A Poor Man 27MHz Magnetic Loop Antenna
Maybe my opinion is true… the average radio amateur in Indonesia will know about CB 27MHz radio, which was booming in the 80s.
I was one of the many radio amateurs who delivered CB 27MHz before getting to know other favorite HF bands like 80M, so it is no exaggeration to say that to this day I still have junk (some of which are still functional) left over from CB-er's glory in Indonesia. .
Recently, if we took the time to tune back to this band in the city of Bandung, I can confirm that on channel 30 (27.305MHz) there are still quite a few Partners working on that frequency. Personally, I feel that this band is the magic band, to use the term AR friends, propagation in this band is rarely open, so DX communication is also quite difficult. However, when the propogation is opening, AR in a country outside the fence can be heard clearly and big like our neighbors… hehehe.
OK, I won't comment much about the uniqueness of this CB (Citizen Band) band, I just tried to share my experience in making a small antenna, but it seems that it is rare to use it in the 11 Meter Band, namely 27MHz Magnetic Loop.
I deliberately wrote the title "A Poor Man" ... because I made this antenna with makeshift materials, so that physically it has no aesthetic value hehehe ...
Before making this antenna I will remind you that the voltage generated on the Varco (Capacitor) can be up to thousands of volts, even though the TX power used is only 10W ...
"CAUTION VERY DANGEROUS ... CAN THREAT YOUR LIFE" ... "RISK THERE ARE IN YOUR HANDS OF EACH" ... !!!
Maybe the Magnetic Loop Antenna is one of the strange antennas and is like magic, according to the experts he only works on the Magnetic Field component, sorry I am also still confused by the experts' explanation, so please don't ask me about this explanation huh ... hehehe.
Some of the interesting things about this antenna are:
The physical size is very small, where the largest dimension of the outer loop is <0.25 lambda, making it suitable for ARs with limited space.
Reception is quite silent, because BW is small so the adjacent frequency can be muted properly.
Because it is able to dampen the adjacent frequency well, it will provide another feature, which is free from TVI (or interference to the TV broadcasting band) - clean operation.
The efficiency of the antenna is quite large, with an outer loop diameter of about 80 cm at 27.305Mhz and it can get an efficiency of about 82% (a half-lambda dipole with perfect material can reach about 100% efficiency, but its size is about 5.5 meters).
Anyone can make this antenna well, even if they do not have sufficient carpentry skills and antenna knowledge.
Big bandwidth ... Well looo ... above just said bandwidth is small, how come now says big bandwidth? What does it mean? For a certain capacitor value, the BW antenna is indeed very narrow, but if we complete the Varco it can be tuned remotely, then this antenna can be used for several bands at once.
Well ... if like this who is not interested in the small antenna "Magnetic Loop" ... ???
OK, it would be unfair if we only applaud the advantages of this antenna, I will try to convey the shortcomings of this antenna as follows:
Small bandwidth ... BW is like 2 blades, one side can be an advantage, the other side can be a drawback. So, because the bandwidth is small, we need a Varco that can be tuned from the operator's desk so that it can operate to other bands.
Efficiency is still below the standard 1/2 lambda dipole antenna. But this is a natural law… You Get What You Paid… or Ono Rego Ono Rupa… hehehe
Magnetic Loop Antenna Support Software and Materials 27MHz
There are many free magnetic loop antenna design software on the internet, one that I like is from ARRL which can do calculations online, please try it here: http://www.66pacific.com/calculators/sm ... _calc.aspx
The design results for the frequency 27.305 MHz (Channel 30 CB) with the help of ARRL software and field oprek are as follows:
The circumference of the outer circle is 2.5 meters, the software provides a guide for this frequency, the circumference of the outer circle must be between 1.33 meters to 2.66 meters. The larger the circumference of the outer circle, the higher the BW and efficiency of the antenna.
The diameter of the conductor used for the outer circle, I use the RG8 coax, which is 1 cm (10 mm), the larger the diameter of the material used, the greater the BW and antenna efficiency.
Varco Logam 0-250pF, it takes a capacitance of around 10-15pF, because the Varco I use is too dense, so I replaced it with a capacitor of the coax RG8 along 15 cm (tunning the length to the best SWR). When I used the RG58 as a replacement for the Varco, it couldn't hold it