antenna trimming?
in [Repair]
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From: Michael A. Terrell on 29 Nov 2009 19:08 Jeff Liebermann wrote: > > 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. RG/62, 93 ohm coax. The same as what IBM used for their early computer networks. This, according to the Delco engineers that taught their annual automotive electronics seminars. I drove them crazy with design questions, and was surprised that so many bad ideas came from marketing fools, and the UAW. > These daze, the antenna is built into the windshield or side window of > the vehicle. Some are, but my current vehicle has a real whip antenna > 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. Exactly! use an extra stage to hide a problem, rather than deal with it properly. You know that a broadband, untuned input circuit has lower gain and more noise that a tuned circuit designed to pass little more than a couple channels at a time. The older, tuned input design reduced the design by at least one gain stage, along with the extra tracking problems. Better designs used moving slug tuners with three or four coils. Lets remember that most people don't give a damn about listening to AM radio anymore, and certainly not on long drives. They might listen to the local news, or a ball game, but everything else is FM, Cd or MP3. The best Am receivers I've owned were a solid stated ARN-6 DRF, and a couple frequency selective voltmeters with a tuned antenna and preamp. You've talked about designing two way radios, and I've worked with telemetry designs on multiple bands, along with C-band & Ku band sat TV equipment. No one in their right mind would use an untuned antenna system and expect good performance. The noise floor, intermod and co-channel interference would be horrible. -- The movie 'Deliverance' isn't a documentary!
From: Jeff Liebermann on 29 Nov 2009 20:20 On Sun, 29 Nov 2009 19:08:16 -0500, "Michael A. Terrell" <mike.terrell(a)earthlink.net> wrote: >> 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> > Exactly! use an extra stage to hide a problem, rather than deal with >it properly. I beg to differ somewhat. The problem with anything below about 7 MHz is that atmospheric noise far exceeds the field strength of any possible weak signal worth receiving. It makes no sense to have an ultra-low-noise receiver front end on a lower frequency receiver. However, note that this chip was also made for receiving short wave DRM and HD Radio, which can be up to 30MHz. The level of complexity necessary to build a tracking filter, with automagical impedance matching is a bit much for consumer electronics. The result of using a simple amplifier is lousy image rejection, overload problems, sub-harmonic problems, intermod problems, and lousy NF which has an effect on >7 Mhz reception. An untuned amplified front end for impedance matching might seem like an inferior solution, but it's certainly the cheapest, which drives the consumer car radio market. > You know that a broadband, untuned input circuit has lower gain and >more noise that a tuned circuit designed to pass little more than a >couple channels at a time. Current or voltage gain? In this case, it's current gain, which seems quite high (35dB). It's prime purpose is to transform the rather high antenna impedance down to something the chip can digest (50 ohms???). Certainly an untuned front end has a worse NF than one that is tuned and matched. However, the atmospheric noise at BCB frequencies will swamp out any alleged improvement produced by the better sensitivity. >The older, tuned input design reduced the >design by at least one gain stage, along with the extra tracking >problems. Better designs used moving slug tuners with three or four >coils. These daze, the slug tuners would cost more than the extra stages. Transistors are one of the cheapest parts in the radio. >Lets remember that most people don't give a damn about listening >to AM radio anymore, and certainly not on long drives. They might >listen to the local news, or a ball game, but everything else is FM, Cd >or MP3. See Ibiquity, DRM and HD Radio: <http://en.wikipedia.org/wiki/Digital_Radio_Mondiale> <http://en.wikipedia.org/wiki/HD_Radio> <http://www.ibiquity.com/hd_radio> BCB and SW stereo is kinda strange, but does work in strong signal areas. >The best Am receivers I've owned were a solid stated ARN-6 DRF, and a >couple frequency selective voltmeters with a tuned antenna and preamp. <http://jproc.ca/rrp/rrp3/argus_arn6.html> My guess(tm) is that the reason they're "better" is not because of improved sensitivity, but because of improved overload handling or dynamic range. Much of the garbage and trashy sound is nothing more than overload or intermod from strong nearby stations. >You've talked about designing two way radios, and I've worked with >telemetry designs on multiple bands, along with C-band & Ku band sat TV >equipment. No one in their right mind would use an untuned antenna >system and expect good performance. The noise floor, intermod and >co-channel interference would be horrible. True. I suppose someone could conjure an proper BCB receiver. Most of the better ham radio HF transceivers would certainly quality, such as Elecraft K3: <http://www.w1vd.com/ElecraftK3.html> Well, maybe not. AM is certainly low on the priority list. Still, the test numbers are far better than the typical AM/FM receiver. However, even if the receiver were superior, the real problem is hiding in the antenna system. One big improvement for BCB only would be to install a big fat loading coil at the base of the antenna. It wouldn't improve the received signal strength, but it would provide a lower impedance to the coax and receiver front end, eliminating the front end impedance matching stage. The bottom line is always cost. The next incremental improvement in performance is going to require substantial hardware in the receiver front end. It's not a trivial change and will be expensive. I don't think that Joe Sixpack will pay the price. -- 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
From: mm on 30 Nov 2009 04:13 On Sun, 29 Nov 2009 19:08:16 -0500, "Michael A. Terrell" <mike.terrell(a)earthlink.net> wrote: > Lets remember that most people don't give a damn about listening >to AM radio anymore, It's certainly heartwarming to think that conservatives aren't most people, but even 10% of the market is usually enough to get some attention by manufacturers. In NY and Philly are AM stations I would listen to. Not sure about the rest of the country. > and certainly not on long drives. They might >listen to the local news, or a ball game, but everything else is FM, Cd >or MP3.
From: Dave Plowman (News) on 30 Nov 2009 05:58 In article <fNdMSLb73vELFwlB(a)g3ohx.demon.co.uk>, Ian Jackson <ianREMOVETHISjackson(a)g3ohx.demon.co.uk> wrote: > >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. I've not researched it so was only a guess. But I've not noticed quality radios without a trimmer having inferior MW reception than before. Of course I'm sure there are plenty of cheap units around that are poor - but then they'll be poor in other ways too. And not surprising given just how cheap they are these days. I read somewhere 10 dollars is an average for factory fitted units... However, don't most cars these days have an 'active' aerial? So any matching would be done by that pre-amp? Can't think of any new UK supplied car with the older telescopic type. -- *Happiness is seeing your mother-in-law on a milk carton Dave Plowman dave(a)davenoise.co.uk London SW To e-mail, change noise into sound.
From: David Brodbeck on 1 Dec 2009 17:12
Michael A. Terrell wrote: > RG/62, 93 ohm coax. The same as what IBM used for their early > computer networks. I've got some 93 ohm coax lying around from when I used some to make a matching section. Never knew what the original application was for it. Very interesting. |