Sense and NONsense about "a Magloop".
A tuned, 1/4 wave length circumference loop enables :
FACTS to remember : Dimensions and effectiveness of a
capacitive tuned loop.
The maximal circumference of a capacitive tuned loop is just under 1/4 wavelength. If such a loop is to be tuned to more than one frequency band, that loop has a circumference of 1/4 lambda for the highest frequency band to be tuned to. It has maximal efficiency on that frequency band, and enables a cheap small diameter radiator. When that same circumference (in meters length) loop is used on lower frequency bands, its relative circumference (in wavelength) is smaller. And its feed point impedance drops to a lower value. Its efficiency also tends to drop, causing the need for a thicker and more expensive radiator in order to minimize losses.
Some people state that a loop radiator should be constructed out of one piece, and very thick copper. Every tenth of an Ohm series resistance should be avoided for losses reasons. Even tin-soldering should be avoided. That is not always true.
A very small loop (f.i. 1.3m diameter = 4m circumference = 1/20 lambda for 80m) is useable for reception. Even a perfectly constructed small loop (having f.i. an 8cm thick copper radiator), is showing disappointing properties when used as a transmitting antenna. With 100W power It generates large currents (75Arms) and high voltages (10kVp). Its losses are 10dB or more !
A maximal big 1/4 lambda circumference loop generates the lowest values of currents, voltages and losses. A 20m circumference loop for 80m, having a thin radiator of only 1cm diameter, shows low losses of only 1.8dB.
Why are losses in small loop antennas so high?
I think :
The cause is the relative short length of radiating circumference. Such a loop only can radiate energy, by running very high loop currents. But these currents are limited by the RF-surface resistance of the loop (skin effect). The current is mostly heating the copper surface resistance, instead of being radiated.
So the loading to a small circumference loop is mainly done by dissipating heath in the loop RF resistance. This RFresistance is determined by the specific material resistance, and the thickness of the radiator.
The radiating and electric properties of a (very) small tuned loop are thus determined by the dominant losses caused by the RF resistance of the loop. The damping caused by radiation is relatively small.
With unwanted results :
- very high Q
- unpractical small bandwidth
- very high currenst and voltages
- large heath losses
A high Q is wanted for non-radiating coils.
But a transmitting loop antenna must radiate as good as possible. A large radiating surface, thus a large circumference is
therefore needed. The loop-circuit will then be more damped by radiating its energy, resulting in :
- lower circuit Q.
- wider bandwidth
- lower currents
- lower capacitor voltage
- far less heath losses
A 66pacific-calculation for a very thick but very small loop radiator :
circumference 1/20 lambda (1,3m diameter),
radiator maximal thick !! 8cm !! dia. copper,
- The maximal possible efficiency is only 11% (-9.6 dB), compared with a 1/2 lambda
- Its Q is enormous high (f.i. 5400).
- Resulting in enormous RF voltages at the tuningC (with 100W up to 10kVp).
- Even vacuum and high voltage ceramic capacitors can show flash-over.
- Its bandwidth is only 0.7kHz, unsuitable for SSB.
- Accurate tuning on 80m is impossible. One kHz detuned means to high SWR.
- The loop current is enormous high (with 100W up to 75A).
- It is heating the loop with 89W. Radiating only 11W.
- The feed point impedance is unpractical low (f.i. 0.018 Ohm).
- A matching transformer should have an impossible Pri : Sec ratio between 53:1 to 26:1.
- Matching to 50 Ohms must therefore be done by a loop or capacitive, causing unbalance (TVI/BCI).
G0CWT measured currents around a 1/4 lambda loop. These currents varied, had NOT a constant value
The 66pacific-calculations only give good results for loops having a nearly constant current around their circumference.
Calculation example for a relative big tuned, thin copper (1cm dia) loop :
According to the 66pacific-calculations, the efficiency of a loop, compared with a 1/2 lambda dipole is :
a. 1/4 lambda loop : -2dB. In practice it can be a little better as the surface currents are not constant.
b. 1/8 lambda loop : -7dB. This is 3dB better than a very thick 1/20 lambda loop !
Compared with a smaller loop, a 1/4 lambda circumference loop shows the following better properties :
- the Q is much lower
- loop currents are much smaller resulting in
- less losses
- 7dB better RF radiation
- much lower capacitor voltages
- much larger tuned bandwidth
- the feed point impedance is much higher (up to 22 Ohm) resulting in
- easier matching
- smaller copper thickness
- much lower costs
Very small loops are very good for receiving purposes, not for
With positioning the tuning capacitor at the lowest point of the loop,