Question for Antenna Gurus

Lurf

Not wanting to get incredibly technical but still having this desire to "know how things work", could someone write a short tutorial on how to interpret the radiation patterns which seem to be a great sales tool for each antenna model. For example, why is one antenna termed a "dog" above channel 50 but "excellent" below channel 30? How would I know that just by looking at the radiation pattern? What makes one antenna great for long range while others are not? Is being more directional better than not? Thanks. I think this would be very helpful to many of us.

rbinck

You are kidding, right? Tiger?

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coyote

IMO it's just common sense.

Assuming a company isn't putting out absolute junk, any outdoor TV antenna will fall into one of 3 categories:
VHF
UHF
VHF/UHF

The bigger the antenna**, the longer reception range it will have - and the more directional it will be. So it's a trade-off. If you live within 10 miles of NYC you'd want a fairly omnidirectional antenna as there are multiple signal sources in different places and all within close range. But if you're 50 miles away you'd want a fairly directional antenna, as all those sources would look like one from your antenna's point of view.

That's all there is to it.

**big = length + # of elements

tigerbangs

Rather than reinvent the wheel, have a look at http://www.hdtvprimer.com. I think the information you are looking for can be found there...somewhere.

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maicaw

-this site has been referred to in this forum many times and should best answer your questions at a non tech level - Fundamentals of Antennas.
the list at the bottom of this page RADIATION PATTERNS links to radiation patterns for 41 different types of antennas -so you can compare them all
or more gnerally ARRL Antenna Handbook which gets a little technical - but it is written for hams - not academic folks - and it's been around for over 50 years - used book store maybe or library probably - The Arrl Antenna Book (19th Ed./Bk&CD-ROM)by R. Dean Straw that edition has grown to 800 pages and over $50 with CD-ROM --
I'd look for the older ones - mine is about 250 pages - mostly not TV band - more shortwave and mobile band - but the principals are the same - except that TV signals are only horizontally polarized and mobile/marine is usually vertical polarization -
This later edition is cheaper - but no inside view

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maicaw

I just discovered something neat on Amazon - If you go down to this part of the page and click on concordance - then the word in the concordance you are interested in- say "radiation" - you can sign in to amazon and it will let you read all the pages with that word - try it using the word pattern in the concordance - -I think you can read all the pages this way - hope they don't mind me posting an image I purposely degraded and noised up [it does say copyrighted]from a screenshot of the upper part of one of the pages with pattern in the text - the pages they provide - 100's in this instance -are crystal clear - here's the back cover blurb BTW]
way to go amazon

Attached Images

pattern.gif (10.4 KB, 283 views)

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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

Lurf

Reg:
Thanks so much. I asked specific questions and you gave specific answers. I spent about two hours reading the links suggested by you and others yesterday. I can indeed see that the net gain graph seems to indicate that the DB-8 might be a better antenna for my purposes. As I said, I don't have a preamp but the RG6 from the antenna connects to a splitter in the attic about 25' distant and then goes about another 40' to the main HDtv. A shorter run of about 20' of RG6 comes off the splitter to a bedroom HD tv. I understand that the splitter is reducing the gain by some amount. Should this be significant?
Thanks again for taking so much time for your good explanations.

tigerbangs

Any gain advantage that the Antennasdirect DB8 shows above channel 55 is a function of having it's receiving elements shorter than those on the Channel Master 4228. It's hard to recommend the DB8 when it is literally twice the price of the 4228. I have documented in previous posts that if anyone needs higher gain at higher frequencies on a 4228, Channel Master has provided scoremarks on the tines of each bow-tie, which when cut off, raises the gain of the 4228 as much as 7 DB at channel 69. Channel Master used to document this in the literature, but, it isn't often an issue except where channels above 60 are used, as in Philadelphia . Since channels above 52 are going away after the conversion to digital is complete, I think that Channel Master chose the right course of action in keeping it's antenna bowtie tines longer. The other advantage is that the 4228 is effective on VHF at channel 7 and above, which makes it a good one-antenna choice for those folks with high-band VHF digital signals in their area.

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Lurf

What about the weight issue? Is galvanized steel a better antenna receptor than aluminum? I read that the 4228 is 15 lbs.+ Seems like a lot of weight to have swaying in the wind.

maicaw

It's 8 pounds -out of the box -- mine was still laying out in the back - too rainy to put it up - so I just weighed it - the box probably weighs more than the antenna Warren Electronics will sell you one for about $55 including shipping - big CM dealer (reliable and fast)

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RegGuheert

That's something I didn't know! I'm glad I stuck with your advice and orderd the CM 4228. It's clearly the best of the lot for the DTV channels that will remain for the long term. Thanks again!

coyote

I have the CM 3016 VHF/UHF/FM antenna. I'm approx. 20 miles from NYC, with decent line-of-sight (some trees & other minor obstacles) to the Empire State Bldg.

I'm getting GREAT reception on all available HD channels: CBS, NBC, FOX, ABC, UPN, WB, PBS. I'm also getting a variety of other digital channels, analog channels, and a second PBS HD feed (NJN) different from the first (NET). In addition I'm getting good FM reception.... this antenna was the best choice for me.

Lurf

Maicaw:
Check the link below. The author of this 4228 review must have his antenna leads crossed.

http://www.hdtvprimer.com/ANTENNAS/cm4228.html

maicaw

yep - says 15# alright but he's wrong - I stepped on my digital bathroom scales with it in my hands and then without and it is 8# - and it is a real Channel Master 4228 - says so right on the box - which doesn't list the weight - but it weighs as much as the antenna- so visualize what standard flimsy cardboard box that size would weigh- so they are probably using gross shipping weight - mine is steel conduit and steel mesh -so they couldn't make it weigh more than 8# unless it was made out of lead or gold - besides your rotor,mast and tripod or guy wires will be at least 20# even for a 8# antenna - I'd worry more about having it shift in the wind -which for a bulky high/gain antenna like that can be the problem with a weak mast or a cheap rotator -can you say RS - that can't point in the same direction twice or won't hold it's direction against wind gusts