[Antennas] AM Broadcast RX Antenna?

Fri Nov 13 22:03:12 EST 2015

As Hue has stated, the car radio may be designed for essentially an open circuit (short antenna that looks like a small capacitor).  So a network that matches to 50 ohms may not be the right thing.

First, most receiving antennas at these frequencies are designed to provide adequate signal without a 50 ohm match, so they don't need to be tuned.  Even an 8 foot whip will probably have a tuned bandwidth of a couple of KHz, so it's impractical for a consumer radio.

Most importantly, you only need enough antenna receiving area (gain) to be able to hear the atmospheric noise level, which will be very high at 660 KHz.  So the first thing to try is what you did- substituting the 8 foot whip for the current antenna.  If your noise level drops when you disconnect the antenna from the radio, then you're limited by external noise and a bigger, or matched antenna won't improve your range.

The next best improvement is to put a capacitive  "hat" on the 8 foot whip, to make it effectively higher. This may be impractical.  After that, try a series inductor or L network.  Since we don't know what the receiver input is, this may not help (if the receiver is designed to work with a short whip).

Ed  W2EMN

-----Original Message-----
From: Hue Miller <kargo_cult at msn.com>
To: antennas <antennas at mailman.qth.net>
Sent: Fri, Nov 13, 2015 8:12 pm
Subject: Re: [Antennas] AM Broadcast RX Antenna?

1000 miles out, year round, daytime too? That is a stretch.
You already know,
of course, that there's not much of a "match" possible 
to the car radio's
AM
antenna. The input to the car radio is for a low capacitance (short in

terms of wavelength ) antenna.
It's interesting i think that you got
substantially better results with 
the CB type whip length.
When i was
working, i drove the hills on the Oregon coastal range. In DX 
season i could
listen from
Newport, Oregon, to CBC Vancouver 690, KIRO Seattle 710, KFBK in

Sacramento ( i think ), and a
all-comedy station on 1160 ( IIRC ) from
Alberta. The 690 Vancouver 
would sometimes be fighting
it out with XETRA on
690 from Mexico, and a few times Vancouver was 
nowhere to be heard and i
had
good copy on the Mexican. This is with a stock radio in a Ford 
Ranger
truck.
When i told the service order writer at the dealer that i wanted the AM

radio noise situation in my
Dodge Caravan taken care of, he told me i needed
to go to an auto 
electrics specialty place and
besides, no one uses AM any
more, just a few old folks. The noise in my 
van, i think from the fuel
pump,
is so bad i cannot DX with it, just the local stations. The 
service people
must hate working on
the fuel pump because they all told me just to wait til it
fails, not to 
be preemptive about it.

One thing i've wondered about, is in
the old Whitney car parts catalog 
was a "car antenna booster"
which was
simply some kind of coil that inserted at the base of the 
antenna. However
just mentioning
this makes me wonder how you'd install it. The claim was that
it would 
pull in more & better. Seeing
as the car radio expects only a
certain capacitance hi-Z input, i don't 
see how a coil in the antenna
would
help. There's no magic to a coil. Maybe the benefit was 
'psychological'.
-Hue
Miller

------ Original Message ------
From: "KD7JYK DM09"
<kd7jyk at earthlink.net>
To: antennas at mailman.qth.net
Sent: 11/13/2015 3:54:32
PM
Subject: Re: [Antennas] AM Broadcast RX Antenna?

>The station my wife
wants to hear is about 1,000 miles out.
>
>We did notice an improvement when
holding the 102" whip against the 
>factory
>antenna, greater signal strength
and better signal to noise ratio, I 
>thought
>we could do even better if it
was matched to the primary frequency she
>listens to during her morning and
evening commute.  She doesn't listen 
>to
>local FM at all, so it's either 660
AM or the satellite radio.
>
>I have a close-body CB mount to modify to match
the contour of the 
>truck bed
>and some 1/4" aluminum to make a secondary
mount about a foot above 
>that to
>really stabilize things.  On the back
side, inside the fender, I can 
>add a
>matching network, then either T into
the existing co-ax or remove the
>factory antenna entirely and use just the
102" whip.  We're in a fringe 
>area
>so the longer antenna would no doubt
help with reception in general, 
>but the
>primary concern is the DX AM
station.
>
>Kurt
>
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