Hi Jeff
For 160m? I suggest you think about the on the top band hams reflector.? The way WB5WPA did it allows you to make a quick test on a fixed frequency without a lot of cost and if you have trees it can be a fairly stealthy antenna as well.? If you like it, change to a motorized cap to make it frequency agile.? I've built a couple of these for 80 and 160 and though my contesting skills are poor (detest contests) have had good results with QRP on 160m so that says something about the antenna's efficiency.? The main idea is that very small antennas like the classic 'magloop' because they enclose such a small space, the radiation resistance is in miliohms and that means to get any decent efficiency you would like to keep loss resistance about 10 times less, which is why magloops need big conductors, wide straps connecting to the tuning cap and also a cap with no sliding connection ie you need a split stator or VVC.? This all applies to loops that are defined as small being 1/10th or so of a wavelength in circumference. On the other hand the ARRL antenna book says the smallest "regular" loop antenna is 1/2 wave in length.? This loop I am suggesting is ~1/4 to ~5/16 wave long and has some really neat advantages.? As it encloses a larger volume of space the radiation resistance comes way up and you no longer have to keep loss in the low milliohms to get any efficiency.? This means you can use regular wire for the conductor.? A regular low cost capacitor with sliding contact will also be ok for the same reason.?
The details are in the link so I don't need to repeat it all here but a few guidelines are worth mentioning;? there will be interaction between the bottom wire of the rectangle and earth, and earth losses get reflected into the impedance you will be matching to, so you want the bottom of the antenna a good 2m above the ground or more if possible.? That also makes it good for not clotheslining yourself when you are walking around the yard looking at the person next door washing her car. :)? Another rule of thumb is the longer the wire the higher the impedance for matching.? So this means you can tweak the antenna's match by either changing the wire length or simply adjusting its height above ground.? I find this last to be very handy for portable 160m operation (yes!) where ground characteristics will be different in different locations.? Some collapsible fiberglass poles allow you to support the wires and adjust the height easily.? Also for consideration:? You can certainly build a classic magloop for 160m but if you build it well with low loss it will end up being rather heavy and still fairly large even with two turns and your neighbours will certainly notice it.? It won't be cheap!? It will only be useful for 160m after all that.? On the other hand, activity on 160m tends to be rather sparse and winter oriented with the majority of it going on in the Stew Perry contest weekends.? With this antenna you can put it up all by yourself in about 20 minutes and take it down even faster and afterwards have a couple of 12m poles you can use for other antennas like a nice 40m half square at other times of the year etc. or, use another wire and move the poles closer to get on 80m with it.? I really love this antenna and use my VE2AO magloop tuner for keeping it tracking my VFO.? Had one up the whole winter a couple of years ago and it was very reliable and consistent in use.
Best regards...Joe
VE7VXO