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Author Topic: Alpha Loop for Directional Finding  (Read 101397 times)
KM4FVI
Member

Posts: 37




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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: 18072




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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: 37




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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: 18072




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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.
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ONAIR
Member

Posts: 3687




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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: 37




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

Thanks guys!

Frank
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WB5WPA
Member

Posts: 16




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« Reply #6 on: February 13, 2019, 07:12:20 AM »

[The FCC uses multiple automated receiving sites where the timing of arrival can be precisely measured

Cite please?  Surely this isn't just conjecture.

Until a few years ago their system was much simpler, and connectivity to their remote sites were by a pair of (2) POTS lines via dial-up.

There was an RFQ a few years back to run the equivalent of a couple T-spans to these sites, so an upgrade in capabilities is on the way.

 
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WB5WPA
Member

Posts: 16




Ignore
« Reply #7 on: February 13, 2019, 07:21:57 AM »

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.

This isn't quite true either. I'll post a video a little later to demonstrate how DF on ionospheric incoming can be done.

Right now I have a wideband, wide-area "arcing" noise source that I have to visit with a camera (since it was too dark for photography last night.)

More info on the noise here: https://forums.qrz.com/index.php?threads/lots-of-broken-or-missing-ground-wires-on-local-power-poles.646286/page-5

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NK7Z
Member

Posts: 2383


WWW

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« Reply #8 on: February 13, 2019, 09:33:51 AM »

As always Dale, your posts are most interesting.  I was not even aware of an Adcock  array! 
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Thanks,
Dave
Amateur Radio: RFI help, Reviews, Setup information, and more...
https://www.nk7z.net
NK7Z
Member

Posts: 2383


WWW

Ignore
« Reply #9 on: February 13, 2019, 09:34:20 AM »

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.

This isn't quite true either. I'll post a video a little later to demonstrate how DF on ionospheric incoming can be done.

Right now I have a wideband, wide-area "arcing" noise source that I have to visit with a camera (since it was too dark for photography last night.)

More info on the noise here: https://forums.qrz.com/index.php?threads/lots-of-broken-or-missing-ground-wires-on-local-power-poles.646286/page-5


Looking forward to your video as well!
Logged

Thanks,
Dave
Amateur Radio: RFI help, Reviews, Setup information, and more...
https://www.nk7z.net
WB5WPA
Member

Posts: 16




Ignore
« Reply #10 on: February 13, 2019, 01:04:45 PM »

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.

This isn't quite true either. I'll post a video a little later to demonstrate how DF on ionospheric incoming can be done.

Right now I have a wideband, wide-area "arcing" noise source that I have to visit with a camera (since it was too dark for photography last night.)

More info on the noise here: https://forums.qrz.com/index.php?threads/lots-of-broken-or-missing-ground-wires-on-local-power-poles.646286/page-5


Looking forward to your video as well!

Here's a quick demo I made sometime back - it's not very long, but the point was there is a very definite null that can be had when you reach the angle of the incoming RF ... so you CAN find an azimuth bearing on an HF signal for DF purposes! The stations were in Dallas and as far south as Houston on 80 meters in the morning when prop was still good.

https://www.youtube.com/watch?v=uzaKUmhyCXI

I've done this at 7 MHz too, and the effect was the same. I want to try it on 160 meters too, but the overall angle on 160 looks like its shallower. I remember driving back to Dallas on I-30 from Texarkana and like halfway to Dallas there is a "fading wall"already! Fading wall is the term used when groundwave interferes with skywave - or vice versa usually.

73, Jim


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WB6BYU
Member

Posts: 18072




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« Reply #11 on: February 13, 2019, 08:54:34 PM »

Quote from: WB5WPA

[The FCC uses multiple automated receiving sites where the timing of arrival can be precisely measured


Cite please?  Surely this isn't just conjecture...
 



I don't remember the article, but it was several years ago (around the time they were consolidating
operations so all the field sites worked remotely, rather than having operators at each location).
The article included a discussion of the principles, but not a lot of detail of the exact equipment.
It might have been in QST, or a professional journal.

Basically they could accurately measure the relative propagation delays to multiple monitoring
stations, then they used ionospheric profiles to solve backwards given the relative path lengths
and ionospheric conditions for each path to narrow down the target area.  I suspect it is done
by carefully digitizing the signal at each location with accurate time stamps and crunching the
numbers in a central location.

That would be in addition to the phased array systems that can accurately determine the
azimuth and elevation of the arriving wave from the relative phase shifts across an array of
antennas mounted in a grid, and I would assume that the direction information would be
used to narrow down the initial position that is fed into the propagation model.  Knowing
the relative distances (via the ionosphere) from the monitoring sites to the target station
provides a more accurate way of averaging among them than just using angular data alone.
Logged
WB6BYU
Member

Posts: 18072




Ignore
« Reply #12 on: February 13, 2019, 09:07:55 PM »

Quote from: WB5WPA

Quote from: WB6BYU

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.


This isn't quite true either. I'll post a video a little later to demonstrate how DF on ionospheric incoming can be done.




I didn't say that it couldn't be done, only that it was more difficult to get a useful
bearing.  (By comparison, and Adcock antenna will still give a good null on waves
arriving at high angles.)  You have to rotate the loop in two dimensions and think in
3 rather than rotating it in one direction and plotting in two dimensions.  The angular
accuracy required to find a station coming in via skywave is tighter because a small
variation in angle means a large difference in distance.  I remember reading a story
from Norway where they flew a ham with a hand-held DF receiver around the country
trying to locate a pirate transmitter, but never got close enough to get a bearing
on ground wave.  (Apparently they never landed within 100 km of the station.)

It's not something that you can hang a simple loop on a broomstick out the car window
and expect to locate a station interfering on 80m that may be anywhere within 200
miles of you.


(Even the Adcock stations that the FCC used in WWII had to be calibrated against
known stations:  a spider web across the open wire feedline was enough to cause
a measurable error, and at one site along the coast (New Jersey?) the new DF
station just couldn't get good bearings, until they realized that the site had been
platted out for a town, and metal water pipes had been laid under the streets.  Those
buried pipes were enough to throw off the bearings.)
Logged
NK7Z
Member

Posts: 2383


WWW

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« Reply #13 on: February 13, 2019, 09:08:12 PM »

See:

https://www.rtl-sdr.com/kiwisdr-tdoa-direction-finding-now-freely-available-for-public-use/

for something similar, and working today...  All at your disposal to use.
Logged

Thanks,
Dave
Amateur Radio: RFI help, Reviews, Setup information, and more...
https://www.nk7z.net
WB5WPA
Member

Posts: 16




Ignore
« Reply #14 on: February 14, 2019, 07:51:50 AM »

Quote from: WB5WPA

[The FCC uses multiple automated receiving sites where the timing of arrival can be precisely measured


Cite please?  Surely this isn't just conjecture...
 



I don't remember the article, but it was several years ago (around the time they were consolidating
operations so all the field sites worked remotely, rather than having operators at each location).
The article included a discussion of the principles, but not a lot of detail of the exact equipment.
It might have been in QST, or a professional journal.

Basically they could accurately measure the relative propagation delays to multiple monitoring
stations, then they used ionospheric profiles to solve backwards given the relative path lengths
and ionospheric conditions for each path to narrow down the target area.  I suspect it is done
by carefully digitizing the signal at each location with accurate time stamps and crunching the
numbers in a central location.

That would be in addition to the phased array systems that can accurately determine the
azimuth and elevation of the arriving wave from the relative phase shifts across an array of
antennas mounted in a grid, and I would assume that the direction information would be
used to narrow down the initial position that is fed into the propagation model.  Knowing
the relative distances (via the ionosphere) from the monitoring sites to the target station
provides a more accurate way of averaging among them than just using angular data alone.

Nothing like TOA was in use as far as I have been able to determine. Factors against this are trifold: the era/timeframe in which the legacy system was developed, the comms to each site involved only TWO dial-up pots lines (as I previously indicated), and three, onsite eyes-on examination of their facilities.

What I can confirm in the way of facilities involves stepped baseline/multiple antenna interferometry with EACH set of antennas on three 120 degree radials from a central point. Even a simple examination (had you done this a few years back) using Google earth would confirm this at the various remote field sites, but I had the luxury of on-site inspect while on a hike on the property of the Allegan Mi site. (It was not at the time fenced off, nor marked prohibiting access from adjacent, adjoining property.)

I am not revealing any big secrets here, as they been have working on system-wide upgrades to the HF-DF system given publicly accessible Request For Quotes (RFQs) issued over the past 4 or 5 years now and I don't know what the final capabilities will be, and I would well expect they entail what you have covered, but, the legacy implementation is not near as capable as you described.






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