequency used will be given and will not change, so we
may design an antenna with our desired frequency as center frequency,
using YagiCad. By varying the overall length of the antenna we find
out that length influences gain. Positioning the centre element, the
active dipole, we notice that the position influences the
forward-to-back ratio. Because everything influences everything else
on an antenna, the observations we make will not be as clear from the
beginning , but after a while of experimenting we get the feeling. Let
YagiCad do the optimising. Compare the results with the recipes given
by Cebik. Note, that both sources assume a non-conducting boom, and
the obtained numbers refer to isolated elements
in air.

What happens when we use an antenna a few megahertz above or below the
nominal design frequency? Gain will not change much. Impedance will
change somewhat, but we are less concerned about when doing fox hunts.
What changes most, is the F/B ratio, actually it changes
catastrophically if only a few hundred kilohertz off center. That
quickly renders an antenna useless, and especially beginners then
claim to hear signals "from all directions". Impossible to distinguish
between direct signal and reflections and the reception from 180°
backwards. And do you really expect your home made antenna to be as
good as the data from your calculations? Do you expect the wire
skeleton to resonate precisely at your target frequency? Remember,
even the coax cable to the rear has an influence, and what if your
boom is constructed of metal?

Optimizing F/B ratio on real antennas.

You guess, just make the center element, the driven dipole, moveable.
The direct advancement is that we can equal out manufacturing
tolerances and practically unknown parameters of small influence. But
the main achievement is that we can precisely adjust the front to back
ratio for our desired frequency. We will never be able to reach the 57
dB which theory promises, but we get the best out of our real existing
home made marvel. Then we may be able to do a nice observation.

A tape measure antenna with metal boom, working at the correct design
frequency, and adjusted to best F/B ratio at 150 MHz.
Compare the setup with the Leggio and with the Cebik images.
To the left (out of sight) is a noise generator, transmitter PA and a
transmitting log-periodic antenna. The analyzer displays what the
receiving antenna delivers.

What does this tell us? Obviously the
Leggio Antenna is detuned from its
center design frequency. It may work
or not, if you copy the design for your
purpose. Make the center T on the PVC
boom moveable, success guaranteed!


Final remarks.

'Nuff said, just a few additional pictures.

The slider, top view. Side view,
elements mounted.


Do we need an attenuator? Absolutely!
In some cases an offset attenuator may be recommended, especially if
you know the precise frequency of the target, the "fox", but sometimes
we are better off with direct frequency equipment, especially when
searching for radio interference sources.

Element mount (director)


To the right and below: Complete fox hunt equipment, antenna,
attenuator
and receiver.

By Helmut Wabnig, OE8UWW, Sept. 2007.