Monday, February 11, 2019

The HF folded dipole

Today's antenna, the HF folded dipole, doesn't seem to be that widely used among amateurs.

Maybe because it needs twice as much wire. Instead of just one wire split in the middle at the feedpoint, folded dipoles have two closely spaced wires joined at the ends. One wire is split at the feedpoint while the other is continuous. On the other hand having all of the element at ground DC potential is sometimes seen as an advantage with users lauding its quietness as a receive antenna.

Folded dipoles have a 300 ohm impedance.  Unlike other types of dipoles and verticals this makes them unsuitable for directly feeding with 50 or 75 ohm coaxial cable without a 4:1 balun. On the other hand the higher impedance can be beneficial for long feed line runs as loss is lower. Why? High impedance means relatively high voltage and relatively low current. The voltage drop caused by a given line resistance is less significant at higher than lower voltages. For a similar reason this is why electricity grids operate at many thousands of volts over their long distance transmission lines before being stepped down to the normal domestic AC voltage.

If resistors are installed at certain places a folded dipole can be made to operate over a wide HF frequency span without an antenna coupler. Performance-oriented amateurs don't like this due to the extra losses introduced.  Commercial and military users, who primarily use HF for strong-signal single hop communication, often prefer versatility and usability over efficiency.  For them a broadband folded dipoles, such as a T2FD, may be more appealing than it would to a distance-oriented amateur DXer or efficiency-minded QRP enthusiast.

I haven't worked with folded dipoles so can't speak from practical experience. But these people have so their accounts are worth reading.

Folded dipole antennas Video by W1GV
Notes on the terminated wideband "folded dipole" Evaluation by W4RNL (PDF)
Terminated Tilted Folded Dipole by PA0FRI - Dutch
The folded dipole W8JI finds negligible benefits in using them

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1 comment:

  1. Hi Peter
    Folded dipoles are a good option at HF, I have also been interested in large surface area HF antennas since reading up on quantum electro dynamics (QED). From that perspective capture area seems important and the increased conduction surface of a folded dipole could provide a greater area for the generation and interception of HF photons.


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