Q re: Scanner Zin vs. frequency

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qed479

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I have built a "slim-jim" folded wire antenna for 160.7 MHz (approximate center of the railroad frequency band) for my Uniden BC92XLT scanner from 300 ohm ladder line, modifying a 2-meter design from the Internet. I used the W7AY cocoaNEC2 antenna simulator program (see www.w7ay.net) to optimize the design and achieve a theoretical 50 ohm feed point impedance at 160.7 MHz, but I have no way to confirm the antenna impedance. The antenna seems to be working well, but my question is: Exactly what scanner input impedance should I be trying to match to the antenna? The BC92XLT spec lists a nominal 50 ohm input, but how much does Zin vary with frequency? RF transistor data sheets show input conductance will vary considerably with frequency. Have the scanner designers managed to flatten the Zin. vs. Frequency curve, or is the input impedance at 160.7 MHz likely to be significantly different from the published 50 ohms. How do you determine Zin of a scanner at a specific frequency?
 

ko6jw_2

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You can assume 50 ohms for all practical purposes. Transistor data sheets are fine if you're designing an RF amplifier, but the scanner has tuned circuits (track tuned in most radios) between the antenna and the RF amplifier that will be designed to match the 50 ohm antenna to the radio. No it won't be absolutely constant, but it won't matter. In any case, what will you do about it?

The better course of action is to get your antenna as close to 50 ohms as you can. I have an antenna analyzer that measures impedance and SWR. They're not cheap, but if you are designing antennas they're invaluable.

Matching impedance on a receive antenna is not as important as it is for transmitting. A folded dipole is a balanced load and requires a balun to be fed by coax. You can easily make a 4:1 balun from coax, but if you do the antenna will need to be around 200 ohms and most folded dipoles are in the 200-300 ohm range.

Have you considered a J-pole? Basically the same gain as a folded dipole, but easier to mount and less influenced by nearby objects.

The feed line of a folded dipole should run at right angles to the antenna for at least a quarter wave length.
 

qed479

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Thank you for your reply

I chose to build the slim-jim partially because it was described in on-line amateur publications as having a max gain lobe with very low elevation angle, which I hoped would translate into improved contact distance. The nec-2 simulation for my antenna puts the max gain lobe elevation angle at 4 degrees. The J-pole is another interesting configuration and I may use my remaining ladder line to build one.

For the moment I have neither a (grid-) dip meter nor antenna analyzer, so I'm reduced to depending on the nec-2 simulations plus hours of contact logging in an attempt to optimize the feed point.

Thank you the feedback regarding scanner Zin being approximately constant at 50 ohm. Of course if I knew the complex values of Zin at 160.7 MHz then I would try to tune the antenna for a conjugate match.
 

Mike_G_D

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Generally speaking you do not need to get too aggressive about creating a perfect conjugate match for designing an antenna for a wideband receiver.

From the point of view of the receiver, the antenna will only truly be non-reactive (purely resistive) at one frequency anyway and will stray off of that as you deviate from the center. Also, as the antenna is placed in various locations with different nearby metallic or otherwise conductive objects its impedance will change anyway. As it and the feedline ages the impedance may change. As gunk accumulates on the antenna itself the impedance may change. As the receiver's components age its input load impedance may change. All of this means, essentially, just try and hit that 50 Ohm resistive point with your desired center frequency and let it go at that. Especially if you're not going to do any transmitting it isn't worth getting too tangled up in nitty gritty conjugate match details.

The 50 ohm point is a common target to hit for communications equipment using unbalanced coaxial lines; it was decided on long ago as it was a decent balance between loss characteristics versus frequency and power handling (for transmit uses) and because it provided a good common design goal for equipment manufacturers. The television receiver industry centered on 75 ohms as that impedance for coaxial transmission lines yielded slightly better loss versus frequency characteristics than using 50 ohm lines and the need for power handling was not important (receive only or low level signals as in cable systems).

A receiver designer (like all designers) needs design goals to shoot for. One of those is the input impedance desired by the system. You look at what the expected feedline impedance is and usually go from there. As the other poster mentioned, the transistor characteristics are important when designing the input RF amp block of the receiver chain but that is the job of the designer - to build a front end which can effectively make use of an ideal 50 ohm resistive source impedance across the frequency range of the receiver. That designer is the one who has to wrestle with the conjugate matching across a certain frequency spread. Everything in engineering is a compromise. Things vary and change over time (and of course, fail). Antenna and feedline designers also use that 50 ohm resistive impedance as a design goal. Again, stuff varies and guarantees of exact nature are not expected, certainly not across a multi-octave frequency range!

That's why you see specs for antennas which say things like "50 ohms NOMINAL" for the feedpoint impedance. "Nominally" they are "50 ohms" which is to say, the antenna was designed for use in a 50 ohm system for best performance and will yield that approximate impedance at the center point of its frequency range with no significant reactive components; outside of that all bets are off. Performance wise, within the designed limits you won't experience enough degradation on the edges of the spec'ed frequency range to be an issue, especially for receive only use; don't worry about that reactive component that much.

Most "nominally 50 ohm" receivers won't hardly notice a problem with looking at a 75 ohm source and vice-versa. In fact, if you look at a wideband receiver's input with a calibrated network analyzer across the entire range of its coverage you probably won't see a nice resistive 50 ohm flat line (or "spread" on a Smith Chart, etc.)! About the only time you would see that in common practice is if the front end had a resistive attenuator on its input - that's actually one reason in test scenarios it is common to stick attenuators on inputs and outputs of things being tested since the test equipment is calibrated for 50 ohms! The main deal here is what was the receiver designed to best work with in terms of expected source impedance - usually 50 to 75 ohms. If you're in that range or close to it and you can keep the reactive component down from completely nuts across the range you are most interested in then you're essentially good to go. Don't sweat it further.

-Mike
 
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