I posted a few questions about loop antannas but they didn't get
answered over on Yahoo loop antenna group so I will try here. I've
condensed post some. Loops I have been playing with so far are
strictly for 500kHz -1600kHz. I know this is SW group so I think maybe
the info would be good for others that may want to experiment with
loops for SW too.

1) I understand that the common method to couple a portable radio to
the loop is to just stick the radio into the center of the loop.
Doesn't this method then tend to have directivity in two orthagonal
directions? In other words the loop will have a node in the North-South
direction while the ferrite rod will tend to be directive in the
East-West direction, because of the orientation of the ferrite rod
within to the Loop. So now the nice null that you have along the axis
of the loop is destroyed because the ferrite is aligned there. Is this
correct? Maybe it might be good to couple signal in another fashion.


2) Where should the tuning capacitor be located physically relative to
the loop? I would suppose the best place to put the capacitor would be
directly on the coil. The capacitor is a heavy unit from a Heatkit
SB220 Linear amplifier, so it just doesn't want to hang anywhere. I
would like to mount it to the base of the loop and run some sort of low
inductance wire to the loop. Maybe a light gauge sheilded cable or lamp
cord - which would minimize the stray inductance between the tuning
capacitor and loop would be good. Total distance would probably be less
than a foot. I have found that alligator leads want to be twisted
tightly together in order for this thing to work. So is this a good
plan?

3) I have three wiring options as far as the variable capacitor goes.
One gang, two gang in parallel, or two gang in series. Right now I use
two in parallel near the bottom end of the band, and one gang nearer
the top. Should I add more turns to the loop and use a smaller
amount of capacitance. Literature I've read says use more turns and
this will also give more antenna gain. I know that more capacitance
will give me more Q, but not sure at this point if I need any more Q.
I know that V = N * dflux/dtime so maybe more turns means more volts.
I guess it's probably time to go review some detailed literature on
loop antenna design.

5) I am considering going from 20 Gauge magnet over to litz wire. I
would like to hear comments regarding this approach, and also who is a
good litz wire vendor if approach is worthwhile. I think maybe the
crystal radio peeps may have some input on this one.

6) I am also considering the varactor diode tuning approach. Comments?


Thank-you for any guidance you can give with this project.

Regards,
NEO

----- Original Message -----
From: "N9NEO"


I posted a few questions about loop antannas but they didn't get
answered over on Yahoo loop antenna group so I will try here.

That particular Yahoo group is generally very responsive and knowledgeable.
I find that if you ask too many questions at one time, the reponse is lower
than if you keep the questions down to 2 or 3. Break down your questions and
you will probably get a better response. As I said earlier, they are good.

So now the nice null that you have along the axis
of the loop is destroyed because the ferrite is aligned there. Is this
correct?

If you want to use both antennas, place your receiver into the loop in such
a way that the two antennas are mutually aiding each other (they should be
locked together in this position) and keep them this way. In other words, if
you rotate one antenna in one direction, the other one should also rotate in
the same direction.

2) Where should the tuning capacitor be located physically relative to
the loop? I would suppose the best place to put the capacitor would be
directly on the coil. Total distance would probably be less
than a foot. So is this a good plan?

Yes, keep it close to the coil but don't go crazy trying to keep it extra
short. Try your plan plus or minus a foot and you will not hear a
difference.

3) I have three wiring options as far as the variable capacitor goes.
One gang, two gang in parallel, or two gang in series. Right now I use
two in parallel near the bottom end of the band, and one gang nearer
the top.

Sounds complex. Do you have to tune both capacitors in order to resonate the
antenna? Simplify by keeping the capacitor(s) near the easy access point
only. If you can tune your antenna across the range you want (which may be
difficult with just one variable capacitor) then you have accomplished your
goal.

5) I am considering going from 20 Gauge magnet over to litz wire.

I suppose in theory there should be an improvement, but nothing that you
will be able to hear. It's like comparing a million dollars to a
million-and-one dollars. Mathematically the million-and-one is better, but
what can you do with that extra dollar that you can't do with a million?


6) I am also considering the varactor diode tuning approach. Comments?

I also have a loop antenna, for the LF band below 550kHz, that I would like
to tune remotely and have considered varactors. The negative comments I read
about varactors is that because they are diodes, there will be some
rectification effect on incoming signals resulting in unwanted signals. I
have not tried them so I don't know how they will work. If you come up with
a workable solution, please let us know.

Thank-you for any guidance you can give with this project.
Regards,
NEO

No problem. Keep us posted on your findings.

Al KA5JGV
San Antonio, TX
http://home.satx.rr.com/ka5jgv/

Thanks for the comments AL.

I am the first to admit that I am long-winded and tend to write Magnum
Opus posts. I am actually a writer stuck in an engineer's body. Yes,
those that hang over at the loop antenna group have a lot on the ball.
I'll make it a point to try and keep my posts smaller.

I'll do some more experimenting today with the orientation of the
ferrite inside the loop. I found the best coupling between the loop
and the ferrite was when they were at 90 degrees to each other.

The three wiring options are just a fact when you have two gangs on one
shaft. It's not complex. I suspect I just didn't get my point across.
I think each gang is about 450pf, so that options are 900pf, 450pf and
225pf depending on how I wire to them. The capacitor came out of a
linear, which I probably already said, so it is way bigger than it
needs to be. I think when all is said and done I will find a smaller
capacitor and use more turns on the loop.

Ok, I'll stick with magnet wire for now and save the litz experiments
for later. Ditto with the varactor. I've got way too many irons in
the fire now.

Thanks for the positive feedback

73
NEO


Al wrote:
----- Original Message -----
From: "N9NEO"


I posted a few questions about loop antannas but they didn't get
answered over on Yahoo loop antenna group so I will try here.

That particular Yahoo group is generally very responsive and knowledgeable.
I find that if you ask too many questions at one time, the reponse is lower
than if you keep the questions down to 2 or 3. Break down your questions and
you will probably get a better response. As I said earlier, they are good.

So now the nice null that you have along the axis
of the loop is destroyed because the ferrite is aligned there. Is this
correct?

If you want to use both antennas, place your receiver into the loop in such
a way that the two antennas are mutually aiding each other (they should be
locked together in this position) and keep them this way. In other words, if
you rotate one antenna in one direction, the other one should also rotate in
the same direction.

2) Where should the tuning capacitor be located physically relative to
the loop? I would suppose the best place to put the capacitor would be
directly on the coil. Total distance would probably be less
than a foot. So is this a good plan?

Yes, keep it close to the coil but don't go crazy trying to keep it extra
short. Try your plan plus or minus a foot and you will not hear a
difference.

3) I have three wiring options as far as the variable capacitor goes.
One gang, two gang in parallel, or two gang in series. Right now I use
two in parallel near the bottom end of the band, and one gang nearer
the top.

Sounds complex. Do you have to tune both capacitors in order to resonate the
antenna? Simplify by keeping the capacitor(s) near the easy access point
only. If you can tune your antenna across the range you want (which may be
difficult with just one variable capacitor) then you have accomplished your
goal.

5) I am considering going from 20 Gauge magnet over to litz wire.

I suppose in theory there should be an improvement, but nothing that you
will be able to hear. It's like comparing a million dollars to a
million-and-one dollars. Mathematically the million-and-one is better, but
what can you do with that extra dollar that you can't do with a million?


6) I am also considering the varactor diode tuning approach. Comments?

I also have a loop antenna, for the LF band below 550kHz, that I would like
to tune remotely and have considered varactors. The negative comments I read
about varactors is that because they are diodes, there will be some
rectification effect on incoming signals resulting in unwanted signals. I
have not tried them so I don't know how they will work. If you come up with
a workable solution, please let us know.

Thank-you for any guidance you can give with this project.
Regards,
NEO

No problem. Keep us posted on your findings.

Al KA5JGV
San Antonio, TX
http://home.satx.rr.com/ka5jgv/

Al wrote:
----- Original Message -----
From: "N9NEO"


I posted a few questions about loop antannas but they didn't get
answered over on Yahoo loop antenna group so I will try here.

That particular Yahoo group is generally very responsive and knowledgeable.
I find that if you ask too many questions at one time, the reponse is lower
than if you keep the questions down to 2 or 3. Break down your questions and
you will probably get a better response. As I said earlier, they are good.

So now the nice null that you have along the axis
of the loop is destroyed because the ferrite is aligned there. Is this
correct?

If you want to use both antennas, place your receiver into the loop in such
a way that the two antennas are mutually aiding each other (they should be
locked together in this position) and keep them this way. In other words, if
you rotate one antenna in one direction, the other one should also rotate in
the same direction.

2) Where should the tuning capacitor be located physically relative to
the loop? I would suppose the best place to put the capacitor would be
directly on the coil. Total distance would probably be less
than a foot. So is this a good plan?

Yes, keep it close to the coil but don't go crazy trying to keep it extra
short. Try your plan plus or minus a foot and you will not hear a
difference.

3) I have three wiring options as far as the variable capacitor goes.
One gang, two gang in parallel, or two gang in series. Right now I use
two in parallel near the bottom end of the band, and one gang nearer
the top.

Sounds complex. Do you have to tune both capacitors in order to resonate the
antenna? Simplify by keeping the capacitor(s) near the easy access point
only. If you can tune your antenna across the range you want (which may be
difficult with just one variable capacitor) then you have accomplished your
goal.

5) I am considering going from 20 Gauge magnet over to litz wire.

I suppose in theory there should be an improvement, but nothing that you
will be able to hear. It's like comparing a million dollars to a
million-and-one dollars. Mathematically the million-and-one is better, but
what can you do with that extra dollar that you can't do with a million?


6) I am also considering the varactor diode tuning approach. Comments?

I also have a loop antenna, for the LF band below 550kHz, that I would like
to tune remotely and have considered varactors. The negative comments I read
about varactors is that because they are diodes, there will be some
rectification effect on incoming signals resulting in unwanted signals. I
have not tried them so I don't know how they will work. If you come up with
a workable solution, please let us know.

The diodes have a reverse DC bias, so there is no rectification of the
RF signal. Now the Q of the varactor might be an issue.


Thank-you for any guidance you can give with this project.
Regards,
NEO

No problem. Keep us posted on your findings.

Al KA5JGV
San Antonio, TX
http://home.satx.rr.com/ka5jgv/

N9NEO wrote:
1) I understand that the common method to couple a portable radio to
the loop is to just stick the radio into the center of the loop.
Doesn't this method then tend to have directivity in two orthagonal
directions? In other words the loop will have a node in the North-South
direction while the ferrite rod will tend to be directive in the
East-West direction, because of the orientation of the ferrite rod
within to the Loop. So now the nice null that you have along the axis
of the loop is destroyed because the ferrite is aligned there. Is this
correct? Maybe it might be good to couple signal in another fashion.

Every small antenna arrangement responds identically, and has the same null
and peaks. Two loops simply function as a single loop oriented in some
other direction.

The complication is that there are two signals out there, the electric field
and the magnetic field. Loops respond mostly to the magnetic field, and
have an infinitely deep null for the magnetic field. They also respond
somewhat weakly to the electric field, and it's the electric field response
that ``fills in'' the infinitely deep nulls. The careful construction of
loops is to, by balancing things out, try to eliminate as much electric field
response as possible. The depth of the magnetic field null is already
infinite and is unaffected by careful construction.

The reason the E and M fields don't combine like two M fields into a single
antenna's response is that they're 90 degrees out of phase, and so can never
cancel each other by reorienting the antenna.

Whips have a null at endfire that's similarly infinitely deep, except for
magnetic field response filling _it_ in.

--
Ron Hardin


On the internet, nobody knows you're a jerk.