Help tuning homebrew UHF turnstile

SatNOGS
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1

Hello everyone!

I’m slowly working towards hopefully getting a SatNOGS station setup in the future. A little strap for cash at the moment, so I decided to build a Turnstile antenna following this guide (which is in use in this station by @mwheeler and this station by @IZ7BOJ). Here’s my rendition:

20200425_155303
20200425_1553033024×4032 198 KB

I borrowed a N1201SA Antenna Analyzer to measure the performance of the antenna. At 437.2 MHz, which is the frequency I’m most interested in, the VSWR was ~2.3 (not great…). Impedance was ~48 Ohm, which I think isn’t too bad. Not sure how to interpret the other results (resistance and reactance), here they are:

4372MHz
4372MHz3024×4032 1.95 MB

And here are some pictures of the VSWR over several frequencies:

narrowband_2
narrowband_22641×2145 1.36 MB
wideband
wideband2943×2689 2.06 MB

As you can see, great VSWR at ~600 and ~1300 MHz :frowning: haha But on the 70 cm band, VSWR is between 2.2 and 2.4.

I’ve been able to at least hear some satellites with this antenna, here’s an example of GRIFEX:

20200425_200806
20200425_2008062595×2181 2.27 MB

What do you guys think of these values? For a homebrew antenna, is this what’s to be expected or could it be improved? And if so, any ideas on how to improve it? I’ve yet to buy an LNA, which would be the natural next step, but I’d like to squeeze more performance out of this if possible…

Any help will be kindly appreciated.

Best wishes,

Dan

P.S. Some additional pictures of the measurement setup with the N1201SA and of the interior of the antenna:

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20200424_1506473024×4032 464 KB
20200425_111322
20200425_1113223024×4032 379 KB
20200712_114201
20200712_1142011512×2016 1.19 MB

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See IMPORTANT! Features of construction and installation.

At the points where the RG-58 is connected to the copper pins of the dipoles, the leads should be as short as possible.

You can check:

  • SWR separately for each dipole + appropriate RG-58 cable

  • the perfomance of the RG-59 transformer using 25 Ohm dummy load at the point of connection RG-58 (instead of RG-58).

2 Likes
3

it has been suggested to me to be sure to ground the outside of the coax to a real ground when using a VNA. I’ve tried this myself and it makes a big difference.

–Roy
K3RLD

1 Like
4

Hi Dan,
I never measured VSWR of my antenna because my miniVNA works up to 200MHz.
Your impedance matching should be better, I suggest to do the tests proposed by @uy0ll, in order to understand wheter the problem is the matching network or the radiating elements.
Alfredo

1 Like
5

Hello Dan,
I built one of these during a quiet time at Christmas from those instructions and managed to get the SWR to something quite workable:

Turnstile 436
Turnstile 4361920×1080 158 KB
I’d not heard about grounding the coax, and those measurements are ungrounded. I had to redo the impedance matching components several times to work out where the critical measurements were, that and a little clipping of the threaded bars to tune it. It’s now the best antenna I have for this frequency.

Toby

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Hi all!

I am sorry for the very late reply, but I finally got back to the project. I haven’t been able to tune the antenna further, but I will attempt to do so following your guidelines. For now, I wanted to let you know that I sucessfully installed the antenna with and RPi and conducted some testing. It’s set up as Station 831! At least, I’ve been able to hear some birds (although too faint to decode). Here’s an example of GOMX-1.

What do you think?

Best,

Dan

CC: @uy0ll @K3RLD @IZ7BOJ @tgk

7

Looks like your antenna are adequate for powerful satellite, but probably need with a bit of LNA help.

1 Like