HI Paul: This is one that will probably catch a lot of people with their
opinions up. My take on your loop is first, by formula, your loop will be a
full wave just below 7 mHz, and should load fairly well on 60M and very well on
40 and up.
On the 3.5 mHz band your loop is close to a half wave OA, and tuning is going
to be touchy. Extremely touchy. So touchy you often run into the same problems
you get on 160.
I suppose it's possible to load a quarter wave loop on 160 but I have not had
any luck doing so. You can "load up the tuner" giving yourself an apparent
match since the MFJ tuners are notorious for that - but all you are really
doing is heating up the tuner. And tuners don't make good artificial antennas.
The cost of replacement parts is pretty high.
I would reccommend both coax and a "balun" at the loop feedpoint. The first
because it will make a better match to your loop on 40/60/80 than most open
lines and I have not had wonderful luck with the typical built in toroidal
impedance matching coil in most tuners. Especially feeding comparitively low
impedance loads.
There's some controversy about the "current balun" at the feedpoint. Some say
there's no need of any sort of choke if your antenna is balanced, and a loop is
about as balanced as you can get. However, I well remember my old professor
telling the class that we need not memorize all the formulas in Terman, we only
had to know where they were. And the second bit of advise was to measure the
critical stuff, not infer results from secondary measurements.
>From my measurement of RF current on the coax shield, a coax fed loop has
>quite a bit of RF on the shield. Enough to cause problems if they aren't
>choked off at the loop feedpoint. You can use 10 to 15 turns of coax solenoid
>wound on a 4 inch, 100 mm, tube, which should work well for 60 M through 30.
>And again, some swear by scramble wound coils, or bunched coils, but my
>measurements lead me to avoid such.
Being a safety type, a "belt and braces man," I generally use a choke balun at
the antenna feedpoint and then back that up with a ferrite balun somewhere
fairly close. No RF in the shack, and the tuner likes the result much better.
I hope this helps;
73 Pete Allen AC5E
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