Yo! Scott:
Tuesday August 13 1996 08:52, Scott Hoffman wrote to All:
 SH> I just bought a new antenna (which is meant for FM radio) but I use
 SH> it for my scanner anyway and it seems to work pretty good.
Indeed, such an antenna ought to be functional for scanner frequencies over 
twice or more of the bandwidth for which it was intended, perhaps from about 
50 MHz to 174 MHz, if you're lucky.  It probably will not serve well at UHF.
 SH> Although I am concerned about impedance mismatches and the fact that
 SH> I might not be working my antenna to its full potential because if
 SH> this.. My problem is... that the antanna comes with a 'built in' set
 SH> of twin-lead cables (which is labled 300 Ohms)..
Ok, that probably means the antenna has a 300 ohm input impedance.
 SH> My problem is how do I Interface 300 Ohms to 50 Ohms which most
 SH> scanners use?  I know that their are plenty of  300 --> 75 Ohm
 SH> transformers which are used for TV's,  but I have NEVER ever seen a
 SH> 300 --> 50 Ohm transformer, does on even exist?  where can I get one?
As far as I am aware, there are no 300-50 ohm baluns or matching transformers 
commercially available.  I think you would have to wind your own, 
instructions for which can be found in the Radio Amateur's Handbook from 
RL.
 SH> So what I'm using now is just a 300-->75 transofrmer and I'm
 SH> inputting the 75 Ohm into my scanner.. Does feeding the 75 Ohm input
 SH> into the 50 Ohm scanner input really make "that much" of a difference
 SH> or is the difference pretty much minimul almost non-existant?
Minimal.  The input impedance of the scanner is 50 ohms.  By connecting a 
75-ohm cable to the receiver, an SWR of 1.5 : 1 is created.  THAT is very 
good for most communications applications.  A 1.5 VSWR has a loss of 4% 
associated with it, for .17-dB, a minuscule, insignificant number, period.
At the other end, the 75-ohm cable finds a perfect match into the matching 
transformer (balun) where the impedance is transformed to 300-ohms at the 
feedpoint of the antenna.  Now if the antenna's input impedance is 300 ohms, 
then there is another perfect match.  This reduces the loss considerations to 
just two:  the 4% loss we already talked about, and then the cable loss, 
which is so many dB per foot.  The cable loss will be much more than the loss 
caused by a 1.5 : 1 VSWR.
 SH> One last question.. Are antennas really 300 Ohms, I'm not sure I get
 SH> this concept..
It is a difficult one.  The input impedance of a dipole is 300 ohms.  Many 
antennas, even though not dipoles, are based on the dipole design.  Like TV 
antennas....an array of dipoles.  Basically you don't know the impedance of 
an antenna unless it is stated by the manufacturer or you conform to standard 
designs in building your own.
Antenna impedance is a lumped parameter that is a composite of the antenna's 
capacitance, inductance, radiation efficiency, and physical resistance, and 
is beyond the scope of this forum to fully analyze.  It is best to study it 
independently, and otherwise pretty much accept the manufacturer's 
specification.
No, not all antennas are 300 ohms.  Discones are 50 ohms.  Antennas expressly 
made for scanners are usually internally matched in some way to offer a 50 
ohm input impedance.  Whenever in doubt, check the manufacturer's specs.
 SH> Couldnt I just splice my low-loss RG-8 50Ohm cable right onto the
 SH> antenna without even using a 300-->75/50Ohm transofrmer?
Sure.  You could.  But that results in a 300/50 impedance ratio or an SWR of 
6:1 for a much more serious loss of something like 67% or worse.
Soooo, your best bet is to use the lowest loss cable that you can afford or 
find, and then feed that cable between scanner and a matching transformer (if 
needed) at the antenna.  In other words, find a way to fit an F connector 
onto the end of your RG-8 cable to mate with the matching transformer, 
OR..... disassemble the matching transformer, and fit a Type N female to it, 
and a Type N male on the end of the RG-8 coax.
Or....just use RG-6 satellite cable and be done with it.
Bill Cheek | Internet: bcheek@cts.com | Compu$erve: 74107,1176
Windows 95 Juggernaut Team | Microsoft MVP
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