2 Meter QUAD - Balun / SWR question
 

I recently built a decent 3-element 2 M Quad that fits inside my insulated
attic (just barely) and is all connected to my shack. Worked great... The
problem now is that the SWR is very high (3:1) on all freqs above the 145
MHz channels.

I think that the Quad, when tested inside the house had a great match across
the band from what I recall.

This thing is fed with RG-8 and I purposely put a whole bunch of turns into
a 5" coil near the feed point, and I'm wondering if I should have done that
at all now. I'm sure that the match would be different regardless since
it's now located in a different space, and I understand how that can change
things.

It's dark and itchy up there - before I go up there and unwind the whole
BALUN, can you guys tell me whether or not I even need that thing, or have I
got another problem.

FWIW it works great - decent gain and nicely directional - but the SWR is
too high, and it resets all the programmable thermostats clocks in the house
when I key up (interesting RFI huh?) which my attic ground-plane DOES NOT
do.

help?

Kevin VE9-XYZ

2 Meter QUAD - Balun / SWR question
 

On Mon, 12 Jun 2006 19:17:44 GMT, "K. Hastings"
I think that the Quad, when tested inside the house had a great match across
the band from what I recall.

Hi Kevin,

You have just defined its environment.

This thing is fed with RG-8 and I purposely put a whole bunch of turns into
a 5" coil near the feed point, and ... it resets all the programmable thermostats clocks in the house
when I key up (interesting RFI huh?)

Do you find the hidden clue when all the excess verbiage is cleared
away?

A whole bunch of turns is not very specific except to indicate you
probably overdid it and negated any benefit you might have expected.
Would it help to say fewer turns on a smaller form?

That, however, is probably not what is responsible for the SWR. You
are going to have to tune the antenna in place AFTER you successfully
isolate the drive line.

73's
Richard Clark, KB7QHC

2 Meter QUAD - Balun / SWR question
 

"Richard Clark" wrote in message
...
On Mon, 12 Jun 2006 19:17:44 GMT, "K. Hastings"
I think that the Quad, when tested inside the house had a great match
across
the band from what I recall.

Hi Kevin,

You have just defined its environment.

This thing is fed with RG-8 and I purposely put a whole bunch of turns
into
a 5" coil near the feed point, and ... it resets all the programmable
thermostats clocks in the house
when I key up (interesting RFI huh?)

Do you find the hidden clue when all the excess verbiage is cleared
away?

A whole bunch of turns is not very specific except to indicate you
probably overdid it and negated any benefit you might have expected.
Would it help to say fewer turns on a smaller form?

That, however, is probably not what is responsible for the SWR. You
are going to have to tune the antenna in place AFTER you successfully
isolate the drive line.

73's
Richard Clark, KB7QHC

Hi Richard

As you know, I have been getting good results with the high permeability
ferrite tubes for decoupling the coax feed line on my balanced antenna. I
bought a bunch of inexpensive ferrites that were intended for absorbing RFI,
from All Electronics. Would it be practical for Kevin, the orginal poster,
to get decent results with a few ferrites covering the coax where it
seperates from the boom??

Jerry

2 Meter QUAD - Balun / SWR question
 

On Mon, 12 Jun 2006 21:46:31 GMT, "Jerry Martes"
wrote:
Hi Richard

As you know, I have been getting good results with the high permeability
ferrite tubes for decoupling the coax feed line on my balanced antenna. I
bought a bunch of inexpensive ferrites that were intended for absorbing RFI,
from All Electronics. Would it be practical for Kevin, the orginal poster,
to get decent results with a few ferrites covering the coax where it
seperates from the boom??

Hi Jerry,

Sure, but you have the advantage of being able to measure the Z of
those ferrites. However, buying them rummage style and hoping they
will work does not always bring a satisfactory solution.

73's
Richard Clark, KB7QHC

2 Meter QUAD - Balun / SWR question
 

"Richard Clark" wrote in message
...
On Mon, 12 Jun 2006 21:46:31 GMT, "Jerry Martes"
wrote:
Hi Richard

As you know, I have been getting good results with the high permeability
ferrite tubes for decoupling the coax feed line on my balanced antenna.
I
bought a bunch of inexpensive ferrites that were intended for absorbing
RFI,
from All Electronics. Would it be practical for Kevin, the orginal
poster,
to get decent results with a few ferrites covering the coax where it
seperates from the boom??

Hi Jerry,

Sure, but you have the advantage of being able to measure the Z of
those ferrites. However, buying them rummage style and hoping they
will work does not always bring a satisfactory solution.

73's
Richard Clark, KB7QHC

Hi Richard

I realize that I place way more emphasis on "Amateur" in the HAM concept.
I'm not inclined to depend on using only the best component. But, it seems
that any of those ferrite tubes that were intended to be used to minimize
RFI will work well at 144 MHz. So, since they are cheap and available, I
thought it might be worth trying some inexpensive, high permeability tubes
instead of coiling the coax into a coil.
I did measure the impedance across a loop of RG 223 with 4 tubes around
the outside. The problem I come up with is my ignorance of *whats needed*
for adequate decoupling of the balanced antenna from the outside of the
coax.
I depend on you to shed light on alot of the things I try.

Jerry

2 Meter QUAD - Balun / SWR question
 

On Mon, 12 Jun 2006 23:40:19 GMT, "Jerry Martes"
wrote:

I did measure the impedance across a loop of RG 223 with 4 tubes around
the outside. The problem I come up with is my ignorance of *whats needed*
for adequate decoupling of the balanced antenna from the outside of the
coax.

Hi Jerry,

Just think of the antenna as the "source" and consider its
characteristic Z. Now consider the common mode circuit of the
transmission line and its Z. So often this is an unknown, and if we
were always lucky, it would be a half wave multiple to present a very
high Z indeed. Thus 'twould be the end of the story.

How high is high enough? I offer three values for choking:
3 times antenna Z at a minimum;
5 times is practical;
10 times is lab grade.

So, tell us what those 4 tubes measured?

73's
Richard Clark, KB7QHC

2 Meter QUAD - Balun / SWR question
 

On Mon, 12 Jun 2006 23:48:48 -0700, Richard Clark
wrote:

On Mon, 12 Jun 2006 23:40:19 GMT, "Jerry Martes"
wrote:

I did measure the impedance across a loop of RG 223 with 4 tubes around
the outside. The problem I come up with is my ignorance of *whats needed*
for adequate decoupling of the balanced antenna from the outside of the
coax.

Hi Jerry,

Just think of the antenna as the "source" and consider its
characteristic Z. Now consider the common mode circuit of the
transmission line and its Z. So often this is an unknown, and if we

Richard, does Z mean the characteristic impedance of the line in the
"differential" mode or "common" mode?

Owen
--

2 Meter QUAD - Balun / SWR question
 

Richard Clark wrote:
Just think of the antenna as the "source" and consider its
characteristic Z.

"Characteristic impedance" is not the impedance associated
with choking requirements. That probably should be
"feedpoint Z" (source Z for receive).

For standing wave antennas the characteristic impedance
and the feedpoint impedance are usually quite different.
For a #14 wire horizontal 1/2 WL dipole at 30 ft., the
characteristic impedance is in the ballpark of 1200 ohms
while the feedpoint impedance is in the ballpark of 60
ohms.
--
73, Cecil http://www.qsl.net/w5dxp

2 Meter QUAD - Balun / SWR question
 

"Richard Clark" wrote in message
...
On Mon, 12 Jun 2006 23:40:19 GMT, "Jerry Martes"
wrote:

I did measure the impedance across a loop of RG 223 with 4 tubes around
the outside. The problem I come up with is my ignorance of *whats
needed*
for adequate decoupling of the balanced antenna from the outside of the
coax.

Hi Jerry,

Just think of the antenna as the "source" and consider its
characteristic Z. Now consider the common mode circuit of the
transmission line and its Z. So often this is an unknown, and if we
were always lucky, it would be a half wave multiple to present a very
high Z indeed. Thus 'twould be the end of the story.

How high is high enough? I offer three values for choking:
3 times antenna Z at a minimum;
5 times is practical;
10 times is lab grade.

So, tell us what those 4 tubes measured?

73's
Richard Clark, KB7QHC

Hi Richard

I have been thinking of the "string of ferrites" as a method of presenting
a high impedance to Conducted currents along the outside of the coax from
the antenna down to the radio. It is clear that a "high impedance" may
actually couple well to the outside of the coax if the length of coax is
some special length that presents a high impedance up to where the choke is
located,
I think I could have saved time if I'd asked you more questions when I was
doing the work of learning about baluns.

I didnt keep any of the data I recorded with any of the coils and
ferrites. It is easy to measure the Z of any high impedance circuit so I
could easily measure choke impedance at 2 meters.

Jerry

2 Meter QUAD - Balun / SWR question
 

On Tue, 13 Jun 2006 07:05:51 GMT, Owen Duffy wrote:
Just think of the antenna as the "source" and consider its
characteristic Z. Now consider the common mode circuit of the
transmission line and its Z. So often this is an unknown, and if we

Richard, does Z mean the characteristic impedance of the line in the
"differential" mode or "common" mode?

Hi Owen,

The Common mode. The remainder of this discussion will undoubted be
known to you, so it is largely meant for our otherwise silent original
poster, Kevin. The ferrites will be more or less transparent to the
differential mode when the ferrites wrap around both conductors for
twin lead, or around the coaxial cable.

As this discussion has been largely coaxial based, the outer
conductive surface of the coax is the primary imbalance to a dipole
through its "third wire" connection at the dipole feed point. It
appears as one of three wires to the abstract source established at
that feed point and it presents an ad-hoc Z load. The value of this
load is rarely determined, except when one deliberately attempts to
make their feed line 1/2 wave long (or some multiple). Of course,
that means 1/2 wave for the velocity factor of the outside conductive
path of the coax. This is often accomplished through cut and try
rather than modeling or measuring currents, but these too would be
good first pass approximations.

However, it is simpler to add ferrites as their Z is well known and
rarely subject to externalities. They also allow for multiband
operation. They are simple to apply. If you aren't using hi power,
they are "set and forget."

In closing, it bears mentioning that this treatment should be repeated
a quarterwave down the coax and away from the drive point. This, in
effect, enforces a one band solution by its description, so choose the
band that counts the most. I would expect it would improve other
bands to some degree. I've simply distributed the same number of
ferrites over a 20 foot length of coax as the second treatment instead
of repeating the same lumped isolation. I cannot vouch for this
method's effectivity in maintaining a null pattern in the lobes of the
dipole as I never confirmed that.

73's
Richard Clark, KB7QHC

2 Meter QUAD - Balun / SWR question
 

On Tue, 13 Jun 2006 15:58:00 GMT, "Jerry Martes"
wrote:

I have been thinking of the "string of ferrites" as a method of presenting
a high impedance to Conducted currents along the outside of the coax from
the antenna down to the radio.

Hi Jerry,

This may be what I've already described at the end of my response to
Owen.

73's
Richard Clark, KB7QHC

2 Meter QUAD - Balun / SWR question
 

On Tue, 13 Jun 2006 07:05:51 GMT, Owen Duffy wrote:
Just think of the antenna as the "source" and consider its
characteristic Z. Now consider the common mode circuit of the
transmission line and its Z. So often this is an unknown, and if we

Richard, does Z mean the characteristic impedance of the line in the
"differential" mode or "common" mode?

Hi Owen,

It occurred to me that there are two Zs mentioned and you did not
really differentiate as I had.

The antenna Z will be something we all have expectations of attaining,
and we quite often measure it. For the dipole we merely assign a
value of 70 Ohms for this discussion.

Now, we have the Z of the "third wire" which is almost never measured,
especially for every application. If it presented an equal 70 Ohms,
it would then hog half the power - however, this is unlikely in the
scheme of things. Let us just say it will exhibit some value that may
tend to absorb and radiate power. This capacity both upsets antenna
patterns and contributes to RFI and feed line radiation.

If the source is 70 Ohms and we add 200 to 700 Ohms of resistivity in
line (through the application of ferrites as common mode series
resistances), then we have diminished its capacity to hog power. As
1000 Ohms of ferrite resistance is easily applied, this has generally
satisfied many conditions.

73's
Richard Clark, KB7QHC

2 Meter QUAD - Balun / SWR question
 

On Tue, 13 Jun 2006 09:51:43 -0700, Richard Clark
wrote:


As this discussion has been largely coaxial based, the outer
conductive surface of the coax is the primary imbalance to a dipole
through its "third wire" connection at the dipole feed point. It
appears as one of three wires to the abstract source established at
that feed point and it presents an ad-hoc Z load. The value of this
load is rarely determined, except when one deliberately attempts to
make their feed line 1/2 wave long (or some multiple). Of course,
that means 1/2 wave for the velocity factor of the outside conductive
path of the coax. This is often accomplished through cut and try
rather than modeling or measuring currents, but these too would be
good first pass approximations.

Richard,

There is a focus on evaluating baluns as a passive component in a
bench test jig designed to characterise them by a simple equivalent
circuit to show their imperfections in certain scenarios (eg impedance
range, frequency range, common mode impedance with different balance
points on the load and floating loads). Collectively, we seem to have
done that to death. To date, I don't think anyone has discovered a
practical balun that is close to ideal in all applications.

Baluns (being any device that is designed to facilitate the transition
from un-unbalanced load (might be perfectly balanced, might not be) to
an unbalanced transceiver (one terminal approximately grounded) seem
to be usually employed to:
- reduce common mode RF currents entering the shack where they may
disrupt operation of equipment, and more recently may be a health
concern;
- reduce the feedline's participation in radiation or reception
(pattern distortion, unwanted noise pickup, EMC / proximity to other
equipment).

It seems to me that NEC models are a worthwhile tool in developing
insight into the effects that occur. I think NEC modelling of a coax
fed, centre fed dipole at various heights as a centre fed conductor
with generator at the centre of two wires, and the "third wire" to
ground through some loss resistance is quite revealing. (The coax
example is not to suggest that this phenomena is peculiar to coax feed
alone.)

You mention measuring currents. It seems that discussion on common
mode currents has spawned a new market for clip on RF current probes.
I even see suggestion that common mode current be continuously
monitored in much the same way as VSWR is monitored at a spot adjacent
to the tx.

NEC modelling reveals that the currents on the "third wire" (common
mode current) varies with position (no surprise there), and that
depending on the topology, can be insignificantly low at some points
while it is significantly high at other points. Measurement of common
mode current at just a single point does not necessarily provide
enough information to detect or assess a common mode current problem.
Spot measurement is a superficial approach.

So if there are maxima and minima in the common mode standing wave
current on the feedline, the influence of chokes on current in all
parts of the antenna system (hence pattern) and loss of chokes will
depend on where they are located.

That is not to suggest that it is all too complicated. I think there
are good reasons to routinely deploy baluns of appropriate type an
location, but they aren't a cut and dried idealised solution and
further work may be required to identify and rectify residual
problems.

Owen
--

2 Meter QUAD - Balun / SWR question
 

If the Swr is better below 145 than above the radiator must be over
length. You need to trim it or match it. If you are measuring through
lot of coax the match at the antenna is a lot worse than 3:1.

Any kind of choke arrangement amounts to working on the wrong problem
to me.

On Mon, 12 Jun 2006 19:17:44 GMT, "K. Hastings"
wrote:


John Ferrell W8CCW

2 Meter QUAD - Balun / SWR question
 

Thanks John - I may have thought the balun assembly was somehow involved.
I'm new to the group and a fairly new amateur, and your respone was the only
one that dealt with my question.

What I thought was a relevant description was mis-read as "excess verbiage"
I guess. I'll try trimming the loop, and hope also to cure the electronic
thermostat problem which is fairly significant.

73

Kevin

e"John Ferrell" wrote in message
...
If the Swr is better below 145 than above the radiator must be over
length. You need to trim it or match it. If you are measuring through
lot of coax the match at the antenna is a lot worse than 3:1.

Any kind of choke arrangement amounts to working on the wrong problem
to me.

On Mon, 12 Jun 2006 19:17:44 GMT, "K. Hastings"
wrote:


John Ferrell W8CCW