directional vs. multi antennas

Archived from groups: alt.tv.tech.hdtv (More info?)

MasterPaul1@webtv.net wrote in
news:16134-41CAE256-827@storefull-3157.bay.webtv.net:

> I live on the fringes of city limits, and receive ota signals pretty
> good with a pair amplified rabbit ears. Also I am fortunate to live
> high on a hillside and can receive signals as far away as 38
> miles!...with the gain off. Believe it or not the more local stations
> are more of pain to get.(10 miles away, and probably from tree
> interference). My question is this...I would like a more appealing
> look of a multi-type antenna, than a typical winged type on the
> outside of the house. 1) Do they work equally well?
> I saw a ampliefied version for about $75.00, and seemed from me from a
> common sense point of view, to be able do the job, without overkilling
> the problem. But I don't want to do it wrong either. vhf and hhf
> required.

There are two things to look for in an antenna system. Directivity and
gain. In your situation, gain is probably secondary, since you already
have plenty of signal. Directivity may help you with multipath
interference, though, which is certainly a problem for digital reception.
Gain usually accompanies such directivity (the antenna collects signals
only from the favoured direction and often has a virtual aperture that is
considerably larger than the physical antenna, especially in the case of
Yagi designs.

You may not even NEED an amplifier in your situation, though. Indeed,
I'd set up without one and see how it goes. It's entirely possible,
especially if several large sources of analog and digital signal are in
the same direction to have altogether too much signal, in which case an
amplifier will just compound an already bad problem. An attenuator might
actually be the answer there. If you have some way of checking signal
strengths across the UHF spectrum, that would tell you more.

You see, if there are in-band signals that combine in to produce in-band
third order products that afflict channels you are actually trying to
receive, then a 10db attenuator will knock these products down by 30db
while only reducing the primary signal by 10db, thus giving you an
improvement of 20db less interfering signal compared to the one you're
trying to watch. On the other hand, if you're already suffering from
this kind of interference (it generally occurs when your receiver's front
end is overloaded with in-band off-channel signals) and you put a 20db
gain preamp at the antenna and you have an antenna of modest gain, say
10db, then you're looking at 90db more interference! That's often enough
to make signals go away entirely. It's a balancing act. An antenna
amplifier may well help if the coax run is at all long and a GOOD antenna
amplifier (if you can get one with a GaAsFET go with it) should not
introduce third order products even with fairly strong signals. This is
more likely to be an urban problem, where you are a short hop from some
loud transmitters. Those of us way out in the suburbs can often find UHF
mobile service signals close enough to the TV bands to cause third order
problems, though. Fortunately, those UPS trucks, etc. tend to move on
and it's a temporary problem unless you live near the dispatcher's
transmitter. Note that it takes at least two and often three really loud
signals in or near the band to cause this effect.

LPDA (log periodic dipole arrays) and the simple bow-tie/reflector are
favoured choices for UHF (which is where most digital TV lives). Each
has its advantages and disadvantages. The LPDA is more apt to exhibit
side lobes that can bring reflected signals into play if they are strong,
whereas the bow-tie is apt to have less gain unless you stack them, but
should exhibit a cleaner (if wider) pattern.

Digital TV is a RADIO system, just like any radio system and, in order to
engineer it properly, you have to understand how signals behave at every
point in the system. For antenna design, this means knowing what is
happening in the space around the antenna and what the actual problems
are. Weak signals are only one kind of problem, and at reasonable
distances from the transmitters are not as common as people think.

On UHF, height and directivity are usually a plus, though you may have to
use an attenuator to get signals down to a level your receiver can
handle. One form of attenuator that actually works quite well is an
extensive length of CHEAP RG-59. I once did something similar when I
needed to attenuate the 14watt output of a radio into the input of a
1000-watt tube amplifier on VHF. I just stuck a length of RG-58 in the
amplifier's switched input line (and switched it out to receive).

--
Dave Oldridge+
ICQ 1800667

A false witness is worse than no witness at all.