On Mon, 15 Aug 2011 15:09:52 -0700, Jim Lux
wrote:

.............


A Yagi with fixed element positions can only be used for 1 frequency
(within a very small range) and therefore may be impractical
in many situations. The CEBIK tape measure antenna with their
water-pipe construction could be easily modified with a moveable
center element.

this is just not true (otherwise the SteppIR wouldn't work.. it has
fixed element spacing and works over a huge (1 octave) frequency range)
with fairly good performance.

One can also do similar with fixed spacing, and fixed length, and
reactive loading of the elements at the centers.


In theory ( from the simulations) the Yagi is rather narrowband.
Especially the F/B ratio is very frequency dependend.

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.

For the same practical reasons I do it the other way round,
move the driver element in the 3 element yagi, and leave
everything else as is. This is intended for mobile direction finding.
Based on the Cebik pages and improved by making it tuneable
by moving the driver along the boom.

There is a story behind.
ARDF is a sporting endeavour, where people run around in
a restricted area searching for a defined signal, or several
foxes, as they are called. The antennas are optimized for the
fox frequencies. That's child play compared to real world.

One night I got a phone call from a desperate guy in Bosnia.
They had held a model flight competition there, class F1C,
and his model airplane flew ten kilometers away.
They put crash transmitters on the models and they
range in frequency from 140 to 160 Mhz without any approved
frequency raster, just what they can get and not to disturb
the neighbour model transmitter. So they need equipment for
a 20 MHz range. Most of them use amateur radio handhelds.
The transmitters weigh about 5 grams.

The Yugoslavian Balkan war was just over, where the killed 200000
people by shooting and cutting throats, and the mine fields were
still active and most of them still are today. They put fences around
and automatic siren warnings. That desperate guy's model airplane
was in or near such an area and night doomed. He has searched
for the whole day , was exhausted and could not locate it.
"Signals from everywhere and from all directions", he said.
That model was his only one for the competition and very expensive,
they cost a few thousand dollars.

"Signals from everywhere and from all directions."
That is a very interesting statement. Did you ever experience such
a situation? I had to place foxes and go after them to find out what
the problem was which the guy described. I expected an easy job,
because I am such an old fart in radio engineering. It was late summer
and corn standing two meters high, railway routes nearby,
high voltage lines criscrossing the landscape, buildings, hills,
mountains, fences, and now imagine it's getting dark and you are
in a foreign country where you never had been before,
between lakes, rivers, wet ditches and waste water canals.
I never knew that 2 or 3 meter high corn fields act as an antenna
array. The power lines and railways are wave ducts of extraordinary
quality, every metal fence collects and reflects and adds to the
reflections from the rocky mountains. Electromagnetic interference
overlays the fox signal until you only hear noise and crackles.
There you go. I hope the reader can imagine what is the difference
to a well planned amateur sport radio fox hunting event.

Quickly I recognized that a fixed-frequency ARDF antenna is
not suited for the task and one must be able to tune the F/B ratio
for the different frequencies the transmitters use, in combination
with a conventional attenuator. If your antenna receives from the
back, you will hear "signals from all directions".


w.