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INTRODUCTION

What antenna tuners do & don't, can and can't (ref. 1, 2):

  • Antenna tuners do not tune the antenna, in the sense of shifting the resonance frequency of an antenna to the operating frequency of the transmitter.
  • Antenna tuners cannot change the feedpoint impedance of an antenna.
  • Antenna tuners cannot change the impedance along the feedline between the tuner and the antenna.
  • Hence, an antenna tuner cannot change the SWR along that feedline between the tuner and the antenna.
  • An antenna tuner can, however, change the load impedance that appears at its connector to the transmitter. Hence, it changes the matching at the transmitter.
  • Conversely, an antenna tuner can and does change the source impedance that appears at its connector to the feedline to the antenna. Hence, it changes the matching at the feedpoint of the antenna.

The function of a "tuner" is to provide impedance matching for the purpose of maximizing power transfer between the transmitter/transceiver and the antenna system ( = antenna + feedline).

More precisely: maximum power transfer requires conjugate impedance matching:

Mag Loop

Simply put: cancel out the non-resistive (i.e., inductive, capacitive) part of its load impedance, and transform the resulting purely resistive impedance to 50 ohm ( = "matching"). So, it would be more appropriate to call a tuner/coupler an "impedance matching unit".

In principle, a "tuner" can be placed anywhere along the feedline(s) between the transmitter and the antenna. If it is placed directly at the antenna, it is often referred to as an "antenna coupler". Here, the "tuner" matches the impedance of the antenna to the characteristic impedance of the coax-feedline between the tuner and the transmitter. This minimizes feed-line losses. Obviously, if the tuner is located at the antenna, it must have remote control. Typically such remote tuners/couplers are automatic: the transmitter outputs a low-power carrier, and the tuner does its thing.

When the tuner is located closer to, or at the transmitter, it may very well be able to provide perfect impedance matching between the transmitter, and the "feed-line + antenna" system. The feed-line may be balanced (ladder line, twin-lead), or unbalanced (coax). Whereas the transmitter may be very happy in this configuration and see SWR 1:1, feed-line losses may actually be (very) high.

Listed and described below, are the "tuners" that I have acquired over the years.


MFJ-945E MOBILE TUNER

Until mid-2011, I have been using a simple manual antenna tuner directly at the transceiver. I have used it with coax, and with 300/450 Ω ladder line (sometimes via 1:1 and 4:1 baluns at the antenna).

antenna tuner / coupler

HF + 6m antenna tuner MFJ-945E



QRPprojects ZM-4 Z-MATCH TUNER

In december of 2008, I built a small QRP antenna tuner kit (10 W max): the ZM-4 from QRPproject in Germany (ref. 2C). It has both a balanced and an unbalanced antenna input, and an LED indicator instead of an SWR-meter. It is a very compact Z-Match. Ref. 2D.

antenna tuner / coupler

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antenna tuner / coupler

The inside of the ZM-4


ALINCO EDX-2 AUTOMATIC ANTENNA COUPLER

Early 2011 I decided to have a go at an "Automatic Antenna Tuner" (ATU) antenna coupler. My rig is an Alinco DX70TH, so I started looking for the matching Alinco EDX-2 tuner. Ref. 4. The EDX-2 supposedly "tunes" just about any "element" that is over 10 feet (3 m) long, from 3.5-30 MHz (80-10m) band, and anything over 40 feet (12 m) from 1.8-30 MHz (160-10m). Note that the ability of the coupler to "load" a random piece of metal at a certain frequency does not imply whatsoever, that the "antenna" will radiate efficiently. The EDX-2 is very similar in design and construction to the Icom AH-4. The effect of the automatic tuning is fun to watch - on the S-meter of my old MFJ-tuner (yes, in bypass mode).

antenna tuner / coupler

The EDX-2 with the installation hardware that is included


antenna tuner / coupler

The inside of the EDX-2


The 4-conductor control cable between the EDX-2 and the transceiver has the perfect length for capturing interference from the antenna while transmitting. That would not be good! So I looped the control cable several times through two FT140-43 ferrite rings: one at the EDX-2, one at the transceiver.

I have also installed a current choke at the EDX-2 end of the coax - just in case. I used a large (!) clamp-on ferrite of material type 31- supposedly significantly better below 5 MHz than the #43 material. The core of this "split round cable assembly" (or "Round Cable Snap-It") is about 2.2" (≈5.6 cm) tall and across.

antenna tuner / coupler

EDX-2 installed on my terrace - ferrite ring on the control cable, large ferrite clamp on the coax


antenna tuner / coupler

FT140-43 ferrite ring on the control cable, at the transceiver


To make antenna experiments easier, and to be able to quickly disconnect in case of a thunderstorm, I have wired two banana jacks to the terminals of the coupler/tuner:

antenna tuner / coupler

Banana jacks for quicly disconnecting an antenna or swapping antennas

Note: the ground/earth terminal is at the bottom of the box, the antenna-wire terminal at the top....

These days, I connect dipole antennas to this tuner/coupler via a section of 300 ohm twin-lead. To make it easy to connect/disconnect antennas durng experiments, I use a regular 220 Volt household plug instead of banana plugs. Do not use this method near a 110/220 Volt power outlet!

antenna tuner / coupler

The tuning process is quite simple, see the timing diagrams below:

  • T0: the operator activates the "tune" function on the DX-70 transceiver. The transceiver asserts the active-low ( = pull-down) START signal to the EDX-2.
  • In the EDX-2, the START input is pulled up to +5 VDC. Pull-down in the DX-70 is open-collector.
  • T1 = T0 + 15 msec: the EDX-2 responds by asserting the active-low KEY signal to the EDX-2.
  • In the DX-70, the KEY input is pulled up to +5 VDC. Pull-down in the EDX-2 is open-collector.
  • T2 = T1 + 2 msec: in response to the active KEY signal, the DX-70 sends a carrier (3-10 watt).
  • T3 = T2 + Ttune: the EDX-2 tunes (typ. 3-8 sec). upon finishing the tuning process, the EDX-2 relinquishes the KEY signal to the transmitter.
  • T4 = T4 + 8 msec: the transmitter relinquishes the START signal and stops transmitting the carrier.
  • If the transmitter does not send (sufficient) RF, the RDX-2 relinquishes the KEY signal 330 msec after activating that signal.


antenna tuner / coupler

Timing diagrams of the EDX-2 control signals


In 2012, I acquired a very compact QRP HF/VHF/UHF transceiver: a Yeasu FT-817ND. I really wanted to use it with the EDX-2. However, the FT-817 does not have a "tune" button or function! Performing a manual „tune“ takes 5-11 manipulations on the FT-817, depending on the mode that the transceiver is in. Very annoying! But don't despair! I use the Ham Radio Deluxe (HRD) digi-mode software package (I still use the V5.11 freeware version) to fully control the FT-817 via a CAT-interface. HRD has a tune-function ("tools" ─► "tune up"). 

antenna tuner / coupler

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However, HRD has no way to assert a START signal to the EDX-2 or to respond to the KEY signal from the EDX-2. So, no automatic tuning cycle. Not a big problem. There is an easy way to manually assert the START signal to the EDX-2. The EDX-2 control cable is disconnected from the DX-70 transceiver, and a push button switch is installed across the START wire and ground. An LED with pull-up resistor is connected to the KEY wire of the control cable, so we can see what the EDX-2 is doing. See the schematic below. I just activate the tune function of HRD and immediately hit the push button. That's all. Not fully automatic, but hey.... it works like a charm!

antenna tuner / coupler

A simple manual control interface for the EDX-2


antenna tuner / coupler

My EDX-2 manual tuning starter



MFJ-974HB BALANCED-LINE TUNER

In 2013, I expanded my collection of antenna tuners/couplers with an MFJ-974HB "balanced line tuner":

antenna tuner / coupler

REFERENCES


External links last checked: October 2015


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©1999-2016 F. Dörenberg, unless stated otherwise. All rights reserved worldwide. No part of this publication may be used without permission from the author.