Topband: Loop Antenna

[Tom Rauch]

> as i thought it would. This antenna is closer to 80 meters but will it
> work on 160 meters? Would it help if I raised the feed point up to the
> middle of one side ? I am sure there is some feed line swr loss as i
> am feeding it with 50 ohm coax. Will it preform better if I change to

That antenna is both far from resonance and close to ground. Even if 
you managed to feed it as a vertically polarized loop, it would have 
considerable earth loss because of height and feedline loss from the 
large mismatch at the feedpoint.

> open wire feed line? If I need to use open wire feed line I need to
> either build a balanced tuner or use the tuner i have with the ferrite
> balun built in. I cant see how a ferrite balun on the output of a
> tuner is any good as a ferrite wants a feed line close to its
> impedance. 

There is a great deal of incorrect but popular balun folklore. 

The balun configuration primarily determines how a balun works with 
"mismatched" loads. A choke balun (1:1 ratio) does not excite the 
core regardless of mismatch or impedance. Only the common mode 
current excites the core. That's why 1:1 choke baluns are the 
preferred balun for tuners. By the way, moving the choke balun to the 
tuner input does NOTHING to improve balance or balun effectiveness. 
That is another popular but incorrect assumption. 

A typical 4:1 balun or any other *voltage balun* excites the core and 
is impedance critical. Voltage baluns are undesirable is nearly any 
antenna application. They do benefit from being moved to a tuner 
input, but then they are the wrong style balun anyway.

Even modest power levels of a few hundred watts would severely 
overheat a balun that has only a small percentage of loss. Think 
about how hot a 100-watt lightbulb gets, and how much smaller a balun 
core is than the bulb. The largest problem by far is not power loss, 
but rather heat destroying a balun. If a balun doesn't smoke or 
totally melt and you are running more than a few hundred watts, it 
can not be dissipating significant energy.     

A loop, especially one near ground, is a very poor choice for a low-
band DX transmitting antenna (especially if it is not a full wave on 
each band). A high dipole or vertical like an inverted L would be 
much better.

If that is all you can use, then use a balanced feedline with very 
large conductors and keep the feedline short before it gets to a 
tuner or matching system. Expect problems feeding the loop, 
especially with 1500 watts, unless you can make it resonant.73, Tom 
W8JI
W8JI@contesting.com