Measuring End Fed Half Wave and Dipole Currents

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:

WZ7I	N4KGL	14067.5	CW CQ	24 dB	15 wpm	2005z 04 Nov
NY3A	N4KGL	14067.5	CW CQ	22 dB	15 wpm	1959z 04 Nov
K1TTT	N4KGL	14067.6	CW CQ	12 dB	15 wpm	1959z 04 Nov
W4KKN	N4KGL	14067.6	CW CQ	5 dB	15 wpm	1959z 04 Nov
W3OA	N4KGL	14067.5	CW CQ	21 dB	15 wpm	1959z 04 Nov
AA4VV	N4KGL	14067.5	CW CQ	27 dB	15 wpm	1959z 04 Nov
K3MM	N4KGL	14067.6	CW CQ	15 dB	15 wpm	1959z 04 Nov
KQ8M	N4KGL	14067.5	CW CQ	5 dB	15 wpm	1946z 04 Nov
KB9AMG	N4KGL	14067.5	CW CQ	16 dB	15 wpm	1946z 04 Nov
W4KKN	N4KGL	14067.6	CW CQ	8 dB	15 wpm	1946z 04 Nov
K8ND	N4KGL	14067.5	CW CQ	5 dB	15 wpm	1946z 04 Nov
VE2WU	N4KGL	14067.5	CW CQ	15 dB	15 wpm	1946z 04 Nov
W3OA	N4KGL	14067.5	CW CQ	26 dB	15 wpm	1946z 04 Nov
AA4VV	N4KGL	14067.5	CW CQ	23 dB	15 wpm	1946z 04 Nov
K1TTT	N4KGL	14067.6	CW CQ	21 dB	15 wpm	1946z 04 Nov

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

Happiness is a warm tube

I had a nice time at the October 27th Headland, AL tailgate. My best find was a one tube code practice oscillator. I like having something that uses a tube for nostalgic reasons.. In addition, this CPO works great  and will come in handy teaching code at our club. It was designed by Dave Schamarder. His web site is Dave's Homemade Radios. The schematic can be seen at this link. I purchased the CPO from Glenn Addison K5WP. He scrounged the parts and built it himself. The tube is a Type 43. Glen got the tube from  www.tubesandmore.com.

I also set up portable at the tailgate which was on the town square. I used the KX3 and the Alexloop and made a few contacts. There were a number of visitors checking out the KX3. I was caught being under dressed for the cool weather in shorts and a T shirt. Harold KF4YDO bailed me out with a light jacket. In addition to Harold, Don KK4DWC and Phil KK4DWI attended from Panama City. All the Wiregrass Club member made us feel welcome.

Greg N4KGL

Alexloop gives solid QRP performance

On Sunday, I setup portable in my Brother-in-law's backyard in Columbus GA. The rig was the KX3 at 5 watts to the Alexloop Walkham antenna on a camera tripod. The ARCI Fall QSO Party and the SKCC Weekend Sprint provided a great opportunity to test this setup. Using the hunt and pounce method, I made eighteen contacts on 20 meters and eight on 15 meters. This included F6HKA in France and V31JP in Belize on 15 meters. This is my best outing with the Alexloop yet. Everyone I called replied. It is amazing that the most convenient antenna to deploy can work so well when conditions allow. I am enjoying the KX3 with the Alexloop. They make a great portable combination.

20 meters: CA, VA, CT, AR(2), TX, MO(3), MN, FL, WA, NY, MI, WI, IL, MA, MD
15 meters FRA, NH, CA, SD, MO, BIZ, AZ, WA

Alabama Lighthouse Ops October 2012

A sunny October day with a blue sky and a breeze sitting on the porch operating a KX3 with some good ham buddies is just about as good as it gets. Thanks to Craig NM4T and Huntsville team for letting me join in. This was the Alabama Lighthouse event which involved three lighthouses. The goal was to bring awareness of the Alabama lighthouses via the airwaves. So this allowed time to rag chew vice contest ops.We were at Fort Morgan which has the Mobile Point Lighthouse. The Sand Island Lighthouse was in sight from Fort Morgan. The Middle Bay Lighthouse was not in our sight but they had a team there also.

There were two operating positions at the lighthouse keepers house; first a travel trailer with the Icom 756 Pro and second a side building with an Elecraft K3. My KX3 was setup on the porch.  The team had a Alpha Delta multi-band inverted vee supported by a fiber glass pole and numerous end fed half wave antennas also supported by fiberglass poles. Jerry Brown N4EO showed me his tuners for the end feds. They were high power versions of what I have used. He has used them successfully from Hawaii and Iraq.

Craig and I setup my off center fed dipole off the truck mast with fiberglass poles on the ends. Craig and I had success withe the KX3 on 20 and 17 meters. Jerry made 23 contacts on 15 meters with the KX3 in the California QSO Party. He switched to an end fed halfwave to get a better signal into CA. Jerry had a good tip for me. The KX3 was getting clicks from the 100 watt K3 on 20 meters when he was on 15 meters. Switching the KX3 preamp off took care of the clicks. 

This was a long day trip for me from Panama City to Fort Morgan but definitely worth it. Everyone was very friendly. I have a few more Huntsville area friends now including Jerry N4EO, Jamie K4OEY and Ed N4EDT. I wound up the visit with dinner with the group. I am looking forward to a possible return next year and any portable opportunity that Craig NM4T might cook up!

See all photos here.

Sand Island Lighthouse

Lighthouse Keepers House

End fed half wave antennas

N4EO's end fed half wave tuner

Another one of N4EO's end fed half wave tuners

Craig NM4T operating KX3 from porch

K3 operator

Tom KG4CUY operating SSB

You Made My Day AK4LP

I was testing a 20 meter dipole with an Electronics USA MK-67 BNC center insulator and 25 feet of RG 174. The dipole was supported by crappie poles in the front yard. The rig is the KX3 at 5 watts. So does the new antenna work? Well good enough, as I had a short but sweet QSL with Bob AK4LP in Forest VA. The cool thing was he was on a Youkits HB-1B and an Alexloop. The only thing that could be better would be Alexloop on both ends. I have both a HB-1B and and Alexloop as well. That was a good QRP to QRP QSO! Then I was lucky to work SM7ZDI Vic in Sweden. He had the patience to pull me out.

I have setup a shade for the KX3 as you can see below. I can handle sunburn but I am uncomfortable about the sun heating the black case of the KX3. The 20 meter dipole is a light weight option to pack and I guess the loss is not too bad with RG174. I checked with an online calculator and the loss is about .8 watt at 14 MHz and 1.5 SWR.

Here are some Reverse Beacon Spots for 20 meter dipole at about 18 feet high at 5 watts and 25 ft of RG174 feedline.

de	dx	freq	cq/dx	snr	speed	time
KQ8M	N4KGL	14057.5	CQ	3 dB	13 wpm	2213z 23 Sep
KM3T	N4KGL	14057.5	CQ	16 dB	13 wpm	2213z 23 Sep
DL1EMY	N4KGL	14057.5	CQ	11 dB	13 wpm	2212z 23 Sep
AA4VV	N4KGL	14057.5	CQ	34 dB	13 wpm	2212z 23 Sep
K8ND	N4KGL	14057.5	CQ	11 dB	13 wpm	2212z 23 Sep
W3OA	N4KGL	14057.5	CQ	30 dB	13 wpm	2212z 23 Sep
K1TTT	N4KGL	14057.6	CQ	15 dB	13 wpm	2212z 23 Sep
VE2WU	N4KGL	14057.5	CQ	23 dB	13 wpm	2212z 23 Sep
WZ7I	N4KGL	14057.5	CQ	20 dB	13 wpm	2212z 23 Sep
WA7LNW	N4KGL	14057.5	CQ	7 dB	13 wpm	2212z 23 Sep
W3LPL	N4KGL	14057.5	CQ	5 dB	13 wpm	2212z 23 Sep
K3MM	N4KGL	14057.5	CQ	22 dB	13 wpm	2212z 23 Sep
DL1EMY	N4KGL	14057.0	CQ	8 dB	14 wpm	2112z 23 Sep