SKCC 5123T----- FISTS 14979----- Flying Pigs 2331----- NAQCC 3610-----QRP ARCI 14176-----Polar Bear 257

Thursday, November 15, 2012

Dancing in the Tar Pit

It took a while but I am letting go of a misconception I had on what I called an end fed dipole. I am now going to call it an end fed half wave monopole. A big part of the misconception was since the current distribution on the end fed half wave monopole is similar to a dipole, I assumed that there was displacement current between the two quarter waves of the half wave. Well not the case. Rather there is displacement current required for an RF return to a ground plane to complete the circuit as in any other monopole. BTW,  I am still looking for a picture of displacement current for the end fed half wave monopole. Note the maximum current in the wire for the end fed half wave is at the center instead of the bottom as for the quarter wave monopole.

However, the curious and controversial fact is to resonate the end fed half wave monopole there needs to be very little current at the feed point. The end of the antenna is a high impedance point. It must be voltage fed as opposed to current fed for a quarter wave monopole.. The return current (enough) can be satisfied with a short counterpoise per AA5TB and Moxon. Note, I am assuming a tuner similar to the one described by AA5TB. There are two sides of the output circuit one for the antenna and one for the counterpoise. The counterpoise could optionally be connected to the coax shield. This would mean that the short run of coax to the rig is the counterpoise in the QRP portable case. I have measured the outside shield current and it is indeed low for a short length of coax to the rig. I have also tried adding a longer counterpoise and it did not affect tuning or the current in the half wave antenna.

UPDATE: After exhaustive Google searching I found Verticals without Vertigo by W4RNL. Refer to section 5 and figures 10, 11 and 12. The models show that radials at the base of an end fed half wave vertical will have very little effect. The parallel resonant circuit in figure 10 requires a ground or a counterpoise. If you tap the coil the coax shield will be the common and act as a counterpoise. The short counterpoise (coax/ rig) provides enough of a return for displacement currents to satisfy the small current required. The rig is usually not grounded. This works fine for QRP.

W8JI discusses at length feed line currents and the possible downside for a home installation of an end fed  Note, that a long feed line is not part of the AA5TB discussion as it is oriented to QRP portable. It seems to me that the low current at the feed point could become part of a standing wave going back to the rig on the outside of the shield. The current would be high at various points and would become a radiator causing pattern distortion. Likewise, you could encounter a high voltage point and get coupling to yourself or other devices in the shack. If I was installing a home QRO antenna I would lean toward a balanced antenna if possible.

UPDATE: The W5ALT discussion on RF ground is also instructive. If you do not satisfy the imbalance of an antenna there will be a potential between the rig and ground. This will result in current in whatever ground you provide or if none is provided it may use your body as that path to ground.. Ouch!

I call this territory the tar pit. Just Google end fed half wave for discussion in the forums. Even the masters disagree. So we all have tar on our feet. The bottom line is that an end fed half wave monopole is a popular choice for QRP folks. It is easy to deploy and when you are making contacts at 5 watts it is a good enough antenna in my book. So I have stated my understanding at this point but I am open to further enlightenment in the future.

Note a nice end fed half wave monopole tuner is available from Hendricks QRP Kits.