Testing Elecraft T1 autotuner with 10-ft. whip
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FYI
Paul, W0RW asked me to test the new Elecraft T1 mini antenna tuner with a 10-ft. whip and 13-ft. drag line. I didn't have that kind of whip so I tested it with a 10-ft length of thick (12-gauge) stranded insulated copper wire taped to one of those black, expandable plastic fish poles in a vertical position. I don't think those poles have detrimental electrical or RF properties (fill me in if I'm wrong). The feed point of this vertical wire was 5 feet off the ground. I didn't use the 6-inch feed line that Paul said he was using -- I just connected directly to the bottom of the 10-ft. wire. The 13-ft. drag line consisted of the same kind of wire, and was simply laid on the grass. I used my FT-817 running 5 watts and the Elecraft T1 tuner. I used a CW signal for testing. The results using an Elecraft 4:1 balun were very good. For every band (looking at both the high and low ends of the broad bands) from 50 MHz down through 7 MHz (excluding 60m where I can't broadcast) the SWR was 1:1. On the high end of the 3 MHz band the SWR was between 1:1 and 1:2. I couldn't obtain a decent SWR on the low end of the 3 MHz band. I couldn't load on 160m. All the above tunes were accomplished "first time." Without using a balun, I again obtained 1:1 SWRs from 50 MHz down through 14 MHz. On 10 MHz I found an SWR between 1:1 and 1:2. I couldn't load on the bands below 10 MHz. Looks like that 10-ft. vertical and the 13-ft. drag line work very well with the T1 and the balun. -- Bil KD6JUI _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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Very good on the report and I for one appreciate the work and effort into
it. My one question is, "are you able to tell if the RF output was "reduced" to satisfy the low SWR or did the RF output remain same?" There are situations where the tuner matches but the radio outputs at reduced power because the antenna is less than optimum. Ron wb1hga "CW, an esoteric experience" KD6JUI writes: The results using an Elecraft 4:1 balun were very good. For every band (looking at both the high and low ends of the broad bands) from 50 MHz down through 7 MHz (excluding 60m where I can't broadcast) the SWR was 1:1. On the high end of the 3 MHz band the SWR was between 1:1 and 1:2. I couldn't obtain a decent SWR on the low end of the 3 MHz band. I couldn't load on 160m. (snip) Without using a balun, I again obtained 1:1 SWRs from 50 MHz down through 14 MHz. On 10 MHz I found an SWR between 1:1 and 1:2. I couldn't load on the bands below 10 MHz. Looks like that 10-ft. vertical and the 13-ft. drag line work very well with the T1 and the balun. _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
T1 Bicycle Mobile report
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Today I put a CB whip on the back of my bike and cable-tied the T1 to a
6" piece of coax, and then another length to my FT817. The whip was at a 45 degree angle but began to sag towards the back. I also clipped a length of wire to the top and various other positions to try to get it to load on 60 or 75m, but couldn't get it to load below 70m. Matches were easy on the high bands, and seemed a little more lossy on 40m. I ran 2.5W or 5W, but did not make any SSB contacts, though I did get a weak signal report from a KH6 in Honolulu from my QRV location at the San Francisco bay. See pix at http://wa5znu.org/log It was mostly a dry run, as I don't think the 817 has enough power to work SSB bike mobile. Next week I plan to try again, with my K5OOR 50W amplifier for the FT-817, now that Elecraft has given us the test results for intermittent operation SSB in the 35W range. I did not try the 817 connector cable, but when I do I will rprobably need to use some kind of wiring harness, as there were more wires than I was comfortable with already. I know that Bud W3FF has had good experience with tuned grounds on the low bands so I may look into that as well. Also, I need to get the antenna more vertical, whixh may require some metal work. Leigh / WA5ZNU _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
RE: Testing Elecraft T1 autotuner with 10-ft. whip
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In reply to this post by W0rw
One way to get a match with a too-short antenna is to use a less-good
ground! <G> Seriously, as an antenna is shortened below 1/4 wavelength the capacitive reactance gets very high and the radiation resistance goes very low - well under 1 ohm. The T1 needs to correct both conditions: add enough inductance to bring the system to resonance (balance out the capacitive reactance of that short radiator) and convert the resulting resistance to 50 ohms. On the lower bands the T1 can run out of inductance and can't bring a short antenna to resonance. Most antenna tuners have limited inductance for extreme matching on the lower bands. That's why they usually are specified for only 80 meters and up, and have limited matching capabilities even on 80. One way to overcome that is to add a loading coil to the antenna. That's why mobile/portable short antennas have them. You can use a toroid core or, my preference, make a small air-wound coil on something like a 35 mm film container and add it in series at the antenna feed point. It's a cut-and-paste sort of thing. When the T1 can produce a match you've got enough inductance. I tried something a little different with my T1. I changed the T1's toroid coils. I doubled all the values since I didn't care about 6-meter coverage. That was easy. I wound a new L7. I found that a T50-2 core would fit if it was tilted just a bit to clear the case when the unit was reassembled. Fully wound it took 38 turns and provides about 7.5 uH; that's about twice the inductance of the original L7. Then it was just a matter of moving all the toroids over one space. The original L7 was put in the place for L6, the original L6 was put in the place for L5 and so on with the little three-turn L1 no longer used. That improved the inductance range available for the lower bands. It still won't load a wet noodle on 160 meters without an external inductor, but it'll load a whole lot more in the way of short antennas on the lower bands than it did with the original coils. The other place the T1, like any tuner, can run out of range is in the ability to convert the remaining resistive part of the impedance the antenna presents to 50 ohms for the transmitter. Few antennas in the real world show an impedance over a few thousand ohms. Compared to 50 ohms, that's a range of maybe 20 or 30:1 (30 times 50 = 1500 ohms). But a short antenna with an impedance of, say. 0.5 ohm requires the tuner have a range of 50/0.5 = 100:1! That gets tough to do. BAD GROUND TO THE RESCUE! The impedance is only that low if the resistance is only the radiation resistance of the short antenna. However, two other things come into play: the resistance of the conductors and the ground resistance. Conductor resistance can be a couple of ohms in any case. If the radiation resistance is 0.5 ohm and the conductor resistance (including any coils) is, say, 2 ohms, then the efficiency of the antenna is only 25%. It can't get any better because the power is shared between making heat in the conductors and making electromagnetic waves in space: 25% to RF and 75% to heat. (That's why we need superconductor for really efficient short antennas.) Even so, in most end-fed antennas there is another major element that adds resistance, and loss, to the antenna. That's the ground resistance. It's in series with the antenna, so it eats up power too. A 'typical' ground can have anything from tens of ohms to several hundred ohms resistance. A well-tuned, full-length insulated 1/4 wave 'counterpoise' is going to show something about 35 ohms, for example. A typical 'drag wire' is going to be a lot higher than 35 ohms unless you're walking through salt water. As you increase that resistance, you make the matching job much easier for the tuner at the cost of less actual RF being radiated. So if your pedestrian mobile setup won't tune on a lower band frequency, try shortening your drag wire - just fold it back on itself a bit - and see if that doesn't help. Of course, your RF output, typically in the range of a few percent at best under these conditions, drops even more. But that's part of the wonder of it all: just how little actual radiated RF it takes to be heard over very surprising distances! Ron AC7AC _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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