Missing Degrees in Mobile Antennas?
 

There seems to be an assumption by some posters here that
a mobile antenna must be electrically 90 degrees in length.
Here's a simple transmission line example to illustrate why
that is not true in the case of a loading coil used with straight
sections of antenna. In the following example, all transmission
lines are lossless.

We want to build a stub that is electrically 90 degrees long,
i.e. a 1/4WL open-circuited stub. The impedance at the
mouth of the 1/4WL open-circuited stub will be 0-j0 ohms,
a short circuit. The first stage of the stub is made from
Z0=450 ohm transmission line and is 30 degrees long.

0-j0 ----30 deg of 450 ohm line----+----50 ohm line---open-circuit

The question is: How many degrees of 50 ohm line do we need
to add to get 1/4WL in all. Sounds simple, huh? Just add 60
degrees and we will have our stub. But that is NOT what happens.

The impedance at '+' is -j259 ohms. That is -j.0575 on the 450 ohm
Smith Chart but is -j5.175 on the 50 ohm Smith Chart. So it takes
only 11 degrees of 50 ohm line to replace the function of 60
degrees of 450 ohm line. Want to stuff 90 degrees of function into
a 41 degree long stub? Make it a two-stage stub as above and
take advantage of the impedance discontinuity.

There is 60 degrees between -j0.575 and infinity on a Smith Chart.

There is 11 degrees between -j5.175 and infinity on a Smith Chart.

The Z01/Z02 impedance discontinuity has provided the "missing"
49 degrees of the stub.

The stub is only 41 degrees in physical length yet it functions
just like a 90 degree stub.

The same thing holds true for mobile antennas with loading coils.
The difference between the Z0 of the straight section and the Z0
of the loading coil provides the "missing" degrees of the antenna.
There is no requirement that the antenna be physically 90 degrees
long. There is no requirement that the delay through the coil plus
the delay through the straight sections add up to 90 degrees.
The impedance discontinuity provides the "missing" number of
degrees.
--
73, Cecil, W5DXP

Missing Degrees in Mobile Antennas?
 

On Thu, 27 Apr 2006 20:28:12 GMT, "Cecil Moore"
wrote:
There seems to be an assumption by some posters here that
a mobile antenna must be electrically 90 degrees in length.
"Some?" In fact, only one and he cites you (which is notable in its
irony for many reasons) as his authority on this subject. Why don't
you simply email him your contribution?

Missing Degrees in Mobile Antennas?
 

Richard Clark wrote:
On Thu, 27 Apr 2006 20:28:12 GMT, "Cecil Moore"
wrote:

There seems to be an assumption by some posters here that
a mobile antenna must be electrically 90 degrees in length.

"Some?" In fact, only one and he cites you (which is notable in its
irony for many reasons) as his authority on this subject. Why don't
you simply email him your contribution?

Hang on, that was Cecil that said that!?!? Mister "a coil fills in the
missing degrees on the stinger"?

tom
K0TAR

Missing Degrees in Mobile Antennas?
 

Richard Clark wrote:

On Thu, 27 Apr 2006 20:28:12 GMT, "Cecil Moore"
wrote:

There seems to be an assumption by some posters here that
a mobile antenna must be electrically 90 degrees in length.

"Some?" In fact, only one and he cites you (which is notable in its
irony for many reasons) as his authority on this subject. Why don't
you simply email him your contribution?

Or, to put it more "accurately", from Cecil's perspective, it fills in
the missing degrees at the position the coil sits.

tom
K0TAR

Missing Degrees in Mobile Antennas?
 

Tom Ring wrote:
Or, to put it more "accurately", from Cecil's perspective, it fills in
the missing degrees at the position the coil sits.

No, in the case of a base-loaded antenna, the "missing"
degrees are filled in at the impedance discontinuity
*between* the coil and the stinger.
--
73, Cecil http://www.qsl.net/w5dxp

Missing Degrees in Mobile Antennas?
 

Cecil Moore wrote:

Tom Ring wrote:

Or, to put it more "accurately", from Cecil's perspective, it fills in
the missing degrees at the position the coil sits.


No, in the case of a base-loaded antenna, the "missing"
degrees are filled in at the impedance discontinuity
*between* the coil and the stinger.

And that differs from your bugcatcher how?

tom
K0TAR

Missing Degrees in Mobile Antennas?
 

On Thu, 27 Apr 2006 21:58:07 -0500, Tom Ring
wrote:

No, in the case of a base-loaded antenna, the "missing"
degrees are filled in at the impedance discontinuity
*between* the coil and the stinger.

And that differs from your bugcatcher how?


Hi Tom,

His has an event horizon, beyond which all missing degrees disappear
by superimposition.

73's
Richard Clark, KB7QHC

Missing Degrees in Mobile Antennas?
 

Tom Ring wrote:
Cecil Moore wrote:
No, in the case of a base-loaded antenna, the "missing"
degrees are filled in at the impedance discontinuity
*between* the coil and the stinger.

And that differs from your bugcatcher how?

The whole point is that it doesn't differ.
That's how my bugcatcher works.

Here are the three parts to the answer.

1. The base-loading coil furnishes a delay equal to
a certain number of degrees which is nowhere near zero
degrees. Half of a coil self-resonant at 4 MHz would
provide 45 degrees of shift.

2. Using EZNEC to add a stinger to resonate the antenna
on 4 MHz, I find that's 11.5 degrees of straight element.
45 degrees plus 11.5 degrees is 56.5 degrees.

3. 90 - 56.5 = 33.5 degrees which is the "missing"
degrees filled in by the impedance discontinuity.
We can even estimate the ratio of the Z0 of the coil
to the Z0 of the stinger to be 5.0.
--
73, Cecil http://www.qsl.net/w5dxp

Missing Degrees in Mobile Antennas?
 

There seems to be an assumption by some posters here that
a mobile antenna must be electrically 90 degrees in length.


Must be? Must is only death and taxes!

Resonant monopole must be 90 electrical degrees and resonant dipole must be
180 electrical degrees long. But that "must" does not apply to antennas as
chosen by users. You can use light bulb, screwdriver, bedsprings, wet noodle
or any piece of RF conducting material of any length, shape you like, just
question how good of antenna it is.

We used 90 deg. monopole in discussion to show typical loaded or mobile
antenna in order to avoid detours to la-la land (cases when current can be
and is equal) and to stay on the subject of current magnitude along the
typical loading coil.

73 Yuri, K3BU

Missing Degrees in Mobile Antennas?
 

Yuri Blanarovich wrote:
We used 90 deg. monopole in discussion to show typical loaded or mobile
antenna in order to avoid detours to la-la land (cases when current can be
and is equal) and to stay on the subject of current magnitude along the
typical loading coil.

Take a look at the rest of the posting, Yuri. I show where a
1/4WL stub can be made up of 30 degrees of 450 ohm line plus
11 degrees of 50 ohm line. The impedance discontinuity (at
a point) furnishes the "missing" 49 degrees of stub.

At 4 MHz, that would make a 1/4WL stub only 23.4 ft long
or about 10% of a wavelength instead of 25%.

Has anything been published on using multiple Z0's to
shorten the physical length of a stub?
--
73, Cecil http://www.qsl.net/w5dxp