MFJ Enterprises Inc. > Help Desk > Knowledgebase

MFJ-259B usage -- 7 MHz traps

Dear Hans,
I do not believe that you can get the trap resonance frequency with a 80 kHz resolution (7.00 MHz, 7.08 MHz) following the method that you described. Even though the numbers that you got were interesting.
As you know an antenna trap equivalent circuit would be a parallel combination of an inductance (L) and a capacitance (C). The inductance has an intrinsic resistance associated with it. Such a condition is represented by a quality factor (Q).  A 40 meter trap could be for example a 15 pF capacitor in parallel with a Q = 200 , 33 uH inductance. Then the equivalent circuit would be a 7 ohm resistor in series with a 33 uH, being both elements in parallel with a 15 pF capacitor.
 7 MHz Trap: parallel Q=200, L=33 uH, C=15 pF
At the resonance frequency the equivalent impedance would be pretty high (above 20 kohm, as it is shown in the graphic). A variation of 80 kHz would mean an impedance variation of around 4 % (800 ohm) with respect the maximum at 7 MHz. Pretty hard to detect the trap center frequency with that precision.
It is obvious than an LC  equivalent parallel and series resonance impedances have opposite behavior at resonance. While a series LC resonance forces a zero impedance the parallel one makes a big high jump.
Some people says that it is possible to measure the resonant frequency of a trap by connecting a piece of wire between the antenna analyzer output and its ground and then  coupling the wire near to the trap (being the trap disconnected). After setting the analyzer in the Z mode a tuning should be done. A big Z jump around the center frequency should be expected, claim this people. I have not tried this procedure, although I do not believe will work with accuracy, considering the physical nature of the parallel LC resonance and other elements.
The MFJ-259B measures impedance magnitude below 650 ohm. A gross procedure to measure your 7 MHz trap center frequency could be to set the analyzer Z mode and hook it up. Then start reading the Z amplitude while tuning up from 1.8 MHz. As soon the magnitude of the impedance reaches 650 ohm write down the frequency (f1). Keep on tuning up to write down the frequency as soon Z goes below 650 ohm again (f2). This could happen in a pretty big bandwidth (i.e. 3 MHz for the first point, 17 MHz for the second one). The trap center frequency would be (f2 - f1) / 2. Certainly this would be a pretty gross idealistic approximation.
W4RNL created some web pages regarding antenna traps
----- Original Message -----
Sent: Wednesday, July 30, 2003 5:53 PM
Subject: MFJ259B usage
Hi all,
I need some technical advice (moral support?) as to the correct interpretation of
an MFJ259B output when testing traps as its handbook gives no guidance for trap
A commercial and home-brew trap gave the same results. Tests were done as follows:
(All measurements were verified by an additional external frequency counter)
A 50 ohm carbon resistor was used in series with the hot output which connected to
a 2-turn 55mm dia loop of wire that was loosely coupled to a trap for 40m; the
other end of the loop going back to the MFJ ground.
The total loop alone measures R=51 X=14.
Tuning the frequency with the trap in-situ resulted in 3 points of interest:
a) 7.00 MHz R=51  X=0 
b) 7,08 MHz R=60  X=14
c) 7,20 MHz R=120 X=42
Which is the actual parallel resonance of the trap? My guess is point (b) where no
reactance is added or subtracted by the "load" and the trap is not seen by the MFJ.
This is an in-between point that has no sharp tuning like points (a) and (c).
I also assume point (a) is where the loop and trap combination present a pure high
dynamic impedance to the MFJ output.
The same test with a dip-meter and its associated coil only showed a dip at point a)
which I consider close but incorrect. The trap is admittedly series-resonant and
absorbs maximum power from the dip-meter but I contend that testing open-ended traps
may be deceptive as they seem to exhibit both a series and parallel resonance not far
Terminating the open end eliminates point (c) altogether as I proved with a swept
measurement using an IFR analyzer: There is only a minimum response visible on the
scope display around 7,20 MHz (c) which must be a parallel resonance as the trap is
in series with the IFR input. However, the proximity of the trap body to the faceplate
could influence the frequency but NOT the fact that the series resonance has totally
Any comments will be welcome.
Hans ZS6KR
Article details
Article ID: 68
Company: MFJ
Date added: 2017-01-12 14:29:20
Views: 82

<< Go back