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Old November 19th 07, 02:25 PM posted to rec.radio.amateur.antenna
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Posts: 326
Default Low Noise receiving Loop antenna

Tony, you have received a crash course on small loop antennas minus
the calculus...
Let me make a few comments as a country boy engineer who cannot do
fancy math...

My thought for you to test the loop+Drake is for you to put out a
temporary Beverage as a wire on the ground... Go out to the curb in
the wee hours of night and roll out 500 foot of insulated wire on the
grass between the curb and sidewalk, match it to the Drake with a 9:1
transformer and a ground stake - a terminating resistor and a ground
stake at the far end is a good idea, but can be dispensed with in a
pinch...'
By flipping back and forth between the wire and the loop you will get
an idea of how well, or poorly, the loop is performing... By having
everything ready to go to roll it out and then roll it back up you
should be able to do this test without the neighbors being any wiser
in the morning...

A variation of the poor boy Beverage, is a 100' untuned dipole running
along the perimeter of your estate, waist high no higher which will
be omni directional... It might even prove to be a full time
installation given your city constraints...

Even those of us in the rural areas have noise problems.. Electric
fences, etc.. I have a nasty broad band up through 40 meters noise
source somewhere to my NorthEast, which essentially wipes out weak dx
from europe for me... It is miles from me and I have yet to locate
it...

denny / k8do
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Old November 19th 07, 07:32 PM posted to rec.radio.amateur.antenna
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Default Low Noise receiving Loop antenna

Tony,

The key to improving the loop is to
-increase the induced voltage; and / or to
-improve its efficiency.

Increasing the induced voltage means increasing the size of the loop.
With a small loop, doubling the size of the side will win nearly 6dB of
induced voltage, but it also increases the inductance of the loop which
might degrade impedance matching and defeat most of the increased induced
voltage.

Improving the efficiency means addressing conductor / transmission line
losses and better impedance matching. Most of the 50 some dB of loss is
attributable to impedance matching.

I expressed surprise at the shunt tuning capacitor in an earlier posting.
I know it is a popular circuit, and it features in the ARRL Antenna
Handbook, but that doesn't make it a good circuit. Try the variable
capacitor in series with the coax inner conductor, you should improve the
gain by around 20dB.

Then try a shunt capacitor on the receiver side of the variable
capacitor, start with 1000pF, you should see further improvement in gain
but with a narrower bandwidth.

This is not a new circuit, you will find it in books, certainly at least
where the tuning / matching network is right at the loop gap. The
relocation of the capacitors by a length of transmission line does change
things a little, and it is more complicated to solve, but behaviour is
soemwhat similar.

Should you try this, your findings would be interesting.

The mathematically based approach might not be popular, and I am no
mathemetician, but the approach does reveal why the antenna is
inadequate, and suggests what can be done to improve it.

Owen
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Old November 20th 07, 06:40 AM posted to rec.radio.amateur.antenna
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Default Low Noise receiving Loop antenna

Owen Duffy wrote:

Tony,

The key to improving the loop is to
-increase the induced voltage; and / or to
-improve its efficiency.


I agree, but how is the question


Increasing the induced voltage means increasing the size of the loop.
With a small loop, doubling the size of the side will win nearly 6dB of
induced voltage, but it also increases the inductance of the loop which
might degrade impedance matching and defeat most of the increased induced
voltage.

Improving the efficiency means addressing conductor / transmission line
losses and better impedance matching. Most of the 50 some dB of loss is
attributable to impedance matching.

I expressed surprise at the shunt tuning capacitor in an earlier posting.
I know it is a popular circuit, and it features in the ARRL Antenna
Handbook,


and alot of other places also.


but that doesn't make it a good circuit. Try the variable
capacitor in series with the coax inner conductor, you should improve the
gain by around 20dB.


this one I will try, does this not make the loop a closed loop?


Then try a shunt capacitor on the receiver side of the variable
capacitor, start with 1000pF, you should see further improvement in gain
but with a narrower bandwidth.


Don't have anything like this on hand, this is a variable capacitor I would
assume.


This is not a new circuit, you will find it in books, certainly at least
where the tuning / matching network is right at the loop gap. The
relocation of the capacitors by a length of transmission line does change
things a little, and it is more complicated to solve, but behaviour is
soemwhat similar.

Should you try this, your findings would be interesting.


I might be able to get to this in a day or two and I will let you know what
happened.


The mathematically based approach might not be popular, and I am no
mathemetician, but the approach does reveal why the antenna is
inadequate, and suggests what can be done to improve it.

Owen



I was thinking that maybe I didn't have enough capture area with the loop I
currently use.
My next step was to figure out how to increase the size of the loop so it
captures more signal. Since I havn't looked for any info on a loop like
this I was going to ask you if you had anything on a larger loop but one
which isn't too large and can be rotated either by hand or a rotator.

I think you answered my question(s) at the top of this post.
Doubling the size would still make it small enough to rotate by hand or with
a small tv rotator, matching though might be the other issue as you
mentioned.




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Old November 20th 07, 06:54 AM posted to rec.radio.amateur.antenna
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Posts: 50
Default Low Noise receiving Loop antenna

Denny wrote:

Tony, you have received a crash course on small loop antennas minus
the calculus...
Let me make a few comments as a country boy engineer who cannot do
fancy math...

My thought for you to test the loop+Drake is for you to put out a
temporary Beverage as a wire on the ground... Go out to the curb in
the wee hours of night and roll out 500 foot of insulated wire on the
grass between the curb and sidewalk, match it to the Drake with a 9:1
transformer and a ground stake - a terminating resistor and a ground
stake at the far end is a good idea, but can be dispensed with in a
pinch...'
By flipping back and forth between the wire and the loop you will get
an idea of how well, or poorly, the loop is performing... By having
everything ready to go to roll it out and then roll it back up you
should be able to do this test without the neighbors being any wiser
in the morning...

A variation of the poor boy Beverage, is a 100' untuned dipole running
along the perimeter of your estate, waist high no higher which will
be omni directional... It might even prove to be a full time
installation given your city constraints...

Even those of us in the rural areas have noise problems.. Electric
fences, etc.. I have a nasty broad band up through 40 meters noise
source somewhere to my NorthEast, which essentially wipes out weak dx
from europe for me... It is miles from me and I have yet to locate
it...

denny / k8do



this is a good idea, except I can't go 500 ft in any direction without
getting into my neighbors property.
Best I can do is about 175ft and that would be into my neighbors property
who has some dogs which will at any hour of the day or nite bark until the
neighbor comes out to quiet them down.

I am in the middle of a cul-de-sac that about 200 ft long. All my neighbors
on the other side of the street have dogs. The 2 north and south of me
wouldn't give me enough room to put out enough wire to make this test
worthwhile and the neighbors behind me are anti amateur radio types.
I was here first and had antennas up before they even built their houses but
yet I am the bad dude in the neighborhood.

FORTUNATELY, I have never been accused of any interference even running 1500
watts.

Oh this is a "rural" subdivision so to speak, no curbs no sidewalks. But the
lots are 1/3 acre or slightly more.

Damn, you give me a good idea and I can't use it.



  #65   Report Post  
Old November 20th 07, 06:59 AM posted to rec.radio.amateur.antenna
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Posts: 1,169
Default Low Noise receiving Loop antenna

Tony Giacometti wrote in
:

I expressed surprise at the shunt tuning capacitor in an earlier
posting. I know it is a popular circuit, and it features in the ARRL
Antenna Handbook,


and alot of other places also.


Ok, but they are all flawed.

but that doesn't make it a good circuit. Try the variable
capacitor in series with the coax inner conductor, you should improve
the gain by around 20dB.


this one I will try, does this not make the loop a closed loop?


I don't understand what you mean. Just reconfigure the tuning box so that
the capacitor is in series with the inner conductor of the feedline to
the radio. Is that ambiguous?



Then try a shunt capacitor on the receiver side of the variable
capacitor, start with 1000pF, you should see further improvement in
gain but with a narrower bandwidth.


Don't have anything like this on hand, this is a variable capacitor I
would assume.


No, just try a fixed capacitor. You could also try 2200pF, it will have a
little more gain but narrower bandwidth.


This is not a new circuit, you will find it in books, certainly at
least where the tuning / matching network is right at the loop gap.
The relocation of the capacitors by a length of transmission line
does change things a little, and it is more complicated to solve, but
behaviour is soemwhat similar.

Should you try this, your findings would be interesting.


I might be able to get to this in a day or two and I will let you know
what happened.


Thanks.

I was thinking that maybe I didn't have enough capture area with the
loop I currently use.
My next step was to figure out how to increase the size of the loop so
it captures more signal. Since I havn't looked for any info on a loop
like this I was going to ask you if you had anything on a larger loop
but one which isn't too large and can be rotated either by hand or a
rotator.

I think you answered my question(s) at the top of this post.
Doubling the size would still make it small enough to rotate by hand
or with a small tv rotator, matching though might be the other issue
as you mentioned.


In the simple case of an untuned loop loaded with 50 ohms, when the loop
inductive reactance becomes more than becomes more than about 150 ohms,
doubling the size of the loop doubles the induced voltage and doubles the
reactance, so the loop current hardly increases and not much more voltage
is developed in the 50 ohm load. Do you follow?

A lossless loop of the size you use has a source impedance of 0.0002
ohms. For maximum power transfer, you have to do something better than
deliver a load of the order of a hundred ohms.

It is perhaps a difficult concept to grasp. But it explains why small
antennas don't work so well, it is just so hard to deliver or extract
power because the radiation resistance is so small.

Owen








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Old November 20th 07, 08:07 AM posted to rec.radio.amateur.antenna
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Default Low Noise receiving Loop antenna

Owen Duffy wrote:

Tony Giacometti wrote in
:

I expressed surprise at the shunt tuning capacitor in an earlier
posting. I know it is a popular circuit, and it features in the ARRL
Antenna Handbook,


and alot of other places also.


Ok, but they are all flawed.

but that doesn't make it a good circuit. Try the variable
capacitor in series with the coax inner conductor, you should improve
the gain by around 20dB.


this one I will try, does this not make the loop a closed loop?


I don't understand what you mean. Just reconfigure the tuning box so that
the capacitor is in series with the inner conductor of the feedline to
the radio. Is that ambiguous?



I guess I am unsure as to how the loop gets connected.

since the loop and cap are currently parallel, removing the cap
leaves me with 2 ends of the loop. Putting the cap in series with the
center of the coax feedline allows me to have only one connection to the
loop.

Did I miss something?





Then try a shunt capacitor on the receiver side of the variable
capacitor, start with 1000pF, you should see further improvement in
gain but with a narrower bandwidth.


Don't have anything like this on hand, this is a variable capacitor I
would assume.


No, just try a fixed capacitor. You could also try 2200pF, it will have a
little more gain but narrower bandwidth.


I have caps in that range, and since its receive only, the voltage can be
low.




This is not a new circuit, you will find it in books, certainly at
least where the tuning / matching network is right at the loop gap.
The relocation of the capacitors by a length of transmission line
does change things a little, and it is more complicated to solve, but
behaviour is soemwhat similar.

Should you try this, your findings would be interesting.


I might be able to get to this in a day or two and I will let you know
what happened.


Thanks.


As soon as I figure out how it needs to be wired.




I was thinking that maybe I didn't have enough capture area with the
loop I currently use.
My next step was to figure out how to increase the size of the loop so
it captures more signal. Since I havn't looked for any info on a loop
like this I was going to ask you if you had anything on a larger loop
but one which isn't too large and can be rotated either by hand or a
rotator.

I think you answered my question(s) at the top of this post.
Doubling the size would still make it small enough to rotate by hand
or with a small tv rotator, matching though might be the other issue
as you mentioned.


In the simple case of an untuned loop loaded with 50 ohms, when the loop
inductive reactance becomes more than becomes more than about 150 ohms,
doubling the size of the loop doubles the induced voltage and doubles the
reactance, so the loop current hardly increases and not much more voltage
is developed in the 50 ohm load. Do you follow?


yes, wellbrook has a large capture area loop, I wonder how they get that one
to work?


A lossless loop of the size you use has a source impedance of 0.0002
ohms. For maximum power transfer, you have to do something better than
deliver a load of the order of a hundred ohms.

It is perhaps a difficult concept to grasp. But it explains why small
antennas don't work so well, it is just so hard to deliver or extract
power because the radiation resistance is so small.

Owen


I never though this was going to be this complicated, I thought all I needed
do was to build the loop and problem solved.......so I thought......







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Old November 20th 07, 08:15 AM posted to rec.radio.amateur.antenna
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Default Low Noise receiving Loop antenna

On Mon, 19 Nov 2007 21:07:30 -1000, Tony Giacometti
wrote:

I never though this was going to be this complicated, I thought all I needed
do was to build the loop and problem solved.......so I thought......


At the risk of repetition still not resolving what is simple:

On Sat, 17 Nov 2007 23:14:01 -1000, Tony Giacometti
wrote:

No. The main loop will still be tuned to the same frequency, no
matter if direct fed, or with a coupling loop. Many people build BC
loops and use the ferrite bar antennas in the portable radios
themselves to couple to the loop.

how do I calculate the dimensions of the coupling loop?


Hi Tony,

It has 1/5 the diameter and is a single, shorted loop.

Please don't try to make it more elaborate than it should be. One
wire, in a loop, connected to the other side of the feed (i.e. the
wire runs from the center conductor, 'round the circumference, to the
outer conductor).

Of course, this demands that the bigger loop be equally, simply
described. It too is one piece of wire, turned in a circle, each end
connecting to the resonating capacitor. In this case, you want to
optimize for the lowest Ohmic connections and materials. This may
suggest hardline, but, please, don't think of using the inner
conductor for anything at all - that is a waste of time. You could as
easily use copper tubing.

You can put more effort into it, but it will reward you only in the
sense of being the owner of a fine piece of furniture.

73's
Richard Clark, KB7QHC

73's
Richard Clark, KB7QHC
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Old November 20th 07, 08:21 AM posted to rec.radio.amateur.antenna
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Posts: 1,169
Default Low Noise receiving Loop antenna

Tony Giacometti wrote in
news:yKednQUgeNMRGt_anZ2dnUVZ_oaonZ2d@hawaiiantel. net:

....
I guess I am unsure as to how the loop gets connected.

since the loop and cap are currently parallel, removing the cap
leaves me with 2 ends of the loop. Putting the cap in series with the
center of the coax feedline allows me to have only one connection to
the loop.

Did I miss something?


Someone else will probably have to transate the following for you (I must
have an upside down view of the world from down under):

Disconnect both wires from the capacitor.

Now cut the centre wire from the loop the the feed line coax in the tee
box... there is only one wire that fits this description.

Insert the tuning capacitor (two wires) where you cut the centre
conductor , ie connect the capacitor wires to each side of where you cut
the centre conductor

The capacitor is now in series with the centre conductor of the feedline
connection in the tuning / matching box.

Now connect a 100pF fixed capacitor between the feedline centre conductor
and shield in the tuning box.

Now adjust the capacitor for maximum rx noise.

Owen
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Old November 21st 07, 07:01 AM posted to rec.radio.amateur.antenna
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Default Low Noise receiving Loop antenna

Owen Duffy wrote:

Tony Giacometti wrote in
news:yKednQUgeNMRGt_anZ2dnUVZ_oaonZ2d@hawaiiantel. net:

...
I guess I am unsure as to how the loop gets connected.

since the loop and cap are currently parallel, removing the cap
leaves me with 2 ends of the loop. Putting the cap in series with the
center of the coax feedline allows me to have only one connection to
the loop.

Did I miss something?


Someone else will probably have to transate the following for you (I must
have an upside down view of the world from down under):

Disconnect both wires from the capacitor.



sorry, I'm not trying to be an ass here but the diagram I have shows 3
connections to the cap. 2 for the loop and one for the center conductor of
the feedline.

Is my diagram whats confusing the issue?




Now cut the centre wire from the loop the the feed line coax in the tee
box... there is only one wire that fits this description.

Insert the tuning capacitor (two wires) where you cut the centre
conductor , ie connect the capacitor wires to each side of where you cut
the centre conductor

The capacitor is now in series with the centre conductor of the feedline
connection in the tuning / matching box.

Now connect a 100pF fixed capacitor between the feedline centre conductor
and shield in the tuning box.

Now adjust the capacitor for maximum rx noise.

Owen


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Old November 21st 07, 08:00 AM posted to rec.radio.amateur.antenna
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Default Low Noise receiving Loop antenna

Tony Giacometti wrote in
news:bMednQ1HY8JTVN7anZ2dnUVZ_gGdnZ2d@hawaiiantel. net:
....
sorry, I'm not trying to be an ass here but the diagram I have shows 3
connections to the cap. 2 for the loop and one for the center
conductor of the feedline.

Is my diagram whats confusing the issue?


Here is a diagram of what I suggested:
http://www.vk1od.net/SmallTunedSquareLoop/index.2.gif .

The variable capacitor needs to be 0-400pF. The fixed capacitor should be
at least 1000pF.

If you couldn't arrive at this circuit from the word description, and you
aren't being silly, then it questions whether your intial construction
was based on misinterpretation of the original article.

Here is an even better idea:
http://www.vk1od.net/SmallTunedSquareLoop/index.3.gif .

There is only one change here, the inner conductor is bonded to the outer
conductor at the left hand side of the shield gap. Ask yourself why that
improves things... some will suggest it just halved the loop size, but it
hasn't.



Owen
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