X-Band

A scan of the SkyNet-5C transponders.

What can be found on the X-Band?

There is a lot of interesting traffic that can be found on the X-Band. I will be covering the frequency range between 7.2GHz and 8.5GHz which contains the geoband and the DSN band. Next to all the geostationary traffic you can find a few weather satellites and some other low earth orbiting satellites operating in that band. In general there is a lot of interesting traffic, including exotic waveforms (lots of PSK stuff) and some UHF -> X-Band relays. And the best thing – it changes regularly!

What hardware do I need to receive X-Band signals?

 Here is a brief summary of the components you will need:

  • a frequency downconverter consisting of a local oscillator (LO), a frequency mixer and an IF amplifier
  • a good front end LNA (low noise amplifier) with a low noise figure
  • a waveguide antenna for 7 – 8 GHz
  • a big enough satellite dish
  • a bandpass filter for the frequencies of interest
  • high quality low-loss coaxial feeder for the high frequency part

I will now describe every bit of this list in detail down below.

1. The frequency Downconverter and filters

The frequency downconverter is the most complex part to understand in this system. It basically consists of a frequency mixer and a local oscillator. So why does a local oscillator connected to a frequency mixer convert the frequency down? Have a look at this image:

The basic concept of a downconverter is relatively simple once you understand it. You usually need a local oscillator (LO) and a mixer. The mixer usually has 3 ports, one of them being the input from the local oscillator, the second one being the RF input and the third one being the IF output. In this case the local oscillator has a frequency of 7000MHz, if our RF input signal would be 7500MHz for example, both of these signals would mix and create two IF frequencies. One of them would be at 14500MHz and the other one would be at 500 MHz which is exactly want we want. 500MHz is a lot easier to detect with most SDRs or other receivers compared to 7500MHz. To summarise:

Input frequency (RF) +- local oscillator frequency (LO) = output frequency (IF)

Most mixers that are available usually have a conversion loss of 6db (a quarter of the original signal). To overcome this issue it is recommend to use another low noise amplifier right after the IF output port of the mixer. That way you overcome the loss and you get a bit of extra gain for long cable runs. I have used SPF5189z and LNA4ALL units in the past, the latter one appears to be more reliable. 

Filters

The frequency passband of your bandpass filter mostly depends on the frequencies you intend on receiving. For the geoband you’ll need a bandpass filter with a passband of 7250MHz – 7750MHz, for most LEOs including WX sats you’ll need a filter with a passband between 7800MHz and 8400MHz and for the DSN band you’ll need a filter that covers 8400MHz – 8500MHz. Of course you can also include multiple filters in your converter and switch between them with a relay – that’s totally up to you. 

My personal downconverter

2. The font-end LNA and waveguide antenna

In order to pick up the signal from your dish you’ll need a waveguide antenna and a good low noise amplifier. Your satellite dish you be reasonable big, everything that is bigger than 1.6m in diameter should work quite well. My personal recommendation would be to use a prime focus dish as they are lot easier to feed compared to offset dishes. 

The waveguide

For very high frequencies you should start using waveguide antennas as they have a low loss and are depending on the design quite easy to make. For 7GHz – 8GHz you should use 28mm copper pipe which is available at most plumbing shops (at least here in the UK). At some point you will have to convert the waveguide back into coaxial feeder, for that you need to make a waveguide to coax transition. Here is a picture of how that should be made:

To receive circular polarised satellite signals you will need some kind of depolariser. The easiest soltution would be to insert a piece of 2mm thick Teflon at a 45° angle into the waveguide. If you decide to go that route make sure it’s not too long and that you get the angle just right. I was never successful with using a Teflon piece which is why I switched to a CT1DMK depolariser. It is a bit more difficult (and expensive) to build, but it always worked well for me. You can find the instructions here. For prime focus dishes I would recommend using a VE4MA feed. They have a reasonable amount of bandwidth but you need good soldering skills to build them. For offset dishes you could use a W2IMU feed; but remember that they only have a very narrow bandwidth!

The completed waveguide VE4MA feed (for prime focus dishes).

A look inside of the waveguide. You can see the depolariser and the waveguide probe. 

3. The LNA (Low Noise Amplifier)

You will definitely need a low noise amplifier to receive signals on the X-Band. I would recommend an amplifier which has a noise figure of less than 1.5dB and a gain of at least 20dB. The important thing to remember with LNAs is that you can use a low gain and low noise figure amplifier as your first stage and then add more higher gain (with possibly a higher noise figure) amplifiers behind it. The first stage dictates the overall noise figure. 

You can usually find some LNAs on eBay – MITEQ and AMPLITECH units seem to work quite well. 

Some LNAs require cooling – make sure that you have a good heatsink to preserve a low noise figure and a longer lifetime. 

X-Band LNAs tend to be very small and they vary in size, noise figure and gain. 

4. Satellite Dishes

You will need a satellite dish to receive most of the signals on X-Band. It is possible to pick up some very strong beacons by pointing a waveguide directly at the sky, but you will be very limited in what you can see. In most cases you should be able to see a signal with a 1 meter dish. The larger your dish is, the better. I would recommend using a dish with at least 1.4m in diameter. You can use offset and prime focus dishes, but remember that it’s easier to point a prime focus dish and you’ll have more options or building a feed. On offset dishes you might be able to get away by just using a waveguide pointing into the focal point. 

An offsetdish with an open waveguide feed.

A prime focus dish with a VE4MA feed. 

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