multi band super mini loop antenna



multi band super mini loop antenna


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Many antennas claimed to be bi-squares are not bi-squares. The
assumption is if a “loop” is two wavelengths around the perimeter, no matter how
constructed or shaped, it is a bi-square. The next guess or claim is usually,
since the antenna was called a bisquare, it must have a certain gain. Usually
the gain is claimed to be around 4 dBd. Unfortunately, a real optimized
full-size
bi-square only has about 2 dBd gain!

One might wonder why it is necessary to “pick on” certain
designs. The answer is simple. By looking at common mistakes, we can learn very
useful things that prevent us from making mistakes.

Here is an example of an antenna claimed to act as a
bi-square, that really doesn’t act like one. It is called a multi-band super
mini loop antenna. (I will interject notes to call attention to certain errors.)
The antenna is explained in the following:

Antenna covers all bands 80-10 meters + 30, 17, 12
meter WARC Bands

Any
antenna will cover all bands with a tuner. The question always is, “How
does is work?”

This antenna works as a Full Wave Loop on 80 Meters and also works as a
2 wavelength open loop or Bi-Square on the 40 Meter band. The gain is
around 4 dBd on 40 mtrs, but it will seem much higher due to the very
low angle, radiation pattern. Any antenna tuned for 80 Meters should
also work on 20 and 10 meters as well. If an antenna works on 40 it
should work on 15 as well. A tuner will probably be needed for 10, 18
and 24 MHz Band operations.

An optimized shape real
Bi-Square antenna has 2 dBd gain
 

The loop is an inverted vertical triangle with the
base along the top and what would be the apex hanging down. Or it could
be erected horizontally if needed.

If that is done, it will beam straight
up

The feed point at the bottom uses a 3:1 or 4:1 balun
and is then fed with any needed length of 50 ohm coax.

Feed impedance models to be
about 70 ohms. Obviously a 3:1 or 4:1 balun is incorrect

The top center is broken with an insulator and has a
29 feet 10 inch length of 450 ohm ladder line connected across the
insulator.
IMPORTANT – The 450 ohm ladder line is shorted across at the bottom end.

450 ohm ladder line has an
impedance around 400 ohms or less

Here is how I calculated the loop size. Dimensions – One wavelength at
say 7.25 MHz = 1005/7.25 = 138.62 or 138 feet 7.5 inches. If this length
is doubled and used on 80 meters as a loop Then Freq(for a loop) =
1005/(2 x 138.62) = 3.625 MHz.
The 1/4 wave stub = 246 multiplied by (Velocity Factor of your type of
ladder line) /7.25 = about 32 feet.

Ladder line / twin lead velocity factors vary by brand, type,
insulation, etc.
The best method is to use a grid dip oscillator or antenna analyzer to
measure the resonant frequency of the 1/4 wave matching stub during
construction, rather than by just using the mathematical formula to
determine the length.

The perimeter of the 80 meter loop for 3.625 MHz = 277.24 – (2 x 29’
10”)[2 x 29.86 feet = the length of both sides of the wire in the ladder
line] = 217.52 ft. Let’s call it 217.5 feet.

Ladder line in stub or
transmission line mode cannot be treated as antenna length. Ladder line
is a stub, and must be treated as a stub. Not as physical antenna length

If the top horizontal span is made 99.5 feet overall and the legs 59
feet each then the balun will hang about 30.5 ft below the top center
insulator.
The 29’ 10” ladder line can have its bottom – shorted end – tied to the
top of the balun with a short length of fishing line, weed eater line,
string, or cord etc.

On 40 meters the ladder line stub automatically acts as a switch and
opens the connection across the insulator so the antenna works as 2 one
wave length loops fed in phase.

That is true on 40, but on 80
meters it adds roughly about 400-ohms reactance because it is a 1/8th
wave shorted stub. We will see the problems this causes with SWR 

This antenna is a high performance, full size, full wave, 80m loop
antenna.

A full wave loop, compressed in
dimensions has much less than 1 dBd gain. That is hardly high
performance, and the stub destroys harmonic resonances that are normally
useful in loops

On 40m it is a 2 wave length open loop or Bi-Square.
The stub in the top leg of the antenna opens the loop when operating on
40m and selected other bands.This improves the antennas radiation
pattern. Its gain is around 4dB, but it will seem a lot higher due to
its excellent, low angle, radiation pattern.

Untrue. Read what a bi-square
really is
Bi-square Link

The 50 Ohm coax from the balun to transmatch is not critical but
recommended to be 1/2 wave length. ( 99 Feet of RG-8X )

This antenna could also be configured as a 54’ 4.5” square on a
horizontal plane. The 217.5 ft loop can be pulled into almost any shape
but the bigger the “aperture” enclosed area the better it will work. I
have NOT tried this experimental design yet. I believe my calculations
are correct and should work. I have heard rumors that something similar
to this Multi-Band loop antenna is available commercially as a “Super
Loop” from Radio Works.

Laying the antenna horizontal will
make it beam straight up. The saving grace is it is not that directional

 

The diagram or pictorial is on Internet sites as follows:

 

super loop that isn't so super

 

Here is a model antenna:

 

WH2T loop model EZNEC

 

 

This antenna roughly conforms to the dimensions and construction above,
except length is adjusted for 40 meter resonance.

 

 

 

 

 

 

 

 

 

 

super dooper loop SWR curve

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The stub destroys harmonic resonances normally present in loop antennas. In
this case, when lowest 40 meter SWR is on 7.150 MHz, the 80-meter resonance is
on 4.25 MHz. This is because the stub is in the circuit as a reactance on
80-meters.

 

When the shorted stub is resonant on 7.15 MHz, the antenna will ALWAYS have
near-zero current  at the middle of the top:

wh2T loop 40 meters

 

This happens because the stub is nearly an open circuit on 40 meters.

Having a current null at the top actually prevents the antenna from acting
like a bi-square. The is because the center of wire 1 and wire 2 invert phase,
so wires 1 and 2 “fight” themselves. This antenna is a bi-square bent into a
shape that makes it stop working as a bi-square!!

 

To even remotely act like a bi-square, the antenna would have to bend near
the current minimum in wires 1 and 2.

 

It was a very bad assumption to guess that the antenna would be a bi-square,
and a very large exaggeration to claim it had 4 dBd gain.  A real bi-square
only has about 2 dBd gain, and this is not even close to a bi-square.

 

 

 

 

super loop gain 40 meters

 

 

 

 

Gain on 7.15 MHz with the antenna 92-feet high at the very top wire.

Gain is 8.36 dBi, which is exactly the same a dipole at that approximate
height! Gain is, as we might expect, zero dBd. It acts like a dipole on 40
meters.

 

 

 

 

 

 

80-meter gain:

 

80 meter gain compact loop

 

 

 

 

80-meter gain is 5.62 dBi, this is about -3dBd. It has slightly negative gain
compared to a dipole on 80 meters. (A dipole at optimum height is 8-8.5 dBi.)