DIY QFH Antenna troubleshooting

Discuss commercial and home made antennas.
AnomalySmith
Posts: 15
Joined: Sun Jul 10, 2016 1:41 pm

DIY QFH Antenna troubleshooting

Post by AnomalySmith » Sun Jul 10, 2016 2:01 pm

Hi everyone,

I recently planned to build my own QFH antenna matched to 137Mhz to listen on the NOAA weather satellites.
I wanted a copper tube as main material as it seems to be best at thoses frequencies than simple wire/coax, and finally decided following scrupulously this tutorial: http://hobitus.com/en/equipment/antenna/ (hoping this kind of links are allowed :) )

And now it's done and.. not working as expected...
I've done some tests on NOAA passing over me, and no signal at all at the expected satellite frequency even with high elevation (>70°) and clear view to sky.

My equipment are the following:
Receiver: HackRF One
SDR: GQRX
LNA: 27db gain @137Mhz

I use gpredict up-to-date to track NOAA satellites.

I wish have some of your advice and help to locate the problem, as I'm not an expert in this domain.
My last track was on the balun, is it critical?


Thanks in advance!

AnomalySmith.

rtlsdrblog
Site Admin
Posts: 2191
Joined: Mon Nov 19, 2012 11:54 pm

Re: DIY QFH Antenna troubleshooting

Post by rtlsdrblog » Mon Jul 11, 2016 3:19 am

It's hard to say what could be wrong without physically inspecting your antenna.

But does it pick up other frequencies, like broadcast FM? (Even though its tuned for 137 MHz it should still pick up other frequencies).

Could you be experiencing overload from other nearby stations? If there is a very strong signal like at broadcast FM it could block reception of the NOAA signal since the HackRF doesn't have very good dynamic range.

Rule out software problems. Are you sure that your gpredict has up to date kepler files? Confirm with an online tracker that the satellite is where gpredict says it is.

The balun is probably not critical, but it will help you get better reception.

AnomalySmith
Posts: 15
Joined: Sun Jul 10, 2016 1:41 pm

Re: DIY QFH Antenna troubleshooting

Post by AnomalySmith » Mon Jul 11, 2016 8:00 am

Firstly, thank you for your reply!

Indeed I noticed some (very) strong signal on broadcast FM frequencies. I'm asking myself if the LNA is needed in such case with this antenna? The kepler files are up to date and checked online for data coherence.

So you think NOAA signals could be killed due to overloaded antenna? Is it possible to avoid this without changing to a better location (far from most broadcast FM) ?

Is a coax filter could enhance the reception? (I've seen a filter could be made with a coax cable measuring a quarter of the wavelength of the frequency to attenuate, placed just before antenna output connector)

I've also done a test with a basic VHF/UHF antenna, and was able to receive a weak signal (NOAA 18 @137.9125MHz). The QFH was unable to do the same..

Here are some screeshots and picture.
Capture d'écran de 2016-07-11 09-17-58.png
Broadcast FM with LNA
Capture d'écran de 2016-07-11 09-17-58.png (715.25 KiB) Viewed 22153 times
DSC_0133.JPG
QFH antenna
DSC_0133.JPG (2.66 MiB) Viewed 22153 times
DSC_0126.JPG
Balun
DSC_0126.JPG (1.99 MiB) Viewed 22153 times
Thanks!

PS: Sorry for large pictures!

rtlsdrblog
Site Admin
Posts: 2191
Joined: Mon Nov 19, 2012 11:54 pm

Re: DIY QFH Antenna troubleshooting

Post by rtlsdrblog » Tue Jul 12, 2016 11:18 pm

Overload is possible, but from your image it doesn't look like your broadcast FM stations are too strong. You can tell if there is overload as the noise floor at 137 MHz will be very high and perhaps with lots of spurious signals arouhd.

An LNA is not needed, UNLESS you have a really long run of coax cable between the antenna and receiver.

What type of coax are you using and how long is the run?

Hmm, if you basic antenna can receive it then there must be something wrong with the QFH. Have you double checked for bad coax cable, or bad connectors?

AnomalySmith
Posts: 15
Joined: Sun Jul 10, 2016 1:41 pm

Re: DIY QFH Antenna troubleshooting

Post by AnomalySmith » Wed Jul 13, 2016 8:08 am

Hi rtlsdrblog,

Again, thank you for your help.

The noise floor seems to be correct at ~137Mhz.

To start with this antenna, I planned to use a cable as short as I could. Actually, a 1m RG58 is connected to a simple raccording terminal which is directly connected at the balun's outputs (I wanted avoiding soldering on this part. Could this causing cold contacts?).
At the other end of the coax is a soldered SMA 50Ohm female used to connect the HackRF One (with a 50cm SMA cable male/male).

I guess the next step will be to disolder the bottom part of antenna to check the coax cable...

The other connectors and cables have been checked/changed without effect.

Do you think the coax could really be the problem, I mean FM broadcasts seems to be picked up pretty easly, why not NOAA ones?

Anyway I'll be back soon :)

rtlsdrblog
Site Admin
Posts: 2191
Joined: Mon Nov 19, 2012 11:54 pm

Re: DIY QFH Antenna troubleshooting

Post by rtlsdrblog » Thu Jul 14, 2016 12:51 am

Do you have a multimeter with a continuity checker? That is the easiest way to check for bad coax connections.

FM broadcast is very powerful, so even if there is a broken connection it could be powerful enough to capacitively couple through the broken part. NOAA on the other hand is much weaker, so may not make it through. Just a guess however.

Can I see what your noise floor looks like at 137 MHz?

AnomalySmith
Posts: 15
Joined: Sun Jul 10, 2016 1:41 pm

Re: DIY QFH Antenna troubleshooting

Post by AnomalySmith » Thu Jul 14, 2016 1:05 pm

Sure!
sc14.png
Noise floor
sc14.png (530.13 KiB) Viewed 21237 times
The strongest part in the fft display corresponds when the HackRF's internal LNA is on.
With a multimeter I obtain 0.0Ohm between shield and inner copper of the coax cable (with antenna connected) which seems to be nice.

I'll test with a 75Ohm coax cable anyway just to be sure.

rtlsdrblog
Site Admin
Posts: 2191
Joined: Mon Nov 19, 2012 11:54 pm

Re: DIY QFH Antenna troubleshooting

Post by rtlsdrblog » Thu Jul 14, 2016 11:09 pm

You have the RF gain to zero, maybe you could try with the LNA off, and the RF gain turned up a little.

It does look a bit noisy like there is some interference going on with the LNA turned on, but it doesn't seem to be too powerful.

AnomalySmith
Posts: 15
Joined: Sun Jul 10, 2016 1:41 pm

Re: DIY QFH Antenna troubleshooting

Post by AnomalySmith » Mon Jul 18, 2016 12:10 pm

Already tried all kind of configuration between BB, IF, RF gain and LNA.

I finally built another antenna with coax dimensioned from https://www.jcoppens.com/ant/qfh/calc.en.php. I obtained a longer antenna and shorter diamater.. I changed balun for infinite balun as explained on https://www.jcoppens.com/ant/qfh/adapt.en.php

The result are here: I was able to picked up all NOAA (15, 18 and 19) sats signal. It was 13/17 db above noise floor (90db) but seemed insufficient to have a decent image (inspite of post-treatment and normalizing audio file before wxtoimg)...
I guess playing again with BB, IF and RF gain would maybe help me.

I'll try this as soon as I can and keep you abreast.

Thanks :)

rtlsdrblog
Site Admin
Posts: 2191
Joined: Mon Nov 19, 2012 11:54 pm

Re: DIY QFH Antenna troubleshooting

Post by rtlsdrblog » Tue Jul 19, 2016 11:03 pm

Great! The other antenna must have some fault somewhere then.

Play with those gain settings on another constant signal to get the hang of what each one does.

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