There are now a number of options from which to choose for "building" a receiver
in computer software.
In some cases the software and front-end hardware come in a single linked
package, while in others you are free to use the software of your choice.
Hardware includes:
The Icom PCR1000 - 100kHz to 1.3GHz
The Flexradio FLEX-5000 - a complete transceiver HF + 6m
The SDR-14 (0.05 to 30MHz) and SDR-IQ (0.0001 to 30MHz) from RFSpace
and doubtless many others.
Note that as at December 2007 the maximum sampling frequency possible in the A/D converters of SDR receivers limits the highest frequency which can be reached by direct sampling of the signal to around 50MHz, so 144MHz and up require the use of transverters. Doubtless this will change sooner rather than later!
The software packages best known to amateurs are:
Winrad by I2PHD (Windows)
Linrad by SM5BSZ (Linux and Windows versions)
Linrad is a very powerful and flexible receiver package, but its
very flexibility means that it has to be configured properly for any particular application.
Here are some detailed notes by VK2KU on his configuration of Linrad as used for
144MHz EME with an SDR-IQ receiver:
Linrad with an SDR-IQ
The question of which transceiver to buy can be perplexing. Here is a list of current models by the Big 3 manufacturers, with abbreviated specifications: Current VHF/UHF Rigs
So you've decided to add a VHF/UHF linear amplifier to your station. Transistors or Valves?
How much power? Which valve is best? Buy one or build it yourself?
Click here for some notes on a talk given by VK2KU in April 2002 to the NSW VHF
Dx Group: RF Power Options
Ian Cowan VK1BG has collected together a series of modifications to the AM-17 Amplifier for operation on 144MHz.
Click here to read or download Ian's notes:
Notes on the AM-17 Amplifier
There is no doubt that tubes from the Russian domestic market represent the best value for money when building your own Linear Amplifier, and these are now available from a number of sources. For a comprehensive list of tube types and specifications, together with references to a large number of actual amplifiers built with them, visit the following websites:
http://www.nd2x.net/base-1.html
http://www.df6na.de/
RF power tubes, whether triodes or tetrodes, require a
significant flow of cooling air (or water).
Although a few amateurs will disagree, it is generally agreed than axial
(muffin) fans, even with 2 in series, are unable to generate enough pressure to
force a large enough volume of air through the tube or tubes.
The anode normally includes a surrounding structure through which air must be
blown to remove heat from this part of the tube.
Additional cooling is required around the base pins; the tube heater alone
generates enough heat to crack the base seal if this heat is not removed.
The data sheets for each tube specify the amount of anode cooling required, both
the volume per second and the pressure to drive it.
A centrifugal type of blower is needed. The volume of air required and the
pressure at which it must be supplied to drive that volume through the anode
structure dictate in practice the use of a blower wheel of diameter no smaller
than 100mm (or 4 inches); a 3-inch blower is simply not enough, no matter how
impressive the volume per second that it can blow in the absence of any back
pressure.
The most difficult tubes to blow adequately are in fact the smallest ones,
because of their smaller and narrower anode structures. Thus large tubes such as
the 4cx800 (GU74B) are much easier to blow than the commonly used 4cx250 range
in spite of the larger volume of cooling air required.
The air cooling circuit is analogous to an electrical circuit,
with the blower supplying the pressure (voltage) which drives the air flow
(current) through the anode structure (resistance). The blower also has a
significant "internal resistance", which reduces the actual pressure available
(potential difference) at higher air flows.
Remember too that there are other points in the air circuit at which pressure is
lost, so you need to design for a substantial reserve of pressure.
Inadequately cooled tubes have a short life; well-blown tubes will last a
lifetime.
Eimac have a range of air-flow sockets which allow air to pass
between the top and bottom of the socket in close contact with the pins.
Two basic cooling designs have evolved. In the first, air is blown into the grid
box below the tube and then passes into the anode compartment through the
air-flow base; a chimney around the anode forces this air to enter the anode
cavity through the anode cooling structure after which it is vented through a
mesh grid. One difficulty with this arrangement is that the grid box is often
too small to allow an adequate fan to be mounted there.
The second design avoids this problem by using the fan to blow air directly into the anode cavity. A chimney (rolled teflon sheet) connects the anode structure to the vent hole, forcing the vented air to pass through the anode structure; most of the air follows this path. A small proportion of the air passes downwards into the grid box, either through the air-flow base or through holes in the chassis if an air-flow base is not available. This air is then directed over the tube pins before being vented from the grid box. Although initially criticized, this second scheme is now in wide use because of its convenience.
Finding a suitable fan is one of the more challenging aspects of building a big linear amplifier using tubes. Click here for some notes on obtaining fans in Australia: Notes on Sourcing Blower Fans
W1SL - a classic push/pull design for 2 tubes on 144MHz, first presented in QST in 1971, but described in detail in Chapter 8 of the Green Book*, also in Chapter 4 of the VHF/UHF Handbook*.
W2GN - a parallel tube design for 2 tubes on 144MHz, first presented in Ham Radio in 1977, but described in detail in Chapter 4 of the VHF/UHF Handbook*.
K2RIW - a parallel tube design for 2 tubes on 432MHz, first presented in QST in 1972, but described in detail in Chapter 10 of the Green Book*.
Here are some possible sources of parts for building your linear amplifier:
Surplus Sales of Nebraska:
http://www.surplussales.com/SSIndex.html
RF Parts:
http://www.rfparts.com
For a comprehensive discussion of the design of anode and screen supplies, with particular emphasis on safety of the HV supply and proper voltage regulation of the screen supply, see Chapter 11 of the Green Book*.
* The Green Book - "The VHF/UHF Dx Book", edited by Ian White G3SEK,
published by DIR Publishing Ltd (England) in 1992.
* The VHF/UHF Handbook, edited by Dick Biddulph G8DPS, published by the RSGB (England) in 1997.
We plan further pages/links on:
VHF Rigs vs Transverters