PA0FRI's ACTIVE LOOP ANTENNA
Version 12-apr-2021 

This article is under construction.

THE CHOSEN SYSTEM

 


Under test see below.

 

This active loop antenna is the result of experiments, a selection of which is mentioned later in this article. Despite its relative simplicity, the antenna works quite well from at least 100 kHz to 145 MHz. It can be a very helpfull system for those who listen a lot to radio stations , but do not have the option of installing a high or long antenna.
A 2 × 16.55 m inverted dipole with the top at 12 m was used as a reference for performance. An automatic antenna tuner per amateur band ensured that the system was brought into resonance. An ICOM 7300 served as receiver. My tests were done with the loop 50 cm or 170 cm above the ground and that was not disappointing. It is therefore expected that with a higher position the result will be more than satisfactory. I am thinking of listeners who live in a flat and only have a small balcony available.
If the antenna is set up rotatable, it is possible to reduce an interfering signal considerably and set it to maximum if the signal is weak.
The performance depends on conditions, the time of day and the beam angle of the transmitting station. The reception with the loop can be less than the dipole, but also stronger. The comparison must be done over a longer period of time in order to be able to assess properly.

The impedance of the antenna depends on the frequency. The lower the frequency, the lower the impedance. The maximum impedance will be approximately 130 Ohms at a frequency (≈ 80 MHz) where the loop is a full wave.
A shielded loop has once again been compared to the wire loop. After testing over a longer period, it appears that a shielded loop has less interference.



In the following images nothing was changed to the receiver (IC7300). It was switched from the inverted V antenna to the active antenna as quickly as possible to get both images as identical as possible. Before taking the test, the amateur band or a bit outside of it was tuned to a signal that was as constant as possible. It will be clear that test were not done at the same time of day.
 


Image with inverted V dipole at 12 m.


 Image with active loop on a 1.70 m pole.


Image with inverted V dipole at 12 m.


Image with active loop on a 1.70 m pole.
 

 
Image with inverted V dipole at 12 m.

 

Image with active loop on a 1.70 m pole.


Image with inverted V dipole at 12
m.



 Image with active loop on a 1.70 m pole.

Image with CX4-68 4m vertical on 12 m mast.



Image with active loop on a 1.70 m pole.
 

Image with inverted V dipole at 12 m.


Image with active loop on a 50 cm pole.

Image with inverted V dipole at 12 m.


Image with active loop on a 1.70 m pole.

 
Image with inverted V dipole at 12 m.


Image with active loop on a 1.70 m pole.

Switching between both antennas was a matter of seconds with the click of a tumbler switch.

.

A choke balun (line insulator) was used for output coupling. That is easy to assemble for home brewers and ensures transmission of a very broad spectrum.
A DC power for the amplifier can be supplied through the choke balun, but to avoid pre-magnetization of the coil core, supplying was chosen via two large chokes.

Although I am not in favor of two separate toroids, it is still being tested how it works with one toroid.

An interesting experience was keeping the output floating, because this made the reception quieter. A PNS statement (probably nonsense statement) may be that the ground plane of the PCB, which was connected to the ground of the output connector, despite its small size, acts as a mini whip antenna sothat unwanted signals reach the receiver via the screen of the coax cable.

 EXPERIMENTS WITH AMPLIFIERS

The many active "(mini)whip" antennas in circulation performs well in open field, but in a densely populated residential area, the system picks up too much interference and a loop antenna with less interference is often a better choice.
The experience gained with PA3GZK's active loop antenna has led to experiments with other preamplifiers.
It has become a project together with PD0EEA. We exchange the results of experiments, he also designs the PCBs, has them produced and performs the necessary measurements.
The intention is to design a system that works as simple as possible and that works well with little fuss.

 
 
 
 
Several amplifier systems were tested and these are a few examples. The experiments were performed with an unshielded loopl (fig»).
The left system is a version of a cascode circuit. The second transistor does not operate in a grounded base circuit but with a grounded emitter.
I still had the 2N3866, but another type (2N3904, BFG541, BFG591?) might be better suited.
I have used the transistor alignment may times in all kinds of experiments or designs, because the gain is straight from 1.8 MHz to 30 MHz when the input and output are loaded with 50 Ohm.
The right circuit has advantages: stable, very low input impedance, simple, broadband up to 70 MHz and not too many components. It is actually a clone of a simple "cable TV amplier".
Because the impedance of the loop is very low, the antenna can be fed symmetrically with a line insulator/choke balun of not too great inductance.

 
 
 
 

 
  
 
 
 
 
A («fig) coupling with a toroid core has also been tested, but despite attempts with various transfer ratios, the result in terms of signal strength was less than with direct coupling.
 
 
 
 
 
 
 

 

The advantage of a balance circuit is the suppression of even harmonics.

To my surprise, someone had already experimented with the balanced design.
PDØEEA wrote:

 

“Tonight I made the loop Amp as it is on your site, There were still a number of new RCA 2N5109's from the 1970s!
The amplifier works well, even below 100 kHz it is two S points stronger than my old BC design. Above 3 MHz to 15 MHz, the gain increases even more and should actually be reduced a bit because the S meter already shows 1.5 S unit. On the other hand, all stations are very strong. It sounds like a good S/N ratio, it works better than with the INA amplifier.

The reception from 50 to 500 kHz is simply superb and can compete with an expensive long-wire antenna. The loop (130cm) with the internal conductor as one turn is approximately 4 µH.
When I insert a second winding (double loop) it becomes close to 10 µH and the reception in this range is even better. The signal of BBC on 198 kHz is over S9 and you can hear all beep and whistle beacons below 100 kHz.

I'm probably going to use a relay to choose between one or two turns, because it is really worth it!”.  

 

That the gain increases at the higher frequency is probably because the 1 ÷ 1 transformer (balun) has too small inductance for the lower frequencies or the coupling capacitor across the 390 Ohm resistors must be 100 nF. This is the measurement result of my transformer:
 
OTHER EXPERIMENTS
 
 
 
 
 
 
Experiments were also carried out with a loop of two turns. The strength at lower frequencies increases at the expense of much higher frequencies. Again, it turns out that a 1-turn loop of about 120 cm in diameter is a good compromise between noise/interference. signal strength and directivity. PA3GZK had already established this after many tests with its design.
If you are not interested what can be experienced below 1.8 MHz, a diameter of 100 cm will also suffice.
 
 
 
 

Another not yet tested idea is a grounded base circuit in which a metal support is used as ground.
 
 

THE BEST SYSTEM

This last but one design was chosen because it met our requirements: simple, well-performing, with standard components and suitable for DIY.

   
The previous circuit had too much gain and this design is more suited for an IC7300.                                                      This is the old larger PCB that was well suited for experimenting.



The operation is the same, except that the circuit has been slightly modified for the tiny PCB design. 

Transformers at the input and output cause losses or limit the bandwidth and can have unwanted resonances. Due to (sometimes) many windings with extra capacitance, the question is what remains of such a balancing. That is why I prefer a toroid core as a line insulator/choke balun, where it is possible to separate windings from each other so that they do not lie against each other along their entire length. Moreover, that system is suitable for DIY. The toroid core used here is type 2P80 and with 10 bifilar turns, the inductance is approximately 377 µH measured with a Chinese M4070 LCR meter.
It can be replaced by AMIDON FT50-W.
If you order PCB's from PD0EEA, the white (2P80?) toroid core is included free of charge.

 

If there is too much gain, one can apply a resistor across the choke or use a suitable attenuator at will. The decoupling has become a choke balun or line insulator.

So one is not so dependent on the properties of a transformer or other system as coupling. Strangely enough, unwanted interference was much less. For the time being I assume that asymmetrical interfering signals have less chance of entering the receiver due to an improved balance.

There will be comments of: "The output is not 50 Ohm".
No, that is not 50 Om, but the transceivers or receivers used for testing were not affected. In addition, the impedance at the input of receivers is rarely exactly 50 ohms.
In the meantime, various well-intentioned comments or suggestions have been received along the lines of: "That is not good, why thst not, too much amplification, it could be better, it causes too much noise, the impedance is too low, the collectors are almost short-circuited, etc. " However, the circuit works in practice, is our experience after extensive experiments. The disappointing thing is that all consultants do not present a simple and working circuit that can compete with the design in this article in a comparative practice test. When a better (KISS) pre-amplifier is designed, we as experimental radio amateurs would like to know more about it!



The operation is the same, except that the circuit has been slightly modified for the tiny PCB design. 

The reception range with 2N5109 or 2N3866 is at least 30 kHz to 146 MHz and with BFG135 to 220 MHz.

With the bottom of the loop at 50 cm above ground, the Bergen op Zoom repeater can be heard on 145.625.
The distance to the repeater is approximately 30 km.

The 70.070 MHz beacon at 40 km use a Big Wheel horizontally polarized antenna and the signal is received more strongly than with my vertical antenne on a 12 m high mast. Given the size of the loop, this band clearly shows horizontal polarization.

The result also depends on the beam angle of the opposing station. For example, if two stations close to each other at 40 m have the same strength with a 2 × 16.50 m dipole, then one of the two is stronger with the loop antenna than with the dipole.

ASYMMETRIC OUTPUT

A considerable simplification can be achieved by applying a single ended output. No transformers or baluns are needed, which can be frequency dependent.

Various attempts were made to obtain an even simpler design.
PD0EEA came up with the bright idea to design according to the internal circuitry of many ICs, such as an MC1350. This was and is still used in DIY projects and is mounted in many transceivers.

In the original schematic, you only have to install one capacitor at a collector to ground and insert a common emitter choke. It remains to be tested whether the result is the same as that of the original design. If that works out positively, it will come even closer to our objective: to present a system that works as simple as possible (KISS) and performs with little effort.

It remains to be tested whether the result is the same as that of the original scheme. If that works out positively, it will come even closer to our objective: to present a system that works as simple as possible and that works well with little effort.

The dipole (inverted V) has a 12m mast as its suspension point. The loopmas mounted on a 1.70m wooden pole.

  

 

 

 

  

  

 

 

 

 
BBC Radio 4, 198 kHz and 70.070MHz bacon .
Considering the preliminary test result, this simplified circuit is interesting as well!

 


True size.

 

The current PCB is succeeded by a smaller format 35 × 50 mm, where you can experiment with additional components such as with BFG135 or with other values of the components if desired.
If there is enough interest in this active antenna, PD0EEA can supply the PCB's.

Three complete antennas have already been made on request. 

 

 

 

  
Can be used for 2N3866, 2N5109                                                                                Universal     PCB

 SUPPLY

 

The amplifier even works with a voltage
< 3 volts, but a good average is 9 volts.

 

The current is then approximately 70 mA.
 

PD0EEA has designed a universal 35 × 35 mm PCB for the power supply, so that the circuit can be changed at will.

 

 


35 × 35 mm

 

LOOP CONSTRUCTION

 

 

 

 

 

 

 

We are still working on a sturdy and simple construction of the loop antenna. It could also be a 4 × 100 cm square. That's close to the circumference of 120 cm a circular loop. It can be made from Alu strip or Alu corner profile for strength. An angle up so that it becomes unattractive for birds to sit on.

 

 

 

 

 

 

LOW PASS FILTER

 

 

Sometimes one do not want a broadband system but a range of 1.8 - 30 MHz. This is possible with a filter and a low pass filter is shown.

See  also: https://pa0fri.home.xs4all.nl/Diversen/LPfilter/Hf%20low%20pass%20filter.htm

PD0EEA has simulated a filter with a program and has yet to be tested.


 

 

 

 

IC7300 WITH ACTIVE ANTENNA