On 17/03/2018 01:01, Sal M. O'Nella wrote:
A base-mounted tuner may have difficulty matching certain lengths.
According to my ICOM AT-150 manual, we should avoid multiples of
half-waves. The manual offers the formulaÂ* 300/freq X 0.5 X integers 1,
2, 3, etc.Â* (You figure it separately for each integer but stop when the
product is longer than your antenna is.)Â* For all your desired
frequencies, you do the calculations, then avoid that length.Â* You will
develop a list of lengths that are "bad" and you avoid an antenna of
that length.

I did the calculations and saved them with the manual.Â* I observe from
my own results that 43 feet is in a sweet spot between 39.5 feet (which
is bad for 12m) and 45.8 feet (which is bad for 30m).Â* Never mind who
uses those bands or doesn't.Â* I simply ran the numbers for he
frequencies we can bring up and that's where the chips fell.Â* By the
way, ICOM says the antenna should be more than 7 meters (23 feet).

Here are the "bad" lengths that I got, in meters.
5.05 - 5.30
6.00 - 6.03
6.99 - 7.14
8.25 - 8.30
10.10 - 10.71
12.00 - 12.05
13.99 - 14.85
15.15 - 15.81
16.51 - 16.60

So, obviously, some good lengths fall between those "bad" lengths.Â* Here
are the midpoints of the "sweet spots" that remain in play:
7.75mÂ*Â*Â*Â*Â*Â*Â* (25.42 feet)
9 mÂ*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â* (29.52 feet)
11.5mÂ*Â*Â*Â*Â*Â*Â* (37.72 feet)
13mÂ*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â* (42.64 feet)
15mÂ*Â*Â*Â*Â*Â*Â*Â*Â*Â*Â* (49.21 feet)Â* but this is a minimal sweet spot
16.25Â*Â*Â*Â*Â*Â*Â*Â*Â*Â* (53.3 feet )

The 43' antenna is at one of those sweet spots.Â* CAUTION:Â* If you use a
long pigtail, that could mess things up.

I hope this helps.

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"Gareth's Downstairs Computer"Â* wrote in message
news
What is so special about that magical length, for it does
not appear to be resonant anywhere!

An interesting analsis, thank-you