It’s been a little over one year since SOTA started in Australia, first with VK3 in February 2012. To celebrate, the Morrabin and District Radio Club agreed to host a gathering for us. We will do this close to the actual anniversary at the start of February in future, but this year it turned out to integrate with their usual Saturday morning meeting towards the end of the month.
Ron VK3AFW gave a demonstration of various antennas and activation options to a large gathering that was close to filling the club’s meeting room. It was a great turnout with most of our VK3 activators present, along with Andrew VK1NAM who was in Melbourne for family reasons. He was able to join us for most of the meeting.
After Ron’s presenation, it was out to the nearby parklands to set up a variety of SOTA stations. I brought along a MiniVNA Pro and used this, along with Blue VNA to have a look at what activators are using out there.
The Endfedz is a commercial end fed antenna for 40 and 20 rated to 100 watts. The antenna is well made and the cable is quite robust. The match box at the end is suspected to be a 9:1 balun.
The FT-817 reported no SWR at 7.09. Here’s what the VNAPro found at the feedpoint:
Both ends of the antenna are about 1.5m off the ground. The insulator at the far end has been moved up about 70cm. The centre of the endfed is about 6.8m off the ground at the top of a squid pole.
The interesting thing about this is that the antenna works quite well, but there is still some signal being lost. The match is only just ok right down the bottom of the band, with a SWR a little below 2. When we tested the antenna with about 10m of LMR195 between the feedpoint and the radio, we found the return loss at around 13dB. The extra loss is imposed by the cable. Not all of this loss is going to be the first 3dB, but still we are going to be burning about 30% of the TX power in the cable.
The SWR at the end of the cable was about 1.6 at 7.12MHz. The radio is at least going to want to put out most of it’s power.
Apparently the makers of this antenna stress that this setup is not a vertical with a single counterpoise. It’s supposed to be a dipole half on it’s side. Hmmmm, the conductor going up has a loading coil about a foot and half above the feedpoint, plus then further conductor above that. Seems like verticals I’ve made. The horizontal part of the dipole off the ground seems a lot like a radial to me. It’s broadly the same electrically as the vertical I’ve made, but only one radial. Verticals seem to like at least 4 radials a little in the air to work well.
Anyway, this was the initial result:
This pic shows that the antenna is resonant around 6.84MHz and has the best return loss at 6.85MHz (it is typical for best performance to be a little above resonance if the resistance is less than 50 ohms).
After moving the loading coil up a notch, we moved the resonant point to 7.05, and then played with the counterpoise to further tune the antenna.
The picture makes it look a bit worse as the range is plotted from 6 to 7.5, but the bandwidth is narrower than the end fed. Return loss tops out at about 8dB. It’s not a great match, so this thing could benefit from a tuner near the feedpoint.
This is what things looked like after a little bit of work:
Notice that the main reason for the SWR still remaining at about 1.8 is the feedpoint resistance at resonance of around 27 ohms.
This is after a bit more work:
Notice that because the feedpoint resistance at resonance is well below 50 ohms, about 24 here, the best performance is frequencies a little above resonance.
This is what the above looks like at the other end of the coax, the radio end:
The 24ohms is being transformed to a little above 100ohms (but at a slightly lower frequency because the length of coax is not exactly a quarter wave length). The effects of a near 1/4 length coax can clearly be seen by comparing the two pictures.
Next up was a 2m SlimJim mounted on a 7m squid pole. As can be seen in the pic, the results of this antenna are excellent.
It can be seen that the SWR is low across the whole band. This atnenna has a deep return loss. The low SWR means that most power is transported to the antenna. Loss there is good – it means it got radiated. At 145MHz, it is about 26dB, meaning that only a fraction of 1/100th of the signal is coming back to the radio. It’s still around 20dB at 144.1, and a still very nice 18dB at 146.5.
Homemade Endfed with a counterpoise
Now to finish off, it’s time to look at a homemade endfed. It’s interesting to compare this with the endfed that I am using:
While this is tuned a little high for the usual action at 7.1 and CW at 7.027, clearly the counterpoise adds value. The best return loss is around 15dB, which is a big improvement on the 9 to 10dB of the Endfed without a counterpoise. Might have to think about doing something 🙂
The wrap up
It was interesting to have a look around and see what people are up to. I missed out seeing Rik VK3KAN’s freestanding squid pole, but that’s for another day. The guys spent so much time looking at the various options out there, we even took our time getting up to the BBQ. Now: radio even over food. There is something wrong.
Regards, Wayne VK3WAM