Pages: 1 [2]   Go Down

Author Topic: Old cop cars had loop antenas ?  (Read 802 times)

WB6BYU

  • Member
  • Posts: 20896
    • Practical Antennas
Re: Old cop cars had loop antenas ?
« Reply #15 on: January 03, 2023, 01:32:40 PM »

Quote from: K8AXW

Since the loop antenna is a directional antenna and would be unsuitable for mobile two-way communications, I'm wondering if the loop on the police car was simply "Hollywood?"




The directional characteristics depend on the
polarization of the transmitting antenna.  With
a low dipole using NVIS, the loop has little
directivity.

Even on ground wave, the null of a loop is
relatively narrow, so with random orientation
you probably still get at least 90% coverage.
After all, ferrite loops with a similar pattern
are used on AM BC pocket radios, and loop
antennas were built into most table radios.

KC6RWI

  • Member
  • Posts: 901
Re: Old cop cars had loop antenas ?
« Reply #16 on: January 04, 2023, 08:58:04 AM »

Thanks BYU for clarifying that point, This was a minor detail in this movie, it wasn't about police cars, I noticed it because I saw it once and had my attention peaked to see it a 2nd time and I did. I be those loops were made from bakelite.
Logged

K8AXW

  • Posts: 7391
    • HomeURL
Re: Old cop cars had loop antenas ?
« Reply #17 on: January 04, 2023, 09:38:47 AM »

BYU:
Quote
The directional characteristics depend on the
polarization of the transmitting antenna.  With
a low dipole using NVIS, the loop has little
directivity.

Even on ground wave, the null of a loop is
relatively narrow, so with random orientation
you probably still get at least 90% coverage.
After all, ferrite loops with a similar pattern
are used on AM BC pocket radios, and loop
antennas were built into most table radios.

I have never heard of a police "transmitting antenna" being a dipole.  But, I probably don't understand the reply.

I have used loop antennas for HF DF and I'm very familiar with ferrite rod antennas and their notoriety for being directional.  I presently use two "atomic" clocks which apparently use ferrite antennas.  One is mounted on a N/S wall and will not update automatically.  It has to be taken down and oriented to the E/W before it will update.  My second clock, which is mounted on an E/W wall updates each night automatically.

Some early police cars that used HF radios  mounted their long antennas on either side rear bumper like we hams do.  That could have presented a "directivity" problem but because of the short communications paths it seldom happened.

I have never heard of police "receive only" radios.  As with airplanes, early forays into radio communications was two-way.

The 1920's police photo you saw might actually might have been military.  The Germans were early developers of DF to counter enemy radio communications.  They used portable (?) radios that used loop antennas.  Early photos quite often show these portable receivers hanging around the neck of some hapless grunt with an "officer" supervising the whole operation.

I admit to being somewhat jaundiced with anything Hollywood presents.



« Last Edit: January 04, 2023, 09:54:28 AM by K8AXW »
Logged
A Pessimist is Never Disappointed!

KH6AQ

  • Member
  • Posts: 9292
Re: Old cop cars had loop antenas ?
« Reply #18 on: January 09, 2023, 07:41:07 AM »

I have never heard of police "receive only" radios.  As with airplanes, early forays into radio communications was two-way.
[/quote]

Beginning with post #4 are links to articles describing the first police radios as one-way from the police station to the cars, very similar to pagers. The police receiving the radio message would carry out the radioed order, or telephone the station from a police callbox if need be.
« Last Edit: January 09, 2023, 07:46:50 AM by KH6AQ »
Logged

KC6RWI

  • Member
  • Posts: 901
Re: Old cop cars had loop antenas ?
« Reply #19 on: January 09, 2023, 09:20:03 PM »

I found a tiny clip of the car with the loop..
At 26 minutes in the pursuit starts and there are scenes of the police car, if you back up a bit from 26 minutes in you might see the police car parked and just a small view of the loop

https://www.youtube.com/watch?v=5s-sSHplXRc
Logged

K5LXP

  • Member
  • Posts: 6823
    • homeURL
Re: Old cop cars had loop antenas ?
« Reply #20 on: January 12, 2023, 10:39:15 AM »

Consumer radios of the day had a spot on the AM dial to pick up the police.  From what I've understood early radio dispatch was one way, and field units would respond by using a call box.  Hard to imagine a fleet of police cars running 160M mobile.

https://www.alamy.com/am-shortwave-radio-dial-image346709531.html

(look to the right around 1700kHz)

"Galvin began producing "Motorola" brand mobile AM police one-way radio equipment in 1937, on a large scale, with a modified broadcast automobile radio first called the "Police" and later dubbed the "Police Cruiser."

https://www.wb6nvh.com/Moto42/Moto42.htm

Mark K5LXP
Albuquerque, NM
Logged

K8AXW

  • Posts: 7391
    • HomeURL
Re: Old cop cars had loop antenas ?
« Reply #21 on: January 13, 2023, 09:46:11 AM »

ENOUGH ALREADY!  I stand corrected.  In all of my 87 years, and 75 of that with an interest in radio communications, have I ever heard of "one-way" cop car communications!

I suppose the good thing about this discussion is that I'm still able to learn something.  Thank you all.
Logged
A Pessimist is Never Disappointed!

WA2CWA

  • Member
  • Posts: 710
    • ManualMan
Re: Old cop cars had loop antenas ?
« Reply #22 on: January 16, 2023, 10:58:32 AM »

Slightly off original thread topic:
amateur radio mobile in 1919:

Logged
Pages: 1 [2]   Go Up