lu6etj wrote:
On 12 mayo, 19:00, Owen Duffy wrote:
I read on their web page for some more BS.

In explaining why their traps are lossless:

"Gain is a function of element spacing and boom length and not what
constitutes an element. The published gain figures for our products are
dBd, actual measured gain. Antennas which use other forms of trapping such
as linear loading, poor trap copies, baluns or matching devices have
inherit loss. "

So, all these things have inherent loss, but not their traps.

Fact is that GAIN is reduced by LOSS, anywhere and everywhere in the
system, dB for dB. This includes conductor losses, dielectric losses, and
trap losses which they exclude from their definition.

Owen

As I said: "I wanted to discover what was the supporting idea behind
that
feeding system for its designers"

Defend commercial stuff and his claims It is not my job (I do not
trust enough in all of them), I am an Amateur :) but seems to me that
their concept it is not opposed to my modest theoretical knowledge
neither my simulations exercises in 4NEC2 (their "numbers" partially
agree with mine, although I have not simulated exactly a Mosley
antenna model).
Now I am only interested in validate or negate the especific technical
concept and explanation given, from a scientific perspective.

Proposition =. Is it possible match the impedance of some Yagi
antennas lengthening the driven element and canceling its inductive
reactance splitting the element and adding series caps?
Can be made these caps with special pieces of insulated wire
introduced in element tube?.

A worthy goal.. There's countless schemes for feeding, most developed
empirically, and all probably work about the same, but have some aspect
that is convenient for manufacture, installation, patent avoidance; and
a lot of them are just marketing sizzle (Now! with brigher color paint!
our patented 10 mm bolt with 1.618 mm threads in the Golden Ratio is
available exclusively on our Whiz-Bang model 32 antenna! Worked all 5
boroughs of NY from the top of the Empire State Building on 40m!, etc.)



Try it... lenghten in a NEC simulation the driven element and cancel
inductive reactance = impedance raises and gain stays (I used
"3YAGI20.NEC" from 4NEC2 examples provided with Arie soft, for
testing).
In my test, raisen length from 5.12 m to 5.85 m per leg, Zin raises
from 31.8 + j2 ohm to 50.9 + j150 ohm and gain stays in 12 dBi (with a
fast ground and 15 m height).

Running that model in free space gives a forward gain of 8.13dBi and a
f/b of about 26.5 dB, with a couple of sidelobes at 115 degrees off
boresight at about 29 dB down.

making the change you give, changes the forward gain to 8.11 dBi
(insignificantly different), but the rear lobes get bigger: The side
lobe is now 25 dB down (instead of 29), and the back lobe is at 24dB down.

Still not a bad F/B, though.




Changing the length of the elements changes the relative phases and
magnitudes of the currents in the elements. You can have pretty big
changes without affecting gain very much.

However, what *will* go bad in a hurry is front/back or sidelobe
performance. The "cancellation" of the back and side lobes depends on
the phasing of the currents.

So, if the goal is "reasonable gain" in the forward direction and "easy
to build a matching network for" that's a valid strategy.

Just for grins, here are the currents in the center of each element:
5.85m leg 5.15m
R (wire2,seg62) 0.27 + j0.07(0.30 70) -0.3 +j 0.75 (1.3111)
De(wire1,seg21) 0.39 - j1.45(0.51-75) 1.62 -j 1.03 (3.1-32.5)
D (wire3,seg103) -1 + j0.05(0.45152) -1.2 -j 0.27 (1.9-167)

(magphase)


A couple things are interesting.. the overall magnitudes of the currents
are lower for the 5.85m version, but that's because it's excited with
1Volt, so the higher feedpoint Z means lower total current.

What's interesting is looking at the relative currents and phases:
The 5.85 has almost equal magnitudes, while the 5.15 has more current in
the driven element, and very much lower in the rear element.

The 5.15 meter has about 143 degrees phase shift from reflector to
driven, and then another 135 degrees from driven to director.
The 5.85 has 145 degree from reflector to driven, and 133 degrees from
driven to director.

That's pretty close in phase, which is why the forward gain is pretty
much the same.. the wavefronts line up nicely. It's the different
current magnitudes that are ruining the back lobes. A current magnitude
error of 10% changes the forward gain by about 0.4dB. But that same
error can turn a -30dB side lobe into a -10dB sidelobe.