[Elecraft] Testing traps with a GDO

[DAVID C. OLEAN]

Hello Dan,
    I am the guy who made the CC A3 and A4 tribanders. (A long time ago!)  They use
single band traps for 10 and 15 meters. In addition, we made all sorts of other traps
for other antennas.  We used a spectrum analyzer with tracking generator to check
traps in the lab. The production traps were tuned with a free running oscillator that
was calibrated with a standard trap as part of the tuned circuit, and the output freq
was measured with a counter.
    The lab setup with the Tektronix analyzer would give the parallel resonant trap
frequency exactly. As I recall, the trap freqs were at 28.4 MHz for the A3 and A4. In
addition, the A4 director trap was 28.8 MHz. The 15 meter traps were 21.250 MHz.  The
problem comes when trying to set these numbers with a GDO. Even loose coupling causes
the apparent frequency to drop quite abit. I remember making up a troubleshooting
sheet with all the typical GDO frequencies for each trap but it was never
disseminated because the management figured things might be made worse with people
checking and trying to open their own traps.  As far as I know, that information was
eventually pitched out.
    W1TF wrote a troubleshooting book for antennas, "Troubleshooting Antennas and
Feedlines" in 1992 and published by MFJ at the time. In it, he had my trap testing
stuff listed in part. (He had a copy of the notes I had made.)  The GDO frequencies
were:
    ACTUAL FREQ      GDO FREQ
        28.800                    27.87
        28.400                    27.47
        21.250                    20.17

    If you are measuring a metal encased trap, it is best to remove it from the
antenna and lay it on an insulated table. Couple the GDO into the coil by lining the
axis of the trap and the axis of the GDO coil in line, with the GDO on the inner side
of the trap (towards the tribander boom)  Butt the end of the GDO coil next to the
trap tube. You will see a slight and narrow indication at the parallel resonant trap
frequency. Your readings will be lower than the real number, but as has been
suggested, you can compare traps from each side of the element and determine if
things look different.
    Most problems involved bad connections and no dip at all, or a higher than normal
dip frequency that signified a shorted turn.
Sometimes, wasps built mud houses inside, and the dip disappeared but would return
when the trap was cleaned out and dried out after a thorough washing!
    I hope this helps you abit.  I think the TA33 had dual band traps.  The same
techniques should work. You can also use a noise bridge and run it through the trap
while it sits on a fixture. Connect it up with small coax, and a few BNC connectors,
Terminate each side of the trap tube to ground with a 20-70 ohm resistor, and use
your receiver to find the noise null. This will be your actual trap frequency. (or
close to it!)  It is much the same as we used with the Spectrum analyzer and tracking
generator combination.

    73
Dave K1WHS
K2 #1247 & 915

Dan Holloway wrote:

> Hello,
>
> I know there are some pretty knowledgeable folks here so please allow me
> to tap some of that knowledge.
>
> I have a troublesome beam (TE-33) and I suspect one of the traps.  Being
> a tuned circuit, I would like to try using a GDO to check the trap's
> resonant frequency.
>
> I have found nothing on the Internet on how this is done.
>
> Can anyone tell me or, send me a link, on the correct technique for
> testing a trap?
>
> danl@internode.on.net
>
> Thanks in advance.
>
> 73 de Dan
> VK8AN
> K2/100 # 2469
>
> _______________________________________________
> Elecraft mailing list: Elecraft@mailman.qth.net
> You must be subscribed to post to the list.
> To subscribe or unsubscribe see: http://mailman.qth.net/mailman/listinfo/elecraft
> Elecraft Web Page: http://www.elecraft.com
> Also see: http://www.elecraft.com/elecraft_list_guidelines.htm