On 30 Mar 2005 08:08:27 -0800, "Brian Kelly" wrote:

2. Remove half the transmission line muffling of results by using a
field strength meter to find resonance (another reason for power);

Same as above but with a field strength indicator? Just might work if I
use a 4-digit DVM and a diode.

Excellant choice (add a filter cap too with resistive load for
averaging).

3. Find the Vf (as you put it) by derivation against a wire model
(through the difference in lengths of bare wire model resonance to
real wire resonance);

That would seem to work but I'd expect to still have the flat curves
because of the coax losses.

Hi Brian,

Actually, by using the FSM you entirely remove the transmission line
as disturbance to accurate response readings. Those come from the
external reading which interprets all power being applied AT the
antenna junction. However, it imposes upon you that you be scrupulous
about achieving the same drive levels at all the intermediate
frequencies across the swept band. If you do that, then the
transmission line characteristics for the drive going up to the
antenna junction fall out too.

Careful drive monitoring, and careful response monitoring render the
transmission line transparent to the measurement. Thus response/drive
is the antenna characteristic. Define one point's SWR, and you can
cast that into the suite of readings for a swept SWR curve. Take care
in that "one" SWR determination to anticipate the SWR lowering effect
of transmission line loss.

Then you do the same thing in software, and tailor the characteristic
insulation thickness to match your measurements. Having achieved
that, then you have your standard insulation. This does not give you
Vf until you then remove that virtual insulation and find the native,
bare wire resonance. This last step is satisfying (it answers your
question as to Vf), but the step before is more useful because you can
model other antennas from that standard.

73's
Richard Clark, KB7QHC

Richard Clark wrote:
On 30 Mar 2005 08:08:27 -0800, "Brian Kelly" wrote:

2. Remove half the transmission line muffling of results by using
a
field strength meter to find resonance (another reason for power);

Same as above but with a field strength indicator? Just might work
if I
use a 4-digit DVM and a diode.

Excellant choice (add a filter cap too with resistive load for
averaging).

Yup.

3. Find the Vf (as you put it) by derivation against a wire model
(through the difference in lengths of bare wire model resonance to
real wire resonance);

That would seem to work but I'd expect to still have the flat curves
because of the coax losses.

Hi Brian,

Actually, by using the FSM you entirely remove the transmission line
as disturbance to accurate response readings. Those come from the
external reading which interprets all power being applied AT the
antenna junction. However, it imposes upon you that you be
scrupulous
about achieving the same drive levels at all the intermediate
frequencies across the swept band. If you do that, then the
transmission line characteristics for the drive going up to the
antenna junction fall out too.

"Eureka". You're right. This is the way to go. Or at least to try.

Careful drive monitoring, and careful response monitoring render the
transmission line transparent to the measurement. Thus
response/drive
is the antenna characteristic. Define one point's SWR, and you can
cast that into the suite of readings for a swept SWR curve. Take
care
in that "one" SWR determination to anticipate the SWR lowering effect
of transmission line loss.

Since coax losses don't vary much if at all over any of the individual
HF ham bands a decent inline wattmeter with maybe a 4 inch scale should
allow me to maintain a constant power output over the sweep.

Then you do the same thing in software, and tailor the characteristic
insulation thickness to match your measurements. Having achieved
that, then you have your standard insulation. This does not give you
Vf until you then remove that virtual insulation and find the native,
bare wire resonance.

Agreed.

This last step is satisfying (it answers your
question as to Vf), but the step before is more useful because you
can
model other antennas from that standard.

That's what I need. It'll make a worthwhile weekend project which, if
successful, should result in less futzing around with the cutters and
the soldering iron up the tower. I'll also compare the experimental
results of the bare wire sweeps to the predictions given by the modeler
and "calibrate" the modeler in this respect too. Might lead me to my
own real world ground condx vs. the generic "real ground" in the
modeler which is another big source of modeling non-truths.

73's
Richard Clark, KB7QHC

w3rv

On 30 Mar 2005 19:16:44 -0800, "Brian Kelly" wrote:

Since coax losses don't vary much if at all over any of the individual
HF ham bands a decent inline wattmeter with maybe a 4 inch scale should
allow me to maintain a constant power output over the sweep.

Hi Brian,

I left that unsaid, expecting someone, if not you, would also come to
that conclusion. It does not work across all bands, but within a band
it will suffice. Also, even given the impression of accuracy that
most impart to their power meters, this method demands only "relative
accuracy" which can be exceptional when care is shown (try to maintain
a full scale indication or at least greater than 2/3rds at some
cardinal point on the scale).

At this point, one should reflect that if there is a mismatch, then
power at the feed point will vary somewhat. In other words, the
presumption of constant power (to subtract out the effects of the
transmission line) is violated. However, as a first pass estimation,
the method is still quite productive, and tightening up the method and
the numbers is an exercise left to the experimenter.

73's
Richard Clark, KB7QHC