OT improving radio reception

Jerry,

Thanks for the info I'll look around for a "yagi" plan for the FM and go from there. As far as AM is concerned I can easily run 35' of wire on the exterior wall and access the receivers with a short horizontal run. Can I connect more then one receiver to the antenna? or do I need several runs, and if so could I run say twisted pair or speaker wire and use each conductor as 1 antenna?

Thanks again

Andrew

For one thing, it's a matter of coherent radiation. You need it in transmitting. It may be useful (in the inverse) in receiving, but in receiving, what you need more than anything is a useable signal. So long antennas that may be picking up from numerous directions can be useful in receiving, especially if they pick up more signal strength.

You don't have to worry about harmonic radiation in receiving. You don't have to worry about power reflected to the transmitter. So impedance coupling is less of an issue.

There are other things but it's been a long, long time since I've looked.

-- Ed Huntress

Maybe, maybe not. You may be defeating signal strength, since the "ground plane" doesn't really function as a ground plane.

True enough, but the effect of a small ground plane may or may not be helpful.

-- Ed Huntress

'Don't know, Eric. I'd have to go study it again to have any idea.

-- Ed Huntress

It is a ground plane in that case, in terms of DC, and in terms of preventing certain radiation from penetrating it. What it is NOT is a ground plane in the sense the term is used in antenna theory: as a plane of ground potential that serves as a sort of clamp against which the radiative energy emitted from the radiator reflects.

No, no. the interelectrode distance is much too great for there to be much capacitance. The thickness of a tire reduces the capacitance to a value so low you'd be hard-put to measure it. Do the calculation; it's 'way down there, despite the area of the car. If it were 1/4 inch or so, it would be significant -- if the car was sitting on a sheet of metal that was grounded, which it is not.

My guess is that the whole system is functioning as an unbalanced dipole. I'll see if I can find anything on it later tonight. I don't recall any discussions about it in the various ARRL handbooks.

-- Ed Huntress

Andrew

I have never considered connecting two receivers to one antenna at AM. I'd avoid doing that just because I'm chicken. There are guys around here, like Don Foreman who could comment on the hazards of connecting two receivers together at their antenna terminals.

For FM, do you have a specific station you want to listen to? It gets pretty easy to build a Yagi from old scraps of wire and PVC if you dont have to rotate the antenna and listen to stations from various azimuth angles. I'd be happy to scan some of the sketches i used to build the 6 element FM yagi I made last month. My sister lives about 40 miles South of Palm springs and there is a 8,000 foot mountain in the middle of the path to her house. The signals were very unreliable with the "regular FM antenna" at her house. With the 6 element Yagi the radio was so clear, night and day, that I'd try a 3 element if I had it to do again.

Jerry

Ed

You are way ahead of me with antenna theory. I got thrown off as soon as I read "coherent radiation". Thats a term I never heard of as related to antenna theory. I probably need to get into reading more books. I've been under the impression that antennas behave the same for transmit as for receive.

Jerry

Any good info on yagis will tell you that the characteristic impedence of a yagi is either 300 ohms (loop dipole active element) or 75 ohms (plain dipole active element). These are more-or-less figures. A specific yagi may turn out to be 150 or 50 ohms, depending on element spacing. Any good design you find on the web should specify the impedance.

For the first, you use TV twin-lead. For the second, you have a bit of a problem because, although coax will give you the right impedence, coax is unbalanced and the antenna is balanced. This matters less in receiving than in transmitting but it will attenuate your signal.

All of which is to say, a home made yagi makes a great FM- or single-channel TV antenna (I've made them, using oak flooring planks to hold the elements), but you really have to watch the way you feed the signal into the receiver, or you'll lose so much signal strength that it can all be a waste of time.

A loop-element yagi feeding the receiver with 300-ohm TV twin lead is your best bet. If you want to feed multiple receivers, you need an antenna transformer/splitter. These are sometimes called "baluns," which is a misnomer unless you're going from balanced (twin lead) to unbalanced (coax). They may be very hard to find in all-300-ohm form. A common form is a

300-ohm-in, 75-ohm-out balun (in this case, the term is correctly used). You can find them easily at TV shops or maybe Radio Shack, and you can then use the 300-ohm twin-lead from antenna to balun, and 75-ohm coax from balun to your receiver.

None of this info should be hard to find on the web. None of it is hard to build or buy. The antennas are a snap to make for FM frequencies, especially if you don't have to rotate them for different stations. The baluns are very cheap, and the wire also is pretty cheap.

-- Ed Huntress

Well, that's not a bad place to start. A more accurate way to think of it is that a good transmitting antenna will make a good receiving antenna. But a good receiving antenna may make a miserable transmitting antenna, which will burn up your transmitter in a second.

There is more info about on transmitting antennas than on receiving types. Receiving antennas can be much simpler and work very well at the same time.

-- Ed Huntress

Say what?

Ed, antennas are reciprocal devices. One of the more convienient ways to measure the characteristics of an antenna is to build two identical antennas, point them at each other, using one as receive and one as transmit, and divide the gain. An impedance mismatch on the receive side means some of the energy you are trying to gather will be reflected back out the antenna. The primary difference between transmit and receive is that the transmit structure has to handle more power- bigger conductors with more or better dielectrics to take the voltage. The Ls and Cs dont change.

Kevin Gallimore

Ed

I must have misread your previous comment that ANTENNA THEORY is different for transmit antennas than for receive antennas. I'm pretty sure the Antenna Theory is the same for both Transmit and Receive.

There hasnt been a question that any given antenna might be difficult to use as either a receive antenna or a transmit antenna. I'm convinced that it would very unusual to find an antenna who's radiation pattern or its terminal impedance is different for transmit than for receive. Where have I gone wrong?

Jerry

Jerry,

You are correct. The car, although not an efficient ground plane at AM freqs., does in fact, act as part of the antenna. The basic 50 ohm communications antenna (whip) is a 1/4 wave dipole with 1/2 of its radiation below the whip (probe). That type of antenna is "tuned" by varying it's length until it presents 50 ohms to the transmission line. A whip shorter than 1/4 wave length, AM broadcast on a car for example, can't be tuned to exhibit 50 ohms - i.e., it can't be a 1/4 wave dipole because there is simply not enough room. Such an antenna presents a much higher capacitive impedance. The cable feeding such an antenna is not a transmission line in the truest sense; it is more like a driven conductor inside a shield.

Bob Swinney

Jim sez: It won't be a resonant antenna. But the car's body

Tha's what Jerry has been trying to tell you. If by resonant you mean an antenna that presents a transmission line with its characteristic impedance -- then a whip, even though it is too short to be a 1/4 wave dipole, will still be a better antenna if it has an approximation to a ground plane under it. Jerry's car is part of the antenna!

Bob Sw>>

Bob

I wonder if all this is actually helping Andrew V. I think we've offered him more info than he really wanted. I had convinced myself I was (am) pretty well informed on antenna design and made the mistake of taking offense to being corrected about ground planes. I suspect Andrew would have been better served if I had *not* commented about my use of the term "ground plane".

Andrew does have an interesting project if he is a long distance from AM and FM broadcast stations. But, that is exactly the situation that interests me. I like antennas. I like VHF DX

Jerry

That refers to the fact that you don't want to transmit harmonics if you can avoid it. You want to transmit a single frequency. In receiving, it doesn't usually matter. In fact, antennas made strictly for receiving aren't usually tuned at all.

That's why a car antenna can recieve a bandwith of 3f (500 kHz - 1600 kHz) without tuning. If you tried that with a transmitting antenna, your impedance at the feedline end would be all over the map, and the reflected power would, at some point, probably blow up your output stage.

A transmitting antenna will be capable of transmitting and receiving. A receiving antenna usually is not capable of it, unless it's also designed for transmitting. Car antennas are not.

Again, receiving antennas for the AM band usually work better the longer they are. Most receiving antennas are low-Q designs so impedance matching is less of an issue. Still, they can have a gain advantage due to length (longwire effect), and the gain may outweigh the loss due to mismatch.

In any case, the point is that, in practice, you can get good performance in receiving with very simple antennas that have unknown impedances, which wouldn't work in transmitting without a separate matching circuit. Relatively few receiving antennas are matched, unless they're also used for transmitting.

-- Ed Huntress

Jerry sez:

"I like antennas. I like VHF DX"

Yeah! Better'n cell phone DX.

Bob Swinney

The basic 50 ohm

Are you confusing a dipole with a monopole?

Kevin Gallimore

Oh, boy, now I have to shake 30 years of cobwebs out.

The *basic* theory is the same. In practice, there are both theoretical and practical differences.

In receiving, harmonics don't matter as much, if at all. So you aren't worried about the impedance of the receiving antenna at harmonics to the frequency you're interested in.

Another theoretical matter is that you may experience different polarization in receiving, and a cockeyed wire may actually do better than a highly polarized and tuned antenna. For example, in the daytime, AM broadcast waves, transmitted by ground wave, are vertical-polarized. At night, when the waves are skipped off the ionosphere, the polarization varies in an (apparently) random way. A low-Q, non-directional antenna may be better for receiving in such circumstances. Remember, in transmitting, you determine the polarization, so it's not a factor in efficiency. In receiving, you have to take what you get. On the other side, my vague recollection is that attenuation from mismatched polarization at AM broadcast frequencies is quite low. But you asked about the theory...

Beyond that, in practice, communications receivers and consumer-type broadcast receivers, like car receivers, are quite different from each other. The front end of a broadcast receiver usually is low-Q, and gain in the receiver overall usually is more than you need. You aren't nearly as worried about actual coupling efficiency and the result of a low-Q front end is that there is much less current fall-off in the first stage as you tune the dial, because you aren't tuning the first stage.

That's good, because broadcast receiver front ends (with rf amplifiers, which car radios used to have -- I assume they still do) don't tune. They're broadband amps, if they're amps at all. And their impedance swings widely from one end of the dial to the other. Which is OK, because of the low-Q front end and high gain of most of those receivers.

In other words, you're wasting your time to fine-tune an antenna for one of those receivers. A perfectly-tuned, impedance-matched transmitting antenna

*with gain* will beat a random wire. At the same time, you can get more current flowing through the output end of a long antenna, if the impedance its seeing isn't such that it's reflecting a lot of power back up the antenna, than you can from a short-element transmitting antenna, such as a loaded whip. With low-Q or loosely coupled front ends on the receivers, that's generally the case.

Now, car antennas aren't "long" antennas in the sense antenna engineers use the term, which is, IIRC, 3.5 X the wavelength or more (at 3.5 X wavelength, a long wire has the same gain as a dipole). But, in practice, wire-type antennas for AM broadcast tend to work better when they're longer, particularly when they're feeding crappy receivers.

Since it's been 32 years, I think, since I took my test for First Class Radiotelephone Operator (with radar endorsement), it would be wise to double-check my statements before putting your money on the table. My last edition of the ARRL Handbook is 1990. I don't trust my memory that well, frankly. But that's my recollection. I used to have some good antenna engineering books but they appear to be long gone.

-- Ed Huntress