Vond het volgende stukje in een ander forum...
misschien toch wel intressant
TV Antennas for Scanners?
There is some controversy over the use of TV directional antennas for
scanners and scanning. This article lays out the facts and the
substance of the issue so folks can see, calculate, and decide for
themselves.
So why would you choose a TV antenna for your scanning post? To hear
all there is to hear, for one thing. The GAIN of the antenna alone
will enable you to hear far beyond your present range with an omni
antenna. But there's more! Big city environments are RF garbage
dumps, right? Noise and interference wreak havoc and consternation on
all but the best scanners, and sometimes even them, too.
Say there is a nearby repeater or pager transmitter that overloads
your scanner's front end. The expensive fix is to buy a notch filter
or a tunable filter and notch out the offending signal. Often it
doesn't work. A TV antenna will work! Sure! Just rotate the antenna
until one of its side or back nulls is pointed at the offender, and
the results can be startling! Oftentimes, the offender will disappear
into the noise, even if you're tuned to the sucker! When the offender
is nulled out, the antenna will still receive signals from the forward
and other directions.
A TV antenna has many peak and null lobes around its 360-degrees of
view. The deepest nulls will be on the back while the highest peaks
of gain will be off the front. Nevertheless, there are a whole slew
of minor peaks and nulls on each side of the antenna between front and
back, and these can be used to great advantage when selecting a
desired signal and rejecting one or more undesired ones. The list
goes on, but time and space force me to move on. You get the idea?
If you choose to use a directional TV antenna for scanning, it will
work wonderfully well between about 30 MHz and 1 GHz and better than a
discone down to 25 MHz and up to 1.3 GHz. The caveat here is HOW to
install it......NOT the same as for TV reception.
A TV directional antenna used for scanners and scanning should be
mounted in the VERTICAL plane....that is, with its elements pointing
into the ground and into the sky with the boom retained in the
horizontal plane as for TV reception. A short mast should be rigged
into the normal mount of the antenna....say 2-ft to 3-ft, max. This
side mast must mechanically join the real mast that rises vertically
from the rotator. Any number of methods of making this a rigid,
lasting mount can be employed and will not be presented here.
A TV matching transformer (300 ::75 ohm balun) should be fitted to the
antenna as is customary for coax feeds. Then use RG-6 satellite cable
from the other end of the matching transformer down to the scanner.
Use gold plated Type F connectors on each end of the coax. Down at
the scanner, use a gold plated Type F-to-BNC adapter to mate the coax
to the scanner. Voila! A low cost, high performance directional beam
antenna for your scanning post. Now here is why the sucker works:
Consider:
TV antennas for the USA are expressly designed as follows:
54 - 88 MHz Ch-2 - Ch-6 (34 MHz Passband)
88 - 108 MHz FM Broadcast (20 MHz Passband)
(Good for about 30 MHz to 158 MHz)
174 - 216 MHz Ch-7 - Ch-13 (42 MHz passband)
(Good for about 130 MHz to 324 MHz)
470 - 890 MHz Ch-14 - Ch-83 (420 MHz Passband)
(Good for about 352 to 1.3 GHz)
In order to get specified performance at the band edges, the
antenna has to be "overdesigned" by the mfgr, meaning that positive
gain and useful F/B ratio will be apparent well outside the bands of
design.....i.e., the TV bands.
Now let's look at the gaps in the above spectrum which include
desired scanner frequencies:
25 - 54 MHz (29 MHz gap)
108 - 174 MHz (66 MHz gap)
216 - 470 MHz (254 MHz gap)
890 - 1300 MHz (410 MHz gap)
Understanding that a wideband antenna will still perform with gain and
other specs within 1-octave of its design passband, it can be seen
that all of the above gaps are well within one octave of the
passbands. For instance....one-half an octave up from 216 MHz places
an edge at 324 MHz. One-half an octave down from 470 MHz places a
cutoff at 352 MHz. (I'm using half-octaves to be conservative even
though full octaves are valid).
Therefore, even in the widest gap of 216-470 MHz, there is ample
overlap coverage from the passbands of 174-216 and 470-890 MHz.
The worst coverage in the spectrum will be between 324-343 MHz,
but even there, it will be superior to that of a discone or any other
omni scanner antenna!
The simple conclusion, which can be empirically demonstrated with
minimal effort, is that a TV antenna will perform well above a discone
at 343 MHz, a point halfway in the gap of 216-470 MHz. It doesn't
matter that performance will be slightly down from the TV band specs,
because GAIN, F/B ratio, and side-lobes will still make the antenna a
superior performer to discones, dipoles, and even narrow band yagis,
and for that matter, log periodics which have low gain for their wide
bandwidth....typically 6-8 dB.
A TV antenna will be competitive with the log periodics even in the
gaps of its performance........and as a PLUS, will cost a LOT LESS.
More of a PLUS is that TV antennas are easier to install and maintain
than the larger, high profile log periodics. Finally, neighbors are
less likely to *****, whine, complain, and turn you in to some nasty
authority when all you have on the roof is an innocuous TV antenna,
even if it is mounted "all wrong". Neighbors will just think you are
stupid and leave you the hell alone.......which is what you want,
right?
(c) 1997 <All rights reserved> Bill Cheek
