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Author Topic: Attic dipole  (Read 2898 times)
W1AJO
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Posts: 108




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« on: May 16, 2012, 01:06:56 PM »

I have 52 feet 8 inches of space in my attic. What's the best length for a dipole that will fit for a 40m & 20m?

Rig is an Yaesu FT-817 ND with a 50 watt amp.

Thanks!


Tony
W1AJO
Newnan GA
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WB6BYU
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Posts: 13485




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« Reply #1 on: May 16, 2012, 01:28:48 PM »

I'd put the feedpoint in the center of the attic, against the ceiling.  Run one 40m wire
as far as possible towards one corner of the roof.  If there isn't enough space, bend it
and run it along the end of the attic to use up the length.  (This will require slightly more
wire than a standard dipole due to the bend.)  Repeat for the opposite corner, even if
they aren't exactly symmetric.  Trim the wire length for minimum SWR in the part of the
band you want to operate.  That gets you on 40m.

Then take a pair of 20m dipole wires, connect them to the same feedpoint, and run them
to the other corners.  They won't reach the far end, of course, so put some rope or string
on the ends and tie them off to a convenient support.  Adjust the wire lengths as needed.
(A convenient way to adjust the lengths is to tie the wires off with a foot or so of wire
hanging down at the end - that allows you to trim the hanging wire without having to
readjust the support ropes.)
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KU3X
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Posts: 144




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« Reply #2 on: May 16, 2012, 02:20:10 PM »

That's easy if you want a coaxial fed dipole.
Search google, "2b2040LQ".
It's a dual band 20 and 40 meter dipole only 44 feet long. It will fit perfect.

Barry, KU3X
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W5DQ
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Posts: 1209


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« Reply #3 on: May 16, 2012, 02:39:48 PM »

I have 52 feet 8 inches of space in my attic. What's the best length for a dipole that will fit for a 40m & 20m?

Rig is an Yaesu FT-817 ND with a 50 watt amp.

Thanks!


Tony
W1AJO
Newnan GA

Easy answer ....

If using an untuned antenna with a tuner, max wire length is whatever will fit without too much bending or overlap. Only one doublet dipole.
If using a tuned antenna, cut dipole wire to 1/2 wl plus a bit for tuning, fit to space available and trim for low SWR.

Rig really has no bearing on solution providing your not going to run full legal limit into an attic setup. Too much fire hazard if that power level and not done correctly.

W5DQ
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Gene W5DQ
Ridgecrest, CA - DM15dp
www.radioroom.org
W1AJO
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Posts: 108




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« Reply #4 on: May 17, 2012, 06:28:14 PM »

Thanks all!  I'll look into the dual band 20/40 dipole.
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WX7G
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Posts: 6214




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« Reply #5 on: May 17, 2012, 06:46:32 PM »

MFJ-17754 40/20 meter dipole. 42' long and $60.
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NA7U
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Posts: 72


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« Reply #6 on: May 17, 2012, 07:10:03 PM »

I like WB6BYU's solution, rather than spending 60 bux plus shipping for something ready-made (but then I'm cheap!) and complicated with inductors. Seriously, for 40 meters you only need to bend tails of 10 feet on each end if you run it straight across, less if you go diagonally. It will hardly make any difference in performance. Of course the 20 M will fit easily, but it's best to not run it parallel to the 40 M elements, though doing that is not out of the question. You will probably find that you can work on 15 meters as well with a tweak of the ATU.

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K0ZN
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Posts: 1560




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« Reply #7 on: May 17, 2012, 09:03:47 PM »

I also "second"  WB6BYU's idea.  He has it right.   Keep it simple.

Install a 40 M dipole....bend the ends around to make the 66+ ft. fit in the space available.... trim it to lowest SWR on 40 M....then ADD a 20 M dipole and
tweak the 20 M dipole for SWR. You may need to go back and retune the 40 M dipole after adding the 20 M antenna....that would not be surprising.

Keep the two dipole wires SEPARATED by 12" or more.... put a spacer in (like with in 8" off the feed point) to immediately separate the two dipoles. This will
help reduce interaction between the two antennas. If you run the antennas to opposite corners of the attic that would maximize the space between them.

If that were my antenna going in the attic, I would feed it with a good quality 1:1 current balun.... that would help keep RF off the coax shield and possibly
reduce the pick up of noise off the vertical coax shield.

Fed with coax and a balun, those two dipoles should be simple to tune...NO tuner required.....and work well.

Remember that the ends of the dipoles are the High Voltage points.... make sure they are well insulated and NOT touching any wood. (avoid a fire hazard)

Even though that antenna is in the attic, it would be well worth your time to make sure there is a good DC ground connection to it for static drain and if you have a lot
of lightning in your area, even though it is in the attic, I would put a GOOD lightning ground on it. After lightning has gone through a half mile of AIR.... it could care
less if your roof is between it and the antenna ! I would also disconnect it from the rig during storms and GROUND the coax to a ground that could handle a lighting
hit. Do NOT leave the antenna disconnected and "floating" ! GROUND it if you are not using it.  Bottomline: your roof will NOT stop lightning !!  "Plan accordingly."

73,  K0ZN
« Last Edit: May 17, 2012, 09:06:47 PM by K0ZN » Logged
W1AJO
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Posts: 108




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« Reply #8 on: June 01, 2012, 09:28:19 AM »

So what's the best solution for a ground on the the 2nd floor of my house?  The basement is finished so there's no way to run a copper wire ground.  The windows of the room face the street so I can't run a ground out by the windows.
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W1AJO
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Posts: 108




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« Reply #9 on: June 01, 2012, 09:32:32 AM »

Barry,

I ordered a 2b2040LQ - but it's on back order!  Thanks all for the help!
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M6GOM
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Posts: 993




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« Reply #10 on: June 01, 2012, 11:00:21 AM »

So what's the best solution for a ground on the the 2nd floor of my house?  The basement is finished so there's no way to run a copper wire ground.  The windows of the room face the street so I can't run a ground out by the windows.

You don't need one for a dipole.

cancel the order, buy 100ft of wire. Build your own dipole - one for 40, one for 20 fed with the same coax. Not only will it save you money but the homebrew one should do double duty on 15m band giving you three bands in total.

Nobody should be buying dipoles.
« Last Edit: June 01, 2012, 11:02:22 AM by M6GOM » Logged
WB6BYU
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Posts: 13485




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« Reply #11 on: June 01, 2012, 11:51:11 AM »

Quote from: W1AJO
So what's the best solution for a ground on the the 2nd floor of my house? 


Why do you need a ground?  Until you can explain why, there is no way to answer
your question.

That's because grounds serve several purposes, and what works for one type might
not be suitable for another - or might actually be unsafe. 

For example, equipment running of the standard AC power line should have a safety
ground, generally provided by the third wire on the plug (but not always.) 

A ground for lightning protection requires a lot heavier wire and lower resistance than
one to prevent static buildup on the antenna.

An RF ground system to provide the other half of the circuit for an end-fed wire
antenna needn't actually be connected to earth ground at all.

Grounding a station (by whatever means) sometimes eliminates problems due to
common mode currents on the feedline, but can also increase those currents
in other situations.


My approach is not to bother with such a ground until I know what I need it for,
then design it to suit that purpose.
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