antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

dansawyeror wrote:
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.

Are you aware that a coax cable will change the impedance
from the antenna feedpoint impedance in a spiral to 50
ohms in the limit?
--
73, Cecil http://www.qsl.net/w5dxp

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

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

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

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

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

"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

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

dansawyeror wrote:
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.

Yes, that should be correct.

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.

NEC-2 shows resonance (and minimum SWR) at 3.55 MHz, where R = 16.12
ohms; NEC-4 says resonance is at 3.51 MHz., where R is 16.08 ohms. (I'm
using EZNEC implementations of both.) Although small, I don't usually
see that much difference between NEC-2 and NEC-4. I suspect it's because
of the very low height above ground -- the two programs implement the
Sommerfeld ground somewhat differently. An average gain test shows good
average gain, indicating that NEC isn't having numerical difficulties.

I'm getting pretty convinced that the problem is the use of lumped loads
for the inductors. With this short an antenna, I'd expect the inductor
currents to be quite different at the ends(*), making the lumped load
models inadequate. This can lead to pretty severe errors.

(*) due to inductor radiation and unsymmetrical coupling of the inductor
to the rest of the antenna and to ground.

Roy Lewallen, W7EL

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

Thanks - I will try to figure you how to create a non lumped model for the
inductors. Right now that is 'undiscovered country'.

Dan

Roy Lewallen wrote:
dansawyeror wrote:

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.


Yes, that should be correct.

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.


NEC-2 shows resonance (and minimum SWR) at 3.55 MHz, where R = 16.12
ohms; NEC-4 says resonance is at 3.51 MHz., where R is 16.08 ohms. (I'm
using EZNEC implementations of both.) Although small, I don't usually
see that much difference between NEC-2 and NEC-4. I suspect it's because
of the very low height above ground -- the two programs implement the
Sommerfeld ground somewhat differently. An average gain test shows good
average gain, indicating that NEC isn't having numerical difficulties.

I'm getting pretty convinced that the problem is the use of lumped loads
for the inductors. With this short an antenna, I'd expect the inductor
currents to be quite different at the ends(*), making the lumped load
models inadequate. This can lead to pretty severe errors.

(*) due to inductor radiation and unsymmetrical coupling of the inductor
to the rest of the antenna and to ground.

Roy Lewallen, W7EL

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

Good question. I will play with that. That said, based on Roy's comment at
minimum the load should appear nearly pure resistive.

I just tested the Autek with 6.25, 12.5, and 25 Ohm loads.
25 read 26
12.5 read 12 - 13 - 12 etc.
6.25 read mostly 7 with an occasional 6.

I would say for non-reactive loads it is pretty close.

Tomorrow - will be to experiment with non lumped inductors. That will be a
challenge.

Thanks - Dan


Frank wrote:
"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

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

dansawyeror wrote:
Thanks - I will try to figure you how to create a non lumped model for
the inductors. Right now that is 'undiscovered country'.

EZNEC v. 4.0 users should use Wires Window/Create/Create Helix. You'll
get many choices, including position, orientation, various ways of
specifying the pitch and number of turns, twist direction, and so forth.
(EZNEC demo users can create any size helix to see how it works, but
won't be able to run a calculation unless the helix is extremely
simple.) In NEC, use a GH 'card'.

There should be at least a wire diameter of air space between turns,
preferably several. (That is, the center-center distance between the
wires in one turn and the wires in adjacent turns should be at least two
wire diameters, preferably more.) If air spacing is less than 2 or 3
wire diameters, the calculated loss will be somewhat lower than reality
because NEC (or EZNEC) doesn't account for proximity effect.

Roy Lewallen, W7EL

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

Roy Lewallen wrote:
I'm getting pretty convinced that the problem is the use of lumped loads
for the inductors. With this short an antenna, I'd expect the inductor
currents to be quite different at the ends(*), making the lumped load
models inadequate. This can lead to pretty severe errors.

(*) due to inductor radiation and unsymmetrical coupling of the inductor
to the rest of the antenna and to ground.

Over on qrz.com, W8JI reported that he measured a 60 degree phase
shift through a 100 uH coil at 1 MHz. He also asserted that the
flux density is highest in the middle of a coil. Since the current
is proportional to flux density, that means the current in the
middle of the coil is higher than at the ends. These things are
perfectly consistent with what EZNEC reports when the distributed
network helical coil inductor is used instead of the lumped circuit
load inductor.

Essentially the only time the currents at each end of the coil are
equal is when it is installed near a standing-wave current maximum
point where the slope of the current is already close to
zero whether it be in a wire or in a coil. The phase of the standing-
wave current is relatively constant whether it be in a wire or
in a coil. (The standing-wave current doesn't rotate like a normal
phasor.) The phase shift caused by the coil happens in the forward
and reflected currents, not in the standing wave current which is the
sum of the forward current and reflected current. Not much
changes when part of a wavelength of wire is replaced by a large
loading coil. The current waveform, though warped somewhat by the
high fields inside the coil, still very roughly follows the classic
cosine shape of a wire. After all, no matter what, the current at
the tip of an antenna is zero whether it be a wire or a coil. If
a coil is placed at a standing-wave current node, the phase at each
end of the coil will be opposite, i.e. current is either flowing in
both ends at the same time or out both ends at the same time. Such
is the nature of distributed networks.
--
73, Cecil http://www.qsl.net/w5dxp

antenna impedance - calculated 10 - 20 Ohms - measured 36 Ohms??
 

Roy Lewallen wrote:
EZNEC v. 4.0 users should use Wires Window/Create/Create Helix.

And the detailed results are quite different from the lumped
circuit load inductor.
--
73, Cecil http://www.qsl.net/w5dxp