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Author Topic: Alpha Loop for Directional Finding  (Read 25767 times)
KM4FVI
Member

Posts: 36




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« on: February 07, 2016, 03:33:31 PM »

I found several alpha loop antenna vendors advertising their products for use with directional finding that would cover HF 20 to 80m. I'm looking for recommendations that have superior qualities.

Frank
« Last Edit: February 07, 2016, 03:46:29 PM by KM4FVI » Logged
WB6BYU
Member

Posts: 17176




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« Reply #1 on: February 07, 2016, 06:39:39 PM »

That depends on your specific application.

If you need frequency agility, there are active loops that will cover the
same range with no tuning required. 

For best results the loop has to be mounted outside, away from wiring
or power/phone lines (even under ground.) That means that a manally-
tuned loop won't be as convenient.

Performance also depends on the vertical angle of the incoming wave.
A loop works well on ground wave (like an AM broadcast station) or
space waves (at VHF), but not as well for ionospheric propagation because
the higher the angle of radiation, the shallower the null.  They are very
difficult to use to track NVIS signals.

For receiving, the losses in the loop aren't as important, so you can make
the loop of much lighter materials.  I use several turns of wire inside a
piece of plastic tubing bent into a circle, though you can use a cross frame
of wood or PVC pipe to hold the wires just as well.  You can also mount a
pre-amp at the loop feedpoint and/or use a varactor to tune it remotely if
desired.  My loops for hunting on 80m are only 6" to 10" in diameter, as
they are designed for use on foot while running through a forest.  For a fixed
application you can make them bigger (depending on the desired frequency
range), but even that at 1% or so of the cost of a commercial loop.
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KM4FVI
Member

Posts: 36




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« Reply #2 on: February 08, 2016, 02:50:56 AM »

Dale thanks for your quick response. My application will be in the 40 and 80m HF bands.

Performance also depends on the vertical angle of the incoming wave. A loop works well on ground wave (like an AM broadcast station) or space waves (at VHF), but not as well for ionosphere propagation because the higher the angle of radiation, the shallower the null.  They are very difficult to use to track NVIS signals.


What antenna system/manufacture would work best for location from my QTH? Mobile operations?

Thanks in advance,
Frank



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WB6BYU
Member

Posts: 17176




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« Reply #3 on: February 09, 2016, 09:32:35 AM »

Quote from: KM4FVI

What antenna system/manufacture would work best for location from my QTH? Mobile operations?



What signals are you trying to hunt?

For vertically polarized signals arriving via ground wave, a loop works well, either air core or wound
on a ferrite rod.  I can typically get a good bearing on a 1W transmitter using a wire tossed over
a tree branch from up to 10km away on 80m.  Using one of these:
http://www.urfmsi.org/nm-arts/wp-content/uploads/2010/10/Dale_Hunt_80_meter.pdf

I made a larger loop about 12" diameter for mobile use, and it also worked, as long as I wasn't
parked under (or over) a power or telephone line, etc.

Maximum ground wave distance depends on ground conductivity and power:  the ARRL estimate
is that it has a maximum expected coverage distance of about 50 miles on 80m using 1kW, assuming
vertical polarization at both ends.  Coverage might be half that on 40m.


For NVIS signals, DF is much more difficult, until you can get close enough to hear the station on
ground wave.  An Adcock array is better than a loop, as it maintains the null at high angles.  However,
signal levels will be weak on 80m for any practical rotatable Adcock due to the necessary shortened
elements and the signal coming in from close to the ends of the antenna.  When the signal is coming
straight down from the ionosphere overhead, it is very difficult to resolve the exact angle, but a very
well calibrated loop or Adcock that is rotatable in both azimuth and elevation (and possibly in
polarization) may give you some ideas.  The biggest problem is accuracy:  a 10 degree elevation error
on an NVIS signal can mean 100 miles in actual location, and the signal actually gets more difficult to
track as you get closer (due to the signal coming from directly overhead), until you can pick up the
ground wave.

Not that it can't be done, but if you are trying, for example, to track a signal on a local net it could
be a couple hundred miles away, so taking bearings from multiple locations around the perimeter may
give you a better idea of where to start, then at some point the best you can do is to try to get
close enough to hear the ground wave.  An Adcock array (possibly made with 4 mobile whips, though
it will require careful calibration of the phase shifts) might work, though it would be difficult to use
while in motion.  You'll have to do some calibration against known stations to see how well it works.
You might also manage a pair of phased loops, though getting enough spacing between them will
again generally rule out mobile operation (but not portable use where there is room to set up a
mast and antenna.) 

It is important to keep in mind the required accuracy:  at VHF, a DF system with a 20 degree error
will still get you to the transmitter, albeit via a spiral route.  When the signal is coming from nearly
straight up, such an error could put it 100 miles from you in any direction.  To get the required sharp
null, the elements of an Adcock need to be precisely matched in phase and amplitude, and/or have
suitable corrections applied, in addition to the elimination of common mode currents and other local
anomalies.

The FCC uses multiple automated receiving sites where the timing of arrival can be precisely measured
and compared (along with propagation data) to work out the relative distance from each of the sites,
in addition to other factors.
Logged
ONAIR
Member

Posts: 3536




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« Reply #4 on: March 21, 2016, 12:00:37 AM »

Quote from: KM4FVI

What antenna system/manufacture would work best for location from my QTH? Mobile operations?



What signals are you trying to hunt?

For vertically polarized signals arriving via ground wave, a loop works well, either air core or wound
on a ferrite rod.  I can typically get a good bearing on a 1W transmitter using a wire tossed over
a tree branch from up to 10km away on 80m.  Using one of these:
http://www.urfmsi.org/nm-arts/wp-content/uploads/2010/10/Dale_Hunt_80_meter.pdf

I made a larger loop about 12" diameter for mobile use, and it also worked, as long as I wasn't
parked under (or over) a power or telephone line, etc.

Maximum ground wave distance depends on ground conductivity and power:  the ARRL estimate
is that it has a maximum expected coverage distance of about 50 miles on 80m using 1kW, assuming
vertical polarization at both ends.  Coverage might be half that on 40m.


For NVIS signals, DF is much more difficult, until you can get close enough to hear the station on
ground wave.  An Adcock array is better than a loop, as it maintains the null at high angles.  However,
signal levels will be weak on 80m for any practical rotatable Adcock due to the necessary shortened
elements and the signal coming in from close to the ends of the antenna.  When the signal is coming
straight down from the ionosphere overhead, it is very difficult to resolve the exact angle, but a very
well calibrated loop or Adcock that is rotatable in both azimuth and elevation (and possibly in
polarization) may give you some ideas.  The biggest problem is accuracy:  a 10 degree elevation error
on an NVIS signal can mean 100 miles in actual location, and the signal actually gets more difficult to
track as you get closer (due to the signal coming from directly overhead), until you can pick up the
ground wave.

Not that it can't be done, but if you are trying, for example, to track a signal on a local net it could
be a couple hundred miles away, so taking bearings from multiple locations around the perimeter may
give you a better idea of where to start, then at some point the best you can do is to try to get
close enough to hear the ground wave.  An Adcock array (possibly made with 4 mobile whips, though
it will require careful calibration of the phase shifts) might work, though it would be difficult to use
while in motion.  You'll have to do some calibration against known stations to see how well it works.
You might also manage a pair of phased loops, though getting enough spacing between them will
again generally rule out mobile operation (but not portable use where there is room to set up a
mast and antenna.) 

It is important to keep in mind the required accuracy:  at VHF, a DF system with a 20 degree error
will still get you to the transmitter, albeit via a spiral route.  When the signal is coming from nearly
straight up, such an error could put it 100 miles from you in any direction.  To get the required sharp
null, the elements of an Adcock need to be precisely matched in phase and amplitude, and/or have
suitable corrections applied, in addition to the elimination of common mode currents and other local
anomalies.

The FCC uses multiple automated receiving sites where the timing of arrival can be precisely measured
and compared (along with propagation data) to work out the relative distance from each of the sites,
in addition to other factors.
  For military applications, the government also uses satellite technology.
Logged
KM4FVI
Member

Posts: 36




Ignore
« Reply #5 on: November 05, 2017, 04:57:10 AM »

Thanks guys!

Frank
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