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On Feb 27, 12:54 pm, "KE5MBX" wrote:
I'm planning to get a Yaesu FT-857D and operate mobile HF on as many bands as possible. I see two options so far for my Jeep Wrangler- a 102" whip with a tuner or a screwdriver antenna. Which is the better way to go? Do you need fast tuning from band to band? Can you see the antenna from where you operate? Do you want to operate while actually driving, or just when you're parked. I had a screwdriver on the roof of my VW Passat for a while. It was tedious to adjust because I didn't have feedback about where it was set. I then moved it to the front bumper, and it was a lot easier to do band changes (you can quickly see when you're in the right region). Worked greate for 10,12,15,17, and 20, because you can tune to a visible mark, and then just check SWR. However, it's still a bit tedious to tune for 40 and 75, because it has a pretty sharp resonance, so if you QSY very far, you have to readjust the antenna. There are some newer screwdriver antennas with automatic controllers that make life much easier. They can count turns, or actually look at the reflected power and adjust up and down for the match. However, you still have the "tuning speed" issue.. that motor only moves so fast. And, no matter how you look at it, you're only adjusting one parameter (series L), which limits the best possible match you can get. Granted, with the lossiness of a typical screwdriver, and the usual clever 4:1 transformer with leakage C, you can do pretty well. better than 2:1 from 10-40 is no problem and you can probably do somewhat better. A whip and auto tuner at the base of the antenna, on the other hand, is virtually instantaneous to tune and requires no manual intervention on the part of the operator. Squeeze the pickle and you're matched and ready to go. I suspect that the loss in the tuner is not materially different than in the variable loading coil, particularly when you consider the mismatch from only having one adjustable device, and besides, the dominant efficiency issue is going to be from having a physically short radiator on lower frequencies, which is the same regardless of how you tune it. I didn't have very good luck with a single whip on a SGC tuner, (as far as getting a match on all bands) but when I improvised a dualband whip by wrapping another wire around the first whip (say, 25-30 ft of AWG 16 hookup wire) and connecting them at the base, it works quite well. (this is similar to what's inside the SGC whips and other similar wideband whips... essentially it's like having two verticals in parallel, with widely varying resonant frequencies... when you're close to the resonance of one, the other just looks like a lumped L or C in parallel, and the tuner takes it out) |
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