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Quagi for AO-40



In prep for Field Day I built an AA2TX design pyramidal horn for the 
downlink.  With AA2TX's help with the dimensions, I built it one wavelength 
longer than his cardboard design (limited by commonly available cardboard 
box sizes), and used 1/8 inch tempered hardboard and 1x2 lumber rather than 
cardboard, and covered the inside with aluminum foil.  It works!  I can get 
about an S5 on the downlink, though my DEM 13 ULNA preamp, lightly modified 
Drake, and 50 feet of Radio Shack RG-6 75-ohm cable to my FT-736R.

At my last chance to test before Field Day (early Wednesday morning), I 
dragged out an Arrow Antenna 7-element 70cm beam (designed for the likes of 
AO-27, I think), hoping that it along with my 100 watt amp would get me 
into AO-40, but no luck.

Obviously I needed something better.  I grabbed the design for a 15-element 
Quagi for 432 that can be found in the ARRL hand book or at 
http://commfaculty.fullerton.edu/woverbeck/quagi.htm .

I really like the Quagi, as I've found it much easier to build than an 
equivalent Yagi due to the feed point construction.  No fussy gamma (or 
other) match to build (which usually specs parts that no hardware store in 
my town have).  Just make an open loop, solder the coax center to one end 
and the coax shield to the other, and you're done.  Adjust for SWR by 
adjusting length of the loop.

Using a formula out of the ARRL Antenna book for "re-scaling" an antenna 
design from one freq to another, I whipped up the dimensions for 435.7: 
http://davidarush.com/hamradio/ao40/quagi15.html

I built the thing to exactly the scaled dimensions (for 435.7), using the 
originally specified materials (number 12 copper wire for the DE and 
reflector, 1/8 inch brass rod for the directors, wood for the boom).

Got it all assembled by Thursday night, and dragged it to the back yard for 
SWR testing.  Got a horrible match at the target frequency.  Found it 
matched okay around 446 - way too high.

I experimentally lengthened the driven element (soldered in another chunk 
of wire - not pretty but effective) until I got a good match around 
435.7.  Ended up lengthening it nearly an inch.  Then I lengthened the 
square loop reflector by about the same amount, and packed things up for 
the drive to the Field Day site.

In short, it worked, so I was happy.

However, shouldn't a 15-element beam with 100 watts have been WAY 
over-powered?  My voice signal was "okay", and at best seemed on-par with 
many of the rest of the stations.  A few were louder and more clear than 
I.  To my knowledge I never triggered LEILA (although I heard her several 
times while tuning around).

Yes, this was linearly-polarized, not circular, but I expected I'd have to 
crank back the power on my amp, but didn't.  In fact, on Sunday, I could 
hear my signal but not copy my voice.  Beacon was okay, and a couple of CW 
QSOs worked.

David, ky7dr

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