[Antennas] Think I've got this figured out, Re: AM Broadcast RX Antenna?

[JOJ006 Mail]

Years ago I recall moving the antenna to the rear of the vehicle and I had to buy a special extension cable for the antenna. This had a series capacitor built into it to offset the extra capacitance introduced by the longer coax cable, so Hue's suggestion to add series capacitance would seem to be on the right track.

I used to be an AM DX reception enthusiast and found that by making a longer mobile antenna resonant on the one spot frequency, the antenna would detune when I was near buses trucks etc. so a flatter, more broadly tuned antenna might be more desirable in built up areas. This detuning effect was even worse when I experimented with a tuned preamp at the base of the antenna, but great for static mobile! A broadband medium wave preamp at the antenna base was my final solution, and yes, with a small series capacitor.

OT, at home, my best ever breakthrough for portable DX AM was when I bought a Sony ICF-2001D all band portable radio. It used Single Sideband, upper or lower switchable Sideband , and used the carrier of the AM signal to synchronise the phase of the demodulator properly to achieve higher quality audio than you would get by listening on an SSB receiver designed for suppressed carrier like most all of the modern receivers. I don't think it would have enough front end gain for mobile use, unless a preamp was used.

The beauty of this synchronous detector approach was that it by using just one sideband it reduced QSB distortion quite a bit and you could switch sidebands to eliminate a strong interfering signal near to one side of it. At home, I loosely coupled the external long wire antenna to the radio by winding a could of loops around the radio and on to a ground or central heating radiator fitting. 

Does anyone know of an affordable modern radio available now that uses this synchronous detection method for am broadcast stations? Probably little chance that a mobile version would be available anyhow. The nice thing about modern communication receivers is that they have effective noise blankers, essential for eliminating piezo fuel injector QRM  pulses.

....John, EI1EM



> On 14 Nov 2015, at 6:16 a.m., Hue Miller <kargo_cult at msn.com> wrote:
> 
> No, don't bother with adding inductance. The car radio's input trimmer may be able to be adjusted for the added capacitance.
> If not, a simple series capacitor at the input to the radio will do it; will restore the total antenna + cable capacitance to the
> range the radio's antenna trimmer can handle.  This is not so critical. I am guessing it would be circa 100 pF
> or less. There will be no noticeable loss of signal with the added cap.
> -Hue
> 
> -----Original Message----- From: Richard (Rick) Karlquist
> Sent: Friday, November 13, 2015 9:43 PM
> To: Hue Miller ; antennas at mailman.qth.net
> Subject: Re: [Antennas] Think I've got this figured out, Re: AM Broadcast RX Antenna?
> 
> All modern car radios use a 30 inch whip, because that is
> a quarter wave on 98 MHz, the center of the FM band.
> Meanwhile, on AM, the radio is designed to resonate out
> the total capacitance of the whip and the cable.  Older
> radios had an adjustment so you can change the length of
> the whip and peak for maximum sensitivity.  I doubt
> that you will still find this adjustment.
> 
> If you want to convert to a CB whip, you need to put
> enough inductance in series with it to cancel out the
> additional capacitance the CB whip has.  You don't want
> to tune the whip to resonance, just reduce its effective
> capacitance.  This can only be done for one specific
> frequency of course, but that is all you need.
> 
> Rick N6RK
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