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: Elecraft@mailman.qth.net 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
