[TowerTalk] "different" trap

[jimlux]

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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