[Elecraft] Horizontal Delta Loop Questions

[Jim Dunstan]

At 08:13 AM 1/5/2012 -0700, you wrote:
>Hi,
>
>I'm a CW operator who dwells in the lower end of the HF bands.  My current
>antenna is a 200' horizontal delta loop 40' high, fed with 87' of 450 ohm
>ladder line.  This connects to a 4 to 1 balun (common mode choke?) which is
>attached to my Dentron Jr. Monitor tuner via 5' of RG213.  Looking at a
>spreadsheet, I see that a full wave loop for 80 meters is 287' and 143.57'
>for 40 meters.
>
>A hypothetical question to the wise ones on this list is would I expect any
>practical improvement in performance of my antenna system (on 40 meters and
>above) by reducing my 200' delta loop to 143.57' and feeding it with 71.78'
>of ladder line so that the sum of the antenna length plus twice the ladder
>line length is 287'?   This seems to make theoretical sense since the
>entire system would be resonant at 7mhz and integral multiples thereof.
>However, the performance of my current 200' loop appears to be quite good
>from 80 to 6 meters and my small tuner matches whatever impedance this
>system is presenting to it so I'm hesitant to take a pair of snippers to my
>antenna.

Changing the length of the feed line will not change the dimension or 
performance of the loop.  A loop of dimension (X) will present a feed point 
impedance (Y) at particular frequency (Z).  Attached to the loop at the 
feed point is a 450 ohm balanced feed line (as pointed out in your 
case).  There will be an impedance mismatch which will produce standing 
waves on the feed line which will produce varying RF current/voltages along 
its length.  A matching device at the end of the feed line must convert the 
resultant impedance to the 50 ohm unbalanced impedance accepted by your 
radio.  If the resulting impedance falls outside the range of your 
coupling/matching device changing the length of the feed line may solve the 
problem and bring the impedance back into the range of the matching 
device.  However this will not change the performance characteristics of 
the loop.

However, 1, 2, 3 wavelength loops have interesting characteristics on the 
fundamental and multiple frequencies.  A 1 wavelength loop radiates 
broadside to the loop and more off the sides on multiple wavelengths.  A 
140 ft horizontal loop is a 'cloud burner' on 40m and a much better dx 
performer on 20m and up.  Making the loop 1.4 wave lengths on 40m probably 
won't make much difference on 40m but I am not quite sure what will happen 
to the radiation angle at harmonic frequencies.  Probably not a great deal 
of difference.


>I would like to increase the length of the loop to 287' feet and feed it
>with 71.78' of ladderline but that is another story.  Also, would I expect
>to be able to tune any of the these three antennas on 160 meters if I were
>to purchase either a KXAT100 or KAT500 tuner which presumably have a wider
>tuning ranges than the Dentron Jr. Monitor tuner?

There is no doubt that a 280' loop will provide much better dx performance 
on 40m and up.  Choosing the feed line length with the intent of changing 
the antenna radiation pattern is not fruitful.  However certain line 
lengths present a better opportunity of matching the end to the 
radio.  Check the antenna handbook for suggested line lengths that fall 
into this category.  Another caveat is that a particular antenna tuner may 
not work beyond a certain power level.  It doesn't mean that the impedance 
is different between the different power levels ... but that the resulting 
RF voltages or currents my go beyond the component capabilities ... eg arc 
over or melt hi hi.

73

Jim, VE3CI