I am building my antennas for a new station that will use the SATNOGS rotor design.
I also have the materials to build the circular polarised SATNOGS UHF antenna.
I also have the materials to build the SATNOGS VHF yagi.
Does anyone have a design for a cross yagi that would result in being able to receive both linear polarizations? I understand that the cross yagi design is a circularly polarized antenna. My thinking was that it would receive from any VHF satellite while not being so suceptible to fading due to cross polarization issues with a single VHF yagi.
I manage a local non-profit Maker Space, so I am not too keen to drop $400 Canadian on a nice commercial cross pol antenna.
My last question is this: Are most VHF satellites in one linear polarization? If so, maybe I only need a single yagi.
While pretty much all cubesats are using linear polarisation (there are some exceptions), you also need to remember that they may tumble, and the relative orientation between the sat and your station will change over the duration of the pass.
This means that the effective polarisation of the signal as observed by your station will also change throughout the pass.
If your station is a fixed linear polarisation, this means that you may experience deep fades (>20dB) throughout the pass due to polarisation mismatch. Using a circular polarisation (e.g. RHCP) means that you will always take a 3dB hit (Linear -> RHCP loss), but you will not experience the deep polarisation mismatch fades.
Manually operated stations may switch between different linear (H/V) and circular polarisations to obtain the best signal during a pass, but this is not practical for an automated station. So, I recommend using a RHCP antenna. (Why RHCP and not LHCP? Weather sats use RHCP.)
Does anyone see big issues with this? I don’t want to go down this path is there are going to be issues that I cannot track down if this doesn’t work. I have a VNA that I am going to use to ensure the driven elements are matched, and I can ensure all elements are in the proper plane, but that is about it.
In theory, that’s it. In practice, it’s a little harder. I’m pretty sure a couple of “circularly polarized” antennas that I’ve made are better described as “elliptically polarized” due to small errors here and there.
I’ve edited this post a few times because I’m confusing myself with phasing and antenna positioning - probably part of the issues I have!
I’m not sure this design is going to scale well to what the OP is asking about - a VHF yagi…
It would result in huge disks with lots of wind loading. Even for a 70cm yagi it would be questionable.
I have made some bad circular polarisation cables for UHF, the result was worse that just using a vertical single yagi. So be very careful when making the cables for UHF.
Ok, here is what I think I am going to do. The helcal SATNOGS UHF antenna design has been updated a few times, so I trust its performance.
There are a couple of commercial cross-pol VHF antennas (that result in circular polarized reception if you combine the antennas at the correct phase), but I am going to do a build based on the link I have listed in an earlier message. VHF is a bit more tolerant to small errors, so I am expecting decent performance.
I also have the materials to build a cross-pol antenna and the circularly polarized antenna. I should probably post the results of the builds as I (as a newbie) had a bit of a journey to decide on the designs.
Since I run a Maker Space, purchasing an antenna rubs me the wrong way. I am also a cheapskate.
Please feel free to continue to comment on antennas - it is a bit of an art form that can be tough to grasp and I am sure SATNOGS station builders are not all radio guys!
