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Author Topic: Help with my attic antenna please  (Read 4611 times)
M1EQA
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Posts: 5




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« on: October 18, 2013, 12:06:49 PM »

Dear All
I would like some help on this experimental antenna as I am reading conflicting articles on the internet.

I am just about to move into a top floor flat (which is two floors up from ground level) with an attic space. I am not allowed any outside antennas so I am trying to find the best HF antenna to put up in the attic for multiband operation (40m to 10m).

My initial thought is a slinky antenna:

I thought of building a dipole with one slinky on each side fed with a 450 ohm ladder feeder connected to a home-made 4:1 Balun fed into a MFJ-949E tuner with RG58 to my Kenwood TS-820 transceiver.

The attic has a diagonal length of 35 feet facing roughly North-South.

Each slinky leg would be stretched out 7.5 feet (so a 15 foot dipole in total).
(I understand a slinky contains 65 feet of wire).

Questions:
Is a 4:1 Balun the right choice?
What length of 450 ohm ladder would I need i.e. multiples of half wavelength etc. (or doesn’t it matter)?
What length of RG58 would I need (or doesn’t it matter)?

Or can anyone suggest a better alternative

73's

Nigel M1EQA


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WX7G
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Posts: 6332




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« Reply #1 on: October 18, 2013, 12:47:30 PM »

It would be best to feed your SlinkyTM antenna with 450 ohm line to the MFJ-949. The balun in the tuner is all that is needed and the ladder line length that makes it from the antenna to the tuner is the length to use. 

 
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WB6BYU
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Posts: 13582




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« Reply #2 on: October 18, 2013, 01:09:59 PM »

You certainly will find conflicting information on the internet, even here on eHam,
because there are so many variables that we really can't tell you want will work
best in your situation.  The best we can do is to give you some general
suggestions to try, and tips for maintaining good efficiency.

Yes, there are a number of accounts of folks using Slinky coils for antennas.
Sometimes they work well, other times not so well.  One problem is that the
basic Slinky is steel, which has higher RF losses than copper.  (This is true of
any magnetic material.)  While the difference in efficiency may be small for
a straight half wave wire out in the clear, antennas shorter than 1/2 wavelength
have lower radiation resistance, so conductor resistance becomes more
important in calculating efficiency.

Another point is that, for short antennas in general, the performance is limited
by the physical length more than the number of feet of wire used to build it.
So if your antenna is 15 feet long, it will behave much more like a 15' antenna
than like a 130 foot antenna, in spite of the fact that it contains 130 feet of
conductor (assuming you are using one Slinky on each side.)  In fact the excess
wire length may actually degrade performance, especially on the higher bands.


My suggestion would be to run a wire dipole across as much of the available
diagonal length as possible, then bring the ends back along one of the sides
of the attic to make more of a Z shape.  The closer you can get it to a total
length of 1/2 wavelength on 40m, the better performance you can expect to
see on that band.  It's difficult to say exactly where the optimum point is, but
perhaps 10' or so of wire folded back along opposite edges of the attic would
be a good starting point. (It depends on the shape of the attic, etc.)


The MFJ-949E already has a built-in balun, and you can use that to see how
it works.  It is possible that using a home-made 1 : 1 balun instead may make it
more efficient and/or easier to match on some bands, depending on the feedline
length, but I often recommend the approach of getting it to work with what
you have on hand, then deciding if it is worth trying to improve matters - often
it isn't.

If you have problem matching on a particular band, you can add a few feet of
extra ladder line running around the room to see if that makes a difference.  It is
difficult to predict the exact lengths that may cause problems because they depend
on the details of the antenna itself and how it might interact with wiring, ducting,
and metal flashing in your attic / roof.  So plan to do some experimenting and
adjustment to find something that works well enough for you.

There are other options, of course, but this seems like a reasonable way to start
given the materials you have available, and it can be adapted to your circumstances.
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N5GZH
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Posts: 87




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« Reply #3 on: October 18, 2013, 09:23:40 PM »

Throwing this out for what it might be worth:

http://frrl.wordpress.com/2008/09/14/life-on-hf-the-mfj-1796-6-band-hf-antenna-for-limited-space/
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M6GOM
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Posts: 1014




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« Reply #4 on: October 19, 2013, 02:37:27 AM »



The slinky is terrible as a TX antenna. Been there, played that game and I had mine extended a lot more than you are intending.

OK two options.

1) Fan dipole. Whether or not you'll get 40m in depends on the space you have available. You can use some inductive loading but its still going to be a 40ft plus antenna. It can be bent into the space.

http://www.radiowymsey.org/FanDipole/fandiploe.htm


2) G3TPW Cobwebb for 20-10m and a dipole for 40m. The Cobwebb is 8ft square and is 5 full size dipoles so no performance loss.

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N4KD
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Posts: 145




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« Reply #5 on: October 19, 2013, 05:06:56 AM »

I put up a trap dipole on 40m, trimmed it for 7050, and it works well enough wit a kx3. The I made it a fan dipole wit 10 es 20 meter legs and those work well. I won't push it wit the WARC bands, though. I use a 1:1 balun es rg6 coax.

73 Dave, N4KD
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WA9UAA
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Posts: 321




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« Reply #6 on: October 19, 2013, 05:27:57 AM »

I vote for a loop antenna around the outside walls of the attic and fed with ladder line. A loop antenna will have no high voltage points as a dipole does. HTH
73,
Rob
WA9UAA
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W5DXP
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« Reply #7 on: October 19, 2013, 08:36:14 AM »

To get a feel for what's is possible, I would start out with a 20m or 17m hamstick dipole.
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W5DXP
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« Reply #8 on: October 19, 2013, 09:44:10 AM »

A loop antenna will have no high voltage points as a dipole does.

A loop antenna is a standing wave antenna as is a dipole. The feedpoint is a low-resistance (voltage-minimum/current-maximum) point. 1/4WL from the feedpoint is a high-resistance (voltage-maximum/current-minimum) point and the current goes through a zero-crossing meaning that all the energy at that point is contained in the electric field, just as it is at the ends of a dipole. In fact, since the current goes through a zero-crossing 1/4WL from the 1WL loop feedpoint, one could cut the elements at those two points and have a 1/2WL dipole. Therefore, a 1WL loop has voltage loops (current nodes) much like a 1/2WL dipole.
« Last Edit: October 19, 2013, 09:49:52 AM by W5DXP » Logged
N6AJR
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« Reply #9 on: October 19, 2013, 09:45:38 AM »

fan dipole.
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WB4MDX
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Posts: 11




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« Reply #10 on: October 19, 2013, 05:02:38 PM »

I tried slinky and did not do as well as a trap multi band dipole at apex.  100 watts max though. Fed with coax.  I finally 'grew' an outside antenna for 160-10m.
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KB3MDT
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Posts: 208




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« Reply #11 on: October 20, 2013, 08:39:10 PM »

Hi,
   I suggest a simple Wire Dipole or a Fan Dipole.  It's okay to get creative for the wire pattern, i.e. turn corners, whatever, to make it fit.   Use your tuner for non resonant bands. 

KB3MDT
Ken
73
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KQ6Q
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Posts: 995




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« Reply #12 on: October 21, 2013, 07:53:31 PM »

If you have the budget for it, consider an MFJ Magnetic Loop in the attic. they come in two flavors - 30m-10m, and 40m-15m
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WA2VUY
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Posts: 171




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« Reply #13 on: October 23, 2013, 07:26:22 PM »

I worked OZ5DM today on 40M. You should look at his qrz page to see how he built a 4 band dipole in his attic.
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M1EQA
Member

Posts: 5




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« Reply #14 on: November 02, 2013, 03:28:35 AM »

Thank you all for your help - It was much appreciated - It has given me much to think about.

73's

Nigel M1EQA
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