Date: Tue, 9 Dec 2014 22:24:07 -0600
From: "Matt" <maflukey@gmail.com>
To: <towertalk@contesting.com>
Subject: [TowerTalk] FW: Trap ID
Mosley TA33 trap explanation:
To the best of my knowledge, this is how it works...
Inside the trap there are coils on each end. The inner element inserts into
coil form such that the aluminum trap cover forms a coaxial capacitor along
their overlapping length. This is wired in parallel with the inner coil to
form a resonant trap for the 10m band. On 15m, the 10m trap becomes a net
lump-sum inductance, thereby shortening the resonant length of the 15m section.
The length of the aluminum trap cover is cut to resonant the antenna on 15m.
The outboard element tip inserts into the other end of the coil form such that
the aluminum trap cover also forms a coaxial capacitor along their overlapping
length. This is also wired in parallel with the outer coal to form the 15m
resonant trap. On 20m, both the 10m trap and the 15m trap both become net
lump-sum inductances which shorten the resonant length of the 20m section.
The outer element length is then selected to be resonant on the 20m band.
73
Matt
KM5VI
## heres some real issues. On 10m, the ele ends where the 10m trap is. On
15m, the ele ends where the 15m trap is. On 15m, that makes for an extremely
short ele, only a few inchs longer than 10m ! The 10m trap, is now a loading
coil on 15m.
Problem is, the loading coil is just mere inchs inboard from the 15m trap.
Draw this on paper, and you will see it comprises of a fubar design at best.
You end up with an extremely short ele on 15m, which has its sole loading coil,
(10m coil), right at the
extreme ends of the 15m ele. Run that through any software, and you will fast
see that the required loading coil would have to have a huge value of uh. If
you were to build a monoband loaded dipole for any band, you would place the
loading coil half way out, or slightly inboard of that point, IE 40-50% .
How the heck the mosely et all trap yagis even begins to work is beyond me.
How the heck you end up with 50 ohms at the feedpoint is also beyond me. How
they get one beta match to work on 3 bands is the engineering feat of the
centrury. My guess is the trap losses are sky high, bringing the Z up to 40-60
ohms. Interesting to note, here in town over the years, from 1977 to present,
every time a TA-33 jr or sr owner swapped to a hy gain 204BA, they always saw
a 10 db improvement on the 20m band. That test has been done at least 8 times
now. Brand new mosely, installed correctly vs new or used monbander. Mosely
is not all its cracked up to be. When a 4-5 el monobander literally decimates
a mosely trapped yagi, something is amiss in the trapped yagi design. Most
f-12 2 el yagis will eat any mosely for breakfast. A local fellow, 130 miles
north of me installed a mosely Pro-96, a few years ago. Nobody was impressed
with it, including the owner. Very expensive and heavy ant, with one helluva
lot of traps. It was later removed, and replaced with a f-12. Another fellow
with a 33-jr, complained he had some issues with his new 33-jr. We took it
down, and removed the trap covers. It appeared that they had been subjected to
a propane torch for several minutes, literally melted. His sb-220, with 1200 w
pep out on ssb, and low duty cycle, with perhaps 300 w average power, and no
speech processing, had done the job.
## I spoke to mosely engineers a few years ago, and asked them to explain
the 5kw rating of their pro-96 to me. Turns out the 5 kw rating is pep INPUT
of the amp itself. Mosely engineers explained to me they assumed the amp would
have no more than 60% tank efficiency. IE: 3 kw pep out on ssb. They also
explained to me that they assumed a 20 % max duty cycle on ssb. IE: no more
than 600 watts average power out. On cw, they rated the ant at 2500 watts.
Again that was INPUT power. That worked out to 1500w out, using 60% tank eff.
They explained to me that on CW, they assumed a 40% duty cycle. 2500 x .6 =
1500w. 1500 x .4 = 600 watts average output on cw. On RTTY, they rated the
ant at 1000 w INPUT. They assumed a 100% duty cycle and 60% tank eff. MAX
output is 600 watts on RTTY. So no, you can NOT run 1500w RTTY through their
yagis. That was carefully explained to me during the long ph conversation.
They are a 600 watt AVERAGE output rated yagi, that’s it. Tune the amp up
with a 1.5 kw cxr..and you will fry the yagi. He also explained to me that
the 600W average po rating was where the swr was low on each band..and not at
the band edges. The circulating current through the traps is sky high, which
is what over heats them. Small gauge aluminum wire will not handle high rf
currents. You require a lot of heat to melt the forms the trap coils are wound
on.
## IMO, I would pass on the trapped yagis, and move into the 21st century, its
not 1953 anymore. Even the telrex trapped yagis were fraught with issues,
with the fixed caps having to be replaced on a regular basis.
Jim VE7RF
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