"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.
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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.
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