What is the most critical part of a car? The tyres. (tires in us English)
What is the most critical part of a HiFi (High Fidelity) setup ? The loudspeakers.
[We can argue about what constitutes a HiFi later.]
What is the most critical part of an amateur radio station?
It is the antenna installation. Which includes the matching circuitry in the output stage and the cable connection. We usually refer RAE students to the "maximum power transfer theorem" when discussing this. Forgetting the other bits and pieces that make up the whole installation.
When discussing a 'loudspeaker', we usually refer to it as a 'transducer'. It converts electrical signals into sound or air pressure changes. With a radio station it is similar. The radio frequency energy is transferred to an electrostatic wave (also an electromagnetic wave). In the 'very old' days this was called the 'aether'. This field fades away as you get further away from your transmitter. The magnetic field quite quickly. But the electrostatic field not as quickly.
Figure 1: From: Radio & Electronic Laboratory Handbook by Scroggie
Most times Radio Amateurs get very concerned about 's.w.r.' (Standing Wave Ratio). And the notes also refer to this as a 'bad thing'. Essentially what you need to worry about is getting the maximum amount of power in your transmission 'out there'.
So recently with the supply of 'vector network analysers', it has become possible to measure the antenna with great accuracy. This leads to critical testing of the antenna and a very worried Amateur.
In the early part of the 20th century a current meter in the antenna wire was all that was required. Certainly the s.w.r. was an issue. But the valve power amplifier would ignore this 'reflected power'. Only when the voltage wave became extremely high and broke down insulators, was it 'noticed'!
With the advent of power transistors it became of vital interest to make sure the 'reflected power' did not damage the expensive transistors. Most CB radios in the 60's and 70's had a simple 'reflectometer' in the output connection. These would usually announce a bad s.w.r. in no uncertain terms.
s.w.r - How do you measure it?
Standing Wave Ratio is a ratio of transmitted radio frequency voltage or current, to the reflected voltage or current from the 'load'. The load is usually the antenna system. A 'dummy load' should provide a perfect match for the transmitter. That is no reflection of voltage or current. All of this is complicated by the fact that we are talking about radio frequency voltages. And the phasing of voltages and current will definitely not be 'in-phase'.
What is required is some 'directional' coupling to the circuit. To allow measurement of 'forward' r.f. at the same time as the 'reverse' r.f. . Most radio amateurs reach for a transmission line 'reflectometer'. This works fine over the h.f., 1 to 30 MHz range. As the coupling is usually quite small with regard to the wavelength. The well known example of this is the "Bird Thru-line Wattmeter."
Antenna Measurement
An alternative method is to 'measure' the antenna electrically. With h.f. it is relatively easy to build a 'bridge' which will indicate the impedance of the antenna. Note that I said 'impedance' not resistance. This is simply because most wire antennas 'look like electrically', a resonant LC tuned circuit. This is why an ATU proves very useful in matching an antenna to the transmitter. The bridge type circuit is the most common with transformer types the next most common. Recently though it seems that a lot of amateurs have lashed out and bought a VNA. The VNA has the advantage that it 'sweeps' the frequencies around the antenna resonance. Showing the resultant matching on a display. What about the 'monetary challenged' Radio Amateur? He/She will have to build a test unit to check the antenna.
Bridge Test Unit
The simplest form is a resistive bridge. With some provisos this can be fabricated in a tin box using readily available resistors. The downside to this is the fact that when the 'bridge' balances, nothing 'comes out of it'! So a simple diode detector stops working at the load matching frequency...
In the 'old days' this would use 2 to 5 Watt carbon composition resistors. Which in the 'old days' were readily available from component shops. Usually 'downtown'. Those shops have long ago closed never to be seen again. Component suppliers today will insist on 'minimum order quantities' and prices that bring a tear to your eye!
So what is the alternative? Radio Amateurs and Electronic Enthusiasts have become 'Electronic Scavengers.'
Let us see if we can't make a simple 'bridge'. Some of us and some clubs have a stock of resistors. Either bought over the years or left to the club as part of a deceased estate. So what can we use?
[I actually bought some decades ago 51 Ohm resistors.] It is highly unlikely that you will find them near you! But two 100 Ohm resistors in parallel make 50 Ohms. With twice the power handling of a single resistor. Or four 220 Ohm resistors in parallel make [erk! I had to check using a calculator!] 55 Ohms. Just remember that resistors in parallel have also 'self-capacitance'. Which when you use four in parallel makes four times the self-capacitance...
So this is the circuit of a 'simple antenna bridge'. It is really quite simple. BUT turn the transceiver power output down to a Watt. Otherwise the resistors will get hot! Maybe even burn out. So before checking or testing, connect your dummy load and turn the output power (CW) DOWN!!! [Don't have a dummy load? Another article link!]
The 'balanced' condition when the antenna is approximately equal to the 51 Ohm resistance produces the lowest signal level out of the 'bridge'. Which is why the bridge needs a Watt or so to drive the diode to provide a d.c. reading. A germanium diode is used as it has a low forward voltage for conduction. To give a reading at 'balance'.
There are of course other methods of measuring the antenna impedance. And it doesn't have to be at a set frequency. The source could be a swept oscillator covering the adjacent frequencies. This would show up any 'out of band' resonance. That could be corrected quickly. Also the antenna impedance can be measured accurately. So that the compensating reactance could be connected to get it resistive at the desired frequency.
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NOTE
Even a Watt will go a long way. So other methods of lower power signal sources have been used over the years to reduce the chances of interference. The lower level of d.c. from the diode is usually amplified by an operational amplifier.
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I intend adding to this article. Please let me know in the comments if you would like more information on this subject.
73 John Brock ZS6WL
