OT improving radio reception

Ed

You are missing the point. There is no difference in the *antenna* when used as either a transmitting antenna or as a receiving antenna. You persist in getting involved with how the antenna is used. Antenna Theory is the same for any given antenna whether it is used as a transmitting antenna as or as a receiving antenna. If you want to relieve the specs for a receiving antenna because some aspects of antenna design arent important, thats OK. But, when you post information that indicates that THEORY changes dependent on if the antenna is transmitting or receiving, you are wrong. I have tried to get you to think it over with my previous posts. But you seem to want to bring radiation of harmonics into this 'weak AM signal' discussion. Whats that all about.

And, since there is no need for transmitting radio waves from Andrew's site, the transmitting antenna theory probably doesnt find much application to Andrew's situation.

Jerry

Jerry

Watch out ed, I have a capacitence meter and I know how to use it!

You have your choice of vehicles to measure, a '93 toyota

2-wheel drive short bed pickup, or an 84 toyota camry. Take your pick.

The term 'unbalanced dipole' fits this pretty well I think.

Jim

I fairly regularly send out coherent signals with a 4 MHz bandwidth. You're saying you like to use a narrowband antenna to limit your out of band radiation. Coherence doesn't have a lot to do with it.

In receiving, it doesn't

You might notice that the TV aerial still on your neighbor's house seems to be made to pick up a certain band of frequencies.

Here we get to the crux of the matter. Low frequencies are a special case for receive antennas, in that the receive signal is noise limited, i.e., if you gather more signal you will also gather more noise. Galatic noise is very high a low frequencies, so you use a lousy receive antenna because a good one won't buy you anything. As you go up in frequency (FM broadcast for instance) Galactic noise falls off and you see better antennas.

An antenna is what it is. I could pump a respectable amount of power out of the AM/FM antenna at FM VHF frequencies.

See above

If this were true you could rip out your satellite dish and hook up your longwire in its' place. And I would be out of a job. ;)

Kevin Gallimore

I think maybe what ed was getting at was, if somebody goofs up the impedance matching on a large transmitter output stage, bad things happen. For a receiver, the signal is simply poor in that case.

Jim

Not to nit-pick here, but real quarter wave dipoles don't present 50 ohms to the feedpoint at the center. It's a somewhat higher impedance, more like 300 ohms.

One can tweak this a bit by drooping the *ends* of the antenna down towards the ground, or by making a sloping end fed (zepp) antenna.

This having been said, I always used to feed dipoles without using a balun, with RG-8. Never had any trouble getting the antenna to tune up or trouble making contacts.

Jim

Ed

I read your post and now want to apologize for anything I might have written about your grasp of the theory of antennas and the use of it to design antennas. I didnt realize that you are an old guy and we all know how that can effect our grasp.

Jerry

'Sorry if I took it off-track, Jerry, but I responded to the issue of groundplanes, without having seen the earlier posts. I was just jumping in on that point.

As for the theory, I may have left the wrong impression, but see my comments on polarization and front-end Q as well as harmonics. I didn't know what you know or not, and you asked about theory, so...

I didn't mean to confuse the OP but it looked like this discussion had branched out. As Robert says, there's no way that a car is acting like a ground plane (in antenna design terms) at AM broadcast frequencies. And the discussion seemed to long-gone at that point, anyway.

-- Ed Huntress

I can recall that most of my receivers worked a *lot* better when the antenna was tuned up to get good matching for transmitting. A *lot* better. Like, they didn't really work at all unless the transmatch was set right.

I think that probably most folks who say they are getting good performance from a random length longwire are either lucky or they don't really know how good it *could* be.

Jim

Interesting idea. I'm not quite sure how one would test this. Wouldn't that say that the whip antenna would work best if it were bend over to lie near the body of the car?

Jim

Ed

Now I have really gone wrong. I think I'd better sign off on antennas. I just posted to your previous thread with the impression that you were rambling and probably didnt have a grasp. Now that you write this very coherent post to this thread I realize that I have gone too far with my criticism of you and your comment about the difference between antennas.

I actually beleive that you have alot of info on antennas and matching devices. I could have stopped posting yesterday and be better off than I now am.

Jerry

Preliminary calculations indicate that the number will be somewhere around 1000 pF. Pretty low.

Jim

A thin dipole is about 73 Ohms. If you fold it you will get about 300.

Kevin Gallimore

The ground plane /EFFECT/ of the car body on the signal strength from a 1/4 wave radiator proves to me that it has to be part of a receiving antenna system - otherwise it wouldn't have worked back in the bad old days, when I used to put a 102" whip on the very back center of the car and drive around in tight circles fine-tuning the Squelch and RF Gain settings. Do it in a few widely spaced locations, and you can easily triangulate the source.

Hell, it even worked when I hung that same 102" antenna off the back fender of a Schwinn Corvette 3-speed beach-cruiser bicycle, and adjusted a 23-channel handheld mounted on the handlebars. Just took longer (and a nice aerobic workout) to get to the Hidden Tee...

(One install where you were guaranteed no ignition noise. And totally stealth, as long as you weren't winded.) ;-)

-->--

Jim

I dont know how far you want to go with this antenna thread.

I have submitted that it is more the car thats radiating than the "antenna". Further, I submit that what we normally refer to as the "car antenna" is really something that senses, or probes the field induced in the car body by the passing (AM) radio wave. I supose that if the "antenna could be made about twice the height of the car body, the "antenna" would become more than a probe. But, most car antennas are fairly short, so it is the body that has the major effect on the reception. And the placement of the probe becomes important.

Jerry

Try connecting the radio chassis to an "earth ground", like an outlet box. This is an "earth ground" to 60 Hz power, but there is probably an appreciable run between your bench and an actual earth ground. That wire or conduit may serve as the "car body".

Aack. I'm going to be sorry I got into this. I knew I shouldn't have used the word "coherent" as soon as I sent that message.

Here's what I was talking about: A resonant transmitting antenna, which often is physically tuned to length in the case of commercial broadcast transmitting antennas, has induced and reflected waveforms that are in-phase along its length. A receiving antenna almost never does. The incoming signals are of different frequencies and, except in those specific cases where the antenna is physically resonant (no add-on capacitive or inductive loading), the reflected waves are out-of-phase. Thus, the receiving antenna has to deal with non-coherent waves along its length, while a transmitting antenna, if physically tuned to a single frequency, has only coherent waves

*along the antenna*, with no out-of-phase reflectance from the terminal ends.

Thinking more in terms of physics at the time than of radio terms, I used "coherence" to express the whole mess. And I was thinking about the waves

*within* the antenna rather than the radiation. That was a mistake, as I quickly realized that I had bought trouble when you questioned it. Instead, I should have said that a transmitting antenna, ideally, is designed so it severely attenuates harmonic waves (if it's cut to the half-wavelength resonance or odd multiples of it). Ideally, its induced waves and reflected waves are in-phase: coherent. That, indirectly, also aids in the suppression of harmonics.

A receiving antenna, usually, has to deal with fundamentals, harmonics of so me of those fundamentals, and everything in between. A car antenna has to do precisely that, in fact, when it tunes between 700 and 1400 kHz, for example.

I was trying to avoid all of that in explaining what I meant by "coherent," and so I chickened out in my first attempt to explain it. d8-)

Several bands, actually: two within the VHF band. And that's a function of a highly compromised design. It's very broadly "tuned," in two separate segments. All of that complication, all of those tubes, are necessary to achieve some gain over a range of frequencies from 54 to 216 MHz. If it's a VHF/UHF combination, it has to range up to 890 MHz.

I think if you examined the design electronically it would look more like a coupled set of band-pass filters than a tuned circuit.

AM broadcast radio receiving antennas, as I said, usually aren't even broad-tuned. They're completely un-tuned.

That's part of it, and the much greater ease and lower cost of making better antennas at FM broadcast frequencies is another part of it. And the combination of widely varying input impedances of receivers as they tune a

3:1 frequency range, as in AM broadcast reception, is still another part of it. Finally, the low Q of the front ends of AM receivers is another part of it.

'Depends on what you mean by "respectable," and how much power you had available to put in. Or if you loaded it with the proper inductance or capacitance -- or if you got lucky and happened to hit a current antinode where you fed the antenna. If you hit the wrong spot, you'd just have a very warm antenna.

See above. Yes, I'm aware of the noise issue. But try it. I spent a lot of time fooling with broadcast-band DX as a kid. One of my antennas was 250' long, and it was the best at night. In the daytime, I used one about 30' long.

performance in

That has little to do with antennas at AM broadcast frequencies, as you well know. But here's a related point that some people don't know: A longwire of

5 wavelengths, for example, gives a 5 dB gain over a dipole. As the interest in radio has gone up the frequency scale, that fact has gotten lost in the history.

-- Ed Huntress

I was thinking the same thing! But where do you get the ground to connect the other side of your cap meter? Cold water pipe maybe?

To start the pool, my guess is 500 pF.

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No, I don't think so. One of the RF sages, H.A. Wheeler, said "All small antennas are dipoles, loops, or combinations of these two canonical types". When a dipole (or monopole over a large ground plane) is short the current is essentially linear from feed to end. When the dipole is along the z - axis of a standard spherical coordinate system, E and H fields are produced. The mathamatical description of these fields can be gotten from an engineering reference data handbook.

Bob Swinney