[TowerTalk] Full 40 mts dipole on side of a tower
k4to.dave at gmail.com
Mon Apr 2 10:18:52 EDT 2018
John -- sorry if I wasn't clear on my answer. I measured the SWR with the
test coax, no balun, through 100 feet of RG8X, known to be good. It showed
the lowest SWR at 7200, 3.0:1. It rose to 5:1 at the band edges. All
measurements were made with the coax hanging at a 90 degree angle.
The SWR curves will show a flatter shape, I believe, because the dipole is
made with 1.625 inch diameter tubing for the first 24 feet either side of
center, then 1.5 inch for another 6 feet, then swaged to 7/8" for the
tips. It is made from old Telrex elements, but beefed up in the middle,
because the first time I put it up, it broke in the middle in a 35 mph
wind. It was before I got my stress analysis software. I expected it to
break. I put it up to see where it would break so I would know where to
beef it up. Backwoods engineering at its finest! :-)
At this point, it is not a question of what frequency it shows lowest SWR,
it is a matter of the SWR being unacceptable at the frequency of lowest
SWR. Read my last post (this morning). I think I have a plan that might
work. I will let everyone know when I have results to report.
On Mon, Apr 2, 2018 at 9:47 AM, john at kk9a.com <john at kk9a.com> wrote:
> Good Point. I also thought that the initial SWR was good for a dipole but
> perhaps it was not high or has a hairpin match. I asked where the new
> resonant point is and did not get an answer. If it just moved I would
> readjust the tips. I also wonder if the test coax and balun are the same
> and in the same configuration (running 90 degrees from the center of the
> element) as the permanent.
> BTW his initial post is here:
> John KK9A
> Sent via the Samsung Galaxy 7 edge, an AT&T 4G LTE smartphone.
> AC0DS Wrote:
> Dave …
> I’ve been following this mystery since your first post. Lots of very
> experienced people contributing (including yourself), so I’m sure the
> solution will surface shortly.
> One thing that I just now recalled. I no longer have your initial post
> handy, but remember that the initial test with the dipole hung up in free
> space away from the tower was very good. In fact, so good that it seemed
> suspicious. I think the SWR was something like 1.1 over the entire 40
> meter band?
> Don’t know what height the dipole was, but even a perfect dipole won’t be
> that close to 50 ohms at most heights. And, would expect to see some
> variation over the band.
> How long is the coax and what type?
> You said you had a balun at the feed point. I assume it is a choke with
> the coax wound around ferrite?
> How were you measuring the SWR?
> One possibility is that you perhaps had some unintentional losses (or
> measurement error) in your first test that went away when you mounted the
> dipole to the tower?
> 73 Craig AC0DS
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