Hi Harlock
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You ask'd good questions... the answers to which, I am sure you will find, are filled with a lot of emotion. Hams are quite passionate about antennas..... (smiles
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If you have a really good antenna, one that you know the SWR is low, and can be assembled in a temporary location with such confidence- then your need for an external tuner should not be necessary. Just throw it up and operate. Chances are, it won't work the same as it did in your design environment....
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....So: a Tuner.... but remember, a tuner only cheats the transmitter into thinking it has a 1:1 match with the antenna and the feed line... what happens after the tuner is anyone's guess. Yes, a tuner does improve things, but it a compromise, plain and simple. You should strive to not need one, ever, in your antenna designs.
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To that end, and from one who is hardly an Angel when it comes to Perfect Antennas (I use tuners all the time, both as a 'ham' and professionally)-- I suggest you construct the best dipole(s) for the band(s) you intend to operate. Adjust them for the lowest SWR's, having erect'd them to the the approximate heights they will be used at the temporary locations. Coil them up, take off for the hills-- and then, when you operate at these locations, use your tuner to 'tweak' the SWR to 1:1.
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It really doesn't matter that you are operating low power- the rules apply regardless. Also, don't sweat anything that is SWR 2:1 or less-- in some cases even 2.5:1-- though that ratio is in the 'Stink Zone'-- you should always try to do better-- and you should be able to. If not, something is definitely wrong and you can correct it by looking things over carefully-- feed line connections, solder joints, too close to something RF- active (metallic etc)??... get the SWR down to 2:1 or better.
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I hope this gives you some pointers... good luck!
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.........................................CF