I enjoyed the article about the EH antenna in the "Local Oscillator." The name of the newsletter is both clever and appropriately geeky. It shows that electrically-short antennas DO radiate, but their gain is low unless they are elevated above ground in order to create an additional radiating element.
The newsletter also mentions WDCX in Buffalo. I investigated further, and there is WDCX-FM on 99.5 MHz in Buffalo, and also WDCX on 990 kHz in Rochester. On another thread, the use of official-sounding call letters for Part 15 was recently discussed. In fairness to wdcx on part 15.us, I've listened to the very professional Dade City Radio web stream, and the call letters are not actually mentioned.
"The Kinstar is probably the newest accepted design. But the amount of land required for one is no different than a standard unipole two-antenna array and the Kinstar is non-directional."
Well it's main feature is that it operates at much less height. I guess you might say it's sort of the reverse of an EH antenna ... it needs land, it actually works as advertised. IIRC, the standard Maxwell equations and NEC analaysis apply.
But what about the height requirement for EH antennas where the antenna is high because of land elevation and not a tower with a long vertical transmission line?
A long vertical electrical conductor carrying RF current increases the radiation resistance of an antenna, and therefore its gain. The same does not occur with a short antenna mounted on elevated land, such as on the top of a hill.
the kinstar came about from zoning boards restricting the height of antenna towers in most locations.
it's intention was to lower the height required not the amount of land required.
This seems like a logical place to bring in a question I've been working on since day 1. How does one make an AM signal (3-meter antenna) go uphill?
My building is halfway up a long hill. The front is downhill, the back is uphill. Every antenna I try goes downhill with no effort, but every antenna, whether it's vertical, angled or any of the few variants as possible, gives only a weak noisy signal up the hill, often no signal at all.
See if this helps.
Radio waves propagate off of Part 15 AM antennas in a "donut" shape. This means there is a "cone of silence" straight up and down off the antenna. Near the antenna, the donut isn't very tall and tends to hug the ground tightly around the base of the antenna. So, the donut is pretty squished out.
As you gain elevation up a hill, the radiated wave runs into the hill and at the same time begins to attenuate. Elevation will bring you closer to the cone of silence. Medium wave signals above about 1300 Khz don't crawl over hills very well and are attenuated even more by poor ground conductivity. Dirt and rock will stop micro-power radio waves pretty completely at about 1600 Khz and above. Conversely, when you go downhill, there is less dirt and rock for the signal, propagating out horizontally, to run into. Trying to run signal through reinforced concrete structures, buildings and heavy forests is about the thing.
The further from the offending obstruction, there is a decrease in shadowing of signal that occurs up to the point that the signal can't be received anymore (penumbra effect). At some distant point, the signal may wrap around the obstruction.
One solution might be to raise the antenna elevation closer to equal to that of the hill. That is why some Rangemaster installations are elevated on masts. Elevation of the antenna and staying away from obstructions will help in maximizing the signal area.
Licensed AM BCB signals, of the 10 to 15 watt variety, notice the very same challenges you mention.
Marshall Johnson, Sr.:
Your comments have sparked in the brain first thoughts about raising the antenna. Up till now I've been sticking to the ground.
My favorite pass time is visualizing things. It's so much easier than actually doing something.
Maybe the attic? The side of the building? The chimney?
Not ready yet for getting into open space outdoors. If that were a choice, the antenna could just go at the top of the hill.
The fun has begun anew.
How does one make an AM signal (3-meter antenna) go uphill?
Why not just tilt the antenna so it is normal (perpendicular) to the hill? This would place the broadside lobe maximum in line with the hill.
Obstructions are significant at line of sight frequencies (above about 50 MHz) but less so at BCB and somewhat higher frequencies. I used to work a local ham net on 15 meters (21 MHz) when I lived north of hilly Cincinnati. We had reliable communications (ground wave after dark) all the way into northern Kentucky using modest dipoles and 200 W PEP. Police, fire, taxis, etc. used the 36 MHz band to good effect for mobile communications. Ground wave works for these frequencies despite "obstructions" and hills with the most common problem being daytime skywave interference.
Neil
Neil,
Glad you brought up the idea of tilting the antenna. I thought of that and tried it, even with more than 3-meters (just for a test), but it didn't matter. That made me wonder whether RF radiation sets up a relationship with earth gravity. Is that too nutty an idea?
Playing with NEC, tilting an electically-short antenna is shown to reduce the radiation resistance, and therefore the gain. This makes sense, because tilting the antenna reduces its height above ground. With perfect ground conductivity, there is no preferred direction when the antenna is tilted. Using the "real" ground mode of NEC, there is less directivity in the direction the antenna tilts downward. This is the "uphill" direction. NEC does not calculate the influence of ground very well, and so I don't have absolute confidence in this result; but I think it is more probably right than wrong.
As for gravity, it has been shown to have very little (but not zero) influence on the propagation of electromagnetic waves. The Eddington experiment in 1919 showed that the enormous mass of the sun deflects the light from the celestial bodies as seen from the earth by only about 1.75 seconds (a second is 1/3600 of a degree). Eddington's result challenged the sensitivity of his instruments, and may even have been in error by a considerable percentage. So, one should not expect any noticeable effect on electromagnetic waves by gravity.