Dutchman wrote:

wrote in message
...

Dutchman wrote:

Hi all,

I need some advice (moral support?) as to the correct interpretation
of an MFJ259B output when testing traps as its handbook gives no
guidance for trap testing. Intuitive interpretation leads one up the
garden path?
A commercial and home-brew trap gave the same results. Tests were done
as follows:
(All measurements were verified by another external freq. 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 with no sharp tuning like points a)or
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 apart.

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
disappeared.

Any comments will be welcome!
Thanks
Hans ZS6KR

| What happens if you try this? No link coupling - direct
| impedance reading. Wouldn't resonance be at lowest Z on
| the MFJ readout?
|
| MFJ---Trap---50ohms----+
| S0- |
| 239--------------------+

Thanks for your suggestion but this also gives the same as the
terminated measurement where the only only throughput occurs way off
parallel resonance both sides but using the MFJ the actual parallel
resonance point cannot be measured due to infinite SWR being out of
range of the meter.

Sorry - my mistake. Resonance occurs at highest Z, so
it won't work the way I posted. How about this:

MFJ---Trap--A--50ohms-------+
S0- |
239---------B---------------+

At points A & B you connect an RF probe to measure
the RF voltage across the resistor. At lowest voltage
you are at resonance. I've never tried that, but it
might work.