John,
I will agree with everything chuck said with the the exception of the following:
But for amateur use where few if any are running horizontal polarity, there is no reason to employ CP. And yes, that reduced gain factor has a big impact on reception. I have used CP antennas for reception in emergency situations (because they were already at hand and mounted high) and the results are what you'd expect- not stellar. They worked, but only as a temporary solution.
Please remember, that when a signal bounces - off a building, or off a mountain, or other propagation anomalies, it usually changes polarization.? In other words, what may be transmitted as vertical may not arrive at your antenna as vertical polarization.? It can arrive at your antenna at any polarization other than how it was originally launched. A lot of the mobile flutter you may hear when driving, is not just fading, but polarization flopping about. FM broadcast is rarely if ever circular to circular.? It's almost always circular transmit to linear receive.? There's a good reason why FM broadcasters choose to go circular transmit.
I use the term, "Commutate," meaning that the signal's launched polarization is spinning, not just vertical and/or horizontal.? It's basically rotating at the speed of the frequency.? So for 2-Meters that's 146-million RPM.? The desirable effect of this is that the signal is essentially "launched" at all polarizations.? So as you drive or move about, or hold your handheld radio at some funny angle, the polarization of the received signal is pretty much matched to the polarization of your receiving antenna.? It can also screw its way into places that a linear signal might not be able to penetrate. It's important that the transmit signal "commutates," and is not just vertical and horizontal.? It needs to be all polarizations, and in all directions to really be effective.?
If you have a properly operating circular antenna, you can make interesting measurements by taking a linear dipole on the end of a pole, go out into the far field, even a few miles out.? You probably need to go to some high point, such as the roof of a parking structure, put the dipole out on a long horizontal pole, and with the dipole stuck out facing towards your circular transmit antenna, rotate the dipole through 360 degree.? The maximum to minimum signal you receive is known as the "Axial Ratio."? A good number is 2 dB. Axial Ratio is the secret ingredient.
In the case of the WA6TDD's circularly polarized antenna, the antenna was mounted on a 10' pole above the top of the tower - there was nothing within at least 10 wavelengths in any direction to the sides.
The bottom line is, that a properly working circularly polarized antenna will make listening much more consistent and get into places that a linear antenna might not.? You have to drive it to really appreciate the difference.? It can more than make up for the loss of gain.
Burt, K6OQK
Burt K6OQK