Loop antenna matching question
 

I'm starting up an old project that I never quite finished - my loop
antenna.

When I first put it together, I tried a simple loop of coax to couple
the radio to the loop. Was never really satisfied with that though.

I'm wanting to try a gamma match, possibly something a little like what
Owen Duffy posted a nice version of.

The Gamma match is fairly understandable to me, and I expect it to work
well.

What I am wondering about is in the coax loop coupling system, how is
the best match obtained? Loop size? orientation? Luck?

- 73 de Mike N3LI -

Loop antenna matching question
 

Last I tried a loop, I used a simple wire loop of about 10 cm diameter
(for a 1 m antenna diameter).

The coax version might save you some noise when receiving, but I never
tried coax myself, I use solid tubes.
For transmitting, coax or not should make no difference.

I do have an antenna tuner for the plain dipol. For the loop, you never
use antenna tuner.

For tuning the loop for transmitting, I find it easy to make the
coupling loop variable in size, and just try smaller/large loops, and
then fix the size when satisfied

Christen
oz1aab

Loop antenna matching question
 

Michael Coslo wrote in news:gvhhkj$do4m$1
@tr22n12.aset.psu.edu:

I'm starting up an old project that I never quite finished - my loop
antenna.

When I first put it together, I tried a simple loop of coax to couple
the radio to the loop. Was never really satisfied with that though.

I'm wanting to try a gamma match, possibly something a little like what
Owen Duffy posted a nice version of.

The Gamma match is fairly understandable to me, and I expect it to work
well.

What I am wondering about is in the coax loop coupling system, how is
the best match obtained? Loop size? orientation? Luck?

Mike, you haven't given much information about the loop (size,
frequency). Is loop balance / symmetry important?

If it was a small loop, I would not be thinking about a gamma match
because of the impedance ratios for just one reason.

Owen

Loop antenna matching question
 

Owen Duffy wrote:
Michael Coslo wrote in news:gvhhkj$do4m$1
@tr22n12.aset.psu.edu:

I'm starting up an old project that I never quite finished - my loop
antenna.

When I first put it together, I tried a simple loop of coax to couple
the radio to the loop. Was never really satisfied with that though.

I'm wanting to try a gamma match, possibly something a little like what
Owen Duffy posted a nice version of.

The Gamma match is fairly understandable to me, and I expect it to work
well.

What I am wondering about is in the coax loop coupling system, how is
the best match obtained? Loop size? orientation? Luck?

Mike, you haven't given much information about the loop (size,
frequency). Is loop balance / symmetry important?

It's Octagonal, .75 inch copper tubing, around 8 feet in diameter. It's
designed to just reach into the 80 meter voice segment with it's
capacitor setup. Upper end is either 17 or 15 meters. It's been a few
years since I worked on this, which is the reason for the "around" remarks.

Capacitor is a 4 tube trombone capacitor, each tube roughly a foot in
length. External tube is .75 inch, and internal tube is .5 inch.
Insulation is wrapped Kevlar sheet around internal tube.

Adjustment is via a power screwdriver and a threaded rod setup to move
the trombones in and out.

- 73 de Mike N3LI -

Loop antenna matching question
 

Owen Duffy wrote:

Mike, you haven't given much information about the loop (size,
frequency). Is loop balance / symmetry important?


Oops, I didn't mention anything about symmetry in that post.

Probably because I'm not sure how important it might be. So if it is an
issue, I'm all ears!

- 73 de Mike N3LI -

Loop antenna matching question
 

If I recall, a full wave loop has an impedence of about 100 ohms, so if you
feed it with an electrical half-wave length of 75-ohm coax, you should have
an acceptable match, and a decent vswr.

"Michael Coslo" wrote in message
...
I'm starting up an old project that I never quite finished - my loop
antenna.

When I first put it together, I tried a simple loop of coax to couple the
radio to the loop. Was never really satisfied with that though.

I'm wanting to try a gamma match, possibly something a little like what
Owen Duffy posted a nice version of.

The Gamma match is fairly understandable to me, and I expect it to work
well.

What I am wondering about is in the coax loop coupling system, how is the
best match obtained? Loop size? orientation? Luck?

- 73 de Mike N3LI -

Loop antenna matching question
 

Michael Coslo wrote in news:gvju7l$n034$4
@tr22n12.aset.psu.edu:

Owen Duffy wrote:

Mike, you haven't given much information about the loop (size,
frequency). Is loop balance / symmetry important?


Oops, I didn't mention anything about symmetry in that post.

Probably because I'm not sure how important it might be. So if it is an
issue, I'm all ears!

- 73 de Mike N3LI -

Mike, the symmetry issue is about whether you want the deepest nulls. A
gamma match is not likely to give you the best symmetry and hence the
deepest nulls.

You are talking about a smallish loop, and without modelling it, I wonder
if the gamma feed is as efficient say, as a inner feed loop.

You have gone to a lot of trouble to keep losses in the loop down, it
would be a pity to compromise it too much with the feed system.

Owen

Loop antenna matching question
 

Hal Rosser wrote:
If I recall, a full wave loop has an impedence of about 100 ohms, so if you
feed it with an electrical half-wave length of 75-ohm coax, you should have
an acceptable match, and a decent vswr.

Methinks you meant *quarter-wave* length of 75-ohm coax.
A half wavelength would merely repeat the feedpoint
impedance.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Loop antenna matching question
 

In message , Cecil Moore
writes
Hal Rosser wrote:
If I recall, a full wave loop has an impedence of about 100 ohms, so
if you feed it with an electrical half-wave length of 75-ohm coax,
you should have an acceptable match, and a decent vswr.

Methinks you meant *quarter-wave* length of 75-ohm coax.
A half wavelength would merely repeat the feedpoint
impedance.

The question is, "Do you use a quarterwave of 75 ohm cable to get
something like a 50 ohm impedance at the TX end - even if this means
using more coax than you really need or, to minimise losses, do you
simply use the shortest length of coax - either 50 or 75 ohm?"
--
Ian

Loop antenna matching question
 

Owen Duffy wrote:
Michael Coslo wrote in news:gvju7l$n034$4

Oops, I didn't mention anything about symmetry in that post.

Probably because I'm not sure how important it might be. So if it is an
issue, I'm all ears!


Mike, the symmetry issue is about whether you want the deepest nulls. A
gamma match is not likely to give you the best symmetry and hence the
deepest nulls.

You are talking about a smallish loop, and without modelling it, I wonder
if the gamma feed is as efficient say, as a inner feed loop.

You have gone to a lot of trouble to keep losses in the loop down, it
would be a pity to compromise it too much with the feed system.


I'm following you here, Owen. I'll put together an inner feed loop and
give that a shot. I was looking at the magloop thread you were in on
eHam from a little over a year ago. Some good info there.

I was thinking about using copper tubing for the loop, but with the area
being a variable, I'm considering using something more flexible. Or
maybe finding out the area by experimentation, then going with a final
loop area in a neater form.


Thanks much!

- 73 de Mike N3LI -

Loop antenna matching question
 

"Owen Duffy" wrote in message
...
Michael Coslo wrote in news:gvhhkj$do4m$1
@tr22n12.aset.psu.edu:

I'm starting up an old project that I never quite finished - my loop
antenna.

When I first put it together, I tried a simple loop of coax to couple
the radio to the loop. Was never really satisfied with that though.

I'm wanting to try a gamma match, possibly something a little like what
Owen Duffy posted a nice version of.

The Gamma match is fairly understandable to me, and I expect it to work
well.

What I am wondering about is in the coax loop coupling system, how is
the best match obtained? Loop size? orientation? Luck?

Mike, you haven't given much information about the loop (size,
frequency). Is loop balance / symmetry important?

If it was a small loop, I would not be thinking about a gamma match
because of the impedance ratios for just one reason.

Owen


Owen -

I've had this nagging idea for a couple of years: What if the (single-turn?)
loop passes through a toroid which already has, say, 10 or so turns on it?
That should give an impedance step-up of 100 or so. Would that not be a good
way to at least get the matching closer? In fact, the tuning capacitor could
be connected to the high impedance side to reduce the capacitor's value.

Your comments will be appreciated.

John

Loop antenna matching question
 

John KD5YI wrote:

"Owen Duffy" wrote in message
...
Michael Coslo wrote in news:gvhhkj$do4m$1
@tr22n12.aset.psu.edu:

I'm starting up an old project that I never quite finished - my loop
antenna.

When I first put it together, I tried a simple loop of coax to couple
the radio to the loop. Was never really satisfied with that though.

I'm wanting to try a gamma match, possibly something a little like what
Owen Duffy posted a nice version of.

The Gamma match is fairly understandable to me, and I expect it to work
well.

What I am wondering about is in the coax loop coupling system, how is
the best match obtained? Loop size? orientation? Luck?

Mike, you haven't given much information about the loop (size,
frequency). Is loop balance / symmetry important?

If it was a small loop, I would not be thinking about a gamma match
because of the impedance ratios for just one reason.

Owen


Owen -

I've had this nagging idea for a couple of years: What if the
(single-turn?) loop passes through a toroid which already has, say, 10
or so turns on it? That should give an impedance step-up of 100 or so.
Would that not be a good way to at least get the matching closer? In
fact, the tuning capacitor could be connected to the high impedance side
to reduce the capacitor's value.


This has been done in a variety of ways (notably some papers from 20
years ago where they used this scheme to couple to the windshield window
frame on a jeep.

For Rx only, there's no real issues

For Tx, though, a small loop will tend to have high currents. There's a
LOT of energy stored in the magnetic field (and in the E field of the
capacitor that tunes the system). All that reactive energy will be
flowing back and forth through the transformer, so it will need a high
VA rating.

Consider this.. if the loop has an electrical Q of, say, 1000 (which
isn't unusual), and you're radiating 100W, then there's 100kW of
circulating power in the system.

Loop antenna matching question
 

Jim Lux wrote:

Consider this.. if the loop has an electrical Q of, say, 1000 (which
isn't unusual), and you're radiating 100W, then there's 100kW of
circulating power in the system.

Works on zero point energy?

- 73 de Mike N3LI -

Loop antenna matching question
 

Ian Jackson wrote:
The question is, "Do you use a quarterwave of 75 ohm cable to get
something like a 50 ohm impedance at the TX end - even if this means
using more coax than you really need or, to minimise losses, do you
simply use the shortest length of coax - either 50 or 75 ohm?"

Usually not a problem since 1/4WL of RG-11
at 4 MHz is only ~40 feet - hopefully less
than the height of the antenna. 1/4WL at any
higher HF frequency would be shorter than that.
Excess coax can usually be coiled into a choke.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Loop antenna matching question
 

"Jim Lux" wrote in message
...
John KD5YI wrote:

"Owen Duffy" wrote in message
...
Michael Coslo wrote in news:gvhhkj$do4m$1
@tr22n12.aset.psu.edu:

I'm starting up an old project that I never quite finished - my loop
antenna.

When I first put it together, I tried a simple loop of coax to couple
the radio to the loop. Was never really satisfied with that though.

I'm wanting to try a gamma match, possibly something a little like what
Owen Duffy posted a nice version of.

The Gamma match is fairly understandable to me, and I expect it to work
well.

What I am wondering about is in the coax loop coupling system, how is
the best match obtained? Loop size? orientation? Luck?

Mike, you haven't given much information about the loop (size,
frequency). Is loop balance / symmetry important?

If it was a small loop, I would not be thinking about a gamma match
because of the impedance ratios for just one reason.

Owen


Owen -

I've had this nagging idea for a couple of years: What if the
(single-turn?) loop passes through a toroid which already has, say, 10 or
so turns on it? That should give an impedance step-up of 100 or so. Would
that not be a good way to at least get the matching closer? In fact, the
tuning capacitor could be connected to the high impedance side to reduce
the capacitor's value.


This has been done in a variety of ways (notably some papers from 20 years
ago where they used this scheme to couple to the windshield window frame
on a jeep.

For Rx only, there's no real issues

For Tx, though, a small loop will tend to have high currents. There's a
LOT of energy stored in the magnetic field (and in the E field of the
capacitor that tunes the system). All that reactive energy will be
flowing back and forth through the transformer, so it will need a high VA
rating.

Consider this.. if the loop has an electrical Q of, say, 1000 (which isn't
unusual), and you're radiating 100W, then there's 100kW of circulating
power in the system.


Adding the transformer changes the circulating energy in the system how?
(Excluding losses)

Loop antenna matching question
 

Michael Coslo wrote:
Jim Lux wrote:

Consider this.. if the loop has an electrical Q of, say, 1000 (which
isn't unusual), and you're radiating 100W, then there's 100kW of
circulating power in the system.

Works on zero point energy?

Nope, the 100k joules/sec was stored in the system
during the initial transient key-down state and was
not radiated. It won't be radiated until the transient
key-up state. This is a characteristic of standing-
wave antennas. In a 1/2WL dipole, only about 20% of
the energy stored in the antenna is radiated. In the
small loop above with its astronomically high SWR,
only about 0.1% of the energy stored in the antenna
is radiated during steady-state.
--
73, Cecil, IEEE, OOTC, http://www.w5dxp.com

Loop antenna matching question
 

Michael Coslo wrote:
Jim Lux wrote:

Consider this.. if the loop has an electrical Q of, say, 1000 (which
isn't unusual), and you're radiating 100W, then there's 100kW of
circulating power in the system.

Works on zero point energy?

- 73 de Mike N3LI -

not at all.. that's the definition of Q.. it's the ratio of stored
energy: energy lost in 1 cycle.

Loop antenna matching question
 

John KD5YI wrote:
"Jim Lux" wrote in message
...
John KD5YI wrote:

"Owen Duffy" wrote in message
...
Michael Coslo wrote in news:gvhhkj$do4m$1
@tr22n12.aset.psu.edu:

I'm starting up an old project that I never quite finished - my loop
antenna.

When I first put it together, I tried a simple loop of coax to couple
the radio to the loop. Was never really satisfied with that though.

I'm wanting to try a gamma match, possibly something a little like
what
Owen Duffy posted a nice version of.

The Gamma match is fairly understandable to me, and I expect it to
work
well.

What I am wondering about is in the coax loop coupling system, how is
the best match obtained? Loop size? orientation? Luck?

Mike, you haven't given much information about the loop (size,
frequency). Is loop balance / symmetry important?

If it was a small loop, I would not be thinking about a gamma match
because of the impedance ratios for just one reason.

Owen


Owen -

I've had this nagging idea for a couple of years: What if the
(single-turn?) loop passes through a toroid which already has, say,
10 or so turns on it? That should give an impedance step-up of 100 or
so. Would that not be a good way to at least get the matching closer?
In fact, the tuning capacitor could be connected to the high
impedance side to reduce the capacitor's value.


This has been done in a variety of ways (notably some papers from 20
years ago where they used this scheme to couple to the windshield
window frame on a jeep.

For Rx only, there's no real issues

For Tx, though, a small loop will tend to have high currents. There's
a LOT of energy stored in the magnetic field (and in the E field of
the capacitor that tunes the system). All that reactive energy will
be flowing back and forth through the transformer, so it will need a
high VA rating.

Consider this.. if the loop has an electrical Q of, say, 1000 (which
isn't unusual), and you're radiating 100W, then there's 100kW of
circulating power in the system.


Adding the transformer changes the circulating energy in the system how?
(Excluding losses)



It doesn't.. it's that the circulating energy in a high Q system is
quite large, so you need to size the transformer (core) appropriately.

The transformer size needed to handle a 100W Tx in a resonant dipole is
MUCH smaller than the transformer needed to handle a 100W Tx in a
compact loop with a Q of 1000. The transformer has to handle not only
the input(and radiated) power, but also the circulating power.

Actually, ALL the components have to deal with the circulating power,
but most builders of compact loops are aware of the need for low
resistance in the loop and high voltage ratings in the capacitor.

Loop antenna matching question
 

On May 29, 10:49*am, Jim Lux wrote:
John KD5YI wrote:

"Owen Duffy" wrote in message
...
Michael Coslo wrote in news:gvhhkj$do4m$1
@tr22n12.aset.psu.edu:

I'm starting up an old project that I never quite finished - my loop
antenna.

When I first put it together, I tried a simple loop of coax to couple
the radio to the loop. Was never really satisfied with that though.

I'm wanting to try a gamma match, possibly something a little like what
Owen Duffy posted a nice version of.

The Gamma match is fairly understandable to me, and I expect it to work
well.

What I am wondering about is in the coax loop coupling system, how is
the best match obtained? Loop size? orientation? Luck?

Mike, you haven't given much information about the loop (size,
frequency). Is loop balance / symmetry important?

If it was a small loop, I would not be thinking about a gamma match
because of the impedance ratios for just one reason.

Owen

Owen -

I've had this nagging idea for a couple of years: What if the
(single-turn?) loop passes through a toroid which already has, say, 10
or so turns on it? That should give an impedance step-up of 100 or so.
Would that not be a good way to at least get the matching closer? In
fact, the tuning capacitor could be connected to the high impedance side
to reduce the capacitor's value.

This has been done in a variety of ways (notably some papers from 20
years ago where they used this scheme to couple to the windshield window
frame on a jeep.

For Rx only, there's no real issues

For Tx, though, a small loop will tend to have high currents. *There's a
LOT of energy stored in the magnetic field (and in the E field of the
capacitor that tunes the system). *All that reactive energy will be
flowing back and forth through the transformer, so it will need a high
VA rating.

Consider this.. if the loop has an electrical Q of, say, 1000 (which
isn't unusual), and you're radiating 100W, then there's 100kW of
circulating power in the system.

Cecil had something to say like that( different figure) and I find it
quite interesting!
The idea of a loop antenna acting as a battery I find odd. The loop is
a tank circuit
where the power supplied replaces the power lost in the resistance of
the components
Without the resistance it would be continual motion! So where is this
"radiation" ratio coming from as the power supplied is purely a
replacement of resistance losses which includes the energy supplied
for radiation? The voltage is in the kilo volts but with current
extremely low I fail to see where the 100kW comes from which makes it
a
battery
Art

Loop antenna matching question
 

"Jim Lux" wrote in message
...
John KD5YI wrote:
"Jim Lux" wrote in message
...
John KD5YI wrote:

"Owen Duffy" wrote in message
...
Michael Coslo wrote in news:gvhhkj$do4m$1
@tr22n12.aset.psu.edu:

I'm starting up an old project that I never quite finished - my loop
antenna.

When I first put it together, I tried a simple loop of coax to couple
the radio to the loop. Was never really satisfied with that though.

I'm wanting to try a gamma match, possibly something a little like
what
Owen Duffy posted a nice version of.

The Gamma match is fairly understandable to me, and I expect it to
work
well.

What I am wondering about is in the coax loop coupling system, how is
the best match obtained? Loop size? orientation? Luck?

Mike, you haven't given much information about the loop (size,
frequency). Is loop balance / symmetry important?

If it was a small loop, I would not be thinking about a gamma match
because of the impedance ratios for just one reason.

Owen


Owen -

I've had this nagging idea for a couple of years: What if the
(single-turn?) loop passes through a toroid which already has, say, 10
or so turns on it? That should give an impedance step-up of 100 or so.
Would that not be a good way to at least get the matching closer? In
fact, the tuning capacitor could be connected to the high impedance
side to reduce the capacitor's value.


This has been done in a variety of ways (notably some papers from 20
years ago where they used this scheme to couple to the windshield window
frame on a jeep.

For Rx only, there's no real issues

For Tx, though, a small loop will tend to have high currents. There's a
LOT of energy stored in the magnetic field (and in the E field of the
capacitor that tunes the system). All that reactive energy will be
flowing back and forth through the transformer, so it will need a high
VA rating.

Consider this.. if the loop has an electrical Q of, say, 1000 (which
isn't unusual), and you're radiating 100W, then there's 100kW of
circulating power in the system.


Adding the transformer changes the circulating energy in the system how?
(Excluding losses)



It doesn't.. it's that the circulating energy in a high Q system is quite
large, so you need to size the transformer (core) appropriately.

The transformer size needed to handle a 100W Tx in a resonant dipole is
MUCH smaller than the transformer needed to handle a 100W Tx in a compact
loop with a Q of 1000. The transformer has to handle not only the
input(and radiated) power, but also the circulating power.

Actually, ALL the components have to deal with the circulating power, but
most builders of compact loops are aware of the need for low resistance in
the loop and high voltage ratings in the capacitor.


I must have missed something. I did not realize that Michael intended to use
the loop with a 100 watt transmitter. My assumption was that it was
primarily a receiving antenna.

However, it doesn't matter because your comments are equally applicable to a
short wire antenna. Therefore, I would urge readers not to consider the
small loop any worse than a short wire antenna in these regards.

Loop antenna matching question
 

"John KD5YI" wrote in
:

....
Owen -

I've had this nagging idea for a couple of years: What if the
(single-turn?) loop passes through a toroid which already has, say, 10
or so turns on it? That should give an impedance step-up of 100 or so.
Would that not be a good way to at least get the matching closer? In
fact, the tuning capacitor could be connected to the high impedance
side to reduce the capacitor's value.

Your comments will be appreciated.

I doubt that the transformer you propose would be near as efficient as the
transformer formed by a smaller feed turn. Though it is tempting to explain
a transformer using turns ratio, the case of the smaller turn is explained
by the ratio of main loop flux intercepted by the feed loop, and that is
determined by area... so deforming the feed loop to change its area will
adjust the transformation.

Owen

Loop antenna matching question
 

"Owen Duffy" wrote in message
...
"John KD5YI" wrote in
:

...
Owen -

I've had this nagging idea for a couple of years: What if the
(single-turn?) loop passes through a toroid which already has, say, 10
or so turns on it? That should give an impedance step-up of 100 or so.
Would that not be a good way to at least get the matching closer? In
fact, the tuning capacitor could be connected to the high impedance
side to reduce the capacitor's value.

Your comments will be appreciated.

I doubt that the transformer you propose would be near as efficient as the
transformer formed by a smaller feed turn. Though it is tempting to
explain
a transformer using turns ratio, the case of the smaller turn is explained
by the ratio of main loop flux intercepted by the feed loop, and that is
determined by area... so deforming the feed loop to change its area will
adjust the transformation.

Owen


Thanks for your reply.

So, as I understand it, there is a large main loop that does the signal
capture. Then there is a smaller loop kind of co-axial with the large loop
and the loops are not electrically connected. Yes? The small loop is a
signal pick-off? Similar to a transformer but with no core?

I seem to recall that a small loop has less inductance than a large loop. I
also seem to recall that the transformation between them is a function of
their inductances. So, a small loop would transform the larger loop's
impedance to an even lower impedance. Is this not so?

John

Loop antenna matching question
 

"John KD5YI" wrote in
:

....
I seem to recall that a small loop has less inductance than a large
loop. I also seem to recall that the transformation between them is a
function of their inductances. So, a small loop would transform the
larger loop's impedance to an even lower impedance. Is this not so?

You can think of it in terms of inductance, but visit what inductance
really means (ie the relationship between inductance, current and flux).

I gave you an explanation in terms of area of the two loops. If your feed
loop os 20% of the area of the main loop, it intercepts 20% of the flux,
the voltage induced in the feed loop is 20% (1/5) of the main loop
voltage, the impedance is transformed by 1/(1/5^2)=25, so a 2 ohm main
loop is transformed to a 50 ohm load at the feed loop feed point.

Now you can expand that and write it in terms of inductance... but Mike
can measure the loop dimensions with a ruler and calculate area.
Measuring inductance of the components is fraught with problems.

So, if Mike works out what the radiation+loss resistance is of his main
loop, divides it by 50, takes the square root, it gives him the portion
of the main loop area to be occupied by the feed loop. Initially, one
might make the area of the feed loop 20%-50% larger and squash it to fine
tune the transformation ratio.

Owen

Loop antenna matching question
 

"Owen Duffy" wrote in message
...
"John KD5YI" wrote in
:

...
I seem to recall that a small loop has less inductance than a large
loop. I also seem to recall that the transformation between them is a
function of their inductances. So, a small loop would transform the
larger loop's impedance to an even lower impedance. Is this not so?

You can think of it in terms of inductance, but visit what inductance
really means (ie the relationship between inductance, current and flux).

I gave you an explanation in terms of area of the two loops. If your feed
loop os 20% of the area of the main loop, it intercepts 20% of the flux,
the voltage induced in the feed loop is 20% (1/5) of the main loop
voltage,


I understand this and I agree. It makes perfect sense.


the impedance is transformed by 1/(1/5^2)=25, so a 2 ohm main
loop is transformed to a 50 ohm load at the feed loop feed point.


This is the part with which I disagree. Can you show me why it is 25 rather
than 1/25?

(snipped section with which I agree)

Owen

Loop antenna matching question
 

"John KD5YI" wrote in
:


"Owen Duffy" wrote in message
...
"John KD5YI" wrote in
:

...
I seem to recall that a small loop has less inductance than a large
loop. I also seem to recall that the transformation between them is
a function of their inductances. So, a small loop would transform
the larger loop's impedance to an even lower impedance. Is this not
so?

You can think of it in terms of inductance, but visit what inductance
really means (ie the relationship between inductance, current and
flux).

I gave you an explanation in terms of area of the two loops. If your
feed loop os 20% of the area of the main loop, it intercepts 20% of
the flux, the voltage induced in the feed loop is 20% (1/5) of the
main loop voltage,


I understand this and I agree. It makes perfect sense.


the impedance is transformed by 1/(1/5^2)=25, so a 2 ohm main
loop is transformed to a 50 ohm load at the feed loop feed point.

That is wrong, isn't it? What was I ever thinking?

The main loop isn't anything like 2 ohms at resonance, it is much much
higher, and the voltage is quite high for a given power.

The reduction in flux cutting the feed loop means that feed loop voltage
is reduced proportionately to area, and therefore the impedance (in a
lossless system) would be decreased by the square of the voltage
reduction


This is the part with which I disagree. Can you show me why it is 25
rather than 1/25?

It is, but it is not 2 ohms that is reduced, it is something much much
higher.

Thanks.

Owen

Loop antenna matching question
 

On Sat, 30 May 2009 07:19:26 GMT, Owen Duffy wrote:

The main loop isn't anything like 2 ohms at resonance, it is much much
higher, and the voltage is quite high for a given power.

Hi Owen,

By your latest comment, it appears you have jumped off the rails
completely. Let's look at the spec:

On Wed, 27 May 2009 13:36:53 -0400, Michael Coslo
wrote:
It's Octagonal, .75 inch copper tubing, around 8 feet in diameter. It's
designed to just reach into the 80 meter voice segment with it's
capacitor setup.
which through any of several methods reveals radiation resistance
equals 0.27 Ohms. The need for large conductors, high voltage
components, and loop (which, for 80M would be half a meter) coupling
to obtain the proper match are all found in their proper
relationships.

Upper end is either 17 or 15 meters.
where the ratio of the physical dimensions to the wavelength (roughly
half wave) renders a much higher value.

Reggie covered all of this years ago. Unfortunately, he buried the
best of his legacy in a drift of bantering making it an extreme chore
to mine Google for the relevant postings.

73's
Richard Clark, KB7QHC

Loop antenna matching question
 

Richard Clark wrote in
:

On Sat, 30 May 2009 07:19:26 GMT, Owen Duffy wrote:

The main loop isn't anything like 2 ohms at resonance, it is much much
higher, and the voltage is quite high for a given power.

Hi Owen,

By your latest comment, it appears you have jumped off the rails
completely. Let's look at the spec:

On Wed, 27 May 2009 13:36:53 -0400, Michael Coslo
wrote:
It's Octagonal, .75 inch copper tubing, around 8 feet in diameter. It's
designed to just reach into the 80 meter voice segment with it's
capacitor setup.
which through any of several methods reveals radiation resistance
equals 0.27 Ohms. The need for large conductors, high voltage
components, and loop (which, for 80M would be half a meter) coupling
to obtain the proper match are all found in their proper
relationships.

My reckoning:

I ran up a model in NEC of a vertical circular loop of 2.4m diameter,
made of 19mm dia copper conductor, and located with its centre 3m above
average ground.

The feedpoint impedance is 0.13+j168 at 3.5MHz.

If that was resonated with a lossless capacitor, the parallel load
impedance would be about 220k+j0.

To transform that to 50+j0, the impedance ratio is 4400:1, the voltage
ratio is 4400^0.5:1 =66:1, the areas need to be in a ratio of 1:66, the
diameters (for circular feed loop) 1:66^0.5 =1:8.1. The feed loop would
be about 12% of the diameter of the main loop or about 0.3m.

....
Reggie covered all of this years ago. Unfortunately, he buried the
best of his legacy in a drift of bantering making it an extreme chore
to mine Google for the relevant postings.

Reg did leave us a calculator for this problem (MAGLOOP4) but without
(AFAIK) documentation of the method used. MAGLOOP4 comes up with slightly
different figures to my NEC model, slightly higher loss, slightly lower
main loop // Z, slightly larger feed loop.

Owen

Loop antenna matching question
 

"Owen Duffy" wrote in message
...
"John KD5YI" wrote in
:


"Owen Duffy" wrote in message
...
"John KD5YI" wrote in
:

...
I seem to recall that a small loop has less inductance than a large
loop. I also seem to recall that the transformation between them is
a function of their inductances. So, a small loop would transform
the larger loop's impedance to an even lower impedance. Is this not
so?

You can think of it in terms of inductance, but visit what inductance
really means (ie the relationship between inductance, current and
flux).

I gave you an explanation in terms of area of the two loops. If your
feed loop os 20% of the area of the main loop, it intercepts 20% of
the flux, the voltage induced in the feed loop is 20% (1/5) of the
main loop voltage,


I understand this and I agree. It makes perfect sense.


the impedance is transformed by 1/(1/5^2)=25, so a 2 ohm main
loop is transformed to a 50 ohm load at the feed loop feed point.

That is wrong, isn't it? What was I ever thinking?

The main loop isn't anything like 2 ohms at resonance, it is much much
higher, and the voltage is quite high for a given power.

The reduction in flux cutting the feed loop means that feed loop voltage
is reduced proportionately to area, and therefore the impedance (in a
lossless system) would be decreased by the square of the voltage
reduction


This is the part with which I disagree. Can you show me why it is 25
rather than 1/25?

It is, but it is not 2 ohms that is reduced, it is something much much
higher.

Thanks.

Owen


Thanks for clarifying, Owen.

Also, your reply to Richard Clark was particularly helpful to me as I seem
to learn better by example. I now see what you are doing and how you arrive
at your conclusions. It has all been very enlightening to me.

John

Loop antenna matching question
 

On Sat, 30 May 2009 21:54:44 GMT, Owen Duffy wrote:

Reg did leave us a calculator for this problem (MAGLOOP4)

Hi Owen,

but without
(AFAIK) documentation of the method used.

Hence my statement of his work being buried in his banter in this
group. Using MAGLOOP as a keyword might help uncover some of his
narrative documentation.

MAGLOOP4 comes up with slightly
different figures to my NEC model, slightly higher loss, slightly lower
main loop // Z, slightly larger feed loop.

Which by several methods hews closely to a common answer (and by far
better accuracy than most guesses).

73's
Richard Clark, KB7QHC