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Author Topic: Improving the Receive Angle  (Read 109 times)

KD9FRQ

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Improving the Receive Angle
« on: June 13, 2020, 02:06:21 PM »

My son and I "won" our first Fox Hunt today.

We noticed the the yagi we are using has a larger angle of sensitivity than expected (about 30 degrees).
I also need to work on my attenuater usage.

What can we do to the yagi to narrow its focus?
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W1VT

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Re: Improving the Receive Angle
« Reply #1 on: June 13, 2020, 02:42:27 PM »

I've done two things.  A longer Yagi.  Stacking two short vertically polarized Yagis side by side on a horizontal crossboom.  The crossboom can be metal.

Zak W1VT
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KD9FRQ

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Re: Improving the Receive Angle
« Reply #2 on: June 14, 2020, 10:00:53 AM »

I've done two things.  A longer Yagi.  Stacking two short vertically polarized Yagis side by side on a horizontal crossboom.  The crossboom can be metal.

Zak W1VT
I have another matching yagi, have the splitting cable and know how far the manufacturer says to spread them. However, I was hoping for a more portable unit.

If I made two of the three element Measuring tape yagi for 2m, how far would I need to have them spread apart? About the same as any other 2M yagi pair?
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WB6BYU

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Re: Improving the Receive Angle
« Reply #3 on: June 14, 2020, 10:30:14 AM »

Most yagis have a narrower field for horizontal polarization than for vertical.
This is particularly noticeable with short beams like a Moxon (half power
beamwidth 78 degress for horizontal, 143 degress for vertical).  So the first
suggestion is to use a horizontally polarized antenna on the transmitter.

Rotating the polarization may help in some cases, but makes it more
prone to picking up reflections.

Another approach is to hunt the null in the pattern instead of the peak.
A 20 dB null can be much narrower than the peak in the pattern, and
the most accurate (simple) DF antennas use the null as an indicator
rather than the peak for that reason.  But that requires a lot of
attention to detail.  First, the antenna has to have a defined null
in a particular direction, and second, the null is easily blurred
by reflections.  You also may need extra sensitivity to use the
null on very weak signals that are only audible when peak of
the antenna pattern is pointed at the source.

Yagis can be designed for a very deep rear null (such as the original
WB2HOL tape measure yagi or some HB9CV designs), but null depth
and position often vary with frequency.  It isn't uncommon, for
example, to have two nulls, one on either side of a small lobe in the
back, and the pattern likely will vary between 146.5 and 147.5 MHz. 

So one important step is to be familiar with the pattern of your antenna
on the specific frequency that you will be using.

One way to get a null in a reliable direction is to use a loop or Adcock
antenna, as the null is always broadside to it regardless of frequency.
A small loop can be homemade, and they are available commercially.
And Adcock is basically a pair of dipole antennas in parallel, spaced
1/2 wavelength of so (exact spacing and length not critical), and
fed out of phase.  (This one is perhaps more complex than
necessary.)


The next question is, what are you using for a detector? 

The S-meter on an HT or a mobile radio?  Consider, if you had a
"nominal" S-meter with 6 dB / S-unit, and you had a separate
bar graph indicator for each step, you could change the signal
strength 5 dB and not see a difference on the meter.  No, meters
on FM rigs don't work that way - the ones I've measured have
been more like 12 dB total across the whole scale (which might
only be 5 or 7 segments), so roughly 2 dB per step.

That step size limits the resolution of your antenna.  My usual
3-element yagi, for example, has a 3 dB beamwidth of 100 degrees
(+/-50) for vertical polarization.  If that is the resolution of
my indicator then I'll have to eyeball average to find the
center direction over that range.  But it is +/- 30 degrees for
a 1 dB resolution.  So the resolution on the detector can make
a big difference. 

Note that some antennas designed for a sharp rear null often have
a broader peak than those designed for high gain, so it helps to
choose an antenna that fits your hunt methods.  And people
wonder why I have so many antennas...

Personally, I use a 2-element quad for mobile hunting (though I
have longer quads available).  This doesn't stick up as high above
the roof as a yagi does when hunting vertically polarized signals.
There are other quad designs that are wider and shorter that
will have a narrower pattern, but I haven't tried one yet.  It's
on the list...


One other option is to adjust your attenuator to where the signal
is just below a transition point, which will narrow the range over
which the next step is indicated.

Analog indicators are better in this regard, though you may have
to peer closely at the scale to see the exact peak.  I modified
my IC-28 to bring out the S-meter reading to an unused pin on
the mic jack:  it turned out that the internal bargraph meter
indicated from about 0.5 to 1.5 volts, but the actual range of
the voltage was 0.25 to 2.5 volts.  By using a meter to display
the full range I got better resolution and range.

Actually, in that case I used an audio indicator instead, and that
is my favorite method (especially when hunting on foot through
the forest, where I need to keep my eyes on the ground to keep
from tripping, though it is handy when driving also).  The audio
Voltage Controlled Oscillator in my VK3YNG "sniffer" allows
me to determine the peak within 1/4 dB by ear.  That greatly
narrows down the effective beamwidth of the antenna (+/- 15
degrees for vertical polarization, or +/- 8 for horizontal) without
any changes to the antenna itself.


Otherwise, Zach's suggestions of a longer yagi, or two side-by-side,
will both give you a sharper pattern, with the latter probably giving
more improvement for less effort, but of course both will be larger
than your present antenna, which may be a consideration, especially
if you are hunting on foot.

If you are stopping to take bearings and accuracy is more important
than speed, then a "long baseline interferometer" can provide a bearing
to within 1 degree or so using two identical beams.   You connect them
via lengths of coax to a T connector and then to the receiver.  Set up
one and aim it for max signal.  Set the other about 2' away pointing
parallel to the first and move it towards or away from the tentative
transmitter direction until you get a null in the receiver.  (This procedure
allows for using different lengths of coax to the two antennas.)
Mark that point, move back a foot or two perpendicular to the first
antenna, and find the null again.  By repeatedly finding the null with
small movements of the second antenna and marking the points, you
can trace the incoming wavefront and see the effect of reflections.
You have to move in small increments to stay on the same line.  When
you reach the end of the coax, sight down the line of markers (which
may be wavy rather than straight) and take a compass bearing:  the
transmitter is at right angles to this line.


If the issue is primarily at the end when you get close, then a
6-element beam for 440 tuned to the third harmonic can be quite
effective, or a small loop.


But in the end, even with good equipment, I don't count on bearings
to be better than 10 degrees (unless I'm using a long-baseline
interferometer for Search and Rescue).   I use them to give me an
idea where to head on the map, but I never expect to be able to
triangulate the transmitter location from the start.  Besides, just
one bearing confused with reflections will throw the whole triangulation
into uncertainty until I get several more bearings from different
directions.  So I use the bearings as a clue to where to go next
rather than telling me where I will end up.

KD9FRQ

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Re: Improving the Receive Angle
« Reply #4 on: June 15, 2020, 11:17:28 AM »

WB6BYU,

Thank yo for your reply.  That is a lot for me to digest.

I will work through it and see what happens on the next hunt.

73s, Ed, KD9FRQ
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