If your purpose is to try and null out a specific signal, then rotating
the loop may have some merit. If your purpose is to obtain better
signal to noise ratio, then I think rotating the loop is wasted effort.
The null for the K9AY loop is narrow, and you can tune it to minimum by
adjusting the termination resistor. The shape of the antenna, height
above ground, and the ground characteristics will determine this
resistor value. This null is also first thing to go when conditions
change. The maximum value of this null does not effect the signal to
noise ratio very much. If you have a specific signal to reject or there
is a noise source coming from only one direction, and that noise source
is the dominate noise for the band, then placing it in the null may
improve reception. This is not the way most people use this antenna.
Most of the time, noise sources come from multiple directions
simultaneously. If you are interested in improving the signal to noise
ratio under these conditions, then you need to narrow the receive lobe
for the loop. The K9AY loop does give improved signal to noise ratio by
virtue or rejecting some signals to the rear, but the loop has a very
broad pattern, both in the azimuth and elevation. There is still a lot
of gain in the straight-up direction, which you don't need for receiving
purposes. You will find it difficult to improve this pattern very much
by changing the shape of the loop. Making it a little smaller will
improve the pattern somewhat at the expense of lower gain. Take a look
at the pendant, flag, and EWE antennas. All of these and the K9AY work
on the same principle, but use different shapes, and the performance for
all of them is fairly close. Also if you are really only interested the
amount of the null, you can get a more pronounced null using the
pendant. If you want to make any significant improvement in signal to
noise ratio, you will have to start looking at using multiple elements.
All of these antennas can be used in multi-element configurations. If
you start doing this, it is possible to produce a pattern better than
anything you can achieve with a single beverage, although it will be
considerably more complex than a beverage. A good compromise is two
elements for NS, and two elements for EW, and you can pick any of these
types to use.
There are also other types of receive antenna to choose from. You might
want to stop by Tom's web site. He has lots of tabulated data on receive
antennas.
http://www.w8ji.com/receiving.htm
Jerry, K4SAV
ik3umt@ir3ip.net wrote:
>Hi All
>Here I come again on (unanswered) question about rotatable k9ay loop
>I've never considered this antenna before since I was not really
>interested in topband 'til now (after 14 years of hamradio....hmm).
>So I'm picking up some info about on internet.
>
>I've understood that a single loop has a sharp null on loop plane and only
>in one direction (splitting loop terminals we have opposite direction
>null) and it has to be peaked adjusting Rterm.
>With classic "two loops" configuration we have 4 deep nulls available
>(i.e. N,E,S,W)
>Several articles claim antenna to be 360 degrees elctrically rotatable.
>What they intend ??? I see a cardioid pattern which,yes, I can rotate ,
>but in only 4 position.....
>First time I was thinking that null control was able to rotate somewhat
>cardioid pattern making two loops to interact but I was wrong....
>That's why my question : if i've got to null out a signal coming 45
>degrees from loops plane (from my previous example a NE, NW, SE or SW
>interfering signal) what is antenna behaviour ?? What settings operator
>must use ???
>
>So I came to idea that a single k9ay loop with fixed connections (but
>still adjustable Rterm) can be mechanically rotated with a normal light
>duty rotator to place null exactly where we need.
>
>Will be this solution useful/funcioning ???
>
>Thanks, Federico
>
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