One more antenna question: Antenna pitch?

That's an interesting question. I guess to some extent, it depends on the frequency involved, and how close you are to the transmitter. I feel that the potential losses are likely to be far worse at UHF than VHF. An example that comes to mind is my local ATV repeater. It is sited on a hill, and I clearly remember some of us helping out a new licensee, who was located within line of sight of the transmitter mast, about 3/4 mile away, and at the bottom of the hill. He could barely pick up a signal from it, and could not access it with his own 2W transmitter at all. This was at 23cms (1.2GHz) so a little above the top of the UHF band. We did some field strength checks, and found that there was quite a distinct 'shadow' around the transmitter site, out to a distance of around a mile. The repeater's antenna is a slotted waveguide.

The first thing that we did was to re-site his antenna pair onto a taller mast, which produced a significant improvement to his situation, but was still not good, considering how close he was to the repeater. The final thing that we did was to tilt his antennas up at a similar angle to the hill itself, and this brought about the improvement to solid P5 copy and repeater access, that we had been expecting. I believe I have read that it is common to find this shadowing effect around high power TV transmitter masts, and that it can extend out to several miles in some instances. Michael T could probably comment better on this as he was involved in the industry.

So, I suppose that on paper, the answer has to be that there will be an effect on received signal strength with the antenna tilted down, but it will probably not be enough to notice on a modern TV set, if the signal is not marginal and hovering on the set's AGC threshold in the first place. If you take it to its logical conclusion, if you stood the antenna on its 'nose', it would receive virtually nothing. At some point, horizontal or pointing slightly *up*, you would be receiving a maximum signal. Anywhere inbetween must, in theory at least, be less than the potential maximum.

Of course, now we're talking digital TV transmissions, all bets are off on this ... :-)

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Digital signals travel through space the same as analog signals at the same frequency. The best reception is for the antenna to point directly at the transmitter. If there are reflections, then pointing either upward or downward from the transmitter maight improve reception. Digital formatting of the signal may make reflections more or less of a problem than it was with older analog signal format.

Yes, it matters; the main part of that long antenna is 'director' elements, which create a kind of shadow on the active dipole. The rays get past if you aren't roughly lined up with the line-of-sight to the transmitter.

Also, such a long antenna has high 'gain' which means it MUST be aimed carefully; how can you possibly adjust it if it's bumping into the roof?

Digital signals travel through space the same as analog signals at the same frequency. The best reception is for the antenna to point directly at the transmitter. If there are reflections, then pointing either upward or downward from the transmitter maight improve reception. Digital formatting of the signal may make reflections more or less of a problem than it was with older analog signal format.

I see that you're well into all the media and government hype about how good digital terrestrial TV is then, Bob ...

The (tongue in cheek) point I was making had nothing to do with the format of the modulating signal, which of course has absolutely nothing to do with how well or otherwise, the carrier propagates through space. I was actually referring to less-than-ideal reception circumstances such as those the OP suggests that he may have, which with an analogue signal, may well give perfectly acceptable results, but with a digital signal might result in digital cliff pixellation and freezing. Digital terrestrial TV signals are, in my experience here in the UK, nothing like as robust, or easily received in many transmission regions, as the government - whose primary interest lies in how much money they can make by selling off large chunks of the UHF TV band to cell phone operators - would have Joe public believe through their hyped-up and often misleading media campaign directed at the subject ...

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I agree with everything you said. People tolerate ghosting much better than digital receivers do. I have two digital convertors that I play with, but my household tv is on a cable system that a couple of months ago converted to a special digital transmission on the cable, not the same as the on-the-air digital system that I use the 2 convertors for..

When we had wet weather, I would lose digital reception on any/all combinations of the 3 digital cable convertors that my cable company provided. I complained and got one unit replaced, but still had the problems. I complained again and they sent a "trained" repairman out. He immediately said it was my internal house cable distribution system that was at fault. I replied that unless my house had a bad roof leak, there was no way that the weather should affect my cable reception. He finally went out to the telephone pole in my back yard where the cable came out of the ground and went up the pole to a tap on their cable. There he discovered, right at the tap, that a friendly squirrel had chewed through the outside weatherproofing and that water was getting into the cable. A new drop from the pole to the house solved the problem, and hopefully when the next squirrel gets hungry I won't have such a problem getting things fixed.

I did tv repair work in the 1950's to put myself through college and worked on color tv right after the FCC dropped the CBS color wheel in favor of the RCA system. Almost worked at Hazeltine Labs because they acquired many of the RCA color tv patents, but Bell Labs offered to pay for me to get my Master's degree and so I ended up at Bell Labs. Was there for 44 years and loved every minute of it. Ended up in EMC work which was a great adventure as no two days were ever the same.

You don't want or need a 13 ft beastly antenna that can get channels

2-6. This little guy will be much easier to handle in the attic (though it's better outside) and has reasonable gain. Don't jet fighters release 'chaff' to screw up the enemy? Seems a lot like nails suspended in sheets of plywood. That's one reason to be outside of the 'chaff'.

BTW there are no channels >51 any more.

G=B2

Arfa Daily Inscribed thus:

Well Said ! Yet another make the people pay Government scheme, among other things.

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Best Regards:
                Baron.

I may be confused about what "gain" is. Why would a high gain antenna have to be aimed carefully? If it has high gain, it seems like it would have some gain to spare if it were badly aimed.

If it had low gain, it seems like it would have to be aimed precisely.

High gain means "narrow bundle" , so a high gain antenna needs careful aiming.

So what aobut people on mountains and in valleys, where the transmitter is lower or higher than they are. Should their antennas be tipped down or up to aim at the transimitting antenna?

I never hear anyone recommend that, and afaicr (and I don't spend much time near mountains, but some), every outdoor antenna I've ever seen has been horizontal.

Let's drop back a few yards and punt a bit.

Antennas are sensitive to incoming RF (and convert it into an electrical flow) because the RF induces current flow in the antenna elements. Similarly, if you push power into an antenna (as a transmitter does), the current flow in the elements causes power to be converted from electrical current into RF electromagnetic fields. These fields propagate away from the antenna in all directions.

There's a famous principle in radio known as the Reciprocity Rule. It means, basically, that an antenna's pattern of RF power transmission (when transmitting), and the antenna's pattern of sensitivity to incoming RF, are identical.

Consider the most basic, primitive sort of antenna which one can conceive of. It's equally sensitive to RF arriving from any direction, and when fed power from a transmitter it transmits an equal amount of power in all directions. Looked at in a graphical sense, the transmitted power pattern is a perfect sphere (and so is the receive sensitivity pattern).

This is referred to as an "isotropic" antenna. It's a purely theoretical antenna - no real, physical antenna of any sort can be truly isotropic.

All real antennas have a directional sensitivity to RF - they're more sensitive to RF coming from some directions than from others, and they tend to radiate more power (when transmitting) in some directions than in others.

When one talks about the "gain" of an antenna, one is talking about a ratio.

amount of power transmitted in the "best" direction Gain = --------------------------------------------------- power transmitted by an isotropic direction

This figure is usually expressed in decibels - take the base-10 logarithm of the numerical radio, and multiply by 10. A gain of 10 dBi means that the antenna is transmitting 10 times more power (in its best direction) than an isotropic antenna would. 20 dBi means it's transmitting 100 times more power than an isotropic antenna would.

Now, when transmitting, it should be clear that the antenna cannot possibly radiate more power than it's being fed by the transmitter. It's not an amplifier or an energy-creator - it's just an energy

*transformer*. So, when the antenna has gain over an isotropic antenna, and is transmitting more power in one direction than an isotropic antenna would... and yet the *total* power being radiated is the same... this means that the antenna must necessarily be transmitting *less* power than an isotropic antenna in some other directions.

Similarly, this antenna will be more sensitive (to received signals) than an isotropic antenna, *if* the signals are arriving from the favored direction. Since the transmit and receive patterns are the same (per the reciprocity rule), the antenna will be *less* sensitive than an isotropic to incoming signals from other directions.

In effect, what antenna gain is "doing", is taking the perfectly spherical sensitivity pattern of an isotropic antenna, and "squashing" it like a balloon full of water... it ends up being squeezed in some places (less sensitivity) and bulging out in other places (higher sensitivity).

The most basic "squashing" occurs with the simple dipole antenna - just a single half-wavelength element. The pattern for a dipole looks rather like a donut (rather than a sphere) - it's squashed above and below, and bulges out a bit towards the horizon. A dipole antenna has a gain of 2.15 dBi, plus or minus a bit.

[Some antenna designers describe their antenna gains as "gain, compares to that of a half-wave dipole" - this ratio in decibels is called "dBd". Other designers use dBi (ratio compared to an isotropic), because the figure in dBi is always 2.15 larger than the equivalent figure in dBd, and it makes the marketing easier :-]

The pattern for a typical TV antenna looks a bit like a searchlight beam... it bulges out far towards the horizon in one direction, and is doesn't come out very far at all in other directions. The higher the gain, the narrower (and further-out-sticking) the bulge towards the horizon.

That's the problem with a really high-gain antenna. Of necessity, its sensitivity pattern has a narrow lobe... if you turn it more than a small amount away from the transmitter, then the transmitter falls outside the main lobe, and you can get *less* signal out of the antenna than you'd get with a lower-gain antenna (having a broader sensitivity pattern).

You pay your money and you take your choice... high gain (and narrow directional sensitivity) or lower gain (but a wider pattern). People out in a "deep fringe" reception area sometimes don't have a choice... they must use a high-gain antenna, and aim it very precisely (often with the aid of a remote-controlled antenna rotator).

Folks in most urban and suburban areas can usually get by with a lower-gain, broader-pattern antenna.

Now, things get tricker if you start adding *electrical* gain to the system, by adding an amplifier between the antenna and the TV. This electrical gain will increase the antenna's system's output equally, for signals arriving from any direction. This may be good, bad, or indifferent, depending on the specifics of your actual situation... amplifiers will also amplify incoming noise and interference, they always add some noise of their own, and if they're hit by a strong signal they can overload themselves or overload the TV set and actually reduce the signal quality.

has further information, if you want to go into it in more depth.

--
Dave Platt                                    AE6EO
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It turns a little bit into nit picking, but Yes try to aim as accurately as possible at the transmitter, especially with multi-element antennas. And keep in mind,that an antenna cannot look through a big hill or a mountain. And only with some difficulty through your house/roof or that of the neighbour. For the hill/mountain, you might need a repeater on that hill/mountain top. Or try to find a reliable reflection around those obstacles. In my country the digital tv is polarized vertically, so although we can use the same old antennas, they have to be turned on their sides, and vertical aim is even more important.

attic,

I thought this one didn't get 2-6, but I should go check again.

Good point. Not only that, the roofer used nails that are a lot longer for the second roof than they had used for the first roof. And my hair isn't as thick as it used to be if that matters, but I'm more afraid I'm going to jab my head then I used to be.

Thanks a lot.

Very interesting. Really :)

What about places where there is line of sight between the transmitting antenna and the home antenna, but the home is a lot lower or higher than than the T-antenna. Does the home antenna need to be tipped up to point to a transmitting antenna that's higher? It sounds like that follows from what was said ealier in this thread, but I've never seeen it done or recommended.

Well, you aim carefully at the horizon, and hope that enough radiation is following the earth surface. Some bending down always occurs, and hopefully for you it is enough.

You may get a slight increase in signal by uptilting the receive antenna.

However, this is hard to predict, and usually not a terribly significant factor, for two reasons:

(1) The amount of up-tilt you would need is not likely, in most cases, to be more than a few degrees. It's probably less than the "half-power vertical beam-width" of a typical TV antenna... likely quite a lot less. If, for example, the transmitting antenna is only 2 or 3 degrees above the horizon, and the receiving antenna's vertical pattern has a half-power beamwidth of 10 degrees or more (which would be the case for all but the longest, highest-gain TV antennas), then the amount of power you'd be giving up by not uptilting the antenna is negligible. (2) The signal path from the transmitter to your antenna is complicated by reflections off of the ground, nearby buildings, hills, and so forth. You might actually find a higher-quality signal by pointing your antenna slightly away from the transmitter, if by doing so you picked up a particularly strong reflection, or _avoided_ picking up a side reflection which was causing multipath distortion. The cases in which an antenna needs to be pointed very exactly (both horizontally and vertically), are those in which it has a very high gain and thus a narrow beamwidth. How often will you be far enough away from a transmitter that you need an antenna with this much gain,

*and* be so far below it that its position is a significant distance above the horizon and would need to tilt it up by more than a few degrees?

I don't think this combination of circumstances is at all common. If it's close enough and high enough that it's far above the horizon, then you're almost certainly able to use a low-gain antenna with a very broad vertical pattern, and will get plenty of signal.

--
Dave Platt                                    AE6EO
Friends of Jade Warrior home page:  http://www.radagast.org/jade-warrior
  I do _not_ wish to receive unsolicited commercial email, and I will
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A small amount of tilt won't make a significant difference. Those that suggest tilting the antenna will increase the gain are misguided. The antenna can take advantage of ground reflection up to a theoretical

6db of gain. This gain is seldom reached but the in phase reflected e field that causes this gain is best achieved when the antenna is horizontal.

In any situation, if you have done this before, sometimes it helps and sometimes it doesn't. You are also likely to see similar results rotating the antenna from horizontal. The transmitted waves change the plane as it moves over obsticles, so one station may work better tilted 10 degrees, but it may make the other station worse.

Gain is made by narrowing the beamwidth and providing more front to back ratio. A NASA 85 foot dish has about a 1 degree beamwidth at 2.3 gHz.

greg