On 11/22/19 3:34 PM, Jim Brown wrote:
Right. The gain shortfall is the result of a design that causes the
elements to be shorter, which reduces their radiation resistance, and
the inductive loading provided by the traps. It also modifies the
current distribution in the elements. This is directly analogous to what
happens in a mobile antenna with a base- or center-loading coil.
There was an excellent two-part piece of research on this published in
QEX 4-5 years ago, and included careful measurements of field strength
resulting from various positions of the loading coil and the the use of
top (capacitive) loading.
A change in the current distribution on a "half wave or less" dipole
isn't going to make a huge change in the directivity (gain, ignoring
loss). A half wave has a gain of 2.15dBi, a infinitely small dipole has
a gain of 1.5 dBi.
Putting it in a Yagi isn't going to change that a whole lot (since the
pattern of the yagi is basically the combination of the pattern from
isotropic sources with the element currents multiplied by the gain of
the element by itself).
I think the efficiency problem is more one of higher element currents in
a small antenna, and a larger share of the power going to the resistive
losses.
The shortened element will have less resistive term at the feedpoint
(lower Rrad), which leads to the potential for less efficiency since the
loss resistance is probably the same (or more), and you're looking at
Rrad/(Rrad+Rloss). It might also present a different impedance, so
there might be loss in the matching network.
The loss effect might be aggravated in a Yagi - most Yagis have higher
currents - there's more stored energy in the magnetic field, and the
current is how the coupling among elements works.
The feedpoint impedance of a Yagi without any matching network is often
quite low (one way to think of it is that you are driving a bunch of 72
ohm dipoles in parallel), and that aggravates the effect of resistive
losses.
It's kind of like the problem with a W8JK - the closer you get the two
dipoles, the narrower the beam, but the higher the currents get, so
while the directivity keeps increasing, the gain does not.
73, Jim K9YC
On 11/22/2019 2:22 PM, Joe Subich, W4TV wrote:
The K7LXC and N0AX test protocols have been independently
verified and only the Mosley tribanders showed significant
gain shortfalls.
Believe what you like but the Mosely shortfall is most
obvious on 15 meters where the element shortening is the
most pronounced. If you want to discount the K7LXC/N0AX
results, you need to provide a verifiable hypothesis to
explain the "Mosley anomaly."
73,
... Joe, W4TV
On 2019-11-22 3:39 PM, Glenn Pritchard wrote:
That false narrative about high loss is just not true.
I’ve been working with traps for over 30 years and have never seen
the losses that those two claimed.
Glenn, VA7UO
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