| Home |
| Search |
| Today's Posts |
|
#5
September 5th 06, 12:59 PM
posted to rec.antiques.radio+phono,rec.radio.amateur.boatanchors,rec.radio.amateur.antenna,rec.radio.amateur.homebrew
|
|||
|
|||
Curious about the I.R.E. Standard Dummy Antenna
Maybe the slugs have lost some permeability. Which way do you have to
tune, more or less inductance? Ken Gary Tayman wrote: "Jeffrey D Angus" wrote in message ... Or more specifically, it's to present a uniform high impedance from the generator to the radio. At the time, with the exception of a few lab grade instruments, signal generator outputs were all over the map with regards to output impedance. The "dummy antenna" presented a fairly uniform, and high, impedance to the radio and kept from detuning the front end while doing an alignment while connected to a signal generator. Jeff Ahhh, so for the very first time, I finally hear an answer to one of those things that I've scratched my head about for years! I've often wondered why the literature always says to stick a capacitor inline from the signal generator -- I figured if I had to do this all the time, then why didn't the signal generator maker put one in there? Turns out, usually they do, but it's probably not an impedance match. Still, for most radios I just stick the probe to the antenna and go to town. It's fine for the IF -- enough of it gets past the RF to give me a workable signal. Then for RF I use whatever frequency I need to check/adjust the LO. For final RF adjustments I plug in the antenna and tweak it up on a weak station. Again, I probably differ from most of you because I work mainly with car radios. I already have a pretty decent antenna at the bench -- some $10 special from Auto Zone -- that does a great job. On a conversion there are no RF adjustments; on a repair/rebuild, I can do every adjustment except one, the one which must be done inside the car anyway -- the antenna trimmer. However even this gets checked -- I've noticed that with my bench antenna, the trimmer usually adjusts toward the "high" end, with the screw fairly loose. So if the trimmer tweaks in this area I'm fine. If it tweaks "tight", or not at all, I'll adjust the slug as needed -- or troubleshoot. Actually it is rare that I ever have such a problem -- with one exception: the 58-60 T-Bird radios. Even these are just fine, usually, but I've had a number of them whose front ends are way off. I've spent many hours trying to figure out why, and come up with nothing. I've replaced RF caps, checked the range of trimmers, checked resistor tolerances, and basically gone over the circuit with a fine tooth comb. Everything checks perfect, but the alignment is out. So I adjust the tuning slug -- a pretty fair distance -- to bring it in line. I've had a couple that were so far off that I had to tinker with the others as well. It's really fun because those slugs are WAA-AAY up in there, and difficult to reach. You can't do it with a non-ferocious screwdriver, or even a ferocious one -- it's too tight. You have to reach the shaft with needle-nose. In any case, all this fun aside, I can tinker and bring it into line -- but I still wonder why I have to do it in the first place. I think my next step in this "investigation" is to spend time with the next 58-60 T-Bird I get for conversion. If it's a good radio, when I disassemble I'll measure the tuning slugs, trimmers, fixed coils, etc., and write them down for reference. |
| Thread Tools | Search this Thread |
| Display Modes | |
|
|
Similar Threads
|
||||
| Thread | Forum | |||
| Loop Antennas / minijack works-clips don't / impedence?? | Shortwave | |||
| No CounterPoise - Portable Antenna System | Shortwave | |||
| Question is 'it' a Longwire {Random Wire} Antenna -or- Inverted "L" Antenna ? | Shortwave | |||
| Imax ground plane question | CB | |||
| Poor quality low + High TV channels? How much dB in Preamp? | Shortwave | |||
Threaded Mode
