Home                    Links                   Contact Information

Copyright © 1995 - 2008   John Tait   All rights reserved.

 

TROUBLESHOOTING TRAPS

 

    Below is a document that was produced by staff members at Cushcraft some years ago.. I have reproduced it here, with the permission of Ed Hammond WN1I..  It remains of course, copyright of Cushcraft Corporation...       

  It refers to the one-quarter (1/4) wavelength Cushcraft "AV" series (AV-3; AV-4; AV-5; and AP8) vertical antennas, hence the references to ground radials that are required for 1/4 wave antennas. 

   The Cushcraft "R" series (R5, R7, R6000, R8, etc) are half (1/2) wavelength antennas, and use a counterpoise system instead of ground radials.

"If you fail to get a good VSWR on one band there are three possible problems. One is that the trap is bad or mistuned. Another is that the radials are incorrectly measured or attached. The third is that the length of the radiator has changed, possibly becoming shorter because of a loose clamp allowing one section of tubing to slide into another section.  Check physical dimensions and connections first. Always troubleshoot a trap antenna problem working from the highest frequency to the lowest.. One way to test the radials is to attach temporarily one more quarter wavelength radial that is carefully cut to the correct length for the band on which the problem occurs.  Did the VSWR decrease? If so, then improve the radial system, if it did not, then there may be a trap problem.  

A trap is a high Q parallel resonant circuit. If the antenna works on the next lower band, then the coil of the trap is good, and has good connections to the aluminium tubing. If the next lower frequency does not work then the coil may be open. The balance between inductance and capacitance Is critical, and requires good equipment to assure proper adjustment. Refer to the trap trouble-shooting section for checking individual traps.

SWR CHANGES WITH THE WEATHER

Ice or heavy sticky snow that sticks to the radiator and traps will cause the resonant frequency to shift lower, due to a fatter radiator. If your antenna is ground mounted and you have only a few radials then in wet weather ground conductivity may change and therefore VSWR will change as soil conductivity varies. Any cracked, torn or wrong size plastic caps on the top of traps will allow moisture in, affecting the resonant frequency. Putting any type of sealant on the top of the traps will likely detune them and create voltage breakdown problems since the top of the trap is a high voltage point.

 

VSWR CHANGES WITH POWER

If VSWR varies with power level on one or more band the problem may be in the VSWR bridge. There can be a non linear variation of diode action at different power settings. This is common with inexpensive bridges. It is possible to overload a diode in the forward power mode. The diode is now on a different slope of the curve in relation to the reflected power diode which is not overloaded. The end result is that your VSWR will apparently increase when you go from low to high power. Example: 1.1:1 at 50 watts , 1.4:1 at 800 watts. Observe VSWR as you slowly increase power. If VSWR slowly increases you may be over­loading your bridge. If you see a large jump in VSWR at a specific power level not related to a slow increase in power, you could have voltage breakdown troubles with your antenna

Causes: Poor, or Intermittent connection in the radial system. Poor connection in a trap. High voltage breakdown on a trap, (sniff the end cap to see if burned). High voltage  breakdown in Input coaxial connector or matching network (if supplied).

 

VSWR too high on one or more bands.

Causes: Mistake in assembly. Poor, or no ground or radial system. Defective trap, See trap troubleshooting.

 

TRAP TROUBLESHOOTING  

On the AP-8 antenna check the connections at each trap.. Is the ground screw tight? Are the screws tight at each  strap  connecting  the  radiator  tubing  to  the capacitor tubing?  A poor conection at any  of these points will cause that trap to be detuned and result in poor VSWR on the band for which that trap was tuned. If you  have the AV-5 antenna check each  trap to insure that the cover is tightly secured.   The cover is the 1 5/8" aluminium tubing over the coil, On top of the cover is a plastic cap. Any movement of the cover will cause intermittent VSWR conditions on the antenna. You   may  test for  a  loose  cover  easily   while   the antenna Is still assembled. Grasp each trap in your hand and apply a moderate amount of pressure in a clockwise and then  in  a counter clockwise  direction about the axis of the element. If the cover slips It will require tightening. A hex head screw Is at the base of the trap. Tighten this screw with an appropriate screw driver or spintite.  Be careful  not to apply  so much force as to strip out the sheet metal screw. If the hole is already stripped, or gets stripped  accidentally,  it is an easy matter to fix by substituting a #10 x 3/8" or #10 x 1/2" self tapping screw in the enlarged hole, If all your traps pass the mechanical test, and seem to be installed properly, then a frequency check is in order. The traps should be marked before removal so that proper re-assembly is assured. Remove all of the traps and bring them Indoors for inspection.   A list of Cushcraft traps and resonant frequencies are presented below, so that you can check to see if a trap is near the frequency to which it should be tuned. Use as little coupling as possible so that the dip oscillator Is not pulled in frequency. Use a frequency counter or receiver to determine the frequency of the dip oscillator. (Nowadays we can use our Antenna Analysers of course.. sexier than a GDO..)

TRAP               OPER FREQ           OSC FREQ                      OSC COUPLING

TF                28.8                  27.87                         Capacitive

TG               21.3                  20.17                          Capacitive 

TH               14.2                   12.92                          Capacitive

TJ                 7.20                    5.81                         Capacitive

TR                21.3                  20.23                          Capacitive

TQ                28.7                  26.8                            Inductive

                     24.65               23.5                            Inductive

TS                 21.25                20.1                            Inductive

                     18.11                17.5                            Inductive

TT                 14.47                13.49                          Inductive

TU                 10.19                  9.9                            Inductive

TV                   7.3                    5.8                           Capacitive

The method of coupling to the dip oscillator is important. Traps from the AV series of antennas require capacity coupling because the coil is shielded. Place a trap on an insulated surface (large cardboard box) and couple your dip oscillator meter (GDO) to the trap as shown below. Be careful to follow directions explicitly.

Capacitive Coupling

For capacitive coupling the tip of the GDO coll should be just slightly Inserted into the lower end of the aluminium tubing of the trap. Inductive coupling can be used where the coil is visible except for the TV  trap  where  the  dip  can  be  found   easier  by capacity  coupling. When checking dual  frequency traps   (TQ  & TS)  short the trap  not  under test to prevent obtaining a false reading. It should be noted that  the  dip   meter frequency   is   lower  than   the operational   frequency   of  a  trap.   This   is   caused because the trap will load the dip oscillator and lower it’s  frequency.  You should  use the listed  oscillator frequencies as a guide.   Temperature and   humidity can have a   +/-   100  KHz  effect  on   traps.   If the readings are within 100 KHz of the listed amounts, do not worry, the effect upon the assembled antenna will be minimal, Shorted turns or other serious defects will cause wide   shifts from the norm. One or two megahertz is a definite indication of a bad trap . All coils are sealed and are difficult to repair properly. When all traps are checked and corrected,  reinstall them   in   proper  order,   (as  you   previously   marked them)  and your multiband trapped vertical is now ready for action.   "

Inductive Coupling

 

 

How to measure trap resonance
 
 
 
 
 
 
  The above information was supplied by Cushcraft, as previously stated.
 
 My own favourite method of measuring trap resonance I got from the Towertalk reflector..
 
 
Using an Antenna analyser..MFJ, Autek etc....
 
Put a 50 ohm non-reactive resistor in parallel with the trap and use the frequency button to scan 
it. Look for the zero reactance points. This will be fairly broad, but will show you where the 
resonance is.
 
If you have a higher end analyzer like the AIM 4170, which can measure higher values more 
accurately, use a 450 ohm resistor, run a scan over the 20-10 bands and watch the reactance 
crossovers, which should be at the peak R points.
 
 

 


Home                    Links                   Contact Information

Copyright © 1995 - 2008  John Tait   All rights reserved.

All pages and content there-in, are the intellectual property of the author, and protected by law.

All pages and content, are subject to change at any time, without notice.