ZL2CA wrote:
OK on the elevated loading coil. I'm not convinced that hard results
support your claims:
Bob,
I was confused by your reaction to my assertion that not only had
my increased elevated inductance improved my signal reports, but it
had also increased measured signal strength.
Some of what you say supports me, but some appears to doubt my
results. I concede that some of my attempts to explain the practical
results with theory may be faulty.
If I recall, you said that the changed current distribution was an
explanation for the reduced value of current at the feedpoint, which
was incorrect. Current is dependent on "loop resistance" at
resonance.
Can you explain this concept, please. It is foreign to me.
> Last night, Jim, M0BMU measured my field strength compared to a
> measurement taken two weeks ago. It was 0.9dB up.
That is very similar to some tests I did some time back, at 181 kHz,
with a temporary top loading.
OK, so you agree that toip loading can make an improvement.
> This does not account for the greatly improved reports,
Quite so. 0.9 dB hardly shifts the S meter.
But would you reject a 0.9dB improvement? If I had done so at each
stage over the last few years I would still be struggling to hear my
own signals a few km away. Also, 0.9dB makes a big difference at
the 'knee' of the readability curve, for instance when trying to work
VE.
> but it does
> show that with less current and more resistive losses, the only way
> the ERP could have increased is if the 'effective height' had
> increased.
Yes. The net result of decreased current because of increased loop
resistance (a top loading coil necessarily needs higher inductance
(more wire) than a base loading coil) and the improved current
distribution in the vertical wire. The effective height factor is
just winning out over current reduction, by some 0.9 dB.
Isn't that what I said?
>Field strength is of course directly proportional to current
> squared multiplied by effective height (h) squared.
True for radiated power. Field strength is directly proportional to
the product of current and effective height (amp metres).
Agreed. Field strength involves V, and to get power you need V
squared. My mistake.
> At last here is real evidence that the elevated coil really does
> increase the 'h' part of the equation. Several of us were sure that
> it did, and several have noticed improvements in our signals when
> using elevated coils, but the evidence has always been anecdotal.
And still appears to be the case. The same "gain" should be observed
on transmit and receive (being wary that local noise could be
different at each end of a path).
I am not sure why you are discounting Jim's measurement. Why is
the evidence still anecdotal?
Although I agree with your argument about reciprocity, my antenna
is matched to the Tx and not the Rx so I do not think a receive
measurement would be valid.
There is still the matter of adequately housing the elevated loading
coil and sustaining good insulation at high RF voltages. The weight
of a coil usually means it needs to be supported by a tower or mast.
Antenna modelling shows that if the vertical feed to a T antenna is
near metalwork, it caused shunt capacitance that pulls down the gain
by about 1 dB. Tuning (and re-tuning) of the loading coil is somewhat
inconvenient when it up in the air.
Yes. And that is why I said that I did not expect everyone to rush out
and put up an elevated coil. It is clearly unsuitable for a T antenna,
but very suitable for an L where the mast can take the total weight of
the coil - actually mine is just in the horizontal section but has in the
past been mounted on top of the mast. The coil need not be huge or
heavy. I use a drain pipe (a fizzy drinks bottle can be used) and
enamelled wire recovered from a mains transformer. This has
replaced a much bigger Litz wire coil at the base.
Tuning is easily achieved. I have a 0.5mH variometer at the base,
with a 68000pF heavy duty capacitor in series with it.
One situation not amenable to modelling (with NEC-2) is when there is
clutter around the vertical wire. It seems that clutter causes
disproportionate losses from "soakage" as the high field strength part
of the wire rises above ground. Some experimenters have reported
significant improvement by relocating the "upwire" to being in a
clearer environment. A similar effect would arise with an elevated
loading coil, as the potential on the "upwire" is considerably lower
than for a base loading coil installation. However, all of these
environmental factors should show up in impedance data and gain
measurements.
I am not saying that top loading will work for everyone. If I had a
mast totally in the clear I would expect very little benefit as the
antenna would perform very much as the model. However, practical
LF antennas are hampered by trees, houses and so on, and may
benefit from "improved current distribution" - your words.
I have observed the construction technique a number of LF NDB
stations, from visits and photographs. None have elevated loading
coils. All have clear sites.
Quite.
I'm not against experimentation and developing better antennas that
fit in our back yards.
To summarise the history of this experiment, I have a small garden
and cannot use a large top section to gain 'effective height' on my
vertical section. I have used an elevated inductor for several years.
The idea came from knowing that 160m mobile antennas (in scale a
similar problem to the one we have) worked better in tests when the
coil was half way up the whip. Using some 3mH I instantly got better
results at a distance.
This was met with some doubt from those who relied on modelling,
and from those who could not find a theory that fitted. I have always
argued that if practice and theory do not fit, believe the practice - or
more usefully the wrong bit of theory is being used to explain what is
going on.
Others have tried elevated coils with some success, notably Rik,
ON7YD, who has a similar problem to me - small top section and
local obstructions. He was originally very critical of my claims, but
has become a convert.
I contend that, although I may have my theory incorrect, using
elevated inductance can be worthwhile (even with the increased
resistive losses caused by the total inductance having to increase)
in circumstances where the top section is relatively small and
therefore the effective height is low, or where the lower part of the
vertical section is obstructed. The theory I defend is that the
effective height of the antenna is being increased by this method.
I look forward to further discussion on exactly what theory fits this
practical result.
Mike, G3XDV (IO91VT)
http://www.lf.thersgb.net
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