[PVRCNC] Antenna trap repair/troubleshooting.

[alsopb]

Guys,

Perhaps you can profit from a recent experience in tracing down a bad
SWR problem I had on a Hy-Gain TH-2 trapped tribander.

I thought at first it was a lightning hit which did it in.

Using the Autek RF-1 and a 50 ohm resistor, it was determined that the
feedline and balun was OK.  Thus the problem lay in the antenna.  The
50 ohm resistor was put across the balun after the antenna wires were
disconnected.  The RF-1 at the shack end showed essentially a 1:1
SWR.  You could do this with your xmitter and a suitably high wattage
resistor.

The elements were detached from the boom.

A visual inspection yielded no clues.  DC continuity appeared across
all traps.  Attempts to measure across the traps with the RF-1 yielded
results I couldn't inteprete.

Determining which trap or traps was bad was now the problem.  I
disassembled the two elements into four pieces.

The reflector pieces were suspended a wood table and chairs to form a
split dipole.  The AUTEK RF-1 was used to see if it had resonances
near 10,15 and 20M.  It did but 5-10% or so below the lower band
edge.  SWR or impedance was used to see that all was OK.  Thus the
reflector was OK.

Next the dipole of driven elements were measured.  No good!

Next a dipole of half a driven element and half a reflector were
paired and measured.  One of the driven element halves turned out OK,
the other was bad.
That narrowed it down to two traps.   Swapping traps from the good
half to the bad half determined that the 15M trap was bad.  

OK now what?  Why was it bad?  An ohmmeter showed continuity across
the trap.
Thus the loading coil part was probably OK.  That left the
"capacitor".  I had no clue about the guts of the coil.  For all I
knew, there could have been a discrete physical capacitor which had
been zapped.   

Gulp.  Disassemble the trap.  Doing so was somewhat destructive. 
Inside the trap there was just a coil and no capacitor!  Hmmmm.  The
capacitor was made by the outer metal sleeve of the trap and some
plexiglass insulators around the element stock.  One plexiglass
insulator was completely crudded up with mud wasp leavings.  I cleaned
this out with water and reassembled the trap-- trying to repair the
damage as well as possible.  It now worked.  The wasp crud probably
caused the "capacitor" to be shorted or so leaky that the trap
wouldn't function. 

Had I known the cause of the problem, the trap could have probably
just been submerged in water and washed with a small stream hose! 
Disassembly wouldn't have been necessary.

So if you have a similar problem and the trap shows DC continuity, try
washing it out first!  You won't harm the coil and could just cure the
problem.

By the way, the trap drip holes need to be renamed.  They should call
them "insect entrance doors".  Perhaps wrapping plastic window screen
material around the holes would keep the critters out.

By the way, a replacement trap cost about $20.  Doing the
troubleshooting rather than buying a complete replacement set (8)
traps saved about $140.  Not bad for a few hours work.    

73 de Brian/K3KO