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I've been experimenting with some homebrew direction finding gear,
namely loops and a Wellbrook amp. I have two loops, which for this
discussion will be called large and small. The large loop is about 2ft
on a side, and the small loop is about 1 ft on a side. The loops have a
platform to hold a compass, plus a handle to hold the loop. The handles
and compass platform are plastic. I use a magnifier over the compass to
minimize error. The compass is your basic Silva (about $10 at REI).

The beacon is 374Khz, located in Livermore, Ca. It backs right up to
hills on its east, which I'm sure adds nasty reflections.

The first measurement, point "1", is 2.4 miles from the beacon. I only
had the large loop at the time I did that measurement. The null was
very sharp.

I removed point "2" from the digram as the error was huge. I could not
get a decent null. At the time, I wasn't sure if the loop was the
problem, or the location. That was when I thought building a smaller
loop for comparison would be a good idea.

I used both the large and small loops at point "3". The null was good
with both loops, but clearly the smaller loop had less error.

At point 4, the large loop had a poor null, so I skipped the reading.
The small loop had 1 degree error.

I found that using the radio in CW mode with a narrow CW filter made
finding the null a bit easier.

Any ideas on how to improve the technique would be appreciated. The
goal of this exercise is to locate a beacon that is in a location where
deadly force is authorized, so I'll need accuracy from at least 10
miles away and probably not line of sight in all locations.

Buy an aircraft ADF receiver. The Bendix/King KR87 is an excellent
unit. Don't forget to install the loop antenna on a suitably large
groundplane and be sure to correct for QE (Quadrantal Error).

Brad.

"bradvk2qq" wrote in message
oups.com...
Buy an aircraft ADF receiver. The Bendix/King KR87 is an excellent
unit. Don't forget to install the loop antenna on a suitably large
groundplane and be sure to correct for QE (Quadrantal Error).

Brad.

A loop will not require a ground pane- in fact a local ground plane
(aircraft's conductive body) is what causes quadrantal error.
As long as your loop is well balanced, showing deep nulls, there should not
be a problem.
With my 7' rotatable shielded loop ( shielding serving the purpose of
balance, no improvement in S/N) I can see over 20dB nulls on ground wave AM
stations during the day.

Dale W4OP

I'm quite sure the null is better than the directional antenna

I have put some thought into the S-meter, i.e. detecting the null. It
would be best to have
a meter on the AGC, but I don't have any radios with such a tap from
the factory. If I use the radio in CW mode, I could measure the AC
audio level, but the null is sharp enough that I trust my ear. That is,
I look at the S-meter, but the audio null is just as good.

I was curious about the effect of the size of the loop, with the
smaller loop being much better.

Lumpy wrote:
wrote:

I've been experimenting with some homebrew direction finding gear,
namely loops and a Wellbrook amp. I have two loops, which for this
discussion will be called large and small...

Any ideas on how to improve the technique would be appreciated...

Loop Yagi (quad). Directors will narrow the pattern width.
Reflector will increase the sensitivity. I'd start with
directors since at 10 miles and at low freqs(?) sensitivity
probably isn't as important as beam width. Get more bang
for the buck with narrow vs gain.

What are you using for the readout of the amplitude
of the signal? Some kind of external meter? Analog or digital?
Audio?


Lumpy
--
You were on CHiPs? Did you wear those sexy
tight pants and high boots like Erik Estrada?
No. Swimtrunks.
www.lumpymusic.net

wrote in message
oups.com...
I'm quite sure the null is better than the directional antenna

I have put some thought into the S-meter, i.e. detecting the null. It
would be best to have
a meter on the AGC, but I don't have any radios with such a tap from
the factory. If I use the radio in CW mode, I could measure the AC
audio level, but the null is sharp enough that I trust my ear. That is,
I look at the S-meter, but the audio null is just as good.

I was curious about the effect of the size of the loop, with the
smaller loop being much better.
The difference between a 1' and 2' loop is almost incosequential. Of more
importance is the depth of the nulls ( i.e. loop balance).

Your S meter IS connected to the AGC line- that is what it measures.

Dale W4OP

My s-meter is also a few bars on a LCD, which is why I thought being
able to read the voltage directly would be handy. Now that I think
about it, I can read the S-meter voltage through the serial port.

The difference between the 1ft and 2ft loop is a big deal in that the
smaller loop has deeper nulls and was much more accurate.

Dale Parfitt wrote:
wrote in message
oups.com...
I'm quite sure the null is better than the directional antenna

I have put some thought into the S-meter, i.e. detecting the null. It
would be best to have
a meter on the AGC, but I don't have any radios with such a tap from
the factory. If I use the radio in CW mode, I could measure the AC
audio level, but the null is sharp enough that I trust my ear. That is,
I look at the S-meter, but the audio null is just as good.

I was curious about the effect of the size of the loop, with the
smaller loop being much better.
The difference between a 1' and 2' loop is almost incosequential. Of more
importance is the depth of the nulls ( i.e. loop balance).

Your S meter IS connected to the AGC line- that is what it measures.

Dale W4OP

wrote in message
ups.com...
My s-meter is also a few bars on a LCD, which is why I thought being
able to read the voltage directly would be handy. Now that I think
about it, I can read the S-meter voltage through the serial port.

The difference between the 1ft and 2ft loop is a big deal in that the
smaller loop has deeper nulls and was much more accurate.

There is no theoretical reason why a 1' loop would have any different nulls
than a 2' or 10' loop ( at the freqs we are discussing).
If the nulls are deeper than there is a balance problem with the 2' loop.

I agree- LCD (digital ) S meters are of little value here.
Dale W4OP

I suppose the larger loop can couple to my body easier than the smaller
loop, though technically my body is in the null of the loop. Come to
think of it, my body would be very "near field", so that probably isn't
true.


Dale Parfitt wrote:
wrote in message
ups.com...
My s-meter is also a few bars on a LCD, which is why I thought being
able to read the voltage directly would be handy. Now that I think
about it, I can read the S-meter voltage through the serial port.

The difference between the 1ft and 2ft loop is a big deal in that the
smaller loop has deeper nulls and was much more accurate.

There is no theoretical reason why a 1' loop would have any different nulls
than a 2' or 10' loop ( at the freqs we are discussing).
If the nulls are deeper than there is a balance problem with the 2' loop.

I agree- LCD (digital ) S meters are of little value here.
Dale W4OP

"A loop will not require a ground pane- in fact a local ground plane
(aircraft's conductive body) is what causes quadrantal error. "



True, but an aircraft unit, anticipating a QE, already has a certain
amount of error built in, so even without a groundplane, the antenna
would still need to be swung to check for correct bearings. The beauty
of the system is that it does not rely on the interpretation of nulls
or S meter readings. It simply points directly to the station.

I cannot but think a GPS might be more accurate somehow!

Brad.

I was thinking maybe I can find some coax that will fit tightly in
copper pipe so that I could use a shield. Do you think that would help
the null?

The reason I need copper pipe is that I need the construction to be
very rigid so that I can accurately align the compass to the loop.

Dale Parfitt wrote:
wrote in message
ups.com...
My s-meter is also a few bars on a LCD, which is why I thought being
able to read the voltage directly would be handy. Now that I think
about it, I can read the S-meter voltage through the serial port.

The difference between the 1ft and 2ft loop is a big deal in that the
smaller loop has deeper nulls and was much more accurate.

There is no theoretical reason why a 1' loop would have any different nulls
than a 2' or 10' loop ( at the freqs we are discussing).
If the nulls are deeper than there is a balance problem with the 2' loop.

I agree- LCD (digital ) S meters are of little value here.
Dale W4OP