Click to view the
Resource Credits

A friend of mine (Gary - KD7FVI) wanted to do some portable operating and was looking for something simple, in the line of an antenna, that would allow him to cover as many bands as possible. Among other pieces of equipment, Gary has a Heathkit HW-8 and a FT-817 and wanted to make the most of both. This seemed like a interesting project, so I started gathering information and did a few experiments.

There didn't seem to be a lot of information available on end-fed random length wire antennas. On the internet, you can get a few bits of information, and most books give a paragraph or two, at the beginning of the antenna chapter, but that's all there was. But then I realized that, loading a transmitter into a random length of wire is really a impedance matching issue, and started looking into tuners. Needless to say, there was lots of information available for tuners.

The ARRL Antenna Handbook talks briefly about a random length end-fed antenna. It shows a L-Network that can be used for matching, but then quickly switches to specific lengths and single band use.

One of the items I came across was a copy of an old article named Easy End Fed Z-Match, by G3OGR, published in the January 1972 issue of 73 Magazine. It doesn't go into greate detail, but it's was some where to start.

I then found even more information in a article by Andrew S. Griffith, W4ULD in the January 1995 issue of QST name "Getting the Most Out of your T-Network Antenna Tuner". And even more by digging through the antenna source code for HamCalc by George Murphy, VE3ERP. In fact, some of that code is presented here, in the form of Javascript, rather than Basic.

Tuning Networks

The L-Network is probably the simplest form of impedance matching network. It consists of one Inductor and one Capacitor, both of which are usually variable. The capacitor is usually continuously variable, but the inductor is often a coil with multiple taps, selected by a switch.

Two components give way to 4 possible configurations, each with their own capabilities. The most common one, with a series inductor and a parallel capacitor, is shown on the right. The configuration is a Low-Pass filter, which is good for suppressing harmonics. The position of the Capacitor depends on what kind of load you are trying to match.

If the load you are trying to match is high, relative to the input impedance, the capacitor should be positioned across the load (upper drawing). This would generally be horizontal or diagonal wires that are greater than 30 feet long. But if the load you are trying to match is low, relative to the input impedance, the capacitor should be positioned across the input. Verticals and mobile whips tend to have low input impedances.

If you were using a commercial L-Network tuner, like the MFJ-16010, switching between high and low impedance loads is just a matter of switching the input/output connectors.

But, if you were to actually build a L-Network tuner, or were looking at modifying an existing tuner, you might want to consider adding a switch, so that the capacitor can be moved without a connector swap. This doesn't make it work any better. It just makes it a little more convenient.

The position of the Inductor and Capacitor can also be swapped, changing the network to a High-Pass configuration. This means that this configuration will not suppress harmonics as well as the Low Pass configuration. This is not a real issue, if your working with fairly up to date equipment, which has been designed to strict emission standards. But, you may not want to use this configuration if you dealing with older, or home brew, equipment where you don't have a good idea of the harmonic radiation.

T-Network Tuning Technique

The T-Network tuner, built with appropriately sized components, can match most antennas to 50 Ohms. The only problems that are usually encountered, result from improper adjustment techniques.

For Roller-Inductor Tuners:

1. Set C-Out at maximum capacitance and leave it there.
2. Set C-In to about half scale.
3. Adjust the roller inductor for an SWR dip. (The dip may be barely noticable.)
4. Slichtly increase or decrease the C-In, and readjust the inductor for a dip.
5. If the SWR is lower than it was in Step 3, slightly vary C-In in the same direction as Step 4.
   
   If the SWR is higher than before, adjust C-In in the direction opposite to that taken in Step 4. Alternatively, inch C-In in the Step 4 direction and redip the SWR with the inductor until you obtain an SWR near 1:1.
6. When you've almost reached the match point, the SWR may start to go up as you adjust C-In, but make the change anyway and redip with the inductance.
7. Continue to adjust C-In in the same direction until adjusting the inductor produces a a higher SWR than before. Inch the capacitor back to the previous setting.
8. If you cannot obtain a 1:1 SWR, reduce C-Out and repeat the process, beginning at Step 2. If you cannot acceptably minimze the SWR at some setting of C-Out, the antenna impedance is out of range of the tuner.

For Tapped-Inductor Tuners:

The only disadvantage of a tapped-inductor T-Network tuner is that its limited inductance resolution may not let you set C-Out to its maximum possible value at match. With the tapped-inductor tuner, the inductance becomes the fixed variable.

1. Set C-In and C-Out to midscale. Select an inductance switch position, and rotate the C-Out through it's range to look for an SWR dip. As before, the dip may be very slight.
2. If you don't find a dip, set the inductance switch to another position and adjust C-Out for an SWR dip.
3. When you find a dip, adjust C-In for minimum SWR.
4. Inch C-Out in one direction or the other, and redip with C-In.
5. If the SWR is lower now than it was the previous C-Out setting, continue to inch C-Out in the same direction and redip the SWR with C-In until you obtain a 1:1 SWR.

In some cases, an SWR dip can be obtained with two inductance settings. Choose the setting with the lower inductance to get the larger output capacitance.

General Information/Assumptions

This section simply contains a collection of odd bits of information and assumptions that were used to steer the project. If any of them are wrong, or misleading, I'll find out as I go, and correct them accordingly.

• Insulated wire has a lower velocity factor then bare wire. When tuning for resonance, this may cause a shortening effect of about 3% to 5%.
• Random length wire antennas can be run without ground radials (counterpoise), but some kind of ground system helps stabilize the tuning.
• A single ground rod, or group of them bonded together , is seldom as effective as a collection of random-length radial wires.
• Some magic numbers seem to crop up.
  • 1/4 Wavelength, at the lowest operating frequency. (234/Freq in Mhz = length in feet)
    For 160 Meters a 1/4 wave antenna resonant at 1.9 Mhz would be about 123'-2" long.
    For the 75 meter band a 1/4 wave antenna resonant at 3.85 would be about 60'-9" long.
  • Radiator - 87', Radials - 2 at 16' and 4 at 31'.
• The Inverted-L is a common configuration for a random length antenna. In this configuration, the antenna acts a a top loaded vertica.

Testing Equipment

I have several pieces of equipment, some home made some commercial, that I have picked up over the years, that would fit this project. In the list of equipment I am also listing the additional equipment that my friend had.

First Setup

According to the information I collected, the wire should be as long as you can get it, up to about 150 feet, but no specific length. So for the wire, I went to Radio Shack and purchased a 100 foot spool of 16 gauge stranded, insulated, hookup wire. Our first test

The N2CX HALFER

The Halfer is described on N2CX's web site and is described as a End-Fed Half-Wave Antenna for the 7 or 10 MHZ amateur bands. It is intended to provide a very simple to erect yet effective portable QRP antenna.