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# Antenna Theory for the Thinking Man

A treatment of tuned, wire antennas as used on the HF amateur bands.

by Glenn S. Smith
Georgia Institute of Technology

5.2 ELECTRICALLY SMALL LOOPS. . . . . . . . . . . . . . . . . . . . . . . 5-2

The axial current distribution in an electrically small loop is assumed to be uniform; that is, the current has the same value I0 at any point along the conductor. For single-turn loops and multiturn loops that are single-layer solenoidal coils, measurements suggest that this is a good assumption provided the total length of the conductor (N × circumference) is small compared with the wavelength in free space, typically < 0.1 Lambda
5.3 ELECTRICALLY LARGE LOOPS. . . . . . . . . . . . . . . . . . . . . . . 5-9
As the electrical size of the loop antenna is increased, the current distribution in the loop departs from the simple uniform distribution of the electrically small loop. For single-turn loops, this departure has a significant effect on performance when the circumference is greater than about 0.1 Lamda.
5.4 SHIELDED-LOOP ANTENNA. . . . . . . . . . . . . . . . . . . . . . . . . 5-18
For certain applications it is desirable to position the terminals of the loop antenna precisely so as to produce geometrical symmetry for the loop and its connections about a plane perpendicular to the loop. This can often be accomplished by using the so-called shielded loop.

### Small Loops: Loop circumference < 1/10 Lamda

Physically small antennas can work; special attention must be paid to construction details such as the size of the tuned loop itself AND the tuning capacitor ... low losses in the antennas itself are a MUST if the antenna is used for transmitting.

Practical Small Antenna Efficiency (from Ref 1 below)

• What goes in must come out as RF or heat. Commonsense and physics says so. This applies to all antennas.
• Efficiency (Eff) = Power out (Pout) divided by Power in (Pin).
• An inefficient small tuned loop with 100 watts will get hot.
• Does your loop get hot? If not, then it is efficient.
• An inefficient loop with 400 watts will self-destruct. Plastic will melt. Solder will melt. Wood and paper will self-ignite (above 451°F or 233°C).
• Has your high power loop burnt up? If not, then it is efficient.

Commercial example, loop diameter 4 m:

### Intermediate sized loops: Loop circumference 1/8 to 1/4 Lamda

General:

Paraphrasing:

It consists of 64ft (19.5 meters) of wire.

The ends of this loop are connected to the tuning/matching assembly which consisting of a wide spaced capacitor of about 300 pf in full mesh and a simple ferrite matching.

This transformer is wound through two ferrite tubes with nine turns for the primary winding tapped at three turns, and two turns for the secondary winding the ends of which are coupled one to the loop and the other to the capacitor the other end of the loop is taken to the capacitor.

As it is not a high Q loop the bandwidth is in the region of 40 kHz on 80 m and 14 kHz on 160 m.

Example:

### Full wave Loops: Loop circumference ~ 1 Lamda

Example:

References:

1) Tuneable Coupled (Multi-) Mode Small Antennas — CFA, CFL, EH etc? (pdf file)
By Professor Mike Underhill -G3LHZ

2) All sorts of small antennas – they are better than you think – heuristics shows why! (pdf file)
By Professor Mike Underhill - G3LHZ