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

Monday, November 19, 2012

KX3 Battery Ops

Saturday, I installed the Elecraft internal battery charger option in the KX3. I put in eight AA Eneloop XX NIMH cells. I waited for the charger for battery operation because with the internal charger I do not need to open the case to replace the batteries until they wear out. The charger is working well. It also provides a real time clock that I have set for GMT.

Of course battery operation and QRP go together. The KX3 has a high efficiency mode at 3 watts. So I wanted to try some 3 watt operation. I was in Dothan Sunday and I did some reverse beacon tests. My antenna there is a off center fed dipole. It did pretty good at 3 watts. See the spots below.

Then I wanted to try some QSOs. First was a QSO to Alaska with KL7IDA on 28.050. This is what I call Made My Day QSO! Then I worked KW7D in New Mexico on 17 meters. And last K3WWP John in PA on 14.061. John is a well known QRPer and he was using a KX3 to an attic dipole.

BTW, I don't see any problem charging the batteries via my solar panel. It looks like I can operate off the solar panel as well. If the panel dips the batteries take over. I have a DC to DC converter that will regulate the voltage. I also can use it to boost a source that is less than to 13.8 volts up to the 13.8 volts needed to charge the KX3 internal batteries.

NC7J N4KGL28061.9CW CQ15 dB13 wpm2045z 18 Nov
WA7LNW N4KGL28062.0CW CQ27 dB13 wpm2045z 18 Nov
N0TA N4KGL24906.1CW CQ18 dB13 wpm2044z 18 Nov
PJ2T N4KGL24906.0CW CQ14 dB13 wpm2043z 18 Nov
WA7LNW N4KGL24906.0CW CQ17 dB13 wpm2043z 18 Nov
PJ2T N4KGL21062.0CW CQ3 dB13 wpm2042z 18 Nov
KQ8M N4KGL18096.1CW CQ19 dB13 wpm2040z 18 Nov
W3LPL N4KGL18096.1CW CQ23 dB13 wpm2039z 18 Nov
KQ8M N4KGL18096.1CW CQ19 dB13 wpm2039z 18 Nov
AA4VV N4KGL14062.0CW CQ27 dB14 wpm2038z 18 Nov
N4ZR N4KGL14062.0CW CQ10 dB13 wpm2038z 18 Nov
VE2WU N4KGL14062.0CW CQ12 dB13 wpm2037z 18 Nov
KM3T N4KGL14062.0CW CQ17 dB13 wpm2037z 18 Nov
NY3A N4KGL14062.0CW CQ14 dB13 wpm2037z 18 Nov
W3LPL N4KGL10107.0CW CQ15 dB13 wpm2036z 18 Nov
AA4VV N4KGL10107.0CW CQ22 dB13 wpm2035z 18 Nov
AA4VV N4KGL7055.0CW CQ15 dB13 wpm2032z 18 Nov
W4KKN N4KGL7032.7CW CQ4 dB15 wpm1850z 17 Nov
NY3A N4KGL7032.7CW CQ5 dB15 wpm1850z 17 Nov

The tall pines come in handy for the OCF in Dothan. I use a Balun Desighns 4 to 1 the feed point.

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.

Saturday, November 10, 2012

KX3 Pedestrian Mobile

Today, I used the Elecraft KX3 for pedestrian mobile. The outfitting included the KX3 in a LowePro bag with the lid cut off. I used the belt loops of the bag plus the strap over my shoulder. On my back was an alice pack frame. The frame had a 10 Ah  LiFePO4 battery tied to it which is very light. The frame also had a lap desk plastic sheet with the pillow removed strapped to it. Which in turn had a 13 foot crappie pole strapped to it.

My target band was 17 meters. The antenna was 13 foot of wire taped to the pole and a 9 to 1 balun with two foot of coax to the rig. I kind of knew better but I went with no drag wire. The rig tuned via internal tuner and the spots were good. However, a couple of times I had excessive tuning. It think I'll hang something off the counterpoise terminal next time. The rig and my body were most likely the other half of the antenna. However, the antenna performed better than a buddistick/drag wire arrangement I tested.

As you can see from the pictures it was a beautiful day and and I joined members of the Panama City ARC at a local park beside the bay for portable ops. I worked Pat F5MFO near Paris on CW and PJ2/DL5MFL Curacao on SSB. This was followed by another SSB contact with Bob K0WOJ in Missouri. Bob gave me an S8 and it was a good conversation.

Certainly the size, weight, internal tuner and multiple modes make the KX3 ideal for pedestrian mobile. Likewise for the LiFePO4 battery.

Post Outing Analysis: I found that hanging a 13 foot wire off the counterpoise terminal of the 9 to 1 balun is a good thing. There is about the same on the antenna side. The 9 to 1 balun tends to flatten SWR to a range the KX3 tuner can handle on 40 and up. So I will use that setup the next PM Outing..

WZ7I N4KGL18084.4CW CQ19 dB14 wpm1553z 10 Nov
N0TA N4KGL18084.4CW CQ14 dB15 wpm1553z 10 Nov
WA7LNW N4KGL18084.4CW CQ22 dB15 wpm1553z 10 Nov
W3LPL N4KGL18084.4CW CQ14 dB15 wpm1553z 10 Nov
N7TR N4KGL18084.4CW CQ9 dB15 wpm1553z 10 Nov
WA7LNW N4KGL18080.8CW CQ11 dB13 wpm1528z 10 Nov
PJ2T N4KGL18080.8CW CQ6 dB13 wpm1528z 10 Nov
KQ8M N4KGL18080.9CW CQ17 dB13 wpm1528z 10 Nov
WZ7I N4KGL18080.8CW CQ20 dB13 wpm1528z 10 Nov
KQ8M-98 N4KGL18080.9CW CQ9 dB13 wpm1528z 10 Nov
W3LPL N4KGL18080.9CW CQ19 dB13 wpm1528z 10 Nov

Sunday, November 4, 2012

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 N4KGL14067.5CW CQ24 dB15 wpm2005z 04 Nov
NY3A N4KGL14067.5CW CQ22 dB15 wpm1959z 04 Nov
K1TTT N4KGL14067.6CW CQ12 dB15 wpm1959z 04 Nov
W4KKN N4KGL14067.6CW CQ5 dB15 wpm1959z 04 Nov
W3OA N4KGL14067.5CW CQ21 dB15 wpm1959z 04 Nov
AA4VV N4KGL14067.5CW CQ27 dB15 wpm1959z 04 Nov
K3MM N4KGL14067.6CW CQ15 dB15 wpm1959z 04 Nov
KQ8M N4KGL14067.5CW CQ5 dB15 wpm1946z 04 Nov
KB9AMG N4KGL14067.5CW CQ16 dB15 wpm1946z 04 Nov
W4KKN N4KGL14067.6CW CQ8 dB15 wpm1946z 04 Nov
K8ND N4KGL14067.5CW CQ5 dB15 wpm1946z 04 Nov
VE2WU N4KGL14067.5CW CQ15 dB15 wpm1946z 04 Nov
W3OA N4KGL14067.5CW CQ26 dB15 wpm1946z 04 Nov
AA4VV N4KGL14067.5CW CQ23 dB15 wpm1946z 04 Nov
K1TTT N4KGL14067.6CW CQ21 dB15 wpm1946z 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