[HCDX] Re: loop ant critique

["Sam Dellit" <dellits@xxxxxxxxxxxxxxx>]

g'day mark, (also greetings to the k9ay loop reflector gang)

thanks for taking the time to respond to my
e-mail on the hard-core-dx reflector

again i must say i am still coming up to speed
about the k9ay terminated loop

i have only just joined the k9ay reflector
and am in the process of reading the entire
archive of several hundred e-mails
every one a gem

with that disclaimer, my present level of
understanding is that what you say is generally
correct BUT k9ay's design relies on the exceptions
to the general rule

a good figure of merit for a receiving antenna is the maximum voltage at the
antenna terminals of the receiving antenna (that is the antenna is oriented
for maximum voltage) for a given incident electromagnetic field
devided by the voltage at the terminals of a short monopole of one metre
length (again oriented for maximum voltage) by the same incident field

to a first approximation the figure of merit for an electrically small loop
is 0.02 * N * A * f *Q (where N is the number of turns in the loop; A is the
cross sectional area in square metres; f is the frequency in MHz; and Q is
one for a non-resonant loop or equal to the loaded Q of the loop if
resonated)

thus a small loop is really crappy (except as you have suggested for its
directional characteristics)
for example (and i stress these decibels are with respect to the one metre
vertical monopole; they are not dBi or dBd - both of which would make the
small loops look even worse)

(A) non-resonant single turn loop with a cross sectional area of one metre
@ 500kHz : .01    (that is the voltage at the loop terminals is 40dB down
with respect to a one metre vertical monopole)
@1000kHz : .02    (34dB down)
@2000kHz : .04    (28dB down)
@4000kHz : .08    (22dB down)

(B) if the single turn loop of one square metre cross section is resonated a
Q of 10 is typical so :
@500kHz   : .1         (20dB down)
@1000kHz : .2         (14dB down)
@2000kHz : .4          (8dB down)
@4000kHz : .8          (2dB down)

(C)  below 2MHz, a resonant 5 turn loop of one square metre cross section
with a loaded Q of 50 can generally be achieved so :
@500kHz :    2.5        (8dB up)
@1000kHz :  5.0      (14dB up)
@2000kHz : 10.0      (20dB up)
@4000kHz : 20.0      (26db up)

reality check : i think most experienced mw dxers would agree that a well
made high Q box loop provides about the same signal levels as a 5 metre long
wire (we are talking ballpark here - this is not NEC territory)

(D) for a large non-resonant single turn loop of cross sectional area 100
square metres (k9ay loop ballpark) :
@500kHz :    1.0       (0db)
@1000kHz :  2.0       (6dB)
@2000kHz :  4.0       (12dB)
@4000kHz :  8.0       (18dB)

thus in this region the performance of the loop, particularly at higher
frequencies is comparable to a vertical monopole of the same height - with
the big advantage that it is both broadband (non-resonant) and nulls
broadside to the loop

k9ay seems to have taken this a step further by introducing a "terminating
resistor" to simultaneously unbalance the loop (thereby introducing
common-mode sensitivity i.e. the loop begins to act also as a vertical
monopole antenna) and to adjust the relative amplitudes of the vertical
monopole and loop components to be close ly equal). The result is a
reasonably sensitive, broadband antenna with cardioid directivity.

not to detract from the k9ay design, i do believe that there are better ways
to adjust amplitude levels and to unbalance the loop, also possibly purely
rf mechanisms to relocate the "terminations" back to the receiver, without
the added complexity of active electronics. hopefully more of this later

now how do we go about developing this antenna to give true beverage beating
performance?

73s gd dx de

sam dellit vk4zss





----- Original Message -----
From: <BOEING377@xxxxxxx>
To: "Sam Dellit" <dellits@xxxxxxxxxxxxxxx>
Sent: Friday, February 23, 2001 6:34 AM
Subject: loop ant critique


> Hello,
>
> I have used MANY DF antennas on boats during my days in commercial
fishing.
> Loops are almost always inferior to even four of five feet of longwire as
> receive antennas unless you can use them to null out interference that is
> (ideally 90 degrees) offset from the arrival azimuth of the desired
signal.
> In that instance theory can shine, not due to sensitivity, but rather to
> their steerable insensitivity to an interfering signal.  In all others
they
> are pretty poor as rcvr antennas.
>
> Mark
>

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