Sunday, August 23, 2015

The Attic antenna revisited

I have RF in my Attic (I used to have bats)

Ladder line fed Dipole in attic

Some local QRP folks known as the Knightlites meet regularly on 3.574MHz   They meet on 80m so that nearby folks can participate via skywave effect of 80m.  Well my 40m Windom loads up very poorly on 80m and I'm likely burning a good bit of my meager power heating the coax.   I'm limited at my present QTH to really only having an option for one outdoor antenna at a time run from the peak of my roof and presently that's my 40m Windom.

Update 30-03-2016:  Since I wrote this article I updated my outside Windom to an 80m OCF Dipole.  So I'm no longer using this attic antenna for 80m but it's working great for 40m and above.

I thought if I could install a 40m doublet in my attic I might be able to load it more efficiently (due to less loss into a balanced line) for 80m.  I pieced together wire, laid out on my driveway, from a number of other projects to get me 68ft.  I still had some ladder line from years ago.
There's no better time to work on a project in the attic of your house than a hot summer day in August. 
After crawling under and around duct work in my very, hot and dusty attic I had to go back into the breach to adjust wire lengths a couple of times until it tuned properly. That involved a sort of overweight gymnast routine on the balance beam... um I mean rafters... above one of the bedrooms.  At the time I was thinking through the excuses I might have if I fell through the ceiling (it wouldn't be the first time I've fallen through a ceiling but that's another story).

I have another qualified success.  It's rather more of an inverted "U" than a I'd like but poking fishing poles out the sides of the house with wire attached wouldn't have gone over well with the wife.
Here I've used my finely honed artistic skills to superimpose a drawing of the attic antenna on my house:
Blue is wire. Red is ladder line. Green is Operator
To get the ladder line into my operating position I had to come through a wall.
Ladder line winding its way to the operating position
I stripped some wire bare off the ladder line and snipped the end off of plaster screw anchors to pass it through without making too much of a mess and so that I could hang two very tiny pictures there in the future if I take the ladder line down.  It was too hot on the other side to take the time to solder the passthru piece so I simply used electrical twist type connectors.

Ladder line passing through the wall
Here it is at the operating position.  The ladder line is going into the back of my trusty MFJ Versa Tuner II Super Deluxe (I added the super deluxe bit but you must admit all the knobs look quite spiffy).

Ladder line adding it's beauty to the station
It loads up alright on 80m and with all the bends and height of only 20ft AGL I'm sure it will make the perfect skywave for local communication (update: I received 589 and 599 reports when using it on the local QRP net) .  It will also be a backup to the outdoor Windom but I wouldn't expect it to perform as well.
Addendum (9-18-2015): The Attic 40m Doublet actually outperforms my 40m Carolina Windom to the West (both receive and transmit)  It's about 12db down on receive compared to the Windom to the North and South even though it equals it in transmit performance (according to a few reports where I've asked for A/B comparisons).  With the next outdoor antenna I put up I will do my utmost to get ladder line all the way back to the radio.  I'm becoming a believer in ladder line efficiency, which is especially important for lower power operations.

It also introduces just enough RFI in the house at 5 watts to let my teenage son know when I'm having a QSO via his gaming headphones. He's super excited about that as you may guess.

So lower your power and raise your expectations

73
Richard
N4PBQ (now AA4OO)

Update 11-27-2016
So I've had the Doublet in the Attic for over a year now and it continues to amaze me.  I've also learned a couple of things about Doublet antennas since then...   I noted in my blog that I'd measured the doublet to be a total length of 68 feet.  I've learned since that the length of a doublet is more dependent on total length including the ladder line than simply the horizontal bits and absolute length isn't that important because you don't want it resonant on any band.  I ended up adding a few feet on both ends to improve the tuning across all bands from 40-10.

1 comment:

  1. Don't think that would work at my QTH with the metal roof .... Then again, that roof would make a dandy counterpoise for a vertical.

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