I bought a
MFJ 854 RF Current meter with a snap on core as a sensor. At first I was frustrated on getting the core to open. A piece of metal with two prongs was supplied as a key and it did not work. I found that a small flat head screw driver pushed in one side and a twist will free the first side then keep it open and repeat for the other side. Read on to see how I put the MFJ 854 to work to work.. Note it can measure the current on the outside of a coax shield and it also can measure the current in an antenna element.
I have had an ongoing discussion with Bob WB4BLX on end fed antennas. The crux of the discussion is does the end fed half wave need feedline as a return for displacement current. With a short or no feedline does it act as a dipole? The discussion was lengthy so perhaps real world measurements will help. There are two excellent online resources on end-feds
W8JI and
AA5TB.
Phase A: I started with a 20 meter dipole as a baseline. I measured the coax shield current and the midpoint current on the dipole fed with 25 feet of RG 174 coax. I have that coax because it is very handy to pack for portable ops. I used my KX3 as a signal source at 5 watts. The measurement addresses coax shield current in several configurations as follows.
A-1 No balun at the dipole center and no choke balun at the rig, I got 9 ma on the shield at the rig.
A-2. No balun at the dipole center and a choke balun at the rig: I got 3 ma on the shield at the rig,
A-3. 1 to 1 balun at the dipole center and no choke balun at the rig. I got 30 ma on the shield at the rig.
A-4. 1 to 1 balun at the dipole center and a choke balun at the rig. I got 0 ma on the shield at the rig.
I pulled down the center of the dipole so I could reach it and measured 300 ma as the antenna current near the center.
Phase B: Next I took the 20 meter dipole wires and joined them together to make an end fed half wave. I used various devices to match the the end fed to the KX3 and used about one foot of coax to the rig and the half wave on the other side.
B-1. Par QRP match box the coax shield was 30 ma. The antenna current at the midpoint was 200 ma.
B-2: Balun Designs 9 to 1 balun with no counterpoise. The antenna current at midpoint was 210 ma.
B-3 Balun Designs 9 to 1 with 5 foot counterpoise on the ground. The antenna current at midpoint was 210 ma.
B-4: Balun Designs 9 to 1 balun with 1/4 wave counterpoise on ground the antenna current at mid point was 200 ma.
B-5: Balun Designs 9 to 1 balun with 1/4 wave counterpoise the antenna current at midpoint was 210 ma.
B-6: Hendrix BLT Tuner with no counterpoise the antenna current at midpoint was 200 ma.
B-7 Hendrix BLT Tuner with quarter wave counterpoise the antenna current at midpoint was 180 ma.
Note I used the KX3 internal tuner in combination with the Balun Designs 9 to 1 balun.
Phase C: I setup to use both the dipole and the Par Matchbox at the end of the 25 foot length of RG 174 I used three supports on each so they are deployed in a similar fashion.
C-1 With the Par QRP Matchbox The coax shield current was 90 ma at the rig and 100 ma about 10 feet from the rig. The antenna midpoint current was 175 ma.
C-2 With the Par QRP Matchbox and a Balun Designs 1 to 1 choke at the rig, the coax shield current went down to 40 ma. The antenna midpoint current was 250 ma.
C-3 With the no balun at the center fed dipole and a 1 to 1 choke at the rig, the coax shield current was zero the midpoint antenna current was 250 ma.
There are several observations I can gather from this:
Phase A Using a dipole without a balun or feedline choke has some shield current on the coax. No surprise. Putting a balun at the dipole center helped. However the feedline choke did the best job, I don't consider this a problem doing QRP portable. Some feedline radiation is a not an issue. Using the feedline choke is not too hard and I could go that way and still not have a balun at the dipole feed point.
Phase B. The sloping end fed halfwave was successfully tuned with all the devices I tried. Attaching a counterpoise had no or little effect on the tuning. The antenna current was down from the center fed dipole and that could indicate less efficiency. I did not try a sloping center fed dipole. I guess I should have.
Phase C: I was surprised at how much coax shield current there was. I guess this is just what W8JI and WB4BLX warned me about. However, the choke balun at the rig cut the coax shield current down and boosted the antenna midpoint current to 250 ma which was the same as the last center fed dipole antenna midpoint current measurement I made.
Summary: The coax shield current is known as common mode current. It exists because the end fed is unbalanced. This can cause feedline radiation and RF in the shack. However, for QRP portable there usually is no feedline or a very short length of coax for a vertical or sloping end fed half wave. So no problem QRP but watch out for a home installation with a long feedline. A choke at the rig may be helpful. Note there is a similar issue if you do not have a balun at the feedpoint of a center fed dipole. There are many variables on what the coax shield current will be. W8JI models many best and worst cases. There is no predicting what your luck will be but, QRP levels seem be forgiving.
As far as tuning up a end fed half wave the results seem to agree with Steve AA5TB. The counterpoise seems to make no difference. Now when I say no counterpoise, I am not ruling out that the short coax and the capacitance of the rig to ground may have performed that function. There was a possible reduction in antenna current but to be fair I would need to include a sloping center fed dipole for comparison.
Now I think I can say that an end fed half wave might behave somewhat like a center fed dipole in the best case with a short feedline. I base this on the current distribution being similar to a center fed dipole. However I would not be surprised if there is reduced efficiency. With a long feedline length it could have significant feedline radiation that would combine with the radiation of the half wave radiator. This could lead to an interesting antenna pattern but not necessarily a bad one.
See Center fed Dipole Reverse Beacon Spots at 1959 GMT and Par End Fed Half wave spots in similar configuration at 1946 GMT below:
I think we have seen that feedline current is going to occur with varying degrees with the end fed. But does an end fed half wave need the feedline to be an efficient radiator? The extreme case is an end-fed half wave deployed in a rocket for a beacon. There is no feedline and the ground is at 1500 feet away I know it works see my old posts but is it less efficient. If you are an antenna expert let me know what you think.
Greg N4KGL
N4KGL
