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Old August 15th 06, 06:07 AM posted to rec.radio.amateur.antenna
Jerry Martes Jerry Martes is offline
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First recorded activity by RadioBanter: Jul 2006
Posts: 173
Default QFH Antenna and 72ohm TV Coax


"Cecil Moore" wrote in message
m...
Jerry Martes wrote:
Is it posible that the length of the "stubs" change? I'd have thought
the length of the stub is always the same. 45 degrees should always be
45 degrees, shouldnt it?? An open circuit, 45 degrees back along a 50
ohm line looks like 50 ohms capacitive. That 50 ohms looks like
something like 500 ohms inductive as viewed 45 degrees back along a 600
ohm line.
I'd guess your point is that 500 ohms of pure inductive reactance is
never seen 90 degrees back from an open, no matter what the Zo of the
line


The point I'm eventually going to make is about loading coils
in mobile antennas but let's stick with the above stub example.



Hi Cecil I sure dont want to get involved with any mobil antenna loading
coil discussions, I admit that I'm not qualified.



| 45 deg | 45 deg |
Source====Z01=========Z02====open

Z01 = 600 ohms, Z02 = 50 ohms

If the Z0 were constant and the stub was 90 degrees long,
the source would see zero ohms.


Yeah, but is isnt a line with a constant Zo


Yet in our above example
the stub is physically 90 degrees long and the source sees
+j500 ohms. The above stub is electrically 130 degrees long.


I'd disagree with a conclusion that, just because the impedance seen by
the source is 500 ohms, the line connecting it to a load is 90 degrees
long.


There is a 45 degree delay through the Z01 section of stub.
There is a 45 degree delay through the Z02 section of stub.
There is a 40 degree phase shift at the Z01 to Z02 junction


If I disagree, do I have to get involved with some lengthy mathmatical
discussion? I'm not skilled enough to argue with you Cecil. I'm not even
smart. But, I sure dont see how anyone can conclude there is a phase shift
at the junction of two transmission lines. There is a shunt capacitive
reactance that results from that abrupt change in dimensions, but I wouldnt
have thought it would be enough to result in 40 degrees of phase shift.
..

If we want to turn the above stub into a functional 1/4WL
open stub such that the source sees zero ohms, we can
remove 40 degrees from the Z02 section. If we make the
Z02 section 5 degrees long, the entire stub will be
electrically 90 degrees long, and 1/4WL resonant.
There will be a 45 degree delay through the Z01 section
There will be a 5 degree delay through the Z02 section
There will be a 40 degree phase shift at the Z01 to Z02
junction.


I think I agree with the concept you are using. You are giving the
condition where a purely capacitive reactor of 50 ohm impedance is required
to be resonated by introducing a series inductor. A short length of higher
impedance transmission line will sure do that. The higher the line Zo,
the shorter it needs to be to resonate that 50 ohm capacitor.


--
73, Cecil http://www.qsl.net/w5dxp


Jerry