[TowerTalk] Cushcraft/MFJ Traps

[K9MA]

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 at contesting.com> on behalf of K9MA 
> <k9ma at sdellington.us>
> *Sent:* Thursday, September 12, 2019 2:22:02 PM
> *To:* towertalk at contesting.com <towertalk at 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 at sdellington.us
>
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-- 
Scott  K9MA

k9ma at sdellington.us