Topband: Col-atch-co 160/80 meter antenna

[i4jmy at iol.it]

The Col-atch-co antenna is 42' tall.
On 80m it's purely a capacitively top loaded vertical with a wire hat. Efficiency will be the highest possible for that height, and, of course, function of the used ground plane.
Just above the 80m top loading there is a loading coil for 160m, and above it another set of top loading wires whose basical function is tuning the antenna into the desired band portion.

Common speeches say that a loading coil moved away from base is better, in other words that center or near top inductive loading is more efficient than inductive base loading. 
But this is true when the antenna element is still long enough to have a real current distribution, something that reminds the situation with an unloaded element, not when the current is almost constant.

Beeing infact this antenna very short compared with WL, it's current distribution will be almost equal (constant) below the coil and along the whole element lenght. The coil, wherever placed, is in series with the antenna circuit and its losses will decrease the circulating current. Having the coil at base is needed less inductance to cancel element reactance, but like this, also coil losses will be less. Having the coil at mid element it's required more inductance, thus more coil losses will appear. With the coil near the element top the required inductance will increase exponentially and coil losses will became bigger.
 
In summary, with only 42' in lenght and top loading, the coil position doesn't really make a big difference in terms of efficiency. Less coil, wherever placed, but more top hat would do that.

73,
Mauri I4JMY



> 
> If I were to build either antenna it would be about as much work, but wonder if anyone has used the Col-atch-co antenna and knows how well it works.  
> 
> I am interested to know if the top loaded vs bottom loading type of antenna is prefered.
> 
> Thanks in advance,
> 
> Dave, K4SV
> Asheville, NC
>