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Author Topic: First attempt at a dipole  (Read 5147 times)
WB6BYU
Member

Posts: 16899




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« Reply #15 on: September 15, 2012, 01:44:54 PM »

That's a good starting point.  The metalwork and wiring will affect the bands
somewhat differently.  The good news it that it looks like all the wires are a
bit long, making it easier to tune.

As an estimate, it looks like you'd need to start by shortening the 40m wires
by about 2' on each end.  Pulling more wire through the insulator and twisting
it back along itself isn't quite the same thing, but should show an improved
SWR to confirm you are going in the right direction.  You'll probably have
to pull about 3' of wire though by the time you're done, but start with 2' and
see what happens.  (Changing the length may affect the proximity to other
objects, which will also have an effect.)

The 20m is pretty close:  once you get 40m tuned, check the reading again.  With
the current numbers you may only need an adjustment of 3" or so on each end.
(It is fine at the CW end of the band as it stands.)

Once you've gotten the other two fixed, then turn your attention to 10m.  Shortening
each end by 3" is probably a good starting guess again, and it may take a few tries to
get it where you want it.


You can use tape if you want to secure the turns, but usually it is easier to make a few
gentle wraps around itself to hold the wire in place.  It won't have to hold a lot of strain
in the attic compared to being strung between two trees that are blowing in the wind.


You don't need quite so many samples to plot the SWR curve:  100kHz points are probably
adequate in most cases, especially when you're still at the point where minimum SWR appears
to be out of band.
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N8CMQ
Member

Posts: 683




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« Reply #16 on: September 15, 2012, 03:23:26 PM »

Here is where an antenna analyzer comes in handy.
If the antenna is resonate at a lower frequency, then you shorten the antenna.
If it is higher, then you lengthen it. But if you don't know, what do you do?
All you know is, the antenna SWR is too high.
Odds are, it is too long, but that is only a guess.
If you try shortening the antenna without cutting the excess length off would be best.
Remember, start at the lowest frequency element first, then the higher frequency antenna.
Good luck with your antenna!
And if you get a chance to go to any swap fests, keep an eye open for an antenna analyzer, or even an antenna noise bridge!
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N8CMQ   Jeff Retired...
N4CR
Member

Posts: 1757




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« Reply #17 on: September 15, 2012, 03:58:51 PM »

No, I was wrong when I suggested starting with 10 meters. Start by tuning the 40 meter band.

All of your elements are too long.
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73 de N4CR, Phil

Never believe an atom. They make up everything.
KF7YED
Member

Posts: 37




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« Reply #18 on: September 15, 2012, 04:08:47 PM »

Thanks, guys. Yeah: I was being too conservative when I measured the construction lengths and was too afraid of being too short. Easier to shorten than to lengthen...  And re: the spreadsheet, another guy suggested 25 kHz steps, but yeah I agree. 100 would be coarse enough to judge at this stage.

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KF7YED
Member

Posts: 37




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« Reply #19 on: October 03, 2012, 04:25:04 PM »

Progress--- 10m is right on: 1.5:1 at 28.300 mHz running down to 1:1 at 28.700 mHz.
20m was 2.3:1 to 3.0:1 from 14.225 to 14.350 mHz at the first reading when I started, and after a bunch of adjustments its still 3.2:1 to 2.6:1 across the voice portion of the band. I'll go back and start again from my original length. The wire getting full of Sharpie marks.....  I haven't even started on trimming the 40m wire yet but its at 2.5:1.


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