antenna trimming?
in [Repair]
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From: Dave Plowman (News) on 29 Nov 2009 04:48 In article <tIednfxuCIPUko_WnZ2dnUVZ_vKdnZ2d(a)earthlink.com>, Michael A. Terrell <mike.terrell(a)earthlink.net> wrote: > Most US car radios allowed the adjustment by removing > the tuning knob and using a small screwdriver to adjust the capacitor. A > lot of cheap imported radios had no adjustment, and had very poor AM BCB > performance. My V expensive Blaupunkt doesn't have an aerial trimmer - nor have I seen one for many a year. Thought most had some form of automatic matching circuit these days? -- *To err is human. To forgive is against company policy. Dave Plowman dave(a)davenoise.co.uk London SW To e-mail, change noise into sound.
From: Michael A. Terrell on 29 Nov 2009 13:43 Klaatu wrote: > > You forgot to mention that with age, the antenna coaxial cable would get > water in it, and reception would suffer. Adjusting the trimmer would > make little or no difference in this situation. Replacement was the > only fix. There would be little or no reception by that point, and replacing the antenna was a common repair for me in the '60s & '70s. We kept a used car radio antenna in the service department to test a radio in the car. Unplug the car's antenna and plug in the test antenna. if you picked up some stations, the antenna was bad & had to be replaced. At that time we could get most OEM antennas delivered in two days or under. -- The movie 'Deliverance' isn't a documentary!
From: mm on 29 Nov 2009 15:10 On Sun, 29 Nov 2009 09:48:43 +0000 (GMT), "Dave Plowman (News)" <dave(a)davenoise.co.uk> wrote: >In article <tIednfxuCIPUko_WnZ2dnUVZ_vKdnZ2d(a)earthlink.com>, > Michael A. Terrell <mike.terrell(a)earthlink.net> wrote: >> Most US car radios allowed the adjustment by removing >> the tuning knob and using a small screwdriver to adjust the capacitor. A >> lot of cheap imported radios had no adjustment, and had very poor AM BCB >> performance. > >My V expensive Blaupunkt doesn't have an aerial trimmer - nor have I seen >one for many a year. Thought most had some form of automatic matching >circuit these days? Well that's related to my point. Maybe there is an automatic matching circuit in the new radio that's not working, and he just got lucky that it matched the new antenna. ?? So if he ever takes this radio to another car, again the AM won't work if the matching circit doesn't work and he's not lucky with the new car's antenna. I asked him to read the manual to find out about antenna trimming, but he didn't reply about that.
From: Ian Jackson on 29 Nov 2009 18:04 In message <50c1bbc633dave(a)davenoise.co.uk>, "Dave Plowman (News)" <dave(a)davenoise.co.uk> writes >In article <tIednfxuCIPUko_WnZ2dnUVZ_vKdnZ2d(a)earthlink.com>, > Michael A. Terrell <mike.terrell(a)earthlink.net> wrote: >> Most US car radios allowed the adjustment by removing >> the tuning knob and using a small screwdriver to adjust the capacitor. A >> lot of cheap imported radios had no adjustment, and had very poor AM BCB >> performance. > >My V expensive Blaupunkt doesn't have an aerial trimmer - nor have I seen >one for many a year. Thought most had some form of automatic matching >circuit these days? > In my 'shack', I've got a Philips RD525LEN LW/MW/FM car radio (bought recently for one GBP, from a stall at a charity sale). The aerial is 5 foot of wire in the attic, with maybe 15 feet of 75 ohm TV coax (braid connected to the attic water tank) down to the radio. The coax is much longer than it would be in a car, and it's also the 'wrong sort of coax' (capacitance per unit length will be higher). Nevertheless, it works very well, with no lack of 'liveliness' at the HF end of the medium wave (where you normally expect to set the aerial trimmer, peaking up a weak signal at (typically) 1500kHz. I've got the instruction/installation manual, but there's absolutely no reference to any aerial trimmer. There might be some form of 'automatic matching circuit' but, if there is, what does it consist of? You would need a varicap diode, driven from the AGC line, and some form of servo loop which would automatically adjust and optimise the diode capacitance. It all seems a bit complicated - bearing in mind that a simple aerial trimmer capacitor has been satisfactory for some 60 or 70 years. I just can't see it myself. -- Ian
From: Jeff Liebermann on 29 Nov 2009 18:27
On Sun, 29 Nov 2009 13:43:11 -0500, "Michael A. Terrell" <mike.terrell(a)earthlink.net> wrote: >Klaatu wrote: >> >> You forgot to mention that with age, the antenna coaxial cable would get >> water in it, and reception would suffer. Adjusting the trimmer would >> make little or no difference in this situation. Replacement was the >> only fix. > There would be little or no reception by that point, and replacing >the antenna was a common repair for me in the '60s & '70s. We kept a >used car radio antenna in the service department to test a radio in the >car. Unplug the car's antenna and plug in the test antenna. if you >picked up some stations, the antenna was bad & had to be replaced. At >that time we could get most OEM antennas delivered in two days or under. That was also in the days when the coax cable was attached to the antenna base with a weird connector. I found far more coax cables to be defective than antennas. The cable used was some kind of very low capacitance coax, with a very tiny center conductor floating inside a plastic tube. The center wire would often break from the vehicle vibration. I couldn't find any details on the coax type. These daze, the antenna is built into the windshield or side window of the vehicle. Looking at the data sheet of a modern AM/FM front end chip, the AM section appears to be Hi-Z input: <http://www.atmel.com/dyn/resources/prod_documents/doc4913.pdf> See Page 5. ATR4251 provides an AM buffer amplifier with low input capacitance (less than 2.5 pF) and low output impedance (5ohms). The low input capacitance of the amplifier reduces the capacitive load at the antenna, and the low impedance output driver is able to drive the capacitive load of the cable. The voltage gain of the amplifier is close to 1 (0 dB), but the insertion gain that is achieved when the buffer amplifier is inserted between antenna output and cable may be much higher (35 dB). The actual value depends, of course, on antenna and cable impedance. No mention of any AM trimmer capacitor tuning. My guess(tm) is that older AM front ends were looking at a tuned circuit, instead of a broadband amplifier. The tuned circuit was looking at some specific capacitance in order to be on frequency. The trimmer compensated for the variations in cable and antenna capacitance in order to resonate this tuned circuit. -- Jeff Liebermann jeffl(a)cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |