Bob, WB4MNF wrote:
>(my) Loop is approx 2 wave circumference formed as:
>3/4 wave bottom wire approx 10' above 'average' ground on ridgetop.
>3/4 wave top wire
>1/4 wave vertical wires
>I feed it 1/4 wave from a bottom corner with 450 ohm twin lead that was
>trimmed
>until I could match via a Johnson Matchbox (balanced feed). The feed is on
>the
>order of 3/4 wave (very rough estimate as I didn't measure).
(by direct mail you said it's 2 wavelengths on 40 meters)
This isn't a bisquare. The current distribution is very different from that of
a bisquare and in fact it has a large proportion of vertically polarized
radiation. After a very short time with EZNEC this morning before work I got a
predicted gain of about 3.7dbi in freespace (vs about 2.1 for a dipole). The
azimuthal pattern in freespace is interesting, with four 55 degree wide lobes
evenly spaced at 45 degrees from the plane of the antenna. Over real medium
ground two of the lobes are attenuated a couple
db, leaving two that give about 1.8dbi at an elevation of 22 degrees. A dipole
at the height of your top wire (around 47 feet) models 3.5dbi at 22 degrees
but is down about 2db on the same bearings as the major lobes of your loop. I
didn't have time to look at the feedpoint impedance. Also the enhancement you
get from your ridge is an unknown and mabye a very big unknown that enables
the QRP contacts with VK and ZL.
To me the two keys to a bisquare being a bisquare are the "bi" and the
"square" :-) That is, two distinct elements and having four equal sides. There
are two distinct elements because the top is split in the middle. Without this
split the pattern is drastically altered. Also the feed needs to be from a
split in the bottom middle to keep this a horizontally polarized antenna with
properly distributed currents. I guess it's a mistake to call this antenna a
"bisquare loop" since it causes folks to forgot the gap at the top center and
perhaps equate it with an arbitrary loop design. And then of course some
bisquares in the amateur literature are square but rotated in a diamond
configuration with the split at the apex. I haven't modeled those at all.
A 40m bisquare with four equal size, approx 1/2wl sides fed in the middle of a
horizontal bottom wire and split at the center of a top horizontal wire, 10
feet above the dirt would have around 7.9dbi gain at 26 degrees with a nice
overhead null according to EZNEC. In freespace the gain is around 6.6dbi vs
about 2.1 for a dipole. But these are maximum values with a ton of inductive
reactance. At resonance the gain is closer to 4db over a dipole in freespace
and the gain over real ground is probabably a little lower than 7.9dbi.
The bad news with this antenna 10 feet over real ground like your loop is that
its top wire is over 75 feet up. A halfwave dipole at 75 feet on 40 meters
over real ground has around 7.5dbi gain with a nice overhead null and a launch
angle in the mid 20 degree range. So by removing 3/4 of the antenna to make a
dipole out of bisquare there would be no net loss of performance! The
situation is even worse if you take matching losses into account. Getting from
8000j+0 down to 50 ohms with anything short of open wire balanced line is
murder. All of this drives me to the conclusion that the bisquare and its kin
are high band designs for those of us who are altitude impaired and able (silly
enough) to match unusual loads.
Speaking of transmission line transformers and understanding their losses,
there is a fabulous tool named "TL" (Transmission Line) that Dean Straw N6BV
wrote in file tl.zip at this URL:
http://oak.oakland.edu:8080/pub/hamradio/arrl/bbs/programs/
There is a more recent version included with one of the ARRL books that
provides some refinements but this version on the Net is full featured. It can
be a real eye opener about line losses and impedance transformations and it's
also super for creating custom L and T networks and understanding what they
can and cannot do (I should say "cannot do without glowing in the dark" <g>).
Regards,
Pete
KS4XG
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