Topband: big K9AY-loop + MFJ 10126

[K9AY]

de PA5A:
> In have set up a loop which is twice as big as the normal K9AY loop; a 
> little more noise, but signals are much louder. ... Question is, what will 
> be best size to have the optimum S/N. Does anyone has experience with 
> this?

K9AY quote per Dennis W0JX/8:
  "The loop's published size (25 feet high, +/-15 feet to the corners) is 
about 2/3 the maximum size for 160M. At one point, I was using one 30 feet 
high and +/-20 feet to the corners, which worked well on 160, but had a 
lousy pattern on 80 -- this is with just a resistor for termination. You can 
use the larger loop on 80 with a resistor-and-capacitor in series as the 
termination, but I have done very little study of it."


The maximum size of a K9AY Loop has two answers:

1. For a resistor-only termination, the maximum size is a little more than 
1/4-wave of wire around the loop. The published design of 25' high and +/-15 
feet to the corners is about 83 feet of wire, which is the max. size on 80M. 
In most installations, this size has some F/B up to about 5 MHz. Good F/B 
any higher, such as 40M, is probably due to local variations and will not be 
repeatable.

Note that a loop that is maximum size for 160M will have 3 or 4 dB stronger 
signals than the original design because of the larger capture area -- this 
can make the difference betwen needing an external preamp and using the one 
in you rig.

2. As my note to W0JX said, you can make much larger loops, but for best 
F/B, the termination needs to be reactive (capacitive) to cancel the extra 
inductance of the larger size. Somewhere around half-wave of wire, the loop 
will begin to act more like a quad loop and F/B will be hard to achieve. 
This is why some users have noted pretty strong broadside reception in the 
40M and 30M bands (which may be useful for RX in some cases).

If you are using the original design loop and only want 160M coverage, I can 
suggest extending the horizontal span from 15' to 20' on each side, keeping 
the 25' vertical height. The terminating resistance will increase 7 to 10 
percent and the signal levels will be 2 dB higher, perhaps more, depending 
on local conditions. I did this with loops used for phased array experiments 
this past summer, and the difference in signal levels was measured at 
between 2 and 3 dB, with a very definite increase "by ear."

73, Gary
K9AY