The button bar at the top of the forum Home page now has blue buttons for login and register. These take you to the bottom left of the home page where to login boxes are located.
Eric.

I have just deleted a test account and started a new one.

from email..

iDirect X5 and e8350 and Comtech CDM-625 modems are for sale this Christmas - special offer



If interested please: email sales@satbbc.com

Yes Yes. I figured out how to build this way. There are even some calculators that already tell you all the dimensions.

It is worth doing Google internet search on "helix antenna"  or "helix antenna matching" (choose images option).

There seems good consensus on the design of the basic helix, 1 wavelength for a turn and approx quarter wavelength for the turn spacing.

There are many alternative ideas about the exact way to match the lower end of the helix wire to a 50 ohm cable. Many suggestions involve making the wire substantially thicker for the first quarter of a turn.

If anyone wants to add comments please note that the registration and login links for this forum are at the extreme lower left corner of the forum home page. http://www.satsig.net/cgi-bin/yabb/YaBB.pl
It is difficult to find these text boxes, but even so this forum gets attacked by automated spambots. There have been 403 hacking attempts in the last 3 hours. This rate is quite normal but none were successful....

Eric

Your explanation was very helpful. It is the third forum in which I try to clear this question and everyone only talks in waveguide but forgets the coaxial part.

An LNB with a circular or square waveguide input will normally be able to receive two linear polarisations, at right angles to each other, using a power supply selection voltage 13v or 19v.  This is normal for satellite TV reception.

An LNB with a rectangular waveguide input (as used for Ku band VSATs) will be able to receive one linear polarisation only. The plane of polaristaion is at right angles to the wider dimension.



So this is set for vertical polarisation.  The polarisation is the same as the half dipole pin inside.

An LNB with a coax SMA input has no polarisation. It is simply a low noise amplifier, mixer and IF amplifier.

If such an LNB, with a coax SMA input, is connected to an antenna it is the antenna that defines the polarisation.  If the coax connector is connected to the middle joint of a vertical dipole antenna then the polarisation being received is vertical. If the coax connector is connected to the end of an axial-mode helix antenna then the polarisation being received is circular and the choice of polarisation (clockwise or anticlockwise) is fixed permanently by the physical construction of the helix. This might apply to the reception of maritime satellite L band signals.


UHF and L band satellites normally use circular polarisation as the signal polarisation is dynamically rotated in the ionosphere (called Faraday rotation).  If linear was used the signal levels would be varying dramatically.

Most Ku band satellite TV signals are linear polarisation, but there are a few popular systems in the USA that use circular. 

Most 20/30 GHz Ka band satellites use circular polarisation, to make life easy for the installers.  Having a polariser costs a bit more but eliminates a skill requirement for installers, as adjusting linear polarisation angle accurately is difficult and extremely important in transmit/receive VSAT systems.

In both Ku (10-14 GHz) and Ka (20-30 GHz) band the use of a helix feed would be unusual. Normally such reception is done using a circular or square waveguide between the feed horn and the LNB, with the waveguide section containing a polariser which is either a dielectric plate at 45 deg or sets of castellations and slots on opposite sides inside the guide, also at 45 deg. In such cases what receive signals that appears after the polariser and going into the LNB are two linear polarisations, which may be selected using a normal dual-polarisation LNB using 13v or 19v.

If you have a low noise amplifier (LNA) or LNB, with a coax SMA input, and want choice of two circular polarisations you might have two helix antennas (wound with opposite senses) which you manually dis-connect and swap over.

I hope that helps.  Does anyone have any pictures to contribute, for example: Amateur radio?.

Best regards and Happy Christmas, Eric.

Hello friends.

This week I finished assembling my helix antenna (RHCP).

My goal is to try to get the signal from a satellite. I am using an LNB, connected to the antenna by an SMA cable, but I am having a problem.

I was informed that the LNB only "sees" vertical / horizontal polarizations and would have to convert before.

How can I do this without being a waveguide?

Thanks and Merry Christmas!

In Nov 2019 the USA FCC chairman announced a proposal to sell 280 MHz of satellite frequencies (3.7-3.98 MHz) to 5G operators. The frequency sale auction is proposed starting late 2020.

This means that the only C band satellite range left clear is  4.0 to 4.2 GHz.

If the above comes to pass then a 5G base station transmitting in 3.7-3.98 MHz near a C band receive dish will cause severe interference into a broadband 3.625-4.2 GHz satellite receiver LNB.

You may already be using a pass band filter 3.7-4.2 to reduce WiMAX interference.  All tx/rx VSATs need a 6 GHz stop band transmit reject filters in front of the LNB to prevent damage to your LNB from your own transmit signals.

If you become affected by this interference problem then to stop this 3.7-3.98 MHz interference getting into your C band LNB you will need a new filter, but this will also stop you receiving C band signals below 4 GHz!

Advantech have developed pass band filters 4 - 4.2 GHz and 4.02 - 4 GHz. These are rectangular waveguide devices suitable to insert in front of the LNB. Flange: UDR40 CPR-229 FLAT.  Flange: PDR40 CPR-229 GROOVED.

The spec and pictures are here:
https://advantechwireless.com/wp-content/uploads/2019/12/PB-AWT-IRF-C-001-19339....

What is not clear is if the filter also acts as a 6GHz stop band filter for transmit reject purpose, so ask about this.

I hope this attempt by 5G to grab satellite frequencies only applies to the USA.

Best regards, Eric

I have done a further update of the list today.
See: http://www.satsig.net/sslist.htm

The number of satellites in geo orbit seems to have increased 69 during the past year, which seems wrong as I have only recorded 11 new ones in 2019 as follows:


19001A      CHINASAT 2D (Z 2 D)      130.0 (+ E)
19007B      GSAT-31                      83.0      
19009A      NUSANTARA SATU      146.0
19014A      WGS 10 (USA 291)      179.8
19021A      ARABSAT-6A              30.5
19023A      BEIDOU-3 IGSO-1      113.3
19031A      YAMAL 601              49.0      
19034A      AT&T T-16                     -100.9  (- W)
19034B      EUTELSAT 7C               7.3
19049B      INTELSAT 39 (IS-39)       61.9      
19050A      AMOS-17                       17.0

I think the extra count must be due to the inclusion of many satellites with significant inclination angles of up to 59 deg !

Comments and corrections welcome.