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.

sqrt-sum-of-squares-4 Continue reading Z of EFHW vs. “Counterpoise” Length

3,297 total views, 1 views today

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

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

2,782 total views, no views today

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.”*

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

3,344 total views, 2 views today

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)* ===>

cableloss_unun-vs_direct_feed Continue reading Current Flow Fundamentals for an “End-Fed” Antenna – part 3

3,037 total views, 1 views today