I am hooking up an outdoor antenna to my police scanner. I just
mounted an old car CB antenna on the roof. The cable Is not labelled,
but I assume it RG58 (50ohm). But if I recall, the scanner manual
said something about using RG59 (75 ohm). Will this matter? I also
need to make an extension because the cord barely goes thru the wall.
Should I continue to use RG58, or should I use RG59?

Thanks

Mark

wrote in message
...
I am hooking up an outdoor antenna to my police scanner. I just
mounted an old car CB antenna on the roof. The cable Is not labelled,
but I assume it RG58 (50ohm). But if I recall, the scanner manual
said something about using RG59 (75 ohm). Will this matter? I also
need to make an extension because the cord barely goes thru the wall.
Should I continue to use RG58, or should I use RG59?

Thanks

Mark

The actual impedance match between the antenna and the radio, is not hugely
important in a receive only situation, and it is unlikely, with the type of
listening you will be doing on a scanner, that you will notice a 75 ohm / 50
ohm mismatch.

Much more to the point, I would assume that your police scanner is VHF,
whereas your CB antenna will be cut for 27 megs, so its performance at VHF
will be very poor, and its impedance at those frequencies, will be nothing
even close to either 50 or 75 ohms.

It is generally considered very bad practice to swap backwards and forwards
between cable types when extending.

Arfa

You should use the 75ohm (Cable TV impedance) cable. RG-59 is OK for a
short run, but RG-6U is lower loss and better shielded. If you have the
choice, I'd use the (thicker) RG-6U. Using 50ohm cable provides an
impedance mismatch on both ends so will reflect and partially cancel your
incoming signal. The result may not be noticeable on the scanner depending
on your distance from the transmitter.

Bob

wrote in message
...
I am hooking up an outdoor antenna to my police scanner. I just
mounted an old car CB antenna on the roof. The cable Is not labelled,
but I assume it RG58 (50ohm). But if I recall, the scanner manual
said something about using RG59 (75 ohm). Will this matter? I also
need to make an extension because the cord barely goes thru the wall.
Should I continue to use RG58, or should I use RG59?

Thanks

Mark

wrote in message
...
I am hooking up an outdoor antenna to my police scanner. I just
mounted an old car CB antenna on the roof. The cable Is not labelled,
but I assume it RG58 (50ohm). But if I recall, the scanner manual
said something about using RG59 (75 ohm). Will this matter? I also
need to make an extension because the cord barely goes thru the wall.
Should I continue to use RG58, or should I use RG59?


At a pinch, 50 Ohm cable will work, but if you want to get the best out of
your scanner you need the right impedance cable, and it's not particularly
expensive so why cut corners? I'm surprised though, comms equipment and
scanners are more frequently 50Ohm IME.

Also, a CB aerial is inappropriate for a scanner. It is tuned to 27MHz and
will be very poor at anything in the VHF or UHF range. I'd recommend you get
a proper multiband/wideband scanner aerial, they can be bought for
reasonable money, even brand new. The "Discone" is a popular choice, though
it's rather untidy looking and stands out like a sore thumb for its fairly
small height. There's more 'subtle' aerials out there!

If the CB aerial is a plain 1/4 wave whip (about 9 ft) then you could easily
chop it down so it's resonant at your desired frequency, but it wouldn't be
an ideal solution because it would only work well on one band and optimised
for a narrow frequency range..

Dave

In article ,
says...
You should use the 75ohm (Cable TV impedance) cable. RG-59 is OK for a
short run, but RG-6U is lower loss and better shielded. If you have the
choice, I'd use the (thicker) RG-6U. Using 50ohm cable provides an
impedance mismatch on both ends so will reflect and partially cancel your
incoming signal.

Bad advice, Bob. Extensions should match the feedline.

I agree that impedance should match ...Could you explain why you interpreted
my response that he should mismatch the feed. The OP stated that the unit
he was using called for 75 ohm RG-59 coax in his installation instructions,
but had used 50 ohm coax instead. RG-6U is matched to RG-59 (75 ohms) or so
I had always thought, but has better loss and shielding specifications. I
must have missed something in the original post, so would appreciate an
explanation since I would love to learn from this exchange but your feedback
is not sufficient to understand what I missed or you mean here. Thanks.

Bob

"Ol' Duffer" wrote in message
...
In article ,
says...
You should use the 75ohm (Cable TV impedance) cable. RG-59 is OK for a
short run, but RG-6U is lower loss and better shielded. If you have the
choice, I'd use the (thicker) RG-6U. Using 50ohm cable provides an
impedance mismatch on both ends so will reflect and partially cancel
your
incoming signal.

Bad advice, Bob. Extensions should match the feedline.

Bob Shuman wrote:

I agree that impedance should match ...Could you explain why you interpreted
my response that he should mismatch the feed. The OP stated that the unit
he was using called for 75 ohm RG-59 coax in his installation instructions,
but had used 50 ohm coax instead. RG-6U is matched to RG-59 (75 ohms) or so
I had always thought, but has better loss and shielding specifications. I
must have missed something in the original post, so would appreciate an
explanation since I would love to learn from this exchange but your feedback
is not sufficient to understand what I missed or you mean here. Thanks.

Bob

It depends whether you assume he's going to replace the 50 ohm stuff
which is apparently already in place. To me, the OP is unclear.


"Ol' Duffer" wrote in message
...

In article ,
says...

You should use the 75ohm (Cable TV impedance) cable. RG-59 is OK for a
short run, but RG-6U is lower loss and better shielded. If you have the
choice, I'd use the (thicker) RG-6U. Using 50ohm cable provides an
impedance mismatch on both ends so will reflect and partially cancel

your

incoming signal.

Bad advice, Bob. Extensions should match the feedline.





--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form .

"Bob Shuman" wrote in message
...
I agree that impedance should match ...Could you explain why you
interpreted
my response that he should mismatch the feed. The OP stated that the unit
he was using called for 75 ohm RG-59 coax in his installation
instructions,
but had used 50 ohm coax instead. RG-6U is matched to RG-59 (75 ohms) or
so
I had always thought, but has better loss and shielding specifications. I
must have missed something in the original post, so would appreciate an
explanation since I would love to learn from this exchange but your
feedback
is not sufficient to understand what I missed or you mean here. Thanks.

Bob

"Ol' Duffer" wrote in message
...
In article ,
says...
You should use the 75ohm (Cable TV impedance) cable. RG-59 is OK for
a
short run, but RG-6U is lower loss and better shielded. If you have
the
choice, I'd use the (thicker) RG-6U. Using 50ohm cable provides an
impedance mismatch on both ends so will reflect and partially cancel
your
incoming signal.

Bad advice, Bob. Extensions should match the feedline.


For receiving applications the differance in the mismatch between a 50 ohm
and 70 ohm is not enough to notice in most applications. The antenna and
input to a reciever over a broad range is no where near either of the two
except a few frequencies.

RG 6 cable is normally less lossey than the rg 58 or rg 59 cables. It is
usually cheeper and easy to find at many Home Depot type stores. The only
problem with it is the shield is not usually copper and you can not solder
to it. It will have to be a crimped or friction fit.

Depending on the type of CB antenna, he is probably wasting his time with it
anyway.

In article ,
says...
I agree that impedance should match ...Could you explain why you interpreted
my response that he should mismatch the feed.

Because it could be interpreted that way, and to me it sounded that way.

The OP stated that the unit he was using called for 75 ohm RG-59 coax in his
installation instructions, but had used 50 ohm coax instead.

Just so, and anything he adds to that feedline should match it.
Now if you are saying he should tear out the old feedline and
replace it with 75 Ohm cable, that is another matter, and it is
unlikely to have any noticable effect on scanner performance,
but it will likely render the setup unusable for a CB transceiver.
And no, the CB antenna will not perform well for scanner reception,
but may be better than none. And if he replaces it with a good
scanner antenna (discone?) the feedline should be chosen to match
the antenna, and the receiver probably won't care.

Your post sounded more like use 59 or 6 no matter what.

Sorry for the confusion and thanks for the reply. I was definitely trying
to say that the poster should follow the instructions and use the correct 75
ohm cable not the 50 ohm one he happened to have laying around from an old
CB radio. My assumption was that the antenna that came with the scanner or
that he would be using would be the correct one for the frequencies being
monitored and would be 75 ohm as well since that is what the manual called
for.... I was just adding that the RG6 was better all round compared to the
RG59 and that it really was not much more expensive since it is used heavily
in the CATV business.

Bob


"Ol' Duffer" wrote in message
...

Your post sounded more like use 59 or 6 no matter what.

First double check your scanner manual. I think most of them
are 50 ohm.

For best performance, your antenna, coax, and radio should all
have the same impedance (or baluns to match). You'll have
serious problems with a 50 ohm to 75 ohm mismatch, so replace
the entire cable or use an extension (as a last resort --
connectors will add loss and/or reflections) that's the same
impedance as the rest of the system.

wrote:
I am hooking up an outdoor antenna to my police scanner. I just
mounted an old car CB antenna on the roof. The cable Is not labelled,
but I assume it RG58 (50ohm). But if I recall, the scanner manual
said something about using RG59 (75 ohm). Will this matter? I also
need to make an extension because the cord barely goes thru the wall.
Should I continue to use RG58, or should I use RG59?

Bob Shuman wrote:

I agree that impedance should match ...Could you explain why you interpreted
my response that he should mismatch the feed. The OP stated that the unit
he was using called for 75 ohm RG-59 coax in his installation instructions,
but had used 50 ohm coax instead. RG-6U is matched to RG-59 (75 ohms) or so
I had always thought, but has better loss and shielding specifications. I
must have missed something in the original post, so would appreciate an
explanation since I would love to learn from this exchange but your feedback
is not sufficient to understand what I missed or you mean here. Thanks.

Bob


I just dug out the manual for my Radio Shack PRO-2030 scanner (Don't
laugh, it was free) and it has a 50 ohm input impedance, like every
other scanner I've owned or repaired. The thing that bugs me is that a
lot of scanners use the Motorola plug which was made for 93 ohm coax.
I've never calculated the impedance of these connectors, but they don't
appear to be 93 ohms.

--
Been there, Done that, I've got my DD214 to prove it.

Michael A. Terrell
Central Florida

In article ,
"Michael A. Terrell" wrote:

Bob Shuman wrote:

I agree that impedance should match ...Could you explain why you interpreted
my response that he should mismatch the feed. The OP stated that the unit
he was using called for 75 ohm RG-59 coax in his installation instructions,
but had used 50 ohm coax instead. RG-6U is matched to RG-59 (75 ohms) or so
I had always thought, but has better loss and shielding specifications. I
must have missed something in the original post, so would appreciate an
explanation since I would love to learn from this exchange but your feedback
is not sufficient to understand what I missed or you mean here. Thanks.

Bob


I just dug out the manual for my Radio Shack PRO-2030 scanner (Don't
laugh, it was free) and it has a 50 ohm input impedance, like every
other scanner I've owned or repaired. The thing that bugs me is that a
lot of scanners use the Motorola plug which was made for 93 ohm coax.
I've never calculated the impedance of these connectors, but they don't
appear to be 93 ohms.

That Mot thing is not a controlled-impedance connector; it was designed
for use with very short antennas on car radios for the AM band.
Impedance was not the issue, it was lowest possible shunt capacitance
between the antenna and the front end of the radio. If that cable
happens to be 93 ohms, it's an accident. If it was necessary for that
impedance to be maintained, it would be necessary to make sure the
center conductor was actually in the *center*, which it's not with
"standard" car radio cable.

R-S used those connectors because they're cheaper than anything else on
the market -- except for RCA plugs. And anyhow, a mismatched section
isn't very important if it's short compared to a quarter-wavelength.
Even 450 MHz is not high enough for that to matter much. You're probably
going to get more loss in a reasonable length of coax than anything the
connector will cause.

Bottom line; it's just not very important for the connector's impedance
to match the input impedance of the receiver.

Isaac

Isaac Wingfield wrote:

In article ,
"Michael A. Terrell" wrote:

Bob Shuman wrote:

I agree that impedance should match ...Could you explain why you interpreted
my response that he should mismatch the feed. The OP stated that the unit
he was using called for 75 ohm RG-59 coax in his installation instructions,
but had used 50 ohm coax instead. RG-6U is matched to RG-59 (75 ohms) or so
I had always thought, but has better loss and shielding specifications. I
must have missed something in the original post, so would appreciate an
explanation since I would love to learn from this exchange but your feedback
is not sufficient to understand what I missed or you mean here. Thanks.

Bob


I just dug out the manual for my Radio Shack PRO-2030 scanner (Don't
laugh, it was free) and it has a 50 ohm input impedance, like every
other scanner I've owned or repaired. The thing that bugs me is that a
lot of scanners use the Motorola plug which was made for 93 ohm coax.
I've never calculated the impedance of these connectors, but they don't
appear to be 93 ohms.

That Mot thing is not a controlled-impedance connector; it was designed
for use with very short antennas on car radios for the AM band.
Impedance was not the issue, it was lowest possible shunt capacitance
between the antenna and the front end of the radio. If that cable
happens to be 93 ohms, it's an accident. If it was necessary for that
impedance to be maintained, it would be necessary to make sure the
center conductor was actually in the *center*, which it's not with
"standard" car radio cable.

R-S used those connectors because they're cheaper than anything else on
the market -- except for RCA plugs. And anyhow, a mismatched section
isn't very important if it's short compared to a quarter-wavelength.
Even 450 MHz is not high enough for that to matter much. You're probably
going to get more loss in a reasonable length of coax than anything the
connector will cause.

Bottom line; it's just not very important for the connector's impedance
to match the input impedance of the receiver.

Isaac

When you've spent years working on systems were every connection is
critical, its hard to ignore mismatches. This includes TV stations,
CATV systems and $1M+ turn key telemetry systems. BTW, car radio
antennas were designed to use RG-62 coax, which is 93 ohms.

--
Been there, Done that, I've got my DD214 to prove it.

Michael A. Terrell
Central Florida

In article ,
"Michael A. Terrell" wrote:

Isaac Wingfield wrote:

In article ,
"Michael A. Terrell" wrote:

Bob Shuman wrote:

I agree that impedance should match ...Could you explain why you
interpreted
my response that he should mismatch the feed. The OP stated that the
unit
he was using called for 75 ohm RG-59 coax in his installation
instructions,
but had used 50 ohm coax instead. RG-6U is matched to RG-59 (75 ohms)
or so
I had always thought, but has better loss and shielding specifications.
I
must have missed something in the original post, so would appreciate an
explanation since I would love to learn from this exchange but your
feedback
is not sufficient to understand what I missed or you mean here.
Thanks.

Bob


I just dug out the manual for my Radio Shack PRO-2030 scanner (Don't
laugh, it was free) and it has a 50 ohm input impedance, like every
other scanner I've owned or repaired. The thing that bugs me is that a
lot of scanners use the Motorola plug which was made for 93 ohm coax.
I've never calculated the impedance of these connectors, but they don't
appear to be 93 ohms.

That Mot thing is not a controlled-impedance connector; it was designed
for use with very short antennas on car radios for the AM band.
Impedance was not the issue, it was lowest possible shunt capacitance
between the antenna and the front end of the radio. If that cable
happens to be 93 ohms, it's an accident. If it was necessary for that
impedance to be maintained, it would be necessary to make sure the
center conductor was actually in the *center*, which it's not with
"standard" car radio cable.

R-S used those connectors because they're cheaper than anything else on
the market -- except for RCA plugs. And anyhow, a mismatched section
isn't very important if it's short compared to a quarter-wavelength.
Even 450 MHz is not high enough for that to matter much. You're probably
going to get more loss in a reasonable length of coax than anything the
connector will cause.

Bottom line; it's just not very important for the connector's impedance
to match the input impedance of the receiver.

Isaac

When you've spent years working on systems were every connection is
critical, its hard to ignore mismatches. This includes TV stations,
CATV systems and $1M+ turn key telemetry systems. BTW, car radio
antennas were designed to use RG-62 coax, which is 93 ohms.

Earlier in my career, I, too, "spent several years" in positions that
included designing 50 KW AM and FM transmitters, UHF comms, and HFC
networks.

In the context of the OP's question (scanners), mismatch on one
connector or a short piece of coax just isn't very important.

And AM car radio cable is not "coax", because the inner conductor cannot
be properly centered -- it can't be "coaxial" in that big hollow plastic
tube. The stuff may be about 93 ohms, but that's not important. What
*is* important with an antenna that is *very short* compared to a
wavelength is lowest possible shunt capacitance, and the construction
technique of that cable favors that.

FWIW, that low capacitance bit was far more important with vacuum tube
front ends. With super-high-impedance FETs, things get a bit less
critical. Note that AM car receivers don't always have that "antenna
trimmer" to be peaked at 1600 KHz these days; with tubes, it was
necesssary.

In fact, you can show that for a zero shunt leakage feedline, and an
infinite-impedance RF input, the voltage at the RF amp input is not
dependent on antenna length *at all*. That is very different from the
case where the input is impedance matched.

Isaac

Isaac Wingfield wrote:

Earlier in my career, I, too, "spent several years" in positions that
included designing 50 KW AM and FM transmitters, UHF comms, and HFC
networks.


50 KW? Try a 5 MW UHF TV transmitter site. Ch 55 in Orange City, Fl.
A Comark transmitter, and a 1749 foot tower.

In the context of the OP's question (scanners), mismatch on one
connector or a short piece of coax just isn't very important.

And AM car radio cable is not "coax", because the inner conductor cannot
be properly centered -- it can't be "coaxial" in that big hollow plastic
tube. The stuff may be about 93 ohms, but that's not important. What
*is* important with an antenna that is *very short* compared to a
wavelength is lowest possible shunt capacitance, and the construction
technique of that cable favors that.


Tell that to the people who make the cable. It is listed as "coaxial
Cable" which simply means that one conductor is inside the other. BTW,
IBM used the same RG/62 93 ohm coaxial cable to connect terminals to
their mainframes, and I think it was the early ARCNET networking that
used it as well.

FWIW, that low capacitance bit was far more important with vacuum tube
front ends. With super-high-impedance FETs, things get a bit less
critical. Note that AM car receivers don't always have that "antenna
trimmer" to be peaked at 1600 KHz these days; with tubes, it was
necesssary.


I serviced car radios back in the '70s and all the solid state radios
had a trimmer as well. The trimmer was to trim the capacitance of the
coaxial cable so that it didn't detune the tuned circuit at the input of
the RF amplifier. I have almost every volume of the Sams Photofact AR
series manuals here, and they all used that design. have you noticed
that the newer "No tune" design doesn't work as well? They are easy to
overload, and are useless when you want to listen to a distant station
at night if a local station is a couple channels away.

In fact, you can show that for a zero shunt leakage feedline, and an
infinite-impedance RF input, the voltage at the RF amp input is not
dependent on antenna length *at all*. That is very different from the
case where the input is impedance matched.

Isaac

A car radio antenna is more of a sensing probe than an actual antenna
due to the fact that it is so short.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.

Michael A. Terrell
Central Florida