RegGuheert

Hi Lurf,

I will attempt to answer the specific questions without trying to give a general tutorial:

You are clearly referring to the quote at the bottom of this page:

http://www.hdtvprimer.com/ANTENNAS/w8800.html

While I agree with this statement, I must say that I find it unfortunate that he wrote that statement on the page with *only* antenna patterns on it, particularly antenna patterns which have no gain numbers on them. While you *could* come to that conclusion by looking at antenna patterns, it is quite a difficult task. However, by looking at curve "E" on the "Net Gain" plot of the following link, that conclusion is easily drawn:

http://www.hdtvprimer.com/ANTENNAS/comparing.html

Let me try to explain. While antenna patterns are useful for estimating the ability of an antenna to do important things like reject multipath distortion (which arrives from a different direction than the main signal) or to reject the strong transmissions from nearby transmitters, most of us are *primarily* concerned with obtaining *enough* signal to get it out of the noise that is all around.

To do this, it is most common to *ignore* all of the contour plots except for what is known as the "boresight" of the antenna. That is the direction in which the highest gain occurs. In the top link given above, it is defined as 0 degrees in both elevation and azimuth. This assumption is often quite valid for a well-pointed steerable antenna and can be valid for fixed antennas which have been pointed at a particular transmitter. However, it will, in general, be invalid for other transmitters which are not at, or very near, the boresight.

All that said, calculations and/or measurements of the gain of the antenna are most frequently done at the boresight position and swept over frequency. Frequency is a very important parameter for all wireless systems, but especially for an extremely wideband system like a terrestrial television receiver. Gain versus frequency (actually, channel number) is what is provided in the top two plots of the second link above. Your antenna is represented by curve "E" in both plots. The first plot is a calculation of how much gain your antenna has at its boresight for all the TV channels *assuming it is ideally matched to the impedance of the receiver*. However, in reality, you have connected a 300-ohm/75-ohm balun and a long 75-ohm lossy transmission line. Together, these should look very much like a 300-ohm resistor to the antenna. If the antenna is not well-designed to drive the load which you are providing, then the curves shown in the "Raw Gain" plot will not be accurate. Instead, you should see curves that look more like the "Net Gain" plot.

Considering just curve "E" in those two plots, you can see that the "Raw Gain" (think of it as *potential* gain) of your antenna is outstanding. It is very high and very flat. However, when coupled to a 300-ohm load, it no longer looks so nice at higher frequencies (channels). Specifically, the PR4400, which is simply 1/2 of a PR8800, has *more* gain than the PR8800 at channel 42. In theory, the PR8800 should have about 4-dB *more* gain than the PR4400. (The reason it is not 3 dB is explained in the note here: http://www.hdtvprimer.com/ANTENNAS/cm4228.html )

(Just for completeness, the issue of net gain is a much more "cloudy" problem if you have a preamp in your system. The reason is that the input impedance of the amplifier is almost certainly not as close to 75 ohms as the impedance of your RG6 cable. I have yet to see an impedance plot for the CM 7777 preamp that I am using, so I'm a little concerned about the net gain that I will achieve in my system. If anyone has a plot of that, I would appreciate the input.)

With the above understanding, and looking at the channel frequencies of the Baltimore stations (38, 40, 41, 52, 59, etc.) you might be better able to see why I recommended the AntennasDirect PR-8 for your application. It seems to be the best OTA antenna for the higher UHF frequencies. (I might also end up with this antenna if I find I am getting DC fine and that an additional 2-dB could make Baltimore stations viewable.)

Net gain is the main feature involved.

Yes. In the world of antennas, "more directional" is somewhat synonymous with "higher gain".

Anyway, I hope this is helpful to you and others. These concepts are quite challenging, even for many electrical engineers who have not worked with communication systems. While I've worked with wireless systems for 20 years, my focus has been largely on circuitry. As a result, antennas are still somewhat "foreign" to me. (You must admit they are strange beasts!) However, I've worked closely with some of the worlds *best* antenna designers and have picked up bits and pieces of antenna understanding from this association.

Thoughts?

Reg