🚀 go-pugleaf

In article <3561@ssc-bee.ssc-vax.UUCP> simnet@ssc-vax.UUCP (Mark R Poulson) writes:
>In article <12765@milton.u.washington.edu>, amigo@milton.u.washington.edu (The Friend) writes:
>>      Anyone have some hints to improve TV reception from a TV Antenna?
>Some things to consider:
>
>The higher gain antenna you purchased, the more difficult this becomes. Also
>note that your worry about ch. 12 and 28 being opposite directions is not a
>problem, but being perpendicular is. This assumes that the UHF dipoles are
>oriented the same as the much longer VHF ones. 

Most VHF/UHF antennas that are purely log-periodic antennas will have a
decent front-to-back ratio, so signals coming from the back of the
antenna, particularly those higher in frequency than the frequency the
rear-most elements are designed for, will be attenuated.  THis is
because the longer elements will act like a reflector ala a yagi reflector.
Most VHF/UHF combo antennas have log-periodic elements for VHF, and a corner
reflector yagi for UHF, so the same holds true.  Stations coming in from 
the rear and sides will be heavily attenuated.

>3) Once you have a decent picture on the roof, connect your downlead to inside
>the house. If you don't have similar reception you have:
>
>	a) Used lousy Rat-Shack cabling (you need >400MHz bandwidth).

Bandwidth that varies with brands of cable?  I don't get it.  Cheap cable
may be more lossy, but it doesn't have a self-contained band-pass
filter.  Loss graphs for transmission cables generally follow a linear
plot on log-log paper, so you need only compare the loss figures at
one frequency (e.g. at 200 MHz) to determine which cable will work
better.

>	b) Terminated it poorly or are using the wrong impedance cable
>	    (you'd better be using 75 ohm coax (not 50 ohm!) or 300 ohm twin
>	    lead).

You'll realize little difference between 75 ohm and 50 ohm coax.  When
you consider what a wide range of frequencies you are attempting to
receive, even with a well designed log periodic, the feedpoint 
impedence is going to vary over quite a wide range.  Choose the cable
with the lowest loss that fits into your budget.  Typically, RG59 (75 ohm)
is about the same price as RG58 (50 ohm), but has lower loss figures.
On the other hand, RG8-foam (50 ohm) is probably cheaper and also has
a lower loss than RG-11 (75 ohm).  The only problem you might run into is 
trying to find F connectors that will work on RG8 (I've never seen them).

>	d) Have too weak a signal to make it over the cable (twin lead is
>	    worst with loss).

To the contrary.  Twin lead (aka ladder line or open wire line) is much
lower in loss than most types of coax.  Just be sure you keep the twin
lead away from metallic objects (e.g. aluminum siding), and keep it as
much out of the weather as possible.  If you have to run your cable in
places where it will be near metal or will be heavily exposed to the
elements (e.g. running it across the length of your roof instead of having
it come into your attic below the antenna), get good coax.  Belden makes
a version of RG59 that has an aluminum shield in addition to a few
strands of braid and has good loss figures for the price.  Most cable
companies use this type.  Stay away from Radio Shack cable - get
Belden, Saxton, Times Wire and Cable, or Intercomp.

As a last resort, get a mast-mounted preamplifier.  This isn't going to
make stations that were previously invisible suddenly show up, but
if you have long cable runs (like over 100'), it will make a noticible
difference, particularly on UHF.

							--- Jeff



--
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
 Jeff DePolo  N3HBZ             Twisted Pair: (215) 386-7199                  
 depolo@eniac.seas.upenn.edu    RF: 146.685- 442.70+ 144.455s (Philadelphia)  
 University of Pennsylvania     Carrier Pigeon: 420 S. 42nd St. Phila PA 19104