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Copyright © 1995 - 2008 John Tait All rights reserved.
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.
you fail to get a good VSWR on one band there are three possible problems.
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 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.
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
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
meter frequency is lower
than the operational
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
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
you previously marked them) and
your multiband trapped vertical is now
ready for action.
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
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.