Z of EFHW vs. “Counterpoise” Length

We look at the magnitude of the impedance of an EFHW “antenna” for short counterpoise lengths.

Thanks to Jon AF7TS for the suggestion and discussions that led to this article.

In engineering we sometimes can not directly calculate a value, often when a “divide-by-zero” shows up, as when we try to calculate an impedance wiith a zero length element.  However we can usually still tell what that value will be by sneaking up next to it and determining what it asymptotically approaches.

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Current Flow Example for an “End-Fed” Antenna with a Loss-Less Choke

A simple transmitter, transmission line, and an “End-Fed” antenna WITHOUT a formal “Counterpoise” – We add a loss-less choke at the feedpoint, tune out it’s inductance with a capacitor, and see why common-mode current on the coax shield is the SAME as without the choke.

From Part 1 and Part 2 of the “Current Flow Fundamentals for an “End-Fed” articles, we saw that common-mode current must always flow on the coax shield when we don’t use a formal counterpoise.  And that the value of current on the “counterpoise” is identical to that on the “radiator” at the feedpoint.

In Part 3 we saw that it doesn’t matter what loss-less device(s) we put at the feedpoint – when we re-adjust our transmitter or tuner the SAME current flows common-mode on the shield as before.
choked___transformer_to_-efhw-2-1

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Current Flow Fundamentals for an “End-Fed” Antenna – part 1

Part 1 – A simple transmitter, transmission line, and an “End-Fed” antenna WITHOUT a formal “Counterpoise” – We will see why common-mode current must ALWAYS flow on the coax shield.

End-Fed Antennas have been around since the good ‘ol days and were once most popular.  Yet for some reason, much discord still exists regarding the “counterpoise” – what its behavior is, or if one is even needed.  Not all that surprising since the term “counterpoise” doesn’t seem to have a firm definition.  Hopefully, we can figure out what’s going on despite the semantics, and deal only with easy to understand basic principles.

Principles like this from basic physics:

…charge conservation is the principle that electric charge can neither be created nor destroyed.”*

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Current Flow Fundamentals for an “End-Fed” Antenna – part 3

Part 3 – We place various devices at the feedpoint of an “End-Fed” antenna without a formal “Counterpoise” – We will see why the SAME common-mode current must ALWAYS flow on the coax shield just as in Part 1.

The typical “end-fed” generally has an impedance greatly different from 50 ohms, so it is rarely fed directly with coax, as losses on the transmission line will be undesirably high for lengths of coax greater than a few 10’s of meters.

Note the high loss on several bands when a 42 ft “end-fed” is directly fed with 50 ft of RG8x coax (yellow bars)* ===>

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Current Flow Fundamentals for an “End-Fed” Antenna – part 2

Part 2 – A 40 Meter EFHW with a “Radiator” wire and various length “Coax-as-Counterpoise” wires is analyzed – We will see that the common-mode current is almost always LARGER on the coax shield some distance away from the feedpoint, which is why the “counterpoise” radiates and is called “the other half of the antenna”.

It is sometimes said that common mode current flow on the coax shield of an “End-Fed” antenna system is everywhere low,  simply because the feedpoint current is relatively low.  This is claimed even for systems with no “counterpoise”.  In this article we will see that this is not true.

…charge conservation is the principle that electric charge can neither be created nor destroyed.”*

As shown in Part 1, from this perfectly reasonable principle we now know that there are NO 1-Terminal RF power sources!

nooneterminalacsources2 Terminals – YES!            1 Terminal – NO!

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Non-Resonant 25 ft Vertical with Metal Fence Counterpoise – 7 Bands

With the interest in 43 foot untuned verticals, and some of the less than accurate claims being made, I thought I’d share the results of a 25 foot untuned vertical antenna I developed a few years ago. Hopefully, I can shed some more light on its practical use, as well as show how to gain a bit more performance.

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