[TowerTalk] A3 Traps

[K9MA]

The dimensions of the MFJ version of the A3 are the same as the 
original. (The manual is identical, basically just a copy.) The trap 
design is slightly different, with fewer turns on a slightly larger 
coil. The outside dimensions are the same. Since the rest of the antenna 
is the same, one would presume the trap inductance and resonant 
frequencies would be too, but I've been unable to verify that. I was 
able to measure the resonant frequency, I believe accurately, by very 
lightly coupling a signal generator and scope probe capacitively. I 
tested all the 15m traps that way, and tuned the replacement (MFJ) traps 
to the same frequency as the originals, one of which I suspected of 
being intermittent. No joy.

73,
Scott K9MA



On 9/30/2019 07:38, Marv Shelton wrote:
> I have a 1980’s vintage Cushcraft A4 and just replaced it with a 3 element SteppIR.  I had the 40m add-on as well and never had a problem with it. 15 meters was it’s best band.  Only once did I take apart the traps and service them. Cleaning out bugs, and using scotch-brite and conductive grease with new SS hardware to make sure all the connections were solid.
> I once saw where someone had designed a special jig to measure and test the traps because the labels had worn off.  I do know that MFJ did a redesign and their traps are not made the same way nor at the same frequency as the original Cushcraft. I would compare the assy instructions for the two as I would the guess the element lenghts are different as a result and mixing MFJ’s traps with original Cushcrafts would require some modelling/tinkering to get things working right.
> It sound like in your rebuild that you may have mixed traps in the wrong locations, as 20 meters represents the lowest freq band, it uses the total electrical length of the elements and the order of traps on the element isn’t important. For 10 meters, the electrical length is the shortest, so as long as there is some kind of trap in the 10 meter position, you a likely to get resonance somewhere in the wide 10 meter band.
> You can’t go by the physical length of the traps to determine which one is for which band. You need to build a jig to test them. Due to their physical design inductive coupling to measure them is tricky.
>
> Marv K7IU


-- 
Scott  K9MA

k9ma at sdellington.us