All,

This is a real world "what is going on" question. The antenna is a
'screwdriver' on a 3 foot stand in the back yard. The earth ground is
normal loam, the original soil was forest. The feed cable is about 130
feet 50 ohm coax that measures to be in good shape. The ground system is
8 wire radials between 30 and 60 feet in length.

The measurement tool is an HP 8407a network analyzer and an HP 8601a
sweep generator. The frequencies of interest are from 3.5 to 4 MHz. As
the antenna is moved from resonance at 3.5 MHz to 4 MHz the return shows
a marked difference. It varies between -24 db and -50 db. The pattern is
irregular, however it is repeatable.

The antenna design is a coil that moves up or down across copper
fingers. As the coil moves those fingers contact more or less of the
coil and change the impedance.

What could cause the variation in return across a relatively narrow
band. A return of -24db is relatively small, an impedance very close to
50 Ohms and an SWR about 1.135. A return of -50 db is less then 1.01 SWR
and represents a small change in Z, however a change non the less.

I can think of three potential causes for this change:

1. variations in the coil change Q

2. variations is the finger to coil connection change R

3. variations in the ground radials.

Are there others?

Thanks - Dan

dansawyeror wrote:
Are there others?

What kind of base matching are you using?
--
73, Cecil http://www.w5dxp.com

All,

This is a real world "what is going on" question. The antenna is a
'screwdriver' on a 3 foot stand in the back yard. The earth ground is
normal loam, the original soil was forest. The feed cable is about 130
feet 50 ohm coax that measures to be in good shape. The ground system is 8
wire radials between 30 and 60 feet in length.

The measurement tool is an HP 8407a network analyzer and an HP 8601a sweep
generator. The frequencies of interest are from 3.5 to 4 MHz. As the
antenna is moved from resonance at 3.5 MHz to 4 MHz the return shows a
marked difference. It varies between -24 db and -50 db. The pattern is
irregular, however it is repeatable.

The antenna design is a coil that moves up or down across copper fingers.
As the coil moves those fingers contact more or less of the coil and
change the impedance.

What could cause the variation in return across a relatively narrow band.
A return of -24db is relatively small, an impedance very close to 50 Ohms
and an SWR about 1.135. A return of -50 db is less then 1.01 SWR and
represents a small change in Z, however a change non the less.

I can think of three potential causes for this change:

1. variations in the coil change Q

2. variations is the finger to coil connection change R

3. variations in the ground radials.

Are there others?

Thanks - Dan

Dan, 24 dB return loss is about as good as you can possibly get
for any type of design: matching network, filter, you name it.
Anything better is just pure luck. In any design you always
get some very deep return loss notches. They could just as well
be minor errors in network analyzer calibration, and how
accurately your standards are characterized.

73,

Frank

None. I should be able to use the instrument to figure out the current
input. I am not sure how to figure out values. Is that fine tuning?



Cecil Moore wrote:
dansawyeror wrote:
Are there others?

What kind of base matching are you using?

On Fri, 30 Mar 2007 19:07:23 -0700, dansawyeror
wrote:

Are there others?

Hi Dan,

There is the 130 foot radial (the transmission line's unchoked
shield).

There is also an unreasonable assumption that this slight variation is
meaningful.

You would do better (per the advice of Reggie, in times past) to
invest the wire in those 8 radials to more radials that are not much
longer than the radiator is tall.

Soil from a former forest is not particularly good soil; and it isn't
particularly relevant to the outcome except as a large resistor (which
may more than explain the good match).

73's
Richard Clark, KB7QHC

"dansawyeror" wrote in message
...
All,

This is a real world "what is going on" question. The antenna is a
'screwdriver' on a 3 foot stand in the back yard. The earth ground is
normal loam, the original soil was forest. The feed cable is about 130
feet 50 ohm coax that measures to be in good shape. The ground system is 8
wire radials between 30 and 60 feet in length.

The measurement tool is an HP 8407a network analyzer and an HP 8601a sweep
generator. The frequencies of interest are from 3.5 to 4 MHz. As the
antenna is moved from resonance at 3.5 MHz to 4 MHz the return shows a
marked difference. It varies between -24 db and -50 db. The pattern is
irregular, however it is repeatable.

The antenna design is a coil that moves up or down across copper fingers.
As the coil moves those fingers contact more or less of the coil and
change the impedance.

What could cause the variation in return across a relatively narrow band.
A return of -24db is relatively small, an impedance very close to 50 Ohms
and an SWR about 1.135. A return of -50 db is less then 1.01 SWR and
represents a small change in Z, however a change non the less.

I can think of three potential causes for this change:

1. variations in the coil change Q

2. variations is the finger to coil connection change R

3. variations in the ground radials.

Are there others?

Thanks - Dan

the real important thing though is that the match is near perfect at either
1.135:1 or 1.01:1, so stop playing around and put some power to that sucker
and make some contacts! remember, 'too low an swr can kill you'! or at
least keep you off the air for as long as you are tweaking it.

On Mar 31, 7:22 am, "Dave" wrote:
"dansawyeror" wrote in message

...



All,

This is a real world "what is going on" question. The antenna is a
'screwdriver' on a 3 foot stand in the back yard. The earth ground is
normal loam, the original soil was forest. The feed cable is about 130
feet 50 ohm coax that measures to be in good shape. The ground system is 8
wire radials between 30 and 60 feet in length.

The measurement tool is an HP 8407a network analyzer and an HP 8601a sweep
generator. The frequencies of interest are from 3.5 to 4 MHz. As the
antenna is moved from resonance at 3.5 MHz to 4 MHz the return shows a
marked difference. It varies between -24 db and -50 db. The pattern is
irregular, however it is repeatable.

The antenna design is a coil that moves up or down across copper fingers.
As the coil moves those fingers contact more or less of the coil and
change the impedance.

What could cause the variation in return across a relatively narrow band.
A return of -24db is relatively small, an impedance very close to 50 Ohms
and an SWR about 1.135. A return of -50 db is less then 1.01 SWR and
represents a small change in Z, however a change non the less.

I can think of three potential causes for this change:

1. variations in the coil change Q

2. variations is the finger to coil connection change R

3. variations in the ground radials.

Are there others?

Thanks - Dan

the real important thing though is that the match is near perfect at either
1.135:1 or 1.01:1, so stop playing around and put some power to that sucker
and make some contacts! remember, 'too low an swr can kill you'! or at
least keep you off the air for as long as you are tweaking it.

My experience with a screwdriver antenna is that the bandwidth vs vswr
is not as good as he is showing. My bet is that the ground is acting
pretty much like a dummy load making the SWR look real good. He could
probably use a good ground system before he tries talking . If I were
using a screw driver antenna as a base antenna I would consider
seriously increasing the length of the radiator.

Jimmie

Which portion of this antenna is the radiator? Is it the portion below
the coil or the entire antenna?

Richard Clark wrote:
On Fri, 30 Mar 2007 19:07:23 -0700, dansawyeror
wrote:

Are there others?

Hi Dan,

There is the 130 foot radial (the transmission line's unchoked
shield).

There is also an unreasonable assumption that this slight variation is
meaningful.

You would do better (per the advice of Reggie, in times past) to
invest the wire in those 8 radials to more radials that are not much
longer than the radiator is tall.

Soil from a former forest is not particularly good soil; and it isn't
particularly relevant to the outcome except as a large resistor (which
may more than explain the good match).

73's
Richard Clark, KB7QHC

dansawyeror wrote:
Which portion of this antenna is the radiator? Is it the portion below
the coil or the entire antenna?

The part of the antenna that radiates the most
is the part below the loading coil. On my HS-1600,
that section is 3 feet long. If you want to
dramatically increase your radiated power, add
another three foot section under the existing one.
Since you don't have to contend with the 12.5 foot
mobile antenna limit, make that bottom section
below the coil as long as possible.
--
73, Cecil http://www.w5dxp.com

On Sat, 31 Mar 2007 07:23:23 -0700, dansawyeror
wrote:

Which portion of this antenna is the radiator? Is it the portion below
the coil or the entire antenna?

Hi Dan,

To be precise, EVERYTHING is the radiator. The unchoked line, the
vertical to the coil, the coil, the whip above the coil, AND the
radials.

What is of concern, is what is called the current node (I'm about to
embrace that tarbaby called standing waves) whose placement can have a
very significant impact on the directivity (another tarbaby of "gain"
raises its prospect here) of the antenna.

The higher the coil resides in the vertical element, the more linear
distance beneath it is available to that current node, and from this,
a greater signal out. There is a point of diminishing return (an
economics term, not a radiation term) where raising the coil ever
higher brings no further gain (both a pun to fulfill the economic
basis, and a literal result of RF). You can reclaim more gain from a
high coil by adding a top hat which serves to extend the whip above
the coil. Hence there is a generally acknowledged rule of thumb that
the coil resides somewhere between one half way up to two-thirds the
way up of a simple vertical. You may note that your antenna attempts
to conform to this model in some sense (for instance, it is not a base
loading which is the poorest configuration).

73's
Richard Clark, KB7QHC