Multimeter and AM Question

Where is the picture?

Assuming that it is an old analog volt/ohm meter, could be battery leads.

Are they - one black - one red?

Exactly.

In old Multimeters No PCBs and no AA size cells.

Sometime we had to use external cell(s) as the original was seldom available.

Sounds like the meter is a Micronta which was sold by Radio Shack; not a high end instrument.

Sold by Radio Shack, but not made by Radio Shack. Sort of like Sears/Roebuck.

Well some radio amatures do use λ/4 for reception which is about the optimal antenna length. (λ being the wave length) But I can't think of a way to carry a 50-150 meter aerial on a car.(LOL). Anyway car tuner manufacturers must work with what they have available. Don't we all? S.M.

I have tried add the image, the picture editor comes up I choose the file, push submit, the green bar expands on the screen at the bottom, and wala!!! Nothing.

It is a micranta, with a magnetic deflection dial on it, somewhat older, interesting, nevermind I figured it out, makes me fee real stupid, they go to each other, oh well if you saw the picture you would see, the insulation over the very thin wires slides on and off like a protective covering, unlike insulation that you have to strip off. which is what was misleading to me, it made them look like sensors of some sort, but I noticed the angle on the end of each insulation tip was cut in a angle that matched the other one.

So forget about that how about the electrical length for am radio in cars. The only other thing I could think of would be broadcasting in an IF an it being converted at the radio to be able to dial it in on the tuner, but I've worked on quite a few, and they broadcast in their khz range, which is why we were working 200 and 300 ft towers. I am almost positive the frame of the car can be used for a ground plane. But that still leaves us extremely electrically short, loading coils and other similar accessories can be used to create the illusion of electrical length, but I have never seen any of this on any basic car stereo i've ever seen either.

wala voila

If the wires are red and black, they may go to where the battery used to be. Maybe someone snipped of the contacts.

Well thank you cuba pete, that is what I was lookin for, I in know way thought that you would be able to pick up a 450th harmonic on an antenna like. When we talk about effective or intrusive harmonics usually go only to the 7th or 9th. I know with these antennas they are using for LTE, and UMTS, cellular broadcast, in PIM testing they are generally only concerned with the about to the 3rd or 5th harmonic. I had not thought about until the other day, I had done quite a bit of work on the transmitting side, but none on the recieving side for am. FM wise i have worked on many stacked broadcast antennas(on rather large towers, a few at the 2000 foot level, which is quite an experience in the middle of the night, in a 50m/hr sustained wind.) Anyway when assembling these antennas onto the tower mast, some of them are 10-12 bays long, these are generally separated by 10ft or or three meters, approximate full wavelength, connected with ridgid coax, whenever the the bays are divided in 2 and we ran two separate runs of ridgid, we would have to add coax to the bottom line in order for it to be the same length as the top line to keep the phasing correct. You could get tremendous gain and directivity, with just a fair amount of backscatter.

It still seems difficult to believe that the antenna is capable of picking the 450th harmonic, harmonics generally just disappear into the noise floor after about the ninth or so. It does give me a few(nice) mad scientist if those high harmonics can be utilized like that. I know harmonics can be filtered, multiplied, and used like an oscillator or frequency generator. I would just not think a harmonic that far up can't impress enough voltage on the antenna within the reciever threshold to still be amplified and demodulated. Interesting. Hey, while we are on this topic. Is this high harmonic threshold unique to the lower khz. I would think because of reactance and power being dissipated more rapidly, other than maybe some well engineered microstrip antennas, that the harmonics you could use from the fundamentals in this way would get less and less, the higher the frequency.

thanks

thanks

GA from me, you reaffirmed the way I always understood the AM receiving antenna worked, but I always thought FM meant *ÚÇΚing Magic, dam acronyms, anyhow (lol)

Picture?

Micronta is indeed a RadioShack brand name. I have a Micronta handheld DMM, about 1990 vintage. While not a Fluke (and I do own one), the Micronta has come in handy, especially if I wanted to do quick "sanity" checks. Some RS stuff is just junk, but some is "Mighty Fine Junk".

And I am not sure about some of the statements by "Cuba Pete":

"AM works best with a loop antenna, so little structural length is needed for the circuit. FM works best with a radial/counterpoise setup (in a car)."

It matters not to the antenna what modulation modes is used, AM, FM, digital, etc. If he is referring to the nominal wavelengths of the respective broadcast bands, well, that makes no difference either. Loop antennas are used equally well at VHF/UHF as well as LF, MF (AM broadcast), and HF ("shortwave"). The key to good antenna design in a motor vehicle is that a good RF ground is used to couple the ground side of the feedline to the vehicle chassis (the metal parts). That in turn, is capacitively coupled to earth ground due to the mass of metal and its surface area. And while loops can be very effective for the longer wavelengths, loaded vertical whips can be very effective as well, if they are grounded properly.

It's all about efficiency, impedance matching, polarization, AND the shape of the RF field. Hams use base-loaded verticals very effectively on the 160M band (1.8-2.0 MHz), which is just above the AM broadcast band, because a vertical has an extremely low take-off angle, near the horizon, while loops are often used for VHF/UHF satellite work because they can be stacked easily to form quad-beams for extreme directionality with circular polarization.

Yet, innovative Automotive Electronics engineers do some interesting packaging. My 1971 Olds Cutlass had an in-glass windshield antenna that was essentially a dipole (horizontal polarization), but it had to be base-loaded (probably inside the radio), because the car came only with AM radio! And I could easily pick up broadcast stations here in the Midwest from both coasts at night.

And that bit about 450th harmonic used to pick up a signal on a half-wave antenna? (LOLFOFLMAO) That would mean that you are using a 1/900th wave antenna, right? And I guess you believe in homeopathic medicine too, right? You know, the more diluted the medicine, the stronger it is. Well, that's the theory!

Cuban Pete, stay away from radio and stick with your maracas...

"chick chicky boom, chick chicky boom"

1. I agree the meter is probably a Micronta (unless it is an even cheaper copy!)

2. A car radio antenna is a multi purpose device that is not 'tuned' and does not really make 1/4 wave, even at FM band frequencies. The front end of the tuner is 'matched' to the characteristics of the antenna for all the bands it supplies. In the US you probably don't make any use of LW but most (older at any rate) Eu radios have (had) this band. In this LW/MW case, the same antenna is described as 'aperiodic' or 'not tuned'. I believe the antenna (which exhibits very low radiation resistance on these bands) is essentially brought to a high impedance input stage, buffered, and then tuned in the conventional way within the tuner stages (using varactor diodes).

If you look at the feeder carrying the antenna signals to the radio, you will find the feeder is not like a conventional 50 ohm coax. The inner is very thin and is spaced from the outer as much as possible - often kinked as well. This must help maintain the impedance match to the LW/MW bands and is not really compatible with a 1/4 wave. A standard 50 (or 75) ohm coax would capacitively load the antenna and dump some of the LW/MW signals hard won by such a short antenna.

The whole thing is a compromise, but it is a testament to the designer's craft that the radios work as well as they do with a variety of after-market replacements - including the ubiquitous coat hanger (in all its glory!) It's a long time since I dealt with antenna theory - maybe other forum users from the industry could be more specific.

Steve.

Guys,

I had always thought that it was the Ferrite Rod that was at least partialy resposible for the AM radio tuning ability.

I don't have much theoretical ability below about 1 MHz but one thing is certain, the Ferrite rod provides a lot of direction gain in the ASM radio and obviously helps there. It also, I presume, provides a level of dielectric loading which shortens the perceived wavelength at the antenna - sorry I am not aware of the dielectric constant for Ferrite, perhaps someone else could come up with a number? The Ferrite can also provide tighter coupling between coils on the ferrite rod which could help LW and MW Bands,dependent upon the Receiver design, and primarily reduces the required physical size of the loop antenna which it replaces.

See Wiki: http://en.wikipedia.org/wiki/Loop_antenna

Sleepy

Yes; yes, but this is not a 'portable' radio. A directional (ferrite rod) 'loop' antenna would be useless in this application. That is why the front end is designed to accommodate the electrically short 'rod' antenna in an untuned fashion.

There will be some ferrite in the front end but it will be contained within the tuning coils (i.e. a few mm^3) to provide the inductance.

If a ferrite rod 'loop' were to be introduced to this application, and assuming the roid picks up adequate signals within the vehicle body, the rod antenna signals would add or subtract to the loop depending upon direction and tend to reinforce or null the signals. This is the basis of simple direction finding receivers.

Steve.

P.S. I have just re-read the original post, which referred to a digital tuner. I assume this is a domestic receiver which may just have an FM antenna or feeder problem. That receiver may have a hinged (also directional) AM loop antenna associated with it and that is probably a flat wire loop; not a ferrite one. This is standard in domestic 'hi-fi' tuners.

It's kind strange you mention Automotive antenna's are not tuned, here in the US our older analog tuning radio's always had a trimmer adjustment that was easily accessible, no dis-assembly required, for fine tuning

Yep...I haven't done that for a long time...usually just replace the unit nowadays...

It's been a while for me too. I, myself, not a big fan of digital, you just can't fine tune them_{(without dis-assembly)} like you could with old analog radios.

Don't know why my post #22 was posted as anonymously, I don't like that option unless I have a very remedial question or a very dumb comment, but that was me with #22.

I also use the cheap Radio Shack meters for quick troubleshooting, does the circuit have power and continuity, nothing hi-tech for that.

The front end will cope with a certain range of capacitance (or whatever other parameter is considered.) The trimmer is there to allow the front end to tune within a certain range. If you think of the wide variety of after market antennas thare are (or were) out there, you might understand why.

When I describe an antenna as 'tuned' I would say that it has a defined feed impedance within a certain tolerance band at a narrow range of frequencies (e.g. a 1/4 wave whip will have a resistive impedance at one frequency only. At other frequencies, dependent upon the Q of the radiating element, it will become inductive or capacitive and hence the SWR will change and go silly some point. Not so with car antennas where the front end provides the match to whatever impedance the antenna offers.

AM antennas in portable receivers have been very often made from a ferrite bar (typical length 10cm or 4inch) and a coil wound around.

The resonance is tuned to the wanted frequency by a variable (air-gap) capacitor.

So this is quite different from a lambda/4 antenna but has the same resonant frequency.

So depending on Q-factor (low damping) the gain may be quite high.

The same/similar scheme was used by the very first "radios" near 1920, that worked with a lead-sulphide crystal detector (used as a HF-diode) and a resonant circuit of a air-coil with a variable capacitor.

High impedance headphones were used.

No energy supply was needed for local stations!

In the left photo the coil and the crystal-holder before is seen and the tuning knob for the capacitor below, shown in the right picture.

RHABE

I picked up a Micronta VOM (volt ohm meter) at a sale a few weeks ago. There will be a batter 9volt battery in a compartment at the back if it is the same model. The test leads plug into the recepticals on the front (in most cases, some small digital ones from RS are direct wired). You might be able to pick up the manual or at least the specs from the web if you do a search. It seems to be a fair instrument, not a Fluke, of course but it will do OK around the house and in lower impediance electronic circuits. It will load down high impedience circuits and alter the circuit parameters some what.