NOW AVAILABLE ** New Program "BALCHOKE.exe"

This program is very similar to program CHOKEBAL The main change is to
input data. The ferrite core is rectangular in shape and defined in terms of
thickness and length instead of a circular core diameter. The size of the
core is described more conveniently in terms of the outside diameter of the
ferrite ring instead of the diameter of the mean magnetic path.

The opportunity has been taken to provide, in the author's opinion, a better
statistical way of estimating percent current unbalance in the twin-wire
transmission line. There are also better program operating notes.

Although most numerical results are identical this second version of the
program is recommended.

Program size about 36 kilobytes. Download program BALCHOKE from website
below in a few seconds and run immediately from a desktop icon.
----
.................................................. ..........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. ..........

Reg

How can I determine how effective a balun is?? I am working at 137 MHz
and cant consider a radiation pattern range of any kind.
I have been trying a coil of small coax in a relatively low loss ferrite
toroid and then slipping a high permeability tube of more lossy ferrite on
the coax to the receiver. I thought the higher reactance with a reasonable
Q might minimize the current conducted along the feed line. Then, any
currents that do get by the reactive coil might get disipated in the lossy
and high permeability ferrite tube section.

Since I'm working without much knowledge and almost no test equipment, I'd
sure appreciate any information about how to evaluate baluns at VHF.

Jerry

"Reg Edwards" wrote in message
...

NOW AVAILABLE ** New Program "BALCHOKE.exe"

This program is very similar to program CHOKEBAL The main change is to
input data. The ferrite core is rectangular in shape and defined in terms
of
thickness and length instead of a circular core diameter. The size of the
core is described more conveniently in terms of the outside diameter of
the
ferrite ring instead of the diameter of the mean magnetic path.

The opportunity has been taken to provide, in the author's opinion, a
better
statistical way of estimating percent current unbalance in the twin-wire
transmission line. There are also better program operating notes.

Although most numerical results are identical this second version of the
program is recommended.

Program size about 36 kilobytes. Download program BALCHOKE from website
below in a few seconds and run immediately from a desktop icon.
----
.................................................. .........
Regards from Reg, G4FGQ
For Free Radio Design Software go to
http://www.btinternet.com/~g4fgq.regp
.................................................. .........

"Jerry Martes" wrote in message
...

How can I determine how effective a balun is?? I am working at 137 MHz
and cant consider a radiation pattern range of any kind.
I have been trying a coil of small coax in a relatively low loss ferrite
toroid and then slipping a high permeability tube of more lossy ferrite on
the coax to the receiver. I thought the higher reactance with a
reasonable
Q might minimize the current conducted along the feed line. Then, any
currents that do get by the reactive coil might get disipated in the lossy
and high permeability ferrite tube section.

Since I'm working without much knowledge and almost no test equipment,
I'd
sure appreciate any information about how to evaluate baluns at VHF.

===================================

It all depends on what YOU mean by "effectiveness".

First ask yourself why, in your case, you think you need a balun.

If you are referring to common-mode current on the feedline then you have to
state your requirements in numerical terms and find some means of measuring
it as a fraction of the total line current, without disturbing the antennas
normal environment.

But that's only half the job. Assuming you have some objections to common
mode current you then have to numerically relate common mode current to the
adverse effects it may have on ALL other aspects of antenna performance. And
unless you have some idea of the MAGNITUDE of side effects you don't want,
you can't sensibly talk about it.

Just a caution, nobody ever talks sensibly about power being radiated
specifically from feedlines. There's no such stuff. ;o)

The best way I can think of of discovering what a balun is actually doing is
to entirely remove it from the antenna and see what happens to antenna
performance in terms of the all-important radiation pattern and gain. If
within your limited means of measurement you can detect very little or no
difference then, of course, don't bother to replace it. The usefulness of
baluns is often overated.

Similarly with your ferrite coil and tube experiments. If by doing
something, nothing happens, then don't do it.


Praps you could make a very simple current detector (a current transformer)
which will fit around the feedline. A pair of suitably-shaped ferrite blocks
plus a few turns of wire plus a diode plus a 500pF capacitor plus a 100-ohm
resistor plus a DC microammeter are all that's needed.

But making measurements at 137 MHz is fraught with error even by experts.

If your antenna is for receiving purposes only, then its 99.9 percent
certain you don't need a balun anyway. And a few turns of a few inches of
small diameter coax on a small ferrite ring can't possibly do any harm even
if you decide to use one. It is sure to work as intended even if you've no
idea how well it's supposed to work.
----
Reg.

Reg

The only thing I have been concerned about is the radiation pattern. I
have been trying to "disassociate" the coax's outer conductor with a dipole
so the radiation pattern will be essentially that of the dipole.

I have been trying to think of ways to make impedance measurements so I
might be able to estimate the reactance to currents cunducted along the
outside of the coax.

I really appreciate your comments on the practicallity of winding a few
turns of small coax in a small ferite.. That seems to have provided the
best insertion loss I've been able to acheive.

I've been trying to make a simple antenna for receiving NOAA satellites.
I have been learning that my antenna concept is of limited value for
producing clear pictures from weather satellites. But, I still want to get
more data on antennas so I might be able to get better pictures from a
"simple" antenna.
My principal problem right now is minimizing the nulls in the pattern.
Each null will produce a black line in the picture of te Earth when that
null falls below some threshold.

Thanks again
Jerry



"Reg Edwards" wrote in message
...

"Jerry Martes" wrote in message
...

How can I determine how effective a balun is?? I am working at 137
MHz
and cant consider a radiation pattern range of any kind.
I have been trying a coil of small coax in a relatively low loss
ferrite
toroid and then slipping a high permeability tube of more lossy ferrite
on
the coax to the receiver. I thought the higher reactance with a
reasonable
Q might minimize the current conducted along the feed line. Then, any
currents that do get by the reactive coil might get disipated in the
lossy
and high permeability ferrite tube section.

Since I'm working without much knowledge and almost no test equipment,
I'd
sure appreciate any information about how to evaluate baluns at VHF.

===================================

It all depends on what YOU mean by "effectiveness".

First ask yourself why, in your case, you think you need a balun.

If you are referring to common-mode current on the feedline then you have
to
state your requirements in numerical terms and find some means of
measuring
it as a fraction of the total line current, without disturbing the
antennas
normal environment.

But that's only half the job. Assuming you have some objections to common
mode current you then have to numerically relate common mode current to
the
adverse effects it may have on ALL other aspects of antenna performance.
And
unless you have some idea of the MAGNITUDE of side effects you don't want,
you can't sensibly talk about it.

Just a caution, nobody ever talks sensibly about power being radiated
specifically from feedlines. There's no such stuff. ;o)

The best way I can think of of discovering what a balun is actually doing
is
to entirely remove it from the antenna and see what happens to antenna
performance in terms of the all-important radiation pattern and gain. If
within your limited means of measurement you can detect very little or no
difference then, of course, don't bother to replace it. The usefulness of
baluns is often overated.

Similarly with your ferrite coil and tube experiments. If by doing
something, nothing happens, then don't do it.


Praps you could make a very simple current detector (a current
transformer)
which will fit around the feedline. A pair of suitably-shaped ferrite
blocks
plus a few turns of wire plus a diode plus a 500pF capacitor plus a
100-ohm
resistor plus a DC microammeter are all that's needed.

But making measurements at 137 MHz is fraught with error even by experts.

If your antenna is for receiving purposes only, then its 99.9 percent
certain you don't need a balun anyway. And a few turns of a few inches of
small diameter coax on a small ferrite ring can't possibly do any harm
even
if you decide to use one. It is sure to work as intended even if you've no
idea how well it's supposed to work.
----
Reg.

"Jerry Martes" wrote -

The only thing I have been concerned about is the radiation pattern. I
have been trying to "disassociate" the coax's outer conductor with a
dipole
so the radiation pattern will be essentially that of the dipole.

I have been trying to think of ways to make impedance measurements so I
might be able to estimate the reactance to currents cunducted along the
outside of the coax.

I really appreciate your comments on the practicallity of winding a few
turns of small coax in a small ferite.. That seems to have provided the
best insertion loss I've been able to acheive.

I've been trying to make a simple antenna for receiving NOAA satellites.
I have been learning that my antenna concept is of limited value for
producing clear pictures from weather satellites. But, I still want to
get
more data on antennas so I might be able to get better pictures from a
"simple" antenna.
My principal problem right now is minimizing the nulls in the pattern.
Each null will produce a black line in the picture of te Earth when that
null falls below some threshold.

----------------------------------------------------

Jerry,

Let me say at the outset, whether you use a balun or not, there will be no
detectable effect on the radiation/receiving pattern of the antenna. For
practical purposes it will remain the same as the usual radiation pattern
for a half wave dipole.

You appear to have a 137 MHz 1/2-wave dipole fed via a coaxial line. Ideally
it should be a 75 ohm line and to prevent reflections from the receiver end
it should be terminated at the receiver with 75 ohms. But if reflections
don't matter with whatever type of signals you wish to receive then a 50-ohm
line can be used. But neither impedance will have any effect on the
antenna's radiation pattern.

I can talk in terms of radiation pattern because the receiving and radiation
patterns are always identical for either transmitting or receiving purposes.
This results from the effect known as "Reciprocity".

When transmitted or received currents appear on the outer coaxial conductor
of the feedline they are known as "common-mode" currents which cause the
antenna and feedline to behave as one single, differently shaped antenna.
The radiation pattern of the differently-shaped antenna results in
distortion of the original dipole's pattern.

BUT WHAT MATTERS IS THE MAGNITUDE OF THE CURRENT WHICH FLOWS ON THE OUTSIDE
OF THE COAX COMPARED WITH THE MAGNITUDE OF THE CURRENT IN THE DIPOLE.

The common mode current in the coax flows along the whole of its uncertain
length and so affects the radiation pattern in an uncertain manner.
Furthermore the unknown impedance to ground at the receiver end of the coax
obviously has an effect. The whole business is indeterminate and can be
treated only in a statistical worst and best case manner.

Available from my website is program FEEDPOWR program which deals with the
transmitting case and attempts to estimate the power radiated from the
feedline relative to the power from the dipole antenna proper, versus dipole
length, feedline length and frequency, taking the resistance of the ground
connection at the end of the coax into account. It does not take into
account that the lower part of the feedline, perhaps inside a building, does
not radiate at all well and so the program very nicely over-estimates the
unwanted effects.

If we examine your particular case by entering in the program a dipole
length of 1.09 metres, a frequency of 137 MHz, a feedline length of whatever
you've got, and a ground resistance of 50 ohms for the want of a better
value, we find that that the worst case value of power radiated from the
feedline is about 6 percent of the total transmitter power as line length is
varied. The best case value is about 1 percent as line length is varied.

The change in shape of the radiation pattern is at worst of the order of 0.4
dB which I suggest is negligible and undetectable. It is less than that due
to not bringing the feedline away from the dipole at an angle of 90 degrees
for the whole of its length which can also affect the radiation pattern.
Nearby buildings, phone and power lines, can also affect the radiation
pattern of an ordinary dipole and almost certainly will.

Due to reciprocity the effect on the receiving pattern will be identical.

As I said earlier you don't need a balun. But if you are a perfectionist no
harm will be done by fitting one. You will obtain no useful information by
attempting to make measurements of any sort. A ferrite choke balun is so
simple there's nothing to go wrong with it. Keep the length of small-bore
coax wound on the balun to less than 1/15 wavelengths. Just a few turns on a
1" diameter ring.