Another Double Cross Antenna build (went wrong)

Discuss commercial and home made antennas.
swarl3s
Posts: 9
Joined: Wed Apr 10, 2019 11:45 am

Re: Another Double Cross Antenna build (went wrong)

Post by swarl3s » Mon Apr 22, 2019 11:27 am

snn47 wrote:
Sat Apr 20, 2019 3:08 pm
Did you use this double cross antenna design https://www.qsl.net/l/lu7did/docs/NOAA/ ... MARTES.pdf ?
Hi, thanks for the answer. Yes i used the guide from qsl, beside of some others. Details about the dipole length/etc. is in my first post. The frequency band at 137.9mhz is nearly empty, no interference apart from some noise created by the voltage regulator from the LNA. But this does not affect the signal strength

I'm experienced at soldering. I even pre heated the 4mm2 cooper wire before applying the solder

What confuses me, is the fact that the antenna works great in the northern region, decoding passes with elevation of <10 degrees is possible. But as soon as the satellite reaches the south, the signal strength drops to a unusable value :roll:

I think the next step would be to reposition the antenna? I will have to buy a new coax first, since the old one is not removable from the roof

Ps.: Again, north is marked red on the antenna
antenna.jpg
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snn47
Posts: 147
Joined: Tue Dec 27, 2016 11:00 pm

Re: Another Double Cross Antenna build (went wrong)

Post by snn47 » Wed Apr 24, 2019 11:18 am

Can you extend the pole above the dish and increase the distance to the dish. Assuming that your dish is pointing to South (Astra?), it could be the Impact of the dish. It looks like the dish is within/to close to the antenna elements. If you mount the antenna 180° mirrored in azimuth the Problem should remain, if I'm correct with my assumption.

swarl3s
Posts: 9
Joined: Wed Apr 10, 2019 11:45 am

Re: Another Double Cross Antenna build (went wrong)

Post by swarl3s » Wed Apr 24, 2019 3:31 pm

snn47 wrote:
Wed Apr 24, 2019 11:18 am
Assuming that your dish is pointing to South (Astra?)
Astra 19.2 East, yes. If we assume it's the dish that is causing problems, how could this be possible?

- The Dipoles are only connected on the upper half, the lower one "isn't really connected to anything". I did never understand how dipoles in series work (increasing the impedance)?

Before extending the antenna beyond the dish, i have to check if the pole can handle that. I will turn it 180 degrees first, to see if the null at south remains
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snn47
Posts: 147
Joined: Tue Dec 27, 2016 11:00 pm

Re: Another Double Cross Antenna build (went wrong)

Post by snn47 » Thu Apr 25, 2019 11:25 am

Dipoles consist always of 2 radiating elements of equal length. If the lower part would not be needed, it would have been a waste of material right?

swarl3s
Posts: 9
Joined: Wed Apr 10, 2019 11:45 am

Re: Another Double Cross Antenna build (went wrong)

Post by swarl3s » Fri Apr 26, 2019 6:07 pm

snn47 wrote:
Thu Apr 25, 2019 11:25 am
If the lower part would not be needed, it would have been a waste of material right?
Like i said, i know the lower parts are needed, but did not quite understand why. But that might explain why the satellite dish is messing with the reception. I'll try to extend the pole, it seems to handle that extra stress. Will get you an update as soon as possible

Until then, thanks for helping me out!

Ps.: I just successfully decoded a 4.7 degree el. pass over Russia, so at least the LNA seems to do a great job!

snn47
Posts: 147
Joined: Tue Dec 27, 2016 11:00 pm

Re: Another Double Cross Antenna build (went wrong)

Post by snn47 » Tue Apr 30, 2019 9:09 am

Most antenna pattern you find are calculated and are either for free space, meaning idefinetely high above a reflecting surface, or for a given height and a min. diameter of reflecting surface, normally 1/4 Lambda or more.

Under any other condition reflections from any surface or object will change the antenna pattern. The number of lobes/notches, and the elevation gain/depth will vary significantly.

swarl3s
Posts: 9
Joined: Wed Apr 10, 2019 11:45 am

Re: Another Double Cross Antenna build (went wrong)

Post by swarl3s » Tue Jun 18, 2019 10:04 am

Hey guy's, sorry that it took me that long to answer, i was busy doing other stuff!

So, i did try to extend the antenna away from the satellite dish, sadly that didn't help either. No, it made it even worse! The reception got so bad, that i decided to remove the antenna from the roof. The satellite dish wasn't the problem, more likely the geometry of our roof.

I placed the antenna in our backyard now. Even though decoding is now only possible at a min. elevation of around 12 degrees, all the null's disappeared completely. The signal stays stable over the whole pass, which is all i was ever looking for.

So, thanks again to all of you who helped me out!

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