Tune in Your TX

Loncat ke form komentar

Tulisan yang berhubungan dengan Tune in Your TX ...

If your TX has a series of amplifiers together, you can break in between the stages to tune each one at a time. Do this by unsoldering components and soldering in short bits of coax with suitable plugs to connect to dummy load and VSWR meter.

Harmonics

Harmonics are multiples of the transmitting frequency. For a frequency of 100MHz, the 1st harmonic, known as the fundamental is 100MHz, The second is 200MHz, the 3rd is 300MHz etc. They are produced as side effects in various parts of the circuit, and will interfere with other users of these frequencies if they escape from the TX. This will produce Radio Frequency Interference (RFI), and won't do you any favours with the relevant authorities. Tuned class C amps don't amplify harmonics, as the harmonic frequencies are out of the range the amplifier will amplify. However the use of Class C means that harmonics are generated by the amplifier along with the desired frequency. The strongest harmonics (apart from the fundamental) from such amps are usually the 3rd, then the 5th etc. The amplitude of harmonics is minimised if the output networks are tuned properly, but they're still there. Oscillators and Buffers can also generate harmonics if they're not set up right.

Wavemeters

To detect harmonics we need an Absorption Wavemeter usually called just a Wavemeter. Or we can use a Grid Dip Oscillator (GDO) or a Gate Dip Oscillator, both of which are known as Dip Meters. Most dipmeters have a switch which turns them into wavemeters. A wavemeter has a tuning knob, calibrated in frequency, a meter showing signal strength, and some kind of short aerial. You hold the aerial near a coil in the bit of the circuit you're interested in, and tune the wavemeter. It shows how much signal is present on the frequencies shown on the scale. So you can see what frequencies are being generated in that part of the circuit. Ideally you'll just find the fundamental, unless the circuit is a frequency tripler or something.
If you buy a wavemeter be sure it covers the right range, from below 100MHz to get the fundamental to above 300MHz to get the 3rd harmonic.

The VSWR Meter

Some of you may know that we can use a VSWR meter (also known as Voltage Standing Wave Ratio meter, SWR meter or a Reflectometer) to detect mismatch between TX and the aerial, but the VSWR meter is just as much at home doing this between amplifier stages. VSWR is the ratio of the forward (or incident) and reflected power. Except for the expensive ones, the majority of VSWR meters work the same way. The switch is set to Forward or the set button is pressed. The knob is then adjusted to make the meter read full scale. The switch is then set to Reverse or the button is released. It now indicates the VSWR. A VSWR of 1:1 is perfect (no reflected power) and so unlikely. A VSWR of infinity:1 shows all the power is reflected back into the amplifier, you'll get a reading like this if the VSWR meter is connected to an amplifier output, and nothing is connected to the VSWR meters output (unless its got a built in dummy load). You'll also get this if there's a short circuit somewhere after the VSWR meter. In either case switch off immediately or you can wave goodbye to your power transistor.
The point of all this VSWR business is to get the maximum amount of power out from the amplifier into the aerial, instead of a hot TX and a bad signal.
So how do we go about tuning such an amplifier? We've got to tune it with something connected to it's output, a load, otherwise the tuning won't be right and besides the transistor will probably blow up. We could use an aerial, but this introduces an extra unknown quantity - the characteristics of the aerial. As well as the fact that we'd be broadcasting. What we need is a dummy load.

The Dummy Load

This is basically a resistor, but constructed in such a way that it presents a load to the amplifiers output independent of frequency (unlike the aerial). The three characteristics of a dummy load we're interested in are:
• It should be suitable for the frequency we're interested in, about 100MHz,
• It should be rated to take the power we're trying to produce,
• It should have a resistance of 50R (to match the output network of the amplifier).
Dummy loads designed for the 2 meter band (a common amateur/HAM radio band, centred on 145MHz) will work well in the VHF FM band. Most test gear for this band (dummy loads, VSWR meters, power meters, wavemeters, RF voltmeters, frequency counters etc.) will work on the frequencies we're interested in.
Even if you tune everything correctly you're still going to have some harmonics generated by the last stage. A sensible operator won't let these harmonics reach the aerial. To stop harmonics reaching the aerial we need either a Low Pass Filter or a Band Pass Filter.

Bandpass Filter

A Bandpass Filter (BPF) only allows through a narrow band of frequencies, ie it has a narrow bandwidth, a good one would be less than 1MHz. It needs standard 50R input and output impedance and be able to take the power you want to use. It also has to be tuned to the frequency you want let through. Other frequencies are not stopped completely, but reduced to a much lower level. It also reduces the level of the desired frequency slightly, by an amount known as the Insertion Loss. To keep this loss low, bandpass filters for high output powers are usually pretty chunky numbers.
A well-designed TX will have a filter built into it. It needs to be in a well screened case to stop harmonics leaking out. In fact your whole TX should be well screened to stop any other frequencies inside the TX finding their way out.
As an alternative to a Band Pass Filter, you could use a Low Pass Filter (LPF). This has the advantage that it doesn't necessarily have to be retuned if you change your frequency. Tune it for best match and minimum insertion loss.

Connectors

As you may have guessed, you can't use any connectors at VHF as the connectors have to "match" the amplifier and feeder. Use BNC or UHF series (PL259 plug and SO239 socket). UHF is preferable for higher powers as you can get a wider cable into the plug. N type is also good but more expensive.


Feeders

So you've got your nice clean harmonic-free signal coming out of your bandpass filter - we're on the home run. All that's left is to get the signal up the aerial feeder to the aerial and we're away. However, as always, there's a bit more to it than that. The aerial cable needs to match the TXs output stage at one end and the aerial at the other end. That's right, the aerial cable has a characteristic impedance as well, just like the TXs output, the connectors and the aerial, and to match this, the impedance should be 50R. As well as having the right impedance, it also got to be Low Loss or your watts will be turned into heat. This is not the same as a bad VSWR where you lose energy in the TX, a good VSWR does not necessarily mean the cable is okay. The sort of cable we're talking about is called coaxial cable or coax, which has an inner conductor totally surrounded by a circular outer sheath. Decent coax cables for short runs are UR76 and RG58U. For longer runs or higher powers use UR67. These cables are available in the UK from Maplin, amongst others.

Aerials

At last, the aerial! A badly selected or built aerial can waste all the effort you have put in so far, so I recommend you read a book on aerials. I used to recommend the "The Two Metre Antenna Handbook" by FC Judd G2BCX, a paperback by Newnes Technical Books. This is now out of print, but you may get it in a library. Some of the aerials are not particularly useful for our purposes, but he covers subjects like propagation, matching and VSWR in detail. All the dimensions he gives are for the 2m amateur band centred on 145 MHz. Any other book covering the 2m radio amateur band is worth a look.
To convert designs for the 2m band to other frequencies, all dimensions (including diameter of aerial element, etc) should be multiplied by 145 and then divided by your frequency in MHz. For example 978mm for 2m becomes 978 x 145/103 = 1377mm for 103MHz.