Good Quadrafilar Helix Design Plans?

antennas

#1

Is there any preferred design for QFA helix antennas? I would like to try one compared to my 2m eggbeater, prior to finally building a lindenblad. Already have the lindenblad designs (AA2TX), but finding lots of conflicting info on the QFA’s.

–Roy
K3RLD


#2

Have a look in the QST May 2018 magazine page 38. It has plans for a 2M version. It
uses 1/2 wide X 1/8 in thick bar/strap for the elements. and pvc pipe. It looks much easier
to make than some of the other versions I have seen.

Mike
N6IMFQuad.pdf (2.9 MB)


#3

Thanks, Mike. I’ve ordered/scrounged the parts needed to build this… hopefully it will be ready for testing this weekend.


#4

Great and looking forward to seeing your results. That design stuck in my mind because it looked so easy to build and very clean. Not a lot of support arms and the making the elements.


#5

post pics when you’re done!!


#6

Not done, yet (phasing harness, etc) - but here’s the mechanical bits. The method described in the paper for bending the helices did NOT work for me (not even close). I ended up bending about a 1.5x coil around a 10" piece of pipe, and then stretched it out to fit.


#7

Well its taking shape, I wonder if a slightly thinner aluminum would have made any difference. It
looked so easy and straight forward in the picture.


#8

I actually tried to find some dead soft strip (3003 would’ve been great!), but in this size all I could find was 6061 and 6063 (in either T5 or T6). If I do it again, I will go with the 1/16" thick strip instead of the 1/8". My test system is up and running, but not giving very good results compared to my first station. But the feedline on this helix is over 20ft longer than the other, and using RG-58 instead of RG-8X. I’ve got two more setups to try before moving the antenna (using a 5’ line LMR400UF feedline, and using some combination of a cheapo LNA with the existing feedline).


#9

Station 272 is open for business. Much better reception of the Fox sats for most passes. Still mounted in the attic, so I expect even better performance when moved outside (not sure when that will happen, though). RTL-SDR gain is set to 48.0, no LNA, feedline is 5 feet of LMR400UF.


#10

Good news.
Do you need a feedline with an RTL-SDR?


#11

There has to be some form of feedline, yes. Not sure I understand your question.


#12

I thought that you could directly attach an RTL-SR to the antenna, but I see you need a balun.


#13

Oh, yes - there is a few coils of RG-8X around a piece of pipe. Pretty simple.