Aerial Matching Unit
1. A Classic HF Z-Match with Additions for Top Band
This is a multi-band aerial matching unit (AMU) that can used from 1.8 - 30MHz with the correct components.
This is a multi-
Item
Description
Item
Description
C1a/b
Dual 250-350pF max per section, variable capacitor held off the chassis with insulated mountings. An insulated shaft coupler must be used.
L3
4t 16swg 1.5" ID spaced as for L1
C2a/b
300-500pF per section max value, double gang variable capacitor.
L4
3t 16swg 2" ID spaced as for L1 and mounted concentrically over L3
SK1
50ohm chassis mounting coaxial socket
L5
12t of 18-22swg, tapped every 3 turns with an ID about 0.4-0.5" greater than L6. This inductor assembly must be fixed over the cold end of L6 with insulating spacers and a suitable adhesive.
L1
5t 16swg 2" ID, each turn spaced about 1/6" and mounted concentrically over L2.
L6
37t of 18swg tapped at 17t, 22t, 27t with an ID of 1.5".
L2
6t 16swg 1.5" ID each turn spaced about 1/6"
S1
Heavy duty 4 pole 3 way rotary switch - see text below
Notes
ID = inner diameter. The actual number of turns will depend on the tuning capacitor value and your layout. It is best to prototype and test the coils before making the final versions. Each pair of coils for L1/L2 and L3/L4 should be supported using perspex sheet and some form of low loss solvent based fixative e.g. polystyrene cement. The supports can be attached to the chassis using long, tapped, insulated spacers.
The relatively high number of turns on L5/L6 will require conventional low loss coil formers - ceramic is fine for L6 but fibre-glass or white plastic kitchen waste pipe is also fine for either coil and is more easily available in a range of sizes. In order to allow for alternative tuning capacitor values, both L5 and L6 are shown with taps to make for easier adjustment. Constructors may wish to determine the number of turns required for the tuned winding L6 during a prototyping phase and dispense with the taps. The author used 17t for L6 and 3t for L5 but retained the taps.
The frequency coverage is dependent on the maximum to minimum swing of C1, C2 and the respective inductor values. A three to one frequency coverage requires a minimum of a nine to one capacitance swing so it is recommended that any preset trimmer capacitors on C1 and C2 be set to minimum or preferably removed.
The impedance matching range is primarily dependent on the turns ratio of each pair of coils and the values of capacitors C1 and C2. If you have problems getting a good match then it may be necessary to adjust the turns ratio by increasing or decreasing L1 or L4 during setup. (L1/L2 should be separated from and ideally at right angles to L3/L4 and L5/L6.)
For example, if the Z-Match provides a minimum SWR reading with C1 fully meshed then the impedance being presented is probably very low and the respective secondary winding (L1/L4/L5) should be reduced by one or more turns and tuning retried.
For transmitter powers up to around 25 watts "old fashioned" air-spaced broadcast tuning capacitors from valve radios will probably be fine - transistor radio plastic dielectric tuning capacitors may flash over or run warm at powers over 5W - best to experiment with a prototype first. There is more chance of a flash-over when the impedance presented to the AMU is high.
The entire unit should be built in a metal box of sufficient dimensions to allow a spacing of at least one coil diameter between each coil and the nearest side of the box. Slow motion drives make for easier adjustment and perspex dials calibrated 0-100 allow settings to be recorded for future ease of re-tuning. The constructor may wish to include an SWR Bridge, dummy load and selector switch inside the Z-Match in order to make a self contained unit.
The author has built a dual Z-Match covering 1.8 - 146MHz in a Heath SB series cabinet with the appropriate metering and switching for multiple transmitters and aerials. Two SWR bridges are used, one for the HF bands and one for VHF. The meter and small knobs are from a Heath SWR Bridge and the front panel has been painted and labelled to match other Heath units.
The VHF section uses a very similar circuit to that of the HF unit although the component values are significantly less, C1 is a single section variable capacitor and HF switch sections S1a and S1b and coils L5 and L6 are not required although S1c and S1d are required. C2 is 35 + 35pF. Note that the minimum value of C1 on VHF should be a few pF so some form of selection may be required and the wiring should be a short as possible. L1 = 2t 1.2inches ID spaced 0.75 inches in total, L2 = 3t 1.0 inch ID spaced 1.0 inch overall. L1 positioned concentrically over L2. L4 = 2t 1.2inches ID spaced 0.75 inches in total, L3 = 3t 1.0 inch ID spaced 1.0 inch overall. L4 positioned concentrically over L3. All coils wound with 16swg enamelled copper wire. The layout of the VHF section is much more critical than the HF section so some ajustments may be required for each constructors own version.
Click Wideband Z-Match to see a photograph of the front panel.
Click Internal Construction to see a photograph of the internals.
How to Use a Z-Match
See the connection diagram below
Switch the SWR Bridge to the dummy load
Key up, tune and load the transmitter for maximum output if these controls exist (probably only on older valve rigs). Ensure that the VSWR reading is showing 1:1 or very close i.e. very low reflected power.
Reduce the power output to around 10% of maximum so as not to strain the PA and de-key.
Switch the SWR Bridge to the Z-Match with an aerial connected.
Find a signal on your chosen band. Select the frequency range required on the Z-Match and adjust both Z-Match tuning controls for strongest received signal strength.
Now find a clear frequency, key up the transmitter and adjust both Z-Match tuning controls for minimum reflected power on the VSWR bridge i.e. SWR is 1:1 or very close.
ID = inner diameter. The actual number of turns will depend on the tuning capacitor value and your layout. It is best to prototype and test the coils before making the final versions. Each pair of coils for L1/L2 and L3/L4 should be supported using perspex sheet and some form of low loss solvent based fixative e.g. polystyrene cement. The supports can be attached to the chassis using long, tapped, insulated spacers.
The relatively high number of turns on L5/L6 will require conventional low loss coil formers -
The frequency coverage is dependent on the maximum to minimum swing of C1, C2 and the respective inductor values. A three to one frequency coverage requires a minimum of a nine to one capacitance swing so it is recommended that any preset trimmer capacitors on C1 and C2 be set to minimum or preferably removed.
The impedance matching range is primarily dependent on the turns ratio of each pair of coils and the values of capacitors C1 and C2. If you have problems getting a good match then it may be necessary to adjust the turns ratio by increasing or decreasing L1 or L4 during setup. (L1/L2 should be separated from and ideally at right angles to L3/L4 and L5/L6.)
For example, if the Z-
For transmitter powers up to around 25 watts "old fashioned" air-
The entire unit should be built in a metal box of sufficient dimensions to allow a spacing of at least one coil diameter between each coil and the nearest side of the box. Slow motion drives make for easier adjustment and perspex dials calibrated 0-
The author has built a dual Z-
The VHF section uses a very similar circuit to that of the HF unit although the component values are significantly less, C1 is a single section variable capacitor and HF switch sections S1a and S1b and coils L5 and L6 are not required although S1c and S1d are required. C2 is 35 + 35pF. Note that the minimum value of C1 on VHF should be a few pF so some form of selection may be required and the wiring should be a short as possible. L1 = 2t 1.2inches ID spaced 0.75 inches in total, L2 = 3t 1.0 inch ID spaced 1.0 inch overall. L1 positioned concentrically over L2. L4 = 2t 1.2inches ID spaced 0.75 inches in total, L3 = 3t 1.0 inch ID spaced 1.0 inch overall. L4 positioned concentrically over L3. All coils wound with 16swg enamelled copper wire. The layout of the VHF section is much more critical than the HF section so some ajustments may be required for each constructors own version.
Click Wideband Z-
Click Internal Construction to see a photograph of the internals.
How to Use a Z-
Switch the SWR Bridge to the dummy load
Key up, tune and load the transmitter for maximum output if these controls exist (probably only on older valve rigs). Ensure that the VSWR reading is showing 1:1 or very close i.e. very low reflected power.
Reduce the power output to around 10% of maximum so as not to strain the PA and de-
