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.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 workingfromthehighest frequency
to the lowest.. One way to test the radials is to attach temporarilyone morequarter wavelength radial
that is carefully cut to the correct length for the band on which the problem
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
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
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.
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 voltagebreakdown in Input coaxial connector or matching network (if supplied).
too high on one or more bands.
Mistake in assembly. Poor, or no ground or radial system. Defective trap, See
On the AP-8 antenna check
the connections at each trap.. Is the ground screw tight? Are the screws tight
at eachstrapconnectingtheradiatortubingtothe capacitor tubing?A poor conection at anyof
these points will cause that trap to be detuned and result in poor VSWR on the
band for which that trap was tuned. If youhave the AV-5 antenna check eachtrap
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. Youmaytest foraloosecovereasilywhilethe antenna Is still assembled. Grasp each trap in your hand and apply a
moderate amount of pressure in a clockwise and thenina counter clockwisedirection
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
carefulnot to applyso 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 andbringthemIndoorsforinspection.Alistof 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 receivertodeterminethefrequencyofthedip oscillator.
(Nowadays we can use our Antenna Analysers of course.. sexier than a GDO..)
FREQOSC FREQOSC COUPLING
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 TVtrapwherethedipcanbefoundeasierby capacitycoupling. When
checking dualfrequency traps(TQ& TS)short the trapnotunder test to prevent obtaining a false reading. It should be noted thatthedipmeter frequencyislowerthanthe operationalfrequencyofatrap.Thisiscaused because the trap will load the dip oscillator and lower it’sfrequency.You shoulduse the listedoscillator frequencies as a guide.Temperature andhumidity canhavea+/-100KHzeffectontraps.Ifthe 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 causewideshiftsfromthenorm.Oneortwo 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 theminproperorder,(asyoupreviouslymarked them)and
your multiband trapped vertical isnow
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