Good morning Craig,
You make several good points. As I measured the dipole hanging away from
the tower, there was 100 feet of RG8X used as a feedline. It is known to
be good and I check loss often. It is my "utility" roll of cable, kept for
purposes like this, antenna testing, etc...
The SWR started at 1.1:1 at 7.0 Mhz and rose smoothly to 1.5:1 at 7.3Mhz.
Obviously, the dipole needs to be shortened. But since I operate almost
exclusively on CW and hardly ever above 7.2Mhz SSB, I decided to leave it
there. I didn't use any sort of balun for this test. I have a balun on
the tower, along with the the permanent feedline, a new piece of "buryflex"
cable. The balun is one I made. It is actually a common mode choke, RG392
wrapped six times around a Type 31 core, and then repeated on another core
about six inches later. This is all my best effort to replicate info I
find on K9YC (I think) website and the K7 fellow's website, who sells the
things.
My analyzer is a Comet CAA-500, the original, not the newer one with LCD
screen.
I have also simulated the antenna using accurate diameters and section
lengths using the Antenna Model Software. I ran analysis at 5 feet, 25
feet, 50 feet, 75 and 100 feet. I then took the data at those heights as
I was pulling the antenna up the trolly wire. It behaved just about like
the simulation said it would. It was a great exercise for checking the
accuracy of my model against the "real world".
All in all, it has been a fun project. I built the antenna out of scrap
parts left from my old Telrex yagis. Next step is to build a pair of
Moxon 2 el 40m yagis from some more of the leftovers. This dipole was
mainly a "warmup" exercise to validate the modeling technique. With this
dipole in place and the two Moxons mounted at 145 and 75 feet above and
below it, I will have a good setup to measure gain and F/R of the Moxon
stack compared to the dipole. This will be with only the 40M antennas on
the tower. Later, when I build and mount the next antennas, I can check to
see if there is any degradation of the system.
It is a really fun project and it keeps me off the street and out of the
bars :-)
73,
Dave, K4TO
On Mon, Apr 2, 2018 at 8:36 AM, Craig Smith <ac0ds@sent.com> 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.
>
> Questions:
>
> 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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