QFH for L-Band Construction/Design Help Please!

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Posts: 4
Joined: Sat Jan 25, 2020 10:34 pm

QFH for L-Band Construction/Design Help Please!

Post by Arlo » Sat Jan 25, 2020 11:13 pm

Hi Guys. First time here, long time on rtl-sdr.com.

Firstly I've had a 137mHz QFH working for around 5years now made of commercial 1/2" hardline. Nice performance.

Looking at all of the designs for L-Band QFH antennas I'm a bit confused at the different construction schemes.
I've tried to look at the Vaisala radiosonde gps antennas and see how they are wired, made.
All of the loops are the same length and it looks like all of the bottom loops are shorted to common ground.

I have all materials to start making an antenna and used the calculator. It shows long and short loop lengths.
My intention is to use 3/16" brass tubing for a mast soldered to a TNC fitting ground (shield), or the body of it and pass the center conductor through it. That part is ready to cut and solder.

Here's the confusion. Some designs people are making have all 4 loops soldered to common ground on the mast bottom.
And all 4 loops connected to the center conductor at top.
Others appear to have the long loop and short loop bottoms not shorted together.
Meaning that they are much like my 137mHz APT QFH. Considered as 2 separate loop elements connected at the top.
And the adjacent long and short loop soldered at the top, and other 2 adjacent loops soldered to shield.

Then on others I'm seeing 2 adjacent loops both soldered to the mast (ground)on the top AND bottom, and the other adjacent top loops soldered to the coax center, and bottoms soldered to the mast.

My assumption from jcoppens is to use the calculator and consider the design as 2 different length loops NOT connected (shorted) together at the bottom, like my APT antenna is made.

If this makes any sense at all I sure could use a few tips to save me a little R&D.
I know I could buy a patch antenna, qfh,. But I'd really like to make this myself.

Posts: 4
Joined: Sat Jan 25, 2020 10:34 pm

Re: QFH for L-Band Construction/Design Help Please!

Post by Arlo » Thu Jan 30, 2020 6:29 am

Not pretty but a start.
Went with a US Army publication on L Band QFH design. All 4 helices are soldered to the mast at the bottom. What a bear to do that! One short and long loop soldered to the mast at the top, other 2 are soldered to center conductor.
Used 3/16" brass tubing. A crimp connector with a little drilling to fit the mast and SMA.
Sliver solder to mate 2 the short bottom loops then low temp Alpha electronics solder for the other 2.
It's made for Inmarsat and just got a nooelec lana and a bag of SMA connectors. I was able to grab a signal this evening on a window sill. It's a start.
QFH1.jpg (174.62 KiB) Viewed 245 times

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