[TowerTalk] FW: Trap ID

Jim Thomson jim.thom at telus.net
Wed Dec 10 22:22:38 EST 2014

Date: Tue, 9 Dec 2014 22:24:07 -0600
From: "Matt" <maflukey at gmail.com>
To: <towertalk at 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.


##  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   

More information about the TowerTalk mailing list