Date: Fri, 22 Nov 2019 12:39:45 -0800
From: Glenn Pritchard <gpritchard7000@gmail.com>
To: "Joe Subich, W4TV" <lists@subich.com>
Cc: towertalk@contesting.com
Subject: Re: [TowerTalk] "different" trap
<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
## The full sized Mosely 10m ele becomes aprx 75% FS on 15M. The
10M trap now becomes strictly end loading on 15M.
You require abnormal sky high coil uh values, if a loading coil is
inserted at the ENDS of a shortened dipole. You can easily see
that effect on any loading coil software calculator. The software I
use will depict the required loading coil uh in 7 different locations
simultaneously..... or spit out required coil values for any location.
Current through the end loading coils is very low.
## say u wanted to build a 15M monoband shortened yagi, 75% of FS.
The LAST place you would choose to insert loading coils is the extreme
ends!
Uh vales would be extreme, and the SRF...self resonant freq of the coil
will get u into trbl every time..... its now too low. Mosely
uses low Q traps on 10M.... IE: high L, low C...so they can obtain
the required uh for end loading on 15M.
## In the case of the Mosely, we now have to follow up the 15m end
loading....with a 15m TRAP. Traps are parallel resonant, with extreme
high
circulating currents through the coil, and extreme high peak Vs across
the capacitor. The cap is not like how telrex constructed traps, with
a NPO TX doorknob cap
In parallel with a coil. Mosely derives the capacitance between the AL
cover and coil windings....its distributed capacitance.
## Calculate or better yet, measure the circulating current and u will
gag. The puny 10 gauge AL wire gets hot enough, from localized heat,
to melt and deform
the grooved polystyrene former. One would think the thermal heat would
be heat sinked to the adjacent AL tubing on either side, but it does not,
its localized.
## I can see the same effect on tapped tubing coils on a linear amp.
The active 15M portion of the 20m tank coil gets hot, but the adjacent
un used 20m turns are room temp.
## Swap out a copper tank coil for an identical AL tubing coil, and you
will now find out what HOT is all about.
## 10 gauge AL wire is simply too small in gauge, freq is too high, and
AL only conducts 60% as good as copper... at any freq from DC, 160m,
80, 15 etc.
Run the numbers through brian Beasleys, K6STI coil 3.14 coil program,
and u will see the problem right away.
## toss In the slightest bit of a loss from lousy coil connections , at
each end of each trap, and u just compounded the issues.
## There is a reason Mosely rates their yagis at only 600 watts average
power output...= 3.5A into 50 ohms. One partial work around is the use
of dual driven eles. After melting the polystyrene formers on
several trapped yagis and several trapped verticals, I gave up on traps,
they wont work for my application. HB traps that will work are simply
too large, and too heavy, too expensive, constructed of .25 and .375 OD
silver
plated tubing and paralleled HEC HT-57/ 59 NPO caps.
Jim VE7RF
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