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The "ground" has several effects on antennas, most of them negative (see e.g. Why does an HF "monopole" need dozens and dozens of radials while a VHF/UHF "ground plane" antenna only needs four?). In most cases the further away the whole antenna system can be moved from the actual (physical earth/soil) ground level the better its radiation efficiency and pattern becomes. Yet many amateurs are also led to believe that a "good ground" is something to strive for, not just for the actual electrical safety and lightning protection reasons.

For me this comes to a fine point in the "random wire" and also "end-fed half-wave" antenna guidance. These antennas — at least supposedly — involve feeding only one element at its extrema, using a transformer "matchbox" to compensate for the impedance at that otherwise unlikely feedpoint. But the question then becomes: what to do with the "other side" of the coax, or rather the "unused" side of the transformer windings.

The typical stipulation is that one "needs" some sort of counterpoise (which I'll note is often labeled: "optional"). There's all sorts of diagrams, some involving a second wire as long as and placed in parallel the main antenna, some attaching an arbitrary collection of scrap wire, others suggesting a short lead to a long ground rod being sufficient — and most admitting both that the counterpoise is something of a "stretch goal" and that RF currents will end up on the coax shield regardless!

Antenna and radio system diagram, showing "Feed Choke" placed deliberately near the transceiver rather than the antenna

What was most confusing to me is Palomar Engineer's End Fed Antenna guidance which very explicitly admits to this "common mode current" problem but yet also intentionally encourages the user to put the choke as close to the radio as possible.

The clear intent is for some sort of counterpoise to take up RF current on its own, but what's not clear is why this is necessary. What is clear is that it doesn't seem very desirable, since despite the pretty diagrams this seems to be a classic case of "allowing the coax to become part of the antenna". My reading of Cebik (W4RNL/SK)'s Counterpoise? On the Use and Abuse of a Word (PDF) article confirms my suspicion here but does not propose any particular remedy.

Is it actually possible to "end feed" an antenna? What if I abandon the advice to let my coax be part of the antenna, and put a common mode choke right at the matchbox? This would hypothetically confine the Actual Antenna Part to just the Intended Antenna Part — but then what becomes of the other wire coming out of the transformer? I don't want to add any other elements to my antenna, yet folk wisdom says I need some sort of something to "pull electrons from" attached to the counterpoise lug! Would a capacitor or some sort of resonant circuit inside a shield or even just a volumetric chunk of low-resistance metal be useful here?

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  • $\begingroup$ I found aa5tb.com/efha.html which includes some discussion of this, at least in the context of an EFHW. Still making sure I understand it (and why it uses a variable capacitor in the matchbox, etc.) and whether it applies to non-resonant long wire antennas as well. $\endgroup$ May 14 at 17:03
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    $\begingroup$ Try the experiment of moving the choke to right next to the feedpoint, and compare the impedance you get on different bands. $\endgroup$ May 17 at 2:00

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An end fed isn’t really an end fed, but a highly off center fed, off center fed dipole. The place where a 9:1 or 49:1 transformer balun matches the off center impedance isn’t at the very end, but close to the end. Thus the need for a short counterpoise (so 5 to 15% of the antenna is on the other side of the transformer/balun) to optimize the impedance match.

A choke near the antenna helps reduce EM pickup to the feed line from not being centered in the voltage minimum EM field at the center of a dipole, but near a very high voltage node at the end of any near resonant dipole. The high voltage can capacitively couple to any nearby off center feed line (even a disconnected one!), whether you like it or not. A choke presents an impedance to this voltage so coupled to the coax shield.

Some put the choke a few percent of lambda down the feed line to create a proper length stub for a closer impedance match to the off center ratio of the dipole.

The capacitive coupling around the balun or choke can also try to act as more counterpoise. An antenna simulation including the coax shield is the best way to analyze these parasitics.

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  • $\begingroup$ This is a helpful start, even though it reinforces what I think there's another question brewing in my mind that I should research and/or ask separately here about OCF dipoles. So basically rather than trying to fight some sort of lossy/pattern-skewing counterpoise running along the ground, I could just embrace it as ± part of a dipole and try get it up high, etc. like I would any other antenna element. $\endgroup$ May 16 at 20:47
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Nate.

This isn't as complicated as it seems.

Replace the radio with a battery and the antenna with a resistor. Noting that circuit theory and wave theory are two different things, with battery at one end and antenna at the other end, current flows in the braid. If you disconnect the braid and use the earth as the ground, then the current flows through the earth providing it's conductive enough. If there is no braid or ground then there is no circuit and no current will flow from battery to resistor and the system stops working.

Transmitters always have a two terminal output, and there is always a ground against which the various circuits inside the transmitter operate against. The live or hot output of a transmitter always has a negative connection which has the reverse current of the live output, this being either ground or one side of a balanced output.

If you remove the coax braid connection at the antenna end, then due to skin effect, the negative current from the transmitter will simply flow on the outside of the coax, and then the outside of the braid of the coax becomes part of the antenna.

If you use coax, but don't connect the braid at all, then there is no transmission line and the antenna becomes the center of the coax starting at the radio, and then connecting to the the antenna element at the end of the coax. Then the negative connection of the antenna is the earth of the radio at the radio itself rather than at the antenna.

Everyone knows that if you just connect a piece of wire to your radio, it will work for both transmit and receive, sometimes suprisingly well. With no counterpoise or antenna earth connection, you are just providing the radio with an antenna ground connection at the radio itself instead of at the antenna.

Incidentally, if the radio isn't grounded at all, then you have a tiny antenna negative element comprising the metal in the radio's cabinet, maybe with some capacitance to real ground.

In terms of what happens to the differential RF signal emanating from your transmitter when there is no counterpoise or antenna earth, the impedance will be all messed up, the radiation resistance won't be very good and the SWR will be bad, the antenna probably won't be very efficient and the radiation pattern might be a bit weird, and you also will probably get RF feedback when transmitting, but it will still work at least a bit.

Hope that helps !

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