RadioBanter - View Single Post

April 29th 06, 08:36 PM posted to rec.radio.amateur.antenna

wrote:

I'm going to ask a couple of technical question at the beginning
rather than getting them trimmed and ignored in the body:

1. You seem ready to admit that there is 10 degrees of delay
through a 10 degree long stinger. Yet, if you measured that
delay using standing wave current phase, you would measure
a zero phase shift through the stinger. Why aren't you arguing
that there is no phase shift in the stinger?

2. There is no appreciable standing wave current phase shift from
feedpoint to the tip of the stinger in a 75m mobile bugcatcher
antenna. To be consistent, don't you have to argue that the
75m mobile bugcatcher antenna is zero degrees long?

Cecil Moore wrote:
So you are saying the loading coil is "7's of feet long". Is that
correct?

Compared to a straight wire at 4 MHz, yes, that's what I am
saying. A 75m bugcatcher coil uses about 42 feet of wire.
The delay through that coil is *roughly* equal to about half
that number of feet of straight wire. The reason it is not
equal to 42 feet of straight wire is the flux coupling between
the coils.

Your assertion that nearly 100% of the coils link
nearly 100% of the total flux is unrealistic.

I never said that.

But it would necessarily have to be true for the velocity factor
of the coil to be anywhere near 1.0 and you did say that.

What's your point in making that switch? We were talking about loading
coils, now you are switching to stubs. Why?

THE EFFECT EXISTS WHETHER THE COIL EXISTS OR NOT. Which
indicates it is the nature of standing wave current, not coils,
that you do not understand. The standing wave current phase
is unchanging whether a coil exists or not. ONE CANNOT EVEN
USE STANDING WAVE CURRENT PHASE TO MEASURE THE PHASE SHIFT
THROUGH A WIRE, much less through a coil.

Given that standing wave current phase cannot be used to
measure the delay through a wire, coil, or anything else,
it follows that nobody has provided any valid measurements
for the delay through a coil.

A very similar thing happens with a 75m mobile antenna.
The base loading coil provides tens of degrees of phase
shift.

How do you know that?

The stub involves two different Z0's. The 75m mobile antenna
involves two different Z0's, one for the coil, the other for
the stinger. It's the same principles using the same equations.
The phase shift at the impedance discontinuity depends upon the
ratio of those two Z0's. The higher the ratio, the greater the
phase shift. The ratio of 450 to 50 is obviously 9:1.

A rough estimate of the Z0 of the coil is around 2400 ohms and
a rough estimate of the Z0 of the stinger is around 400 ohms.
That makes the Z0 ratios roughly 6:1.

I'm happy to see you no longer agree with the misplaced notion the
coil replaces missing electrical degrees, but puzzled why you resist
understanding the mechanism that allows the phase shift to change with
coil design.

Speaking of what we both believed two years ago:
I'm happy to see you no longer agree with the misplaced notion that
the coil has equal current magnitudes and phases at each end, but
puzzled why you resist understanding the low velocity factor
associated with helical loading coils. The velocity factor of 75m
bugcatcher loading coils is typically less than 0.1

From the Dr. Corum paper, we have an equation for velocity factor
for coils passing a litmus test. A 75m bugcatcher coil passes that
litmus test with flying colors. The resulting VF is in the ballpark
of 0.04 which is in the ballpark of Reg's VF calculations which is
in the ballpark of Richard Harrison's calculations. Your VF of 1.0
along the length of the coil is the one that is completely out of
the ballpark.

Take your 100uH coil and measure its self-resonant frequency
directly over a large metal ground plane. Keeping everything,
including frequency, the same, cut the coil in half. Add a
stinger to the bottom half of the coil to bring the system back
into resonance at the fixed frequency. We know the delay through
the coil is roughly 45 degrees. We know the stinger is roughly
10 degrees. The impedance discontinuity at the coil to stinger
junction is causing roughly 35 degrees of phase shift. That
tells us that the Z0 ratio of the coil to stinger is about 6:1.

I have been through the above exercise using EZNEC.
--
73, Cecil http://www.qsl.net/w5dxp