All,

This is an attempt to add loaded radials to a loaded vertical. The plan is to
make a large efficient loading coil for the radials and offset the variable coil
in a 'screwdriver' vertical segment.

The antenna is about 4 meters overall with about an load at about 1.5 meters
from the base. The antenna is raised about a meter.

The loaded radials are two 1 inch copper tubes about 1.67 meters long. The
radial loading coil is connected from the coax shield to the radial coil.

The feed is 50 Ohm coax, the shield is connected to the loading coil and the
center to the base.

4nec2 predicts an impedance of between 10 and 20 ohms depending of the assumed
values of R in the impedance loads. The actual antenna measures 36 Ohms at the
feed.

The question is: What is the basis of the difference between predicted and
measured values?

Below is a representation of the nec code used to simulate the antenna.

Thanks - Dan

CM 75 m Vertical 12 ft high
CM base 3 ft up - two radials
CM copper conductivity
CE
GW 1 11 0 0 1 0 0 5 0.025
GW 2 7 0 0 1 0 1.67 1 0.025
GW 3 7 0 0 1 0 -1.67 1 0.025
GE 0
LD 4 1 1 1 5 1500
LD 4 1 6 6 8 600
EX 0 1 2 0 1 0
GN 2 0 0 0 13 5.e-3
FR 0 1 0 0 3.74 0
EN

"dansawyeror" wrote in message
...
All,

This is an attempt to add loaded radials to a loaded vertical. The plan is
to make a large efficient loading coil for the radials and offset the
variable coil in a 'screwdriver' vertical segment.

The antenna is about 4 meters overall with about an load at about 1.5
meters from the base. The antenna is raised about a meter.

The loaded radials are two 1 inch copper tubes about 1.67 meters long. The
radial loading coil is connected from the coax shield to the radial coil.

The feed is 50 Ohm coax, the shield is connected to the loading coil and
the center to the base.

4nec2 predicts an impedance of between 10 and 20 ohms depending of the
assumed values of R in the impedance loads. The actual antenna measures 36
Ohms at the feed.

The question is: What is the basis of the difference between predicted and
measured values?

Below is a representation of the nec code used to simulate the antenna.

Thanks - Dan

CM 75 m Vertical 12 ft high
CM base 3 ft up - two radials
CM copper conductivity
CE
GW 1 11 0 0 1 0 0 5
0.025
GW 2 7 0 0 1 0 1.67 1
0.025
GW 3 7 0 0 1 0 -1.67 1
0.025
GE 0
LD 4 1 1 1 5 1500
LD 4 1 6 6 8 600
EX 0 1 2 0 1 0
GN 2 0 0 0 13 5.e-3
FR 0 1 0 0 3.74 0
EN

Interesting Dan, I get the same results as you, using your code. At 3.74
MHz the input z is 16.8 +j133. The antenna is resonant at about 3.55 MHz.
With your average ground the gain is about -9 dBi.

The only question I have is how certain are you of the accuracy of your test
equipment? Did you use a 1:1 balun at the feed point? You may be getting a
large current on the outside of the coax.

Frank

dansawyeror wrote:
. . .
The question is: What is the basis of the difference between predicted
and measured values?

1. Have you decoupled your feedline? If you're not using a feedline,
have you decoupled your measurement device?

2. Have you substituted a lumped impedance of about 16 + j133 ohms for
the antenna and observed what your measurement equipment indicates?

Roy Lewallen, W7EL

Roy,

No I have not decoupled the feed from the antenna. I will try that tonight. I
have wound a 2:1 balun for testing. (anticipating at least a 25 ohm input
impedance)

I have measured the antenna with two different instruments. One is an Autek
analyzer at the antenna, the second is with an 8405a at the end of 100+ feed of
cable. They both show the same results.

Thanks - Dan

Roy Lewallen wrote:
dansawyeror wrote:

. . .
The question is: What is the basis of the difference between predicted
and measured values?


1. Have you decoupled your feedline? If you're not using a feedline,
have you decoupled your measurement device?

2. Have you substituted a lumped impedance of about 16 + j133 ohms for
the antenna and observed what your measurement equipment indicates?

Roy Lewallen, W7EL

"dansawyeror" wrote in message
...
Roy,

No I have not decoupled the feed from the antenna. I will try that
tonight. I have wound a 2:1 balun for testing. (anticipating at least a 25
ohm input impedance)

I have measured the antenna with two different instruments. One is an
Autek analyzer at the antenna, the second is with an 8405a at the end of
100+ feed of cable. They both show the same results.

Thanks - Dan

Dan, I notice the Autek analyzer only measures the magnitude of the
impedance. With any of these lower cost instruments it is impossible to
find any accuracy specifications. The 8405A is an excellent instrument, but
assume you calibrated it -- short/open/load -- at the end of the 100 ft
cable. This calibration should also be carried out on the antenna side of
your isolation transformer when you install it. Curious as to what kind of
directional coupler you are using for HF. I remember using a small HP
coupler for HF, but cannot remember its model number.

Frank

Frank's wrote:

Dan, I notice the Autek analyzer only measures the magnitude of the
impedance. With any of these lower cost instruments it is impossible to
find any accuracy specifications. The 8405A is an excellent instrument, but
assume you calibrated it -- short/open/load -- at the end of the 100 ft
cable. This calibration should also be carried out on the antenna side of
your isolation transformer when you install it. Curious as to what kind of
directional coupler you are using for HF. I remember using a small HP
coupler for HF, but cannot remember its model number.

Frank

Hm, if the Autek measures only the magnitude of the impedance, how does
Dan know the resistance? The model shows about 133 ohms of reactance,
which is much greater than the resistance.

Roy Lewallen, W7EL

Roy,

Several weeks back, and confirmed by frequency sweep model runs, you indicated
that minimum impedance is close or equal to the resonance point. I tuned the
antenna to the frequency of interest and then used the Autek to verify the
resonance point. That minimum value was 36 Ohms. I am assuming this is or is
very close to the resonance point for the antenna system.

What does your running of the model show for resonance frequency? At resonance
my running of the model shows close to 20 Ohms for the relatively large values
of R used in the model.

Thanks - Dan

Roy Lewallen wrote:
Frank's wrote:


Dan, I notice the Autek analyzer only measures the magnitude of the
impedance. With any of these lower cost instruments it is impossible
to find any accuracy specifications. The 8405A is an excellent
instrument, but assume you calibrated it -- short/open/load -- at the
end of the 100 ft cable. This calibration should also be carried out
on the antenna side of your isolation transformer when you install
it. Curious as to what kind of directional coupler you are using for
HF. I remember using a small HP coupler for HF, but cannot remember
its model number.

Frank


Hm, if the Autek measures only the magnitude of the impedance, how does
Dan know the resistance? The model shows about 133 ohms of reactance,
which is much greater than the resistance.

Roy Lewallen, W7EL

Frank,

The Autek is remarkably close. I have used it to checkout 50 and 25 Ohm loads.
For these two values it is very close. (It is battery level sensitive.)

The couplers are a pair of M-C ZFDC 20-4's.

Dan

Frank's wrote:
"dansawyeror" wrote in message
...

Roy,

No I have not decoupled the feed from the antenna. I will try that
tonight. I have wound a 2:1 balun for testing. (anticipating at least a 25
ohm input impedance)

I have measured the antenna with two different instruments. One is an
Autek analyzer at the antenna, the second is with an 8405a at the end of
100+ feed of cable. They both show the same results.

Thanks - Dan


Dan, I notice the Autek analyzer only measures the magnitude of the
impedance. With any of these lower cost instruments it is impossible to
find any accuracy specifications. The 8405A is an excellent instrument, but
assume you calibrated it -- short/open/load -- at the end of the 100 ft
cable. This calibration should also be carried out on the antenna side of
your isolation transformer when you install it. Curious as to what kind of
directional coupler you are using for HF. I remember using a small HP
coupler for HF, but cannot remember its model number.

Frank

"dansawyeror" wrote in message
...
Frank,

The Autek is remarkably close. I have used it to checkout 50 and 25 Ohm
loads. For these two values it is very close. (It is battery level
sensitive.)

The couplers are a pair of M-C ZFDC 20-4's.

Dan

Thanks Dan, I had forgotten about Mini-Circuits. Their price is hard to
beat. I may pick up one of the "PDC" series dual directional couplers.
Incidentally your code indicates resonance occurs at 3.54 MHz. I wonder how
the Autek behaves when subjected to a reactive load does it actually get
close to the magnitude?

Frank

The first thing I'd try, then, would be to put a 1:1 balun (common mode
choke) at the feedpoint and, when using feedline, a second one about a
quarter wave down the line. The balun can be constructed by winding the
coax (if you're using RG-58 or smaller) 8 - 10 turns on a type 43
ferrite core. Or you can clamp or thread a few large cores over the
coax. You can use the Autek to measure the core impedance -- shoot for
500 - 1000 ohms total -- the angle of the impedance isn't important.
Unlike the 2:1 balun, this won't disturb your basic measurement in any
way, it'll just reduce any common mode current.

If you still get the same result, then there are only two other possible
causes I can think of. One is the modeling of the inductors. I've found
that a lumped model of an inductor isn't good if there's any appreciable
current change in the real inductor from one end to the other due to its
physical length. The solution is to model the inductor as a helix.
You'll have to add some extra R to the model, however, if the turns are
spaced closer than a couple of wire diameters, since the program doesn't
account for proximity effect.

The other possible cause is that there's some source of loss you're not
accounting for in your model. The inductors and coupling to nearby lossy
objects are the most obvious candidates.

Roy Lewallen, W7EL

dansawyeror wrote:
Roy,

No I have not decoupled the feed from the antenna. I will try that
tonight. I have wound a 2:1 balun for testing. (anticipating at least a
25 ohm input impedance)

I have measured the antenna with two different instruments. One is an
Autek analyzer at the antenna, the second is with an 8405a at the end of
100+ feed of cable. They both show the same results.

Thanks - Dan

Roy Lewallen wrote:
dansawyeror wrote:

. . .
The question is: What is the basis of the difference between
predicted and measured values?


1. Have you decoupled your feedline? If you're not using a feedline,
have you decoupled your measurement device?

2. Have you substituted a lumped impedance of about 16 + j133 ohms for
the antenna and observed what your measurement equipment indicates?

Roy Lewallen, W7EL