[TowerTalk] New Cushcraft XM240 Trouble

Robert Pack (NX5M) nx5m at txcyber.com
Wed Apr 12 20:42:02 EDT 2006

I am out of ideas so let me probe the minds here.  I have already been in touch with CC on this issue.

Bought a new XM240 a couple of months ago.  Built it to specs for the center of the band.  The antenna did not present an swr any lower than 1.6:1.
This past weekend the antenna was taken down and up several times.  The coils and balun were checked but nothing seemed to be abnormal.  All connections are good.  Measurements were triple-checked before the antenna went up two months ago but we checked them again just to make sure.  All are correct.  We even changed the tip sections to the upper part of the band setting but still saw the same thing.......best swr 1.6 (only higher in the band as it should be).  The coax was tested and the antenna was even tested with a different piece of coax.  No luck at all.  Finally the balun was bypassed and the coax connected directly to the driven element but that did not seem to make much difference.
What I do find interesting and confusing is that at the lowest swr point, or resonate spot, the analyzer is telling me that it is 29-30 ohms.  This I do not understand.....but it does explain the mismatch.  I also do not understand why the analyzer shows 50 ohms at another point in the band but the swr is over a 3:1. 
Now CC wants me to send the coils back for testing which I guess I will do since the antenna is on the ground anyway and somewhat useless to put back into the air given the swr issue.  All of this even after CC sent me two new coils just in case I found a bad one........which to the best of my knowledge none of them are bad.
There are very few failure points on these antennas so I am really confused and frustrated.
Based on the 29-30 ohm reading I guess I could always feed the antenna with two 130 foot runs of 75 ohm cable in parallel........just being silly.  But spending what I had to spend to buy the antenna seems the thing should be better than this.  The 40-2CD that is on the top of the tower works just fine (flat swr at the design frequency).
To me it has to be a coil or the balun.  

Anyone have any helpful ideas?


Bob NX5M

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