Below is a document that was produced by staff members at Cushcraft some years ago.. I have reproduced it here but it remains of course, copyright of Cushcraft Corporation…
It refers mainly to the old 1/4 wavelength AV series of antennas (12AVQ, 14AVQ etc) hence the references to radials.. The “R” series (R5, R7 etc) are 1/2 wavelength antennas, and the radials are NOT 1/4 wavelength resonant..
If you fail to get a good VSWR on one band there are three possible problems. The first 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.
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 connection 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 accidently, 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 are 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.
For capacitive coupling the tip of the GDO coll should be just slightly Inserted into the lower end of the aluminum tubing of the trap. Inductive coupling can be used where the coil is visable 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.
Below is part of an email that I received from Dick W5TA which contains more “hands on” experience of fixing traps..
Getting ready for Field Day, I repaired an old HyGain tribander which belonged to our local radio club. We found that the connections of the copper wires in the trap coils to the screws connecting them to the aluminium tubing had seriously corroded. With most traps there is one or more retaining screws. After their removal you can pull off the end caps and pull the trap apart. It’s an outer aluminium tube over a plastic inner rod serving as a coil form. Often you will find bug nests, insect carcasses and corrosion bridging turns of the coil as well as corrosion at the terminals and maybe the whole coil. If you rewind the coil, take care to first note the wire size and number of turns. With this, clean up and reassemble and you have a “good as new” trap.
I have to recognize Kees Talen, K5BCQ, who showed me this procedure.