Experiments with and end-fed half-wave antenna

An end fed half-wave antenna would seem the ideal solution for portable operation. In comparison with the dipole it does not need so much feeder and should be simple to set up. It should be possible to feed it from the operating position, though it is noted the feed point needs to be raised from the ground to reduce losses. If tall trees are readily available, and it’s possible to throw the antenna into the tree or climb with it, then there is no need to carry a mast.

A great promise of the half wave is that it needs less of a counterpoise than a quarter wave because RF current through the counterpoise is small into the high impedance. Counterpoise systems for 1/4 wave antennas can be huge and time consuming to set up portable, requiring multiple resonant radials.

The down side is that the voltage at the antenna connection can be huge. If I was delivering 10W to a 5kΩ load then the terminals would have had 220V across them – UK mains but at RF! Care must be taken to avoid contact with the feed point either directly by touching it, or indirectly through conduction back to the radio along the feeder. My antenna should mitigate the latter risk by having no common ground path between radio and antenna, and using a 0.2λ counterpoise. I’d assume that these voltages also exist at the ends of a centre fed dipole, so don’t change links on a linked dipole while transmitting!

Testing with the analyser

I made the tuning circuit using parts available to hand. I tried a couple of different capacitors in parallel with my 150pF variable capacitor. Results here were unexpected. I wonder if this is because the capacitors were not good for RF. One small bead capacitor worked well and allowed me to tune the circuit anywhere between 11MHz and 30MHz. Larger capacitors gave unexpected high results.

Testing with the small bead capacitor in parallel. The resistors present 5kΩ to simulate a resonant end-fed antenna. The two markers on the analyser disiplay at 11.6MHz and 29.2MHz show the tuning range of this circuit. As can be seen it has quite high Q

I chose to target the 20m band, and used the variable capacitor without a padding capacitor. The bandwidth allowed me to cover half of the band, so I could set it up for CW or SSB. During my experiments with the radio I used the analyser to retune it when changing mode and area of band. I wonder if a production version would work well with a smaller trimmer capacitor and a padding capacitor to enable larger input for the required change.

Response with the dummy resistor load covering most of the SSB portion of the band.

Outside the operating area the impedance will be low. The circuit will appear as a short circuit to the antenna so preventing operation outside the tuned area. This would also make it important to check before operating that it was tuned.

I asked myself why the tuned circuit? Why not just a transformer? I guess stray capacitance, especially on my loosely wound transformer, makes anything a tuned circuit. Removing the capacitor from the circuit produced completely unusable results.

Another question that I have not answered is what if I lower Q and make it more broadband by adding series resistance? I guess that will turn into loss due to the circulating currents in the tuned circuit.

Testing with the radio and antenna

An inverted L antenna was set up using my 6m mast. This gives 3 or 4 meters of wire running horizontally from the top. A 10m mast would be great for this! The counterpoise was 2m long laid on the floor. The matching circuit was placed on a low table so as to lift it off the floor and reduce losses.

Antenna as an inverted L. The top goes towards the tree on the right.

This would really seem a poor place to put an antenna, but results were good. Tuning covered the same half of the band. I was able to adjust between the CW and SSB portions using the analyser.

The KX3 reported a match of 1.2:1 with its internal tuner bypassed. The internal tuner allowed me to use parts of the band where impedance was starting to drop and SWR was rising.

These two should both be really fun radios.
Note that the counterpoise is connected to the shaft, not the antenna. Though I wouldn’t touch it while transmitting either way.

Reception seemed noisy, with notable impulse noise in the urban environment. Signals came through though – some contest with a couple of stations calling CQ for general contacts in SSB (voice) mode. I responded to OM3TWM in Slovakia, a distance of about 3,500 miles, and received a signal report of strength “9+10dB”. This is a very strong signal though the receiving station has very good antennas. My transmit power was 10W peak.

I’m not currently able to transmit Morse, having no key at the moment. The MTR3B received the strong signals I tuned to very well. It looks like it would be a fun radio for situations where the KX3 feels too expensive to have with me or I want something truly pocketable. In the picture above I’d tuned to the station using the MTR, then switched to the KX3 to check I had the right frequency without the tuning aids that the KX3 has. The side tone frequency on the MTR-3B does not match the receive frequency as warned in the manual, so tuning is by signal strength and getting used to what to listen for. I may raise the side tone frequency of the KX3 to make both radios similar.

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1 Response to Experiments with and end-fed half-wave antenna

  1. Pingback: Experimentos con una antena de media onda alimentada por el extremo – Puerto Rico Amateur Radio League

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