EFHW Antenna: A Detailed Analysis (NEC4.2)

I ran Rich's model, slightly modified, with the EZNEC Pro/4 NEC-4.2 engine. The modification was to use a current rather than voltage source and to set the feedpoint current at 1 amp. Here's what the current magnitudes look like. Left side of the chart (segment 1) is the far end of the horizontal wire, right side (segment 52) is the bottom of the buried ground rod.



Current at the top of the ground rod is roughly twice that of the feedpoint.

AutoEZ model is in the attached zip.

Dan, AC6LA

 

There's the thing, the outside of the coax has to carry current, or there is no "other half of the antenna ", and therefore no radiation.

If the usual balun, or often recommended "isolator" actually worked like the salesmen would like you to think, the same thing would happen, there would be no current on the outside of the coax, and no radiation.

In practice, the coax shield, and power nuetral and ground wires are connected together in the transmitter, assuming perfect coax,balun, and isolator, using a wire, of any length, connected to the center conductor of the the coax, the RF will use the power nuetral and ground wires as 'the second half of the antenna" on the same idea, any wire connected to the rig may actually operate as "the second half", it's a random crapshoot, explaining why one station swears by his "end fed", and another swears at his

Rege

 

Well stated.

I've used end feds with some short (random length) counterpoise for portable ops and it definitely got me on the air with a simple, easy to deploy antenna. I've also tried them at home with higher power levels and dealt with the typical RFI issues. In all cases I made contacts with them and agree with the folks that say they can and do work but I totally agree with your point about randomness of the RF return path and how some can love them and others can hate them, especially as transmit power rises.

 

Dan,
thanks for posting this. I took your .weq file, and ran it with the Nec2D engine. Since Nec2 is not able to model the ground rod, I could only run your file without wire 3 (the subterranean rod), but with the bottom of the simulated coax shield (wire 2) connected to Mininec ground, everything else being the same. Here is the plot of current in the two wires:


The currents using Nec2 with the Mininec ground plane are about 10% higher than with Nec4 and its model of the ground rod.

If you think that Nec4 is correctly simulating the ground rod, that would mean that the ground-rod has a finite impedance, as compared to Nec2 with Mininec, which thinks that the down-wire connects to the ground-plane with zero resistance.

It occurs to me that all I would have to do to make the plots line up is to put an impedance in series with the down-wire right where it would otherwise connect to the top of the ground-rod. Here is my best fit with a "load" of 400 + j250 in the last segment of the down-wire before it connects to the Mininec ground:



The blue trace is down-wire connected to Mininec ground; Red trace is with 400+j250 in the bottom segment of the downwire. Conclusion, Nec4 must be calculating a more realistic effective impedance of the ground-rod. Nec2 comes up with almost the same current distribution on all the wire segments provided the effective impedance of the ground-rod is inserted in the last wire segment.

I ran the model one more time, this time specifying that the input power is 100W, to see what is the predicted magnitude of the currents in the wire segments:



Note that the highest current in the horizontal wire is 1.037A, while the current into the earth is 391mA

The tapering current in segment 36 of the horizontal wire was bothering me, so I noticed that in Dan's model, the simulated coax jacket down-wire connects one segment from the end of the horizontal wire, so I moved it to the very end. To make the sim more realistic, the coax shield should be modeled as a 13mm dia wire (0.5" coax) instead of 4mm. So here is the current distribution after these changes:




It would be instructive if Dan could run the modified model with Nec4. Remove the 400+j250 load and replace it with the ground rod...

 

The current is small at the feedpoint on both the counterpoise and "radiator" as you say. However just exactly like the intended radiator itself, the counterpoise ALSO has a standing wave, the max magnitude of which depends upon it's length and termination.

Here are the results of a sim of a 40 M EFHW as a function of various coax shield lengths, with shield connected to ground. From "Current Flow Fundamentals for an “End-Fed” Antenna – part 2" ===> http://vtenn.com/Blog/?p=147

A good case where common mode current is somewhat low ===>


A "sick" case where the common mode current max is greater than that of the intended radiator ===>


A chart summarizing various lengths of coax (the height of the antenna) is below ---- "I max R" is the peak value of the current on the grounded coax shield (from the EZNec currents table) , "I max L " is the intended radiator peak current, and dB compares peak value on coax shield (I max R) to peak value on intended "radiator" (I max L).

Note that for a coax length to ground of around 37 feet, current at the ground rod connection is about 3 or 4 dB HIGHER than that in the middle of the intended "radiator".

Well, not exactly - unless a formal counterpoise wire is also used that has a much lower Z than that of the common mode choke.

Putting a choke on a formal-counterpoise-less EFHW simply raises the feedpoint Z by the value of the choke's Z. And of course, the current is STILL the same relative to the intended "radiator" as before the choke was added.

So YES - you are right common-mode coax shield current *can * be controlled by a choke, but only if a formal counterpoise wire (or some other counterpoise scheme) is used....


(By "formal counterpoise" I mean an actual wire counterpoise as opposed to simple using the coax shield as the counterpoise).

 

I think this right here is a lot of the confusion that exists in the EFHW discussions. The commercially-available "end-fed halfwave" antennas, and the one in particular that some appear to want to discredit, are really off-center fed at ~98%. The counterpoise exists as part of the 49:1 transformer assembly. They are end-fed in the sense that there's the coax feedline, the transformer box and the one single wire. The "other half" is in the box.

 

Hence the highly variable reports of common mode issues in practice.

 

Good point. Actually, the common mode issues are well known and somewhat predictable to an extent, but not always by computer modeling. It's partly the issue of an "isotropic environment" and an "antenna in free space". And I don't know of a wire antenna design in which the real RF ground is more important to a successful implementation than the end-fed. Or whatever one calls it- there does not even seem to be an agreement here on the definition.

 

Why not use RBN to compare antennas and see how they do in the real world?

Takes a couple of minutes.

 

Yes. But there are folks who have no problems with them, often QRP. Or they have a counterpoise/ choke situation that works, or radials, and/or the feedline length happens to put the radio at or near a voltage node for the bands they use.