The Cushcraft traps also have a thick aluminum washer on the case
grounding screw. The MFJ ones do not.
BTW, it's very hard to test Cushcraft traps with a dip meter, as you
can't couple to the coil magnetically. The method I'm using is to
position a scope probe and signal generator lead on opposite sides of
the outboard tubing, grounded to the trap case. The probes are attached
to the case with a rubber band, and protrude just enough past the case
to produce a detectable signal at resonance, about 20 mV P-P with a 0
dBm signal. The probe and SG lead are grounded to the case using a hose
clamp near the outboard end. The trap sits on a plastic box to avoid
loading it. I'm seeing very sharp resonances at about 20.2 MHz or the
15m traps. The -3 dB bandwidth is perhaps 40 kHz.
MFJ tells me they measure the attenuation with the trap in series with a
50 Ohm source and load. When I did that, I found a very broad null
centered at about 21.2 MHz. This suggests that the resonant frequency
measured above isn't very critical.
This is really way more than I ever wanted to know about Cushcraft
traps, but maybe the result will be a working antenna.
73,
Scott K9MA
On 9/12/2019 13:34, Wayne Kline wrote:
Scot,
Thanks for the heads UP. I bought an new Chushcraft AW3 antenna. I
got to install it one day B-4 My Major Lightning Strike ( READ UGLY)
Long story short I never did a VSWR sweep because there were storms in
the area and I hate being on a tower or handling coax lines with
thunder heads in the area.
Well testing last week the 12 meter band sweep was 1:23 -1 BUT the
17 meter was 2:2-1 @ 18.070 and 3:5-1 @ 18:165 I took it down
yesterday and am assuming it’s a trap issue.
Again I assume I’ll have to pry the other coil cover up to gain
access to the coil’s stay tuned
I do not want to point fingers to MFJ yet
Wayne W3EA
Sent from Mail <https://go.microsoft.com/fwlink/?LinkId=550986> for
Windows 10
------------------------------------------------------------------------
*From:* TowerTalk <towertalk-bounces@contesting.com> on behalf of K9MA
<k9ma@sdellington.us>
*Sent:* Thursday, September 12, 2019 2:22:02 PM
*To:* towertalk@contesting.com <towertalk@contesting.com>
*Subject:* [TowerTalk] Cushcraft/MFJ Traps
I recently ordered some replacement traps for my old A3S. One was
defective, so I took it apart. I was appalled at the poor quality. While
the original traps had several pop rivets, in addition to the coil
screws, connecting the tubing to the plastic insulator, the MFJ version
had only the two screws in the very end of the tubing. The coil wires
were sometimes wrapped the wrong way around the screws, and some of the
screws weren't even tight. Where the original coil was coated with some
kind of thick lacquer, the MFJ ones were bare.
The moral of the story is don't even think about going to the trouble of
putting up an MFJ/Cushcraft beam without taking apart all the traps and
fixing the above problems.
73,
Scott K9MA
--
Scott K9MA
k9ma@sdellington.us
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Scott K9MA
k9ma@sdellington.us
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