Q: When is an amp not an amp?

A: When it's defined and measured differently, by global consensus.

The scientific community has redefined several System Internationale units of measure, including the ampere, in an historic agreement signed at Versailles in Paris this week.

Previously defined in theoretical terms involving measuring the forces of attraction or repulsion between two infinitely long conductors in a vacuum, the amp is now defined more pragmatically in terms of the passage of a coulomb per second, in other words a specific number of elementary charges (electrons or protons) passing through a conductor in a precisely specified period. 

The charges and the time are both defined as SI units and are measurable, hence the amp is also measurable ... although it's not exactly easy. Whereas the measurement of time is pretty well sewn up by measuring the frequency of energy changes in caesium, scientists are currently developing solid state devices to control and count the passage of individual electrons.

At least, that's my understanding as a radio amateur, a former scientist with an interest in measurement. I'm not a theoretical physicist. I don't count electrons, I put them to use.

Hourglass loop antenna

In "The Hourglass Loop Antenna" (QST December 2018, pages 35-37), John K4ERO describes the construction of two-wavelength wire loop antennas for three VHF/UHF bands. These are tall and thin with an insulated crossover in the middle, making a kind of hourglass shape. As described with the feed point in the middle of the bottom side, they are horizontally polarized.

Although I'm not interested in the VHF/UHF bands, I like loops. John's simple design is said to have a few dBs of gain by compressing the vertical radiation pattern towards the horizon ... which might be useful on the lower bands. If the size is practical, I could hang an hourglass loop from a tree or tower. It's also said to be a good match to 50 ohm coax feeder using just a ferrite choke balun.

John didn't provide any formulae in the article to calculate the dimensions for other bands, so I set about reverse-engineering them from the dimensions given:

By my calculation:

• Wire length = wavelength x 2.05 
• Loop height = wavelength x 0.85
• Loop width = wavelength x 0.18

So, for the 6m band, an hourglass loop would be roughly 5m tall by 1m wide. This would be simple to construct and hoist into place from a tree, using aluminium tube or bamboo spreaders top and bottom. It could be made lightweight enough to risk using, say, a fibreglass fishing pole as a support. Two identical hourglass loops might even be interlaced at 90 degrees to each other, perhaps using two feeders or a relay to connect one feeder to either antenna to switch direction. Alternatively, the antenna could be twisted with the top and bottom spreaders held at 90 degrees to each other, providing a nearly omnidirectional pattern for, say, a 6m beacon.

On 10m, it would be 8.5m tall by nearly 2m wide. This is also feasible for suspension from a tree, using a reasonably strong top spreader. Again, twisting the lower spreader relative to the top one (perhaps using twine to hold them in place) would make it closer to omnidirectional.

Down on 30m, the antenna would be about 25m tall by a bit over 5m wide. It would be trickier to construct and hoist from a tall tree, not impossible but getting impracticable. A one wavelength square wire loop suspended between 2 trees would be simpler and probably about as good, perhaps better being higher off the ground and further from the foliage.

As a trial, I might try making one for the broadcast FM band - 6.21m of wire in an hourglass shape 2.58m tall by 0.54m wide should improve reception of the scratchy stereo radio stations, although vertical polarization might be better with the feed point moved to the centre cross-over.

Thanks for the inspiration, John!