Topband: Inverted L improvements - Part 3 (now with data)

[Guy Olinger K2AV]

Hi Todd,

Have a look at the calculator at
https://chemandy.com/calculators/return-loss-and-mismatch-calculator.htm

This calculator allows me to compute the SWR for your data points, as if
the Z zero of the meter was 32 ohms. This is important because so many
excellent antennas exhibit raw feed R between 15 and 35 ohms. This exercise
will give me SWR numbers the same as if I had put a 32 ohm to 50 ohm unun
in front of your 50 ohm SWR meter. Using the calculator gives a revised and
normalized table:

1820  1.61  29.1  -14.3
1840  1.20  31.0  -5.7
1850  1.04  32.0  -1.3
1860  1.12  33.0   3.6
1880  1.49  35.3   13.0
1900  1.95  38      22.9

This allows me to determine matched to 50 ohm line 1.5:1 SWR points at 1825
and 1880 or a 1.5:1 bandwidth of 55 kHz. A good guess would be that the 2:1
bandwidth is nearly all of 1.8 to 1.9  Both are far too broad, indicating
considerable RF loss yet to be pin-pointed and remedied.

I have a full size inverted L over an FCP, whose feed R at the shack side
of the isolation transformer is 30 to 32 ohms. This conveniently allows me
to use a Balun Designs 16132 Unun (32 ohms in, 50 ohms out) to match the
natural R of the antenna to the 50 ohm feedline to the shack.

At the Unun output, my 1.5:1 points are 1807 and 1832 for a 1.5:1 SWR
bandwidth of 25 kHz.

On the shack side of 82 feet of LDF4-50A 1/2 inch hardline, the 1.5:1
points are 1804 and 1832 for a 1.5:1 SWR bandwidth of 28 kHz at the
operating position.

Over the last two years, this antenna has been carefully designed/worked
over to eliminate loss. Removing the loss will narrow SWR bandwidth.  My
shack SWR at 1880  is 4.8:1.  Switching in secondary matching (ATR30) is
necessary to work in the high 1.8's and above 1.9.

I would be interested in the RigExpert raw feed numbers without the choke
(I have a hard-to-shake dim view of RG58, especially old RG58). It would
also be helpful to have the RigExpert model number, and measurements taken
at 10 kHz points.

Good luck on the project. You have to keep working the problem.

73, Guy K2AV


On Fri, Jan 18, 2019 at 8:23 PM Todd Goins <tgoins at gmail.com> wrote:

> Hello,
>
> I borrowed a RigExpert analyzer and was able to take measurements that
> folks were asking for without AM station overload. I also built the K9YC
> 160m choke (18 turns of RG58 on a type 31 2.4" toroid). That choke is at
> the feed point of the vertical. The analyzer was connected directly after
> the choke.
>
> I have a collection of data and typed it into Excel but I can summarize
> here.
>
> The values are Freq, SWR, R, X, Z
>
> 1810  2.1
> 1820  1.9  29.1  -14.3  32.4
> 1840  1.8  31.0  -5.7   31.5
> 1850  1.7  32.0  -1.3   32.1
> 1860  1.5  33.0  3.6   33.2
> 1880  1.6  35.3  13.0  37.6
> 1900  1.8  38.0  22.9  44.4
> 1920  2.1
> 1940  2.5
> 1960  3.0
> 1990  3.9
>
>  I still only have the 30 x 42' radials attached but can add about 5 x 100'
> more radials (in a non uniform layout) if that might help.
>
> Fair, bad, really bad, horrible, hopeless?  Any advice is appreciated.
>
> 73,
> Todd - NR7RR
> _________________
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>