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Author Topic: workable attic antenna for 40M?  (Read 13345 times)
KK4SUH
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Posts: 15




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« on: December 05, 2013, 07:14:25 PM »

I was originally planning to get an off-center dipole such as the one below and run it from close to the peak of the roof with one element running to the front yard and the other running to the back yard(ends would either be connected to a tree or a spare sections of PVC pipe used as masts.)
http://www.ebay.com/itm/80-6-Meter-OCF-Dipole-Windom-Antenna-W-4-1-Balun-2kw-G5RV-Multi-Band-HF-/130641293121?pt=US_Radio_Comm_Antennas&hash=item1e6ad3ef41

My idea got completely shot down mostly out of fear of it becoming a lightning rod (I did explain that the much higher pine trees on adjacent properties would more likely be struck by lighting than the antenna) and I've been pretty much told my only option is the attic.

The length of the attic is about 33', there are no electric attic vent fans, just some duct work that sits about 1 foot above the ceiling joists. What viable solutions are there for an antenna that can do 40M and higher (I would also like 80M, but I'm guessing that it would be even more difficult to pull off)?

I'm new to HF and my only experience on HF so far has been using remotes.
« Last Edit: December 05, 2013, 07:16:43 PM by KK4SUH » Logged
AA4PB
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Posts: 12689




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« Reply #1 on: December 06, 2013, 02:40:51 AM »

On 40M you could fit a half-sized dipole with loading coils to make it resonant on 40M. The performance of a half-sized dipole is not all that bad as compared to a full-sized (66 foot) dipole. On 20M and up you can fit a full sized dipole (or nearly so).
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K5LXP
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« Reply #2 on: December 06, 2013, 06:21:48 AM »

My idea got completely shot down mostly out of fear of it becoming a lightning rod
I've been pretty much told my only option is the attic.

Just wait until you interfere with all your home electronics due to the close proximity of your antenna inside the house.

I'd stick with the outside antenna plan.  Consider that unless you have underground utilities, you've have suspended power/phone/cable wires connected to your house since it was built.  There are many hams including myself that have antennas all over the house plus a tower in the yard and have never had trouble with lightning for decades.

It's true that a 1/2 size loaded dipole can be a good performer.  But when you put any antenna inside a structure the performance is mostly influenced by the structure.  Attic antennas can work in some circumstances but you can plan on tuning issues, and likely EMI/RFI.  You really won't know until you try it.  Given that you are new to HF, you are setting yourself up for a number of challenges that are easily avoided by using an outside antenna.


Mark K5LXP
Albuquerque, NM
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WB6BYU
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Posts: 13029




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« Reply #3 on: December 06, 2013, 10:04:54 AM »

Quote from: KK4SUH

... out of fear of it becoming a lightning rod...



How much of a problem this is will depend on a lot of factors, especially how prevalent
lightning strikes are in your area and how you handle the antenna feedline.

But often the fear is the problem rather than the actual risk...




Quote

... just some duct work that sits about 1 foot above the ceiling joists...



This will, of course, affect the antenna resonance and performance, as will any other
ducting, wiring, metal flashing, etc.  You can't just cut the wire to formula length and
expect a low SWR.  (That doesn't always work outdoors either.)

Interference to (and from) electronic systems in the house can always be an issue -
you'll have to try it and see.  Sometimes it can be made to work - there are plenty
of hams operating with attic antennas.  Sometimes it may require filtering of alarm
system wires or other problem areas, or turning down the power.  Sometimes it just
becomes too much of a nightmare.


My general approach would be to use a dipole with the feed point in the center of the
roof, running the ends as far as possible along the longest diagonal, then bending them
around the corner and along the side to use up more wire.

There are two general approaches:  multiple dipoles on a common feedpoint, with each
tuned for a specific band, or a single wire fed with twinlead or ladder line to a tuner.
(Many multi-band antennas such as G5RV, trap dipoles OCFD, etc. may be difficult
to tune to account for the nearby metalwork, wiring, etc.)

From the perspective of minimizing the number of trips into the attic, the "doublet"
as long as you can reasonably make it, fed with twinlead, doesn't require any tuning
or adjustment of the wires once it is in place.  I've used TV twinlead and dropped it
down through a slit in the ceiling right to a tuner beside my rig.  That approach also
allows operating all the bands between 40m and 10m (though efficiency might not be
particularly good on 40m.)

You can improve 40m performance by extending the wires around the corners and/or
adding loading coils (I'd probably do both).  But start with the simpler installation and
see how it works.

Multiple dipoles have the advantage of direct coax feed (a 1 : 1 current balun is a
good idea.)  I'd start with one for 20m, which should just about fit.  You'll have to
make a couple trips up and down to tune it for lowest SWR, since the formula length
probably won't be quite right.  See how that works, and if it is promising then add
another wire for 15m to the same feedpoint as well as one along the other long
diagonal with the ends bent (and possibly loading coils) for 40m.)  While this method
works pretty well for the pre-WARC bands, it can get messy when you try to add
12m and 17m because the resonances are close together.


Whichever approach you use, provide sufficient insulation at the ends.  I'd keep the
wires spaced at least a foot away from any metal (preferably more) and use a good
insulator (or at least a foot of synthetic string) at each end or corner.  Light flexible
wire will be easier to work with in a confined space than #12 CopperWeld.
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KK4SUH
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Posts: 15




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« Reply #4 on: December 06, 2013, 02:05:13 PM »

Thanks for the advice.  I learned today that the other person misunderstood exactly what I was planning to put up, and I've been given the green light for it.  I can get the center of it up to around 20ft, of course higher would be better.
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N5GZH
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Posts: 87




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« Reply #5 on: December 06, 2013, 02:12:03 PM »

Thanks for the advice.  I learned today that the other person misunderstood exactly what I was planning to put up, and I've been given the green light for it.  I can get the center of it up to around 20ft, of course higher would be better.

Set up your station as per standard safety principles, throw that puppy up there and enjoy.   Smiley
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KK4SUH
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Posts: 15




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« Reply #6 on: December 06, 2013, 02:26:59 PM »

Thanks for the advice.  I learned today that the other person misunderstood exactly what I was planning to put up, and I've been given the green light for it.  I can get the center of it up to around 20ft, of course higher would be better.

Set up your station as per standard safety principles, throw that puppy up there and enjoy.   Smiley
Will do, I've decided on the Icom 718 now just looking for a power supply, has anybody had any good luck with a Samlex SEC-1223BBM power supply on HF?
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W4CNG
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Posts: 178




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« Reply #7 on: December 07, 2013, 01:09:44 PM »

I have been using an Alpha Delta DX-EE (which is 40-10 no WARC) for 13 years in my Attic and running 1KW into it.  No issues with interferance in the house and it clears all metal and wood by a minimum of 4 feet.  The center is 16 ft up and the ends are 4 feet above the attic floor.  BW on 40 is 100KHZ wide.  Eveything except 10 meters is full BW under 2:1. Done properly antennas and Attics can and do get along well.  PS I have a full sized 75 meter dipole up there also which has two 90's in each end and is almost a loop with the end opening at 15 feet.   
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N3HEE
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Posts: 113




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« Reply #8 on: December 12, 2013, 11:09:18 AM »

What walks down stairs, alone or in pairs and makes a slinkity sound? A spring, a spring, a marvelous thing! Everyone knows it's Slinky!  Grin  Two slinkys each stretched 15-16 feet per side works wonders on 40 meters.  I used one for a long time in my attic and worked almost anything I could hear.  Cheap and easy antenna.  Just make sure it's made of metal !



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KE4HGR
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« Reply #9 on: December 12, 2013, 11:57:12 AM »

I second the slinky...It's not perfect but it puts a lot of wire in a small area...If you have room use the giant original slinky...mine is in the rafters unfortunately I have a lot of noise in my area and nothing is going to help that much.
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K5LXP
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« Reply #10 on: December 12, 2013, 08:57:40 PM »

I second the slinky...It's not perfect but it puts a lot of wire in a small area...

No...what makes an antenna work is RF current over a distance, not how much wire you can fit into a given area.

It turns out that slinkies aren't good for making antennas.

http://www.eham.net/ehamforum/smf/index.php?action=printpage;topic=65811.0

http://www.eham.net/articles/29887


Mark K5LXP
Albuquerque, NM
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VA3AEX
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Posts: 22




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« Reply #11 on: December 17, 2013, 10:50:26 AM »

Not sure how wide your attic is but if there is sufficient room you can always fold a dipole into a Z shape.  I run a trapped dipole, folded into Z shape covering 80/40/20/10M in an attic roughly 30' above ground Z shape.  I run exclusively phone (both AM and SSB); about 100W PEP.  Really no DX for 80/40M (not surprising given the height above ground); when I'm on 40M AM I get into the tv audio, and ring one of the upstairs phones when I key the mic.  However for 20/10M I have a number of DX contacts in the log...  Not ideal but still fun.

Good luck with experimenting!  73  Alex
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