Mast head amplifiers
Chances are your receiver is going to be further than 20 metres from the antenna, so you are going to need a mast head amplifier. This is because of signal loss occurring in the cable itself. The amplifier should be mounted close to the mast head with a very short length of cable between the amplifier and antenna allowing for the minimum amount of loss. The amplifier will need weather proofing and its own power supply.
Whilst you can build your own amplifier, they can be easily purchased. I did a quick google search and found multiple suppliers at around the £20 mark. A dedicated power source can be purchased for less than £15, though many kits come with a power cable. The usual advertised gain is 10db though MiniCircuits.com is advertising gains of 58db, albeit at a significantly higher price.
You can splash out on high quality low loss cable if you want but generally it is not necessary for a home arial. Mid range URM 76 coaxial cable is sufficient. It generally ranges from £40-£90 for 100m. Higher quality RG223 cable is available but expect to pay at least double that of URM 76 cable. BNC connectors at just a few pounds will suffice.
The radio frequency spectrum is crowded especially in the VHF band with multiple high power signals, both digital and analogue. The Graves transmitter in Dijon, France is 600 miles away so the signal will pass through many competing signals to reach your transmitter. A band pass filter will help to mask out these signals.
There is no single solution that will fit all circumstances. The type of filter best
suited to reduce interference depends on the receiver being used, the location
of the site, the strength of particular nearby transmitters, the gain and
beamwidth of the antenna being used and the direction in which it is pointing.
In general a band pass filter centred on 143.050 MHz (or the frequency of
whichever over-the-horizon transmitter is being used for observations) is the
best option. However, commercial narrowband filters are usually bespoke
orders and can be expensive. General filters of the frequency described above can be bought for about £20.
Once you have your aerial connected and all the gizmos and gadgets mentioned above wired up you will need some kind of communications receiver. Commercial receivers are available on eBay for less than £100 pounds, however your laptop will do just fine.
A digital data recorder will store your signal, which can be read by your laptop. Digital data recorders can be as simple as a dongle such as the Funcube dongle. A more expensive digital data logger can produce better results depending on your budget.
We used a program from Spectrum Labs to analyse the data. I’m not going to try and explain the program here but have included a link so you can view it at your leisure.
The completed project
So there you are, a fully assembled and working radio aerial complete with analysing tools. What you do with it is down to you. What we are doing with ours is the subject of other blogs.