Hf Antenna Question - so what's the conclusion?
 

....... bunched coils of coaxial cable,
but I must have missed some posting(s), for I haven't recognized
anything that looks like a CONCLUSION.

* Is "bunched" or "flat" better?


You have recognized a GREAT TRUTH about this group. There NEVER is a
conclusion!
But, there is NEVER a concluding post to a thread.

You need to do some reading.
Try "Conclusions for Dummies", along with
Feynman's "Special Relativity
and the Dipole" and possibly also
"String Theory and
Its Applications to Near Field Anomalies".

funny those things are simple, and pretty much all i understand, the
other stuff?? thats what really confuses me

Hf Antenna Question - so what's the conclusion?
 

Cecil Moore wrote:
wrote:
8 turns loose wound in a 6.625" diameter bundle looks like an effective
single band balun for 80m (approx 500 ohms).

A ladder-line fed dipole might present an impedance of
8000 ohms to the choke. A 500 ohm choke would have
very little effect. However, a self-resonant choke
might have 40K ohms of choking impedance and it
would need to have a lot more turns than 8.


There's a disconnect between these two statements: they make very
different assumptions about the level of performance that is needed.

A couple of days ago, Bill re-quoted the WA2SRQ measurements, which are
the same ones we've been discussing for the past week (seems like more
:-)
http://www.bcdxc.org/balun_information.htm#Ed,%20WA2SRQ

However, it's very interesting to read the whole of that web page, which
is a much longer discussion involving several other designers and users
of feedline chokes (aka choke baluns). In that discussion, there was a
largely unspoken agreement that, to merit being called "effective", a
choke should have an impedance of at least 10 times the cable Zo, ie at
least 500 ohms.

If 500 ohms is all you need, a coiled-cable choke of either the
"bunched" or the "solenoid" type certainly can cover at least two
amateur bands an octave apart in frequency... but a wide range of
ferrite chokes can do the same, and these have the advantage of being
much more broadband so they need no tuning.

In contrast, Cecil is asking for a much higher impedance. If that's what
you really need, then nothing can beat a resonant coiled-cable choke,
which can give a resonant impedance well above 10kohms and possibly as
high as 40kohms [1]. Even the very best ferrite choke can't come
anywhere near that... but the resonant choke will always be a
single-band device and will always have to be adjusted carefully.

[1] Measurements on various kinds of chokes, at:
http://w8ji.com/Baluns/balun_test.htm (table about half-way down the
page).


The performance of a common-mode choke is always going to depend on the
exact situation in which it is used, so the measured impedance can never
be more than an indication of its *potential* performance. For example,
a 500-ohm choke may completely kill the common-mode problems at one
station (or on one band) and have almost no effect in a different
situation.

On balance, it might be better to say that 500 ohms should be considered
the *minimum* value of impedance that can offer some promise of being
useful in a wide variety of different situations.

But maybe Cecil is setting the bar too high by imagining the worst
possible case. If an antenna/feedline system has a common-mode problem
that needs an almost impossibly high value of choking impedance, then
(as I said earlier) the choke isn't really the thing we should be
looking at. It's the root cause of that problem - the antenna and/or
feedline itself - that needs attention.


--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Hf Antenna Question - so what's the conclusion?
 

Ian White GM3SEK wrote:
But maybe Cecil is setting the bar too high by imagining the worst
possible case.

I immediately followed that posting up with another in
which I said 500 ohms of choking impedance is sufficient
for a resonant 1/2WL coax-fed dipole. But assuming the
1/2WL dipole is resonant on 3.75 MHz, 500 ohms of choking
impedance is really not enough at the band edges.

There's another consideration. The location of the common-
mode nodes and antinodes are not necessarily related to the
differential signals because the velocity factor is different
for common-mode signals Vs differential-mode signals. One
could encounter the "worst possible case" and not even be
aware of it.

The moral is: Follow Ian's advice and actually make common-mode
current measurements before and after installing the choke.
I have seen the installation of a choke actually increase the
common-mode current at a point 1/4WL away from the choke.
--
73, Cecil http://www.w5dxp.com

Hf Antenna Question - so what's the conclusion?
 

On Fri, 27 Oct 2006 13:31:57 +0100, Ian White GM3SEK
wrote:

In that discussion, there was a
largely unspoken agreement that, to merit being called "effective", a
choke should have an impedance of at least 10 times the cable Zo, ie at
least 500 ohms.

Hi Ian,

I haven't read to the depth you have, but if your synopsis is
accurate, they are wrong. The generality should be 10 times the
source Z.

If 500 ohms is all you need, a coiled-cable choke of either the
"bunched" or the "solenoid" type certainly can cover at least two
amateur bands an octave apart in frequency... but a wide range of
ferrite chokes can do the same, and these have the advantage of being
much more broadband so they need no tuning.

And if the antennas exhibit a near-match, 500 Ohms is sufficient as
those antennas will be presenting a low (~50 Ohm) source Z.

a 500-ohm choke may completely kill the common-mode problems at one
station (or on one band) and have almost no effect in a different
situation.

This would the consequence of a very high source Z, such as a half
wave end fed design.

On balance, it might be better to say that 500 ohms should be considered
the *minimum* value of impedance that can offer some promise of being
useful in a wide variety of different situations.

Minimum, optimal, best, standard.... All such terms demand a criteria
that is trying to be met. No choke is 100% effective. The two most
compelling of those criteria are
1. RF shock hazard;
2. Antenna null filling.

The shock hazard is often ignored if it is not perceived (can't feel
it, and yet instrumentation might be pegged).

Null filling, or poorer F/B, or F/S performance may only become a
nuisance or a real concern depending upon the individual.

My own standard is 5 times the source Z, or at least 3 times the
source Z. 5 times is suitable for reasonable measurement accuracy,
but filling some other criteria throws us back into the subjective
quagmire.

73's
Richard Clark, KB7QHC

Hf Antenna Question - so what's the conclusion?
 

On Fri, 27 Oct 2006 13:31:57 +0100, Ian White GM3SEK
wrote:

....
A couple of days ago, Bill re-quoted the WA2SRQ measurements, which are
the same ones we've been discussing for the past week (seems like more
:-)
http://www.bcdxc.org/balun_information.htm#Ed,%20WA2SRQ

However, it's very interesting to read the whole of that web page, which
is a much longer discussion involving several other designers and users
of feedline chokes (aka choke baluns). In that discussion, there was a
largely unspoken agreement that, to merit being called "effective", a
choke should have an impedance of at least 10 times the cable Zo, ie at
least 500 ohms.

If 500 ohms is all you need, a coiled-cable choke of either the
"bunched" or the "solenoid" type certainly can cover at least two
amateur bands an octave apart in frequency... but a wide range of
ferrite chokes can do the same, and these have the advantage of being
much more broadband so they need no tuning.


Ian,

I agree that this criteria is oft cited, but it bears examination.

It seems to derive from a bench test of a balun, where the balun
shunts the balanced load (on one or both legs) by some impedance,
often assumed to be purely inductive reactance (though that is not
true from some baluns, eg the common W2DU style), and that if that
impedance is 10 or more times the balanced load impedance, then the
impact of the shunt reactance is negligible, and the common mode
current caused by a single leg shunt is negligible.

This might be a reasonable criteria for a balun in a bench situation
or equipment room situation (eg between instruments or boxes with a
mix of balanced and unbalanced interfaces), it should lead to low
insertion VSWR which might be important if one was VSWR focused or
obsessed.

I suggest that such a criteria is not complete enough in itself to
predict the impact of the balun on common mode current or balun loss
in an antenna + feedline + transmitter + ground scenario.

NEC modelling of some scenarios suggests to me that effect of common
mode chokes at different frequencies depends not only on their
impedance, but also on their location, and that sometimes more than
one choke may be more effective than a single larger choke.

Those models also reveal the standing wave nature of the common mode
feedline current, and the futility of taking a current probe
measurement at a single location to infer any more than the current at
that specific location (if that was important).

Owen
--

Hf Antenna Question - so what's the conclusion?
 

Owen Duffy wrote:
On Fri, 27 Oct 2006 13:31:57 +0100, Ian White GM3SEK
wrote:

...
A couple of days ago, Bill re-quoted the WA2SRQ measurements, which are
the same ones we've been discussing for the past week (seems like more
:-)
http://www.bcdxc.org/balun_information.htm#Ed,%20WA2SRQ

However, it's very interesting to read the whole of that web page, which
is a much longer discussion involving several other designers and users
of feedline chokes (aka choke baluns). In that discussion, there was a
largely unspoken agreement that, to merit being called "effective", a
choke should have an impedance of at least 10 times the cable Zo, ie at
least 500 ohms.

If 500 ohms is all you need, a coiled-cable choke of either the
"bunched" or the "solenoid" type certainly can cover at least two
amateur bands an octave apart in frequency... but a wide range of
ferrite chokes can do the same, and these have the advantage of being
much more broadband so they need no tuning.


Ian,

I agree that this criteria is oft cited, but it bears examination.

It seems to derive from a bench test of a balun, where the balun
shunts the balanced load (on one or both legs) by some impedance,
often assumed to be purely inductive reactance (though that is not
true from some baluns, eg the common W2DU style), and that if that
impedance is 10 or more times the balanced load impedance, then the
impact of the shunt reactance is negligible, and the common mode
current caused by a single leg shunt is negligible.

This might be a reasonable criteria for a balun in a bench situation
or equipment room situation (eg between instruments or boxes with a
mix of balanced and unbalanced interfaces), it should lead to low
insertion VSWR which might be important if one was VSWR focused or
obsessed.

I suggest that such a criteria is not complete enough in itself to
predict the impact of the balun on common mode current or balun loss
in an antenna + feedline + transmitter + ground scenario.


That was very much what I said, farther down the message from which you
quote.

Impedance isn't everything, but it does show the important difference
between "good" chokes that are capable of making a large change, and
"poor" chokes that will always have a much smaller effect.

The next thing is a different point, which can limit the effectiveness
of even the best of chokes.

NEC modelling of some scenarios suggests to me that effect of common
mode chokes at different frequencies depends not only on their
impedance, but also on their location, and that sometimes more than
one choke may be more effective than a single larger choke.

Those models also reveal the standing wave nature of the common mode
feedline current, and the futility of taking a current probe
measurement at a single location to infer any more than the current at
that specific location (if that was important).

Common-mode current is well known to be "standing wave" in nature, in
the sense that it has maxima and minima alternating along the line,
separated by an electrical quarter-wavelength.

If you disrupt this pattern by inserting a common-mode choke at a
current maximum, you're going to force a current minimum at that point.
(That can only happen if the choke has a sufficiently high impedance,
but let's assume it has.) RF currents on the entire antenna-feedline
system will then rearrange themselves to take this new factor into
account.

But something must have caused this tendency to have significant
common-mode currents in the first place - most commonly an asymmetrical
layout of the antenna and/or the feedline, eg when the feedline to a
dipole runs parallel to the antenna underneath one side. A common-mode
choke can treat the symptom (the common-mode current) but it cannot
remove the root cause (the layout). In such cases, inserting a choke
will cause a new common-mode current maximum to pop up, a
quarter-wavelength away.

Now if you insert another choke at this new current maximum, you have
made it extremely difficult for the line to support any common-mode
current between those two chokes - but yet another new current maximum
will pop up, a further quarter-wave away. So on you go, rather like
trying to squeeze down a long balloon...

However, you are actually making progress. Every correctly placed choke
makes it more difficult for common-mode current to exist on the line,
and takes you farther away from the local field of the antenna.

The name of the game is to reduce the common-mode current along the
whole of the feedline, as much as you can, but above all to reduce the
current at the location of the equipment that is being affected (eg the
transceiver). Therefore it's always important to monitor the current at
the victim equipment, and be aware that it can sometimes increase.

In practical terms, the two most important places to try a common-mode
choke a

* At the end of the coax, where it breaks out to feed the antenna, or
into parallel line, because this is where common-mode current can be
launched onto the outside of the coax by a direct hard-wired connection.

* At the victim equipment in the shack, or at some "gateway" point like
the ATU.

Because of the "pop-up" nature of the current maxima, there are always
exceptions for certain layouts and feeder lengths, but these are two
good practical places to start.

It can be difficult, and an RF current probe will be your best friend
and faithful tracker... but it's never futile.





--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Hf Antenna Question - so what's the conclusion?
 

Owen Duffy wrote:
Those models also reveal the standing wave nature of the common mode
feedline current, and the futility of taking a current probe
measurement at a single location to infer any more than the current at
that specific location (if that was important).

In fact, the common-mode currents on the feedline
turn the feedline into a standing-wave antenna.
I suspect that's how an Isotron antenna works.
--
73, Cecil http://www.w5dxp.com

Hf Antenna Question - so what's the conclusion?
 

Cecil Moore wrote:
I also have a GDO. If I get an open circuit dip, would
that be the parallel resonant frequency? If I get a
short circuit dip, would that be the series resonant
frequency?

I have a 20 turn choke wound on a 5.35" diameter
Quaker Oatmeal box at four turns per inch. I used
my Grid Dip Oscillator to determine the open-circuit
(parallel) self-resonance at 11 MHz. The short-circuit
(series) self-resonance is at 23 MHz.

This is a Knight Kit G-30 GDO with the original
6C4 still working. I built it from a kit in 1959.
--
73, Cecil http://www.w5dxp.com

Hf Antenna Question - so what's the conclusion?
 

Cecil Moore wrote:
Cecil Moore wrote:
I also have a GDO. If I get an open circuit dip, would
that be the parallel resonant frequency? If I get a
short circuit dip, would that be the series resonant
frequency?

I have a 20 turn choke wound on a 5.35" diameter
Quaker Oatmeal box at four turns per inch. I used
my Grid Dip Oscillator to determine the open-circuit
(parallel) self-resonance at 11 MHz. The short-circuit
(series) self-resonance is at 23 MHz.

This is a Knight Kit G-30 GDO with the original
6C4 still working. I built it from a kit in 1959.

What kind of wire, what kind of insulation, and how much gap between
turns? It affects the self-capacitance.

I'd like to try that on the VNA, but here in the homeland of porridge,
oatmeal boxes are rectangular! Also we'll soon have visitors arriving
for the rest of the weekend, so if anyone in the land of cylindrical
oatmeal boxes can get to it in the next couple of days, please go ahead.


--
73 from Ian GM3SEK

Hf Antenna Question - so what's the conclusion?
 

Ian White GM3SEK wrote:
What kind of wire, what kind of insulation, and how much gap between
turns? It affects the self-capacitance.

Unfortunately, the spare (75 ohm) coax that I
have to experiment with came free from the local
TV cable company and is unmarked. It is ~0.26"
in diameter with a ~0.04" outer insulation. All
conductors are aluminum. There is an aluminum foil
wrapped around the foam insulation under the aluminum
braid. The choke is fairly close wound with approximately
3.5 turns per inch.

I'd like to try that on the VNA, but here in the homeland of porridge,
oatmeal boxes are rectangular!

Do you have 2 liter plastic pop bottles available
there? I'm going to re-do the experiment with one
of those as a coil form. There's an endless supply
of those coil forms from the day care center next
door.
--
73, Cecil http://www.w5dxp.com