I have noted recently the increasing popularity of the so-called End Fed Half Wave antenna, though the term often includes harmonic operation of the antenna.
It seems that at the heart of common ham understanding of this antenna system is that some kind of two terminal feed device creates a scenario with current on the nominal radiator, and zero common mode current on the feed line. If that feed device is small, its contents bears little influence on the current distribution on the feed line and radiator (the device behaviour approaches that of a simple circuit node).
Above is the kind of current distribution envisaged by many. The equivalent source is shown at the end fed feed point The red curve is the magnitude of current, the horizontal line represents the nominal radiator, and the vertical line represents the common mode conductor formed by the feed line. The feed line is often of arbitrary length, arbitrary route, and it may connect to real ground via an arbitrary impedance. Pretty much everything about this antenna system is random save the length of the nominal radiator.
Note that the current on the radiator adjacent to the source is low relative to the current maximum… but it is not zero, it cannot be zero if there is any transfer of power to the radiator for radiation.
It is true therefore that the current on the other side of the source cannot be zero. Where current flows from the source into the nominal radiator, there must be an equal current flowing into the other source terminal.
The article End fed Zepp showed the current distribution that exists for some different common feed arrangements.
The answer proposed by some is a three terminal feed device, and an additional short conductor to act as a so-called counterpoise (Duffy 2010). The three terminal feed device and counterpoise are accorded magic properties in that they are believed to ensure the current distribution shown below.
The proponents invariably do a lot of hand waving in explaining the perceived operation of this antenna, but do not support their claims with measurement of common mode current on the feed line relative to the nominal radiator.
(Yates 2010) gives the details above of a three terminal feed device. For the purpose of this discussion, the coax is a single common mode conductor.
The behaviour of a small three terminal device is a little more complex than the small two terminal device, but if it is small, the sum of the currents into the device is zero.
To achieve near zero common mode current, the common mode impedance of the tuned transformer must be extremely high… no small challenge. Remember that this transformer is deployed very near the charge maximum in the antenna system, and there will be a very large voltage difference between primary and secondary windings to drive common mode current. Yates states:
Some hams have had great success with this antenna with no feed line imbalance problems (common mode currents) while others have had very bad “RF in the shack” problems due to the feed line radiating as much as the antenna. If you adjust your coupler into a resistive load on the bench first and then adjust the antenna for a proper match then you should have a resistive match that will minimize current through the coupler and through the “counterpoise”.
So, he says it works for some and not for others… but proposes that tuning for a resistive match inside the feed line somehow eliminates common mode current on the outside. This later claim is made without detailed explanation or any current measurements to support his claims, and seems naive. The flawed notion that common mode current on coax is caused by interior (ie differential) mismatch is part of ham lore.
If these so-called solution designs work for some and not for others, it suggests that they are incomplete solutions, ie that they are bad designs.
- Duffy, O. Oct 2010. Counterpoise. http://owenduffy.net/files/Counterpoise.pdf.
- Yates, S. Jan 2010. The end fed half wave antenna. http://www.aa5tb.com/efha.html (accessed 08/09/14).