Tunners / Z Matching

Table of Contents
  • Antenna Matching
  • Antenna Feedlines
  • Remotely Switched Load Coil

Antenna Matching
 
A tuner should be called matcher because it matches the impedance presented to the transceiver to the impedance on the feedline. You could say the tuner takes whatever you have on your feedline and makes it look like 50 Ohms resistive to the transceiver. This is a simplified answer that is not all true, but it is the general idea. Today automatic tuners are the standard; twenty years back the standard was an external manual tuner. Prior to 1980 transmitters normally had internal tuning and loading controls. Think about the controls, controls for matching, this was what you pay extra today is called a built-in tuner!

Schematic of MFJ-941E Tuner


Looking at the schematic above you see many components but few are actually the key to tuning. Most are for the SWR, measuring forward and reverse power. So looking at the MFJ-941E schematic above and the simplified version below you can start to see manufactures use switches to vary the inductance values. Other models use switches for both capacitance and inductance control. Most use series capacitors as this is generally a more efficient design. Capacitors do not contribute any significant loss in the circuit as they have very high Q. Q is the ratio of electrical resistance and reactance at resonance. Capacitor Q is much larger than the Q exhibited by most coils.

Simplified Schematic of MFJ-941E Tuner


Looking at an automatic tuner is even harder to understand the functionality but the automatic tuners work the same way as manual tuners for the same reasons stated above. The automatic tuners use relays instead of switches. The relays are controlled by a microprocessor with a very simple program. The program simply tries every combination of inductance and capacitance until the SWR is lower than some value. The relay positions are then recorder in memory with the frequency so the tuner can try those positions first next time that frequency is used. Nothing is magic or complex just trial and error but faster than a human. So looking at the Collins Radio 180L-3 automatic tuner schematic below is not helpful for understanding tuners but is good for maintenance.

Collins Radio 180L-3 Automatic Tuner Schematic


To understand automatic tuners as an operator most manuals show a functional block diagram like the MFJ-993B diagram shown below. This diagram shows the majority of the functions are needed to control the relays to find the desired combination of inductance and capacitance and then remember it. Some other common automatic tuner functions are worth noting as you look at the diagram. Some designs reverse the inductor and capacitor respective series and parallel positions in the circuit. This is done by using ferrite core inductors with better Q ratios with consideration for the operating frequency range. Additionally you will see a switch that moves the capacitor(s) from the transmitter side to the antenna side saving in hardware costs as well as practical design. It includes other featurees common to many commercial tuners with an antenna selector and current BALUN built-in.
MFJ-993B Automatic Tuner Block Diagram


Now you are ready to start planning your antenna before you build and just take what you get. Here is a great little calculator by G4FGQ that will for a given antenna provide tuner L and C values, feedline losses and overall antenna system efficiency. It is setup for dipoles but can be used for monopoles by doubling the length. This is a DOS program that opens as a window on your PC desktop. You can change all the key parameters of your antenna, feedline and BALUN before you build.

G4FGQ Dipole Calculator Screen


Click Here to download the G4FGQ Dipole Calculator. Click RUN and follow the instructions


Here are some useful resources on antenna matching and problems with HF antenna systems

Antenna Feedlines

Feedline is required to transfer the RF output from the tuner to the antenna and is normally 50 ohm coaxial cable because the transmitters made today are designed for 50 Ohm output. Notice I said the tuner is connected to the feedline not the transmitter. The tuner’s job to match the impedance from the feed input to the transmitter’s 50 Ohm output has just been covered by the previous section. A 50 Ohm coaxial feedline terminated into a 50 ohm antenna at a distance of an exact half wave length are 1 to 1 transformers with very minor resistive loss. Use any other length line and you are no longer terminated in 50 Ohm impedance this also changes the impedance into something else at the transmitter end. The changed value is a function of antenna impedance, line loss, and the length and characteristic impedance of the feedline. There are three feedline characteristics to consider in your antenna system;
  • Resistive Line Loss
  • Antenna Impedance
  • Length Relative to Wavelength

Coaxial cable resistive loss is a one factor in an antenna system that most HAMs consider but not always. In every run, there is some loss of signal strength as the signal travels between the antenna and the tuner. The longer the run of cable, the greater the loss will be is understood. At higher frequencies, the losses in the cable are much greater. We all know not kink the cable, or fit tightly around corners can lead to splits in the jacket and shield, which will lead to a downgrade in performance over time with water intrusion. So we are now good for losses when connecting a 50 Ohm coaxial cable to a resistive 50 Ohm antenna. These are the loss numbers published by the cable manufactures. These are not the values for mismatched loads!


Antenna impedance must be considered to prevent reflections at the antenna from causing standing waves. The feedline must match to the characteristic impedance of the antenna to prevent reflections. It is not difficult to select a feedline to match the antenna but it is often not even considered. Coaxial cable impedance is determined its dimensions and materials used and is a purchased item. Common coax values are 50 and 75 Ohms but an alternative using parallel conductors provides values of 300, 450 and 600 Ohms determined by spacing and materials. An example of the right way is the G5RV design that matches the higher impedance dipole to the 50 Ohm coax with a tuned length of parallel feedline. Another example of the right way is matching a Windom Dipole (OCF) design high impedance to the 50 Ohm coax with a 4:1 BALUM.

Looking at a typical case of a 100W transceiver connected to an 40M antenna with 100 Feet of RG-8U you can see a dramatic difference in antenna RF radiated power by matching at the antenna feed point to the feedline. The dramatic effects of matching are shown in the chart below.

Relative RF Power Radiated vs. Antenna to Feedline Matching

Any length of feedline can be used if you understand feedline SWR. A general rule is to use a feed length or ½ WL and measure the VSWR at the tuner-feedline connection prior to use in all bands. You can go from perfection to dead short by selecting a frequency that yields a wavelength equal to a ¼ WL multiple of the feedline length. Here is the kicker but not always, ¼ WL multiple feedline can also transform a high impedance to a low impedance. The contributions of a ¼ WL can be tricky on a multiband antenna. Sometimes HAMs blame their tuner for not working, some understand enough to add or subtract 1/8 WL from the feedline there is no magic in a ¼ WL just be aware of how it works. The feed impedance at one end is purely resistive, the impedance at the other end will also be resistive, and a random length section can be resistive at one end and yet have a complex impedance at the other end. Remember to measure your multiband antenna in all bands prior to use.


Making a Remotely Switched Load (Matching) Coil
the poor man's remote tuner

Living in a “no antenna” deed restricted Florida community sounds bad for a HAM, but it means you just need to be more creative. I have found this will not stop your DXCC activities and many of my local HAM club members will agree. I have good one hop (2500 Miles) coverage and daily QSOs to the EU (5000 Miles) on 15M, 20M & 40M. I have the same results for distance on 80M and 160M but not much activity when I am on the air. Smaller “short” antennas are less challenging than QRP and the primary restriction. The local soil is sandy and is not very conductive so 32 or more radials are required for a ground plane. The bottom line is the many positive aspects of living in our deed restricted community are a good thing and does not stop me from enjoying amateur radio.

Once you have built a 20 Foot flagpole antenna and the yard is filled with 32 buried radial wires to make a ground system you are ready for a $400 tuner or better to make your own matching system.

Short Antenna Remotely Switched Load Coil


The first step is to decide which bands and frequencies are desired for the tuning system. In most cases the 2:1 SWR bandwidth will be the entire band for 40M, 30M, 20M, 17M, 15M and maybe all of 10M. The 80M 2:1 SWR bandwidth will be 150-180KHz so plan on 1-3 depending on the mode(s) you want to run. This means the design could require 5 relays for 40-15M plus 2 for 10M and 3 for 80M for a total of ten relays. Considering tune the vertical for 40M to work okay as a 3/4-waveIength vertical on 15M and the same for 30M and 10M respectively. In the real world 40M and 15M are the same tap, 10M and 30M are the same tap and most of us only operate in one mode in 80M requiring 1 tap for a total of 6 taps (or 6 relays). Now six relays is a more manageable design. Note: Relays should have a 200 VAC contact rating for 100 Watts RF or 600 VAC for 1000 Watts.

Clip Leads can be used for testing (must be short)


Theoretical Results from Perfect Antenna and Ground Plane
  • 80 M taps have 200 KHz BW set at 3.6, 3.7 & 3.9 MHz
  • 40 M tap has 330 KHz BW set at 7.15 MHz
  • 20 M tap has 700 KHz BW set at 14.20 MHz
  • 15 M tap has 1,100 KHz BW set at 21.00 MHz
  • 10 M tap has 1,700 KHz BW set at 29.00 MHz

I recommend you make your coil from bare #10 or #12 AWG Copper wire winding the coil about 40 turns tightly without spacing around a 3 to 4 Inch form like a soup can. A smaller diameter can be used but the capacitance will increase in the coil. The wire will be stiff so you will need a helper to use a can or a rolling pin if you do not get caught. Back off the tension to remove your form and stretch the coil out so you have 5 to 6 turns per Inch spacing. Now you can add terminal lugs to mount your coil on a sheet of acrylic plastic sheet with the relays. When selecting an enclosure for your remote load coil be sure the box is big enough to keep the coil 1 ½ Inches from plastic and three Inches from metal. Note if you are using a metal box the VSWR will change when opening and closing the cover of the metal box. Most metal boxes have only a minor effect on the VSWR.

Homebrew Coil Winder provides extra hands to hold coil


The dimensions and coil settings given above should produce reasonably low VSWR readings over the entire 10, 15, 20 and 30 meter bands and over at least 250 kHz of the 40 Meter band. Bandwidth on 80/75 meters should be at least 100 KHz for VSWR of 2:1 or less at the low end of the band and may be as much as 200 KHz at the high end of the band, depending on the efficiency of the ground system used, greater bandwidth being associated with poor ground systems. It should be remembered that on those bands where the physical height of a vertical antenna is less than a quarter wavelength, the earth (or the resonant radial system in above-ground installations) will have a good deal to do with VSWR and antenna tuning, bandwidth and overall performance,

Functional Diagram of Tapped Load Coil (Note: Coil always in circuit)


When you are ready to start measuring to set your taps the first measurement should be finding the basic resonance of the antenna. Note VSWR is not best VSWR, resonance is when the antenna is purely resistive, 7 MHz to 11 MHz depending on the element RF length (33 or 22 Feet) selected. VSWR is a ratio to your 50 Ohm cable. Also low VSWR by itself does not mean that a vertical antenna is operating efficiently. If a low wide bandwidth VSWR is obtained likely means the opposite. This condition means you will have improper tuning of antenna can usually be attributed to inadequate (or even reactive) ground systems or interaction with objects in the vicinity of the antenna. For these reasons it is suggested that the antenna be placed as much in the clear area as possible and used with the best ground system that conditions permit.

Taps provide Matching in Band

(Note: 80M requires multiple taps for full band coverage)

It is reasonable to expect your best results on 40M with a VSWR at the base of the antenna less than 2:1 across the band. For taps you should start at 3900 KHz to find the 80-meter relay lead was touched on the turns of the coil until minimum reflected power was indicated. Solder the wire in place on the coil. Next try 7150 KHz and touched the 40-meter relay lead to the coil turns until an SWR of 1:3 is read. While using the same coil tap measure on 21.1 MHz and checked the SWR. If the VSWR is less than 2:1 you are done, but most will need to move the coil tap just ½ to 1 turn you are able to get an SWR of 1.8:1 on 15 meters. Now recheck on 40M is required and this process is repeated until both 40M and 15M Bands are less than 2:1. The same process is required for 30M and 10M. After that you can move on to 20M, 17M and 12M taps. Note if you are using a metal box the VSWR will change when opening and closing the cover of the metal box. Most metal boxes have only a minor effect on the VSWR.

Electrical (Metal & PVC) Enclosures and Sprinkler Control Boxs
make great Matching Coil Cabinets




Remember “High & Dry”
Now hook up your coaxial cable and move to your shack to check each band again. Remember the length of the coaxial cable can cause VSWR mis-match. The 40M ¼ wavelength antenna is dependent on the coaxial feed length. You should use cable lengths of 40-50, 70-80, 100-110 or 130-140 feet and DO NOT use cable lengths of 30, 60, 90, 120 feet.

The first step is to decide which bands and frequencies are desired for the tuning system. In most cases the 2:1 SWR bandwidth will be the entire band for 40M, 30M, 20M, 17M, 15M and maybe all of 10M. The 80M 2:1 SWR bandwidth will be 150-180KHz so plan on 1-3 depending on the mode(s) you want to run. This means the design could require 5 relays for 40-15M plus 2 for 10M and 3 for 80M for a total of ten relays. Considering tune the vertical for 40M to work okay as a 3/4-waveIength vertical on 15M and the same for 30M and 10M respectively. In the real world 40M and 15M are the same tap, 10M and 30M are the same tap and most of us only operate in one mode in 80M requiring 1 tap for a total of 6 taps (or 6 relays). Now six relays is a more manageable design.

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