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Author Topic: Old cop cars had loop antenas ?  (Read 801 times)

KC6RWI

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Old cop cars had loop antenas ?
« on: January 02, 2023, 07:53:04 AM »

I was watching an old movie, Undertow, made in 1949. At first I thought that loop on the back of the cars was an advertisement, like the letter O.
Then I realized that these radios are probably in the HF band. So each cop car had about a 12" to14" loop antenna on the trunk, I'd never seen that before.
I wonder if those antennas were very efficient? I know that early police radio was usually on a low frequency.
« Last Edit: January 02, 2023, 08:01:44 AM by KC6RWI »
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KQ4CMA

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Re: Old cop cars had loop antenas ?
« Reply #1 on: January 02, 2023, 08:22:49 AM »

I’ve been asking about that, but not getting any answers yet. If it’s a benefit, I wouldn’t mind trying it out. At the time, were smaller antennas not available? Was it lack of tuners, cost, earlier technologies?
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KC6RWI

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Re: Old cop cars had loop antenas ?
« Reply #2 on: January 02, 2023, 08:28:45 AM »

Its interesting as low freqs usually require big antennas, I know that some police frequencies used to be just above the Am broadcast band, but if you think about that low of a frequency  it would be interesting to know what types of mobile antenna's were used.
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WB6BYU

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Re: Old cop cars had loop antenas ?
« Reply #3 on: January 02, 2023, 08:30:53 AM »

Quote from: KC6RWI

I wonder if those antennas were very efficient? I know that early police radio was usually on a low frequency.




Adequate, at least, in many cases.  It does
depend on transmitter power, of course, but
I have no problems pulling in a 1 watt signal
from a short vertical antenna at 7 miles
using a 6” diameter loop on 80.

IIRC police back then were often on the
1750 kHz range, and with a loop antenna were
likely receive-only in the cars.  (I have seen
photos of German police on foot in the 1920’s (?)
with a portable receiver for receiving instructions,
but one assumes they would stop at a callbox if
they needed to communicate with HQ.)

But having one-way communications from the
dispatcher to mobile units at least provided a
way to speed up their response to an incident.
« Last Edit: January 02, 2023, 08:33:38 AM by WB6BYU »
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KC6RWI

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Re: Old cop cars had loop antenas ?
« Reply #4 on: January 02, 2023, 08:38:48 AM »

I didn't even think of a receive only antenna, and who knows maybe thats all they had at the time in that PD.
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K7KNX

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Re: Old cop cars had loop antenas ?
« Reply #5 on: January 02, 2023, 09:21:19 AM »


Here is a brief history of LAPD's radio system.

http://users.snowcrest.net/marnells/kma368.htm

Here is CHP's radio history with lot's of pictures of the old equipment.

https://www.wb6nvh.com/CHP/CHP1.htm
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KC6RWI

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Re: Old cop cars had loop antenas ?
« Reply #6 on: January 02, 2023, 09:31:29 AM »

That Lapd post didn't work for me, I'll send you a PM
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KH6AQ

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Re: Old cop cars had loop antenas ?
« Reply #7 on: January 02, 2023, 11:01:05 AM »

Detroit police radio experiment begun 1922 is saved by hams! This is the most comprehensive article I've found so far. In 1949 the FCC authorized VHF and UHF police frequencies.

http://www.theradiohistorian.org/police/police.html



Here is another short article on the beginning phase of the LAPD's use of radio. It was essentially a pager where the police cars had receive only capability. The TX transmitted at 500 watts AM on 1712 kc.

https://www.qsl.net/n6uru/kma367-1a.htm



Another short article on early police radio:

https://statetechmagazine.com/article/2017/06/first-police-radio-stopped-bootleggers-their-tracks



My favorite is this photo of a British 2-way police radio circa 1924.

https://commons.wikimedia.org/wiki/File:First_British_radio_equipped_police_vehicle_1924.jpg
« Last Edit: January 02, 2023, 11:15:21 AM by KH6AQ »
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DL9BDM

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Re: Old cop cars had loop antenas ?
« Reply #8 on: January 02, 2023, 01:39:11 PM »

Posted by: WB6BYU
« on: Today at 15:30:53 »

    Insert Quote

IIRC police back then were often on the
1750 kHz range, and with a loop antenna were
likely receive-only in the cars.  (I have seen
photos of German police on foot in the 1920’s (?)
with a portable receiver for receiving instructions,
but one assumes they would stop at a callbox if
they needed to communicate with HQ.)


[/quote]

See here please, fifth image from the bottom:

http://www.klaus-paffenholz.de/bos-funk/1920-1940.html
« Last Edit: January 02, 2023, 01:42:58 PM by DL9BDM »
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WA3SKN

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Re: Old cop cars had loop antenas ?
« Reply #9 on: January 02, 2023, 05:12:48 PM »

Guys... it's a receive only direction finding antenna used to get bearings on HF/VHF.
You need multiple bearings to get a location.

-Mike.
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WA3SKN

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Re: Old cop cars had loop antenas ?
« Reply #10 on: January 02, 2023, 05:22:32 PM »

Oh... and surplus... "pennies on the dollar", after WW2.
Remember that's when coax became popular too.
Remember the Surplus store?  Lottsa goodies and cheap!

-Mike.
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WB6BYU

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Re: Old cop cars had loop antenas ?
« Reply #11 on: January 02, 2023, 07:29:10 PM »

Quote from: DL9BDM

See here please, fifth image from the bottom:

http://www.klaus-paffenholz.de/bos-funk/1920-1940.html



THANK YOU!

I knew I had seen it somewhere else, but it
is such a wonderful photo I am glad you found
a copy to share with this thread.

G3RZP

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Re: Old cop cars had loop antenas ?
« Reply #12 on: January 03, 2023, 02:09:16 AM »

In the UK in Birmingham in the late 1930s, G5BJ was in  employed by the police amd was looking into low band VHF (around 6 metres) for police communications to mobiles. After September 3rd 1939, this naturally got stopped...
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KD0REQ

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Re: Old cop cars had loop antenas ?
« Reply #13 on: January 03, 2023, 08:45:48 AM »

the bandplan for broadcast TV was robbed of channel 1 to provide public service frequencies from 44-50 MHz in the late 40s. TV thus started in the US at channel 2, 54 MHz. some tabletop radios in my innocent youth had two bands of AM, standard broadcast, and "public service."
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K8AXW

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Re: Old cop cars had loop antenas ?
« Reply #14 on: January 03, 2023, 08:50:17 AM »

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?"
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