[Elecraft] OT: High RF currents in small TX'ing lopps

[Fred Jensen]

Small TX loops are a moderately unexplored section of the radio universe 
by hams.  Impedances are very low, hence very high currents, I^2 * R 
suggests small resistances really count.

I sold my Buddipole, which I've used in many field and SOTA excursions 
and got an AlexLoop.  Much lighter, sets up in 1/10th the time, QSY's 
almost instantly, doesn't need elevation, 1/10th the weight or less, and 
seems to be more or less equivalent to anything I could do with the 
Buddipole.  And, it's pretty much insensitive to "ground."

In the field, it is generally on a tripod over my left shoulder so I can 
just reach up and tune it.  K2, 5W, no tuning drift, and I don't line my 
hat with tinfoil.  Not sure I'd sit that close running power into a loop 
designed for 100+ watts however.  A good friend and retired EE pointed 
out recently, "All the magnetic lines of force from everywhere in the 
universe where I'm heard, go through that small loop." :-))

I tried just about every BuddiPole configuration and came to two 
conclusions:

1.  It doesn't matter how you extend the ends of the elements ... up, 
down, one up-one down, in a "Y'.  If the sections close in to the feed 
point are horizontal, it's a horizontal antenna and elevation matters. 
I guess I knew that from theory but it's nice when you see theory in 
practice.

2.  Any vertical configuration that includes some form of reasonably 
good ground plane [2+ radials?] will be less sensitive to elevation. 
Without a good ground plane, it will behave like a dummy load [on which 
you *can* make a few Q's :-)].

The BP is a good field antenna, very well made, very sturdy, and well 
supported, with lots of options.  The full version is probably more 
"luggable" than "portable."  I think I just got tired of lugging it as I 
get older.

73 and a happy and peaceful New Year to all,

Fred K6DGW
- Northern California Contest Club
- CU in the 2013 Cal QSO Party 5-6 Oct 2013
- www.cqp.org


On 1/1/2013 2:19 PM, Ken G Kopp wrote:
> Small transmitting loops by their very nature involve high levels of
> current flow and this can/will cause "ordinary" variable caps to heat,
> which results in tuning drift.  Most home builders use vacuum caps
> and relatively large copper pipe in their designs.  Mechanical joints
> (clamps, wiping contacts, etc.) are to be avoided.
>
> This tuning drift problem plagued the AEA loops of some years ago.
> (I had the prototype here for early testing.)  Does anyone know how
> well MFJ's loop performs in this respect?